diff --git a/include/avian/codegen/architecture.h b/include/avian/codegen/architecture.h
index b1cd7e8013..47687aefaf 100644
--- a/include/avian/codegen/architecture.h
+++ b/include/avian/codegen/architecture.h
@@ -12,6 +12,7 @@
 #define AVIAN_CODEGEN_ARCHITECTURE_H
 
 #include "ir.h"
+#include "registers.h"
 
 namespace vm {
 class Zone;
@@ -32,16 +33,26 @@ class RegisterFile;
 class OperandMask {
  public:
   uint8_t typeMask;
-  uint64_t registerMask;
+  RegisterMask lowRegisterMask;
+  RegisterMask highRegisterMask;
 
-  OperandMask(uint8_t typeMask, uint64_t registerMask)
-      : typeMask(typeMask), registerMask(registerMask)
+  OperandMask(uint8_t typeMask,
+              RegisterMask lowRegisterMask,
+              RegisterMask highRegisterMask)
+      : typeMask(typeMask),
+        lowRegisterMask(lowRegisterMask),
+        highRegisterMask(highRegisterMask)
   {
   }
 
-  OperandMask() : typeMask(~0), registerMask(~static_cast<uint64_t>(0))
+  OperandMask() : typeMask(~0), lowRegisterMask(AnyRegisterMask), highRegisterMask(AnyRegisterMask)
   {
   }
+
+  void setLowHighRegisterMasks(RegisterMask lowRegisterMask, RegisterMask highRegisterMask) {
+    this->lowRegisterMask = lowRegisterMask;
+    this->highRegisterMask = highRegisterMask;
+  }
 };
 
 class Architecture {
@@ -50,13 +61,13 @@ class Architecture {
 
   virtual const RegisterFile* registerFile() = 0;
 
-  virtual int scratch() = 0;
-  virtual int stack() = 0;
-  virtual int thread() = 0;
-  virtual int returnLow() = 0;
-  virtual int returnHigh() = 0;
-  virtual int virtualCallTarget() = 0;
-  virtual int virtualCallIndex() = 0;
+  virtual Register scratch() = 0;
+  virtual Register stack() = 0;
+  virtual Register thread() = 0;
+  virtual Register returnLow() = 0;
+  virtual Register returnHigh() = 0;
+  virtual Register virtualCallTarget() = 0;
+  virtual Register virtualCallIndex() = 0;
 
   virtual ir::TargetInfo targetInfo() = 0;
 
@@ -67,14 +78,14 @@ class Architecture {
   virtual bool alwaysCondensed(lir::BinaryOperation op) = 0;
   virtual bool alwaysCondensed(lir::TernaryOperation op) = 0;
 
-  virtual bool reserved(int register_) = 0;
+  virtual bool reserved(Register register_) = 0;
 
   virtual unsigned frameFootprint(unsigned footprint) = 0;
   virtual unsigned argumentFootprint(unsigned footprint) = 0;
   virtual bool argumentAlignment() = 0;
   virtual bool argumentRegisterAlignment() = 0;
   virtual unsigned argumentRegisterCount() = 0;
-  virtual int argumentRegister(unsigned index) = 0;
+  virtual Register argumentRegister(unsigned index) = 0;
 
   virtual bool hasLinkRegister() = 0;
 
diff --git a/include/avian/codegen/assembler.h b/include/avian/codegen/assembler.h
index 66e7ed53d6..e1d451a8a9 100644
--- a/include/avian/codegen/assembler.h
+++ b/include/avian/codegen/assembler.h
@@ -15,6 +15,7 @@
 #include "avian/zone.h"
 
 #include <avian/codegen/lir.h>
+#include <avian/codegen/registers.h>
 #include <avian/codegen/promise.h>
 
 namespace avian {
@@ -25,11 +26,11 @@ class Architecture;
 class OperandInfo {
  public:
   const unsigned size;
-  const lir::OperandType type;
+  const lir::Operand::Type type;
   lir::Operand* const operand;
 
   inline OperandInfo(unsigned size,
-                     lir::OperandType type,
+                     lir::Operand::Type type,
                      lir::Operand* operand)
       : size(size), type(type), operand(operand)
   {
@@ -52,10 +53,10 @@ class Assembler {
  public:
   class Client {
    public:
-    virtual int acquireTemporary(uint32_t mask = ~static_cast<uint32_t>(0)) = 0;
-    virtual void releaseTemporary(int r) = 0;
+    virtual Register acquireTemporary(RegisterMask mask = AnyRegisterMask) = 0;
+    virtual void releaseTemporary(Register r) = 0;
 
-    virtual void save(int r) = 0;
+    virtual void save(Register r) = 0;
   };
 
   class Block {
@@ -76,8 +77,8 @@ class Assembler {
   virtual void popFrame(unsigned footprint) = 0;
   virtual void popFrameForTailCall(unsigned footprint,
                                    int offset,
-                                   int returnAddressSurrogate,
-                                   int framePointerSurrogate) = 0;
+                                   Register returnAddressSurrogate,
+                                   Register framePointerSurrogate) = 0;
   virtual void popFrameAndPopArgumentsAndReturn(unsigned frameFootprint,
                                                 unsigned argumentFootprint) = 0;
   virtual void popFrameAndUpdateStackAndReturn(unsigned frameFootprint,
diff --git a/include/avian/codegen/lir.h b/include/avian/codegen/lir.h
index 251e31aadd..de90239b45 100644
--- a/include/avian/codegen/lir.h
+++ b/include/avian/codegen/lir.h
@@ -11,6 +11,8 @@
 #ifndef AVIAN_CODEGEN_LIR_H
 #define AVIAN_CODEGEN_LIR_H
 
+#include <avian/codegen/registers.h>
+
 namespace avian {
 namespace codegen {
 class Promise;
@@ -79,19 +81,8 @@ const unsigned NonBranchTernaryOperationCount = FloatMin + 1;
 const unsigned BranchOperationCount = JumpIfFloatGreaterOrEqualOrUnordered
                                       - FloatMin;
 
-enum OperandType {
-  ConstantOperand,
-  AddressOperand,
-  RegisterOperand,
-  MemoryOperand
-};
-
 enum ValueType { ValueGeneral, ValueFloat };
 
-const unsigned OperandTypeCount = MemoryOperand + 1;
-
-const int NoRegister = -1;
-
 inline bool isBranch(lir::TernaryOperation op)
 {
   return op > FloatMin;
@@ -128,6 +119,21 @@ inline bool isFloatUnaryOp(lir::BinaryOperation op)
 }
 
 class Operand {
+public:
+
+  enum class Type {
+    Constant,
+    Address,
+    RegisterPair,
+    Memory
+  };
+
+  const static unsigned TypeCount = (unsigned)Type::Memory + 1;
+
+  const static unsigned ConstantMask = 1 << (unsigned)Type::Constant;
+  const static unsigned AddressMask = 1 << (unsigned)Type::Address;
+  const static unsigned RegisterPairMask = 1 << (unsigned)Type::RegisterPair;
+  const static unsigned MemoryMask = 1 << (unsigned)Type::Memory;
 };
 
 class Constant : public Operand {
@@ -148,26 +154,26 @@ class Address : public Operand {
   Promise* address;
 };
 
-class Register : public Operand {
+class RegisterPair : public Operand {
  public:
-  Register(int low, int high = NoRegister) : low(low), high(high)
+  RegisterPair(Register low, Register high = NoRegister) : low(low), high(high)
   {
   }
 
-  int low;
-  int high;
+  Register low;
+  Register high;
 };
 
 class Memory : public Operand {
  public:
-  Memory(int base, int offset, int index = NoRegister, unsigned scale = 1)
+  Memory(Register base, int offset, Register index = NoRegister, unsigned scale = 1)
       : base(base), offset(offset), index(index), scale(scale)
   {
   }
 
-  int base;
+  Register base;
   int offset;
-  int index;
+  Register index;
   unsigned scale;
 };
 
diff --git a/include/avian/codegen/registers.h b/include/avian/codegen/registers.h
index f152084605..366fbde2f2 100644
--- a/include/avian/codegen/registers.h
+++ b/include/avian/codegen/registers.h
@@ -16,28 +16,189 @@
 namespace avian {
 namespace codegen {
 
-class RegisterMask {
- public:
-  uint32_t mask;
-  uint8_t start;
-  uint8_t limit;
+class RegisterMask;
 
-  static unsigned maskStart(uint32_t mask);
-  static unsigned maskLimit(uint32_t mask);
+class Register {
+private:
+  int8_t _index;
+public:
+  explicit constexpr Register(int8_t _index) : _index(_index) {}
+  constexpr Register() : _index(-1) {}
 
-  inline RegisterMask(uint32_t mask)
-      : mask(mask), start(maskStart(mask)), limit(maskLimit(mask))
-  {
+  constexpr bool operator == (Register o) const {
+    return _index == o._index;
+  }
+
+  constexpr bool operator != (Register o) const {
+    return !(*this == o);
+  }
+
+  constexpr RegisterMask operator | (Register o) const;
+
+  constexpr bool operator < (Register o) const {
+    return _index < o._index;
+  }
+
+  constexpr bool operator > (Register o) const {
+    return _index > o._index;
+  }
+
+  constexpr bool operator <= (Register o) const {
+    return _index <= o._index;
+  }
+
+  constexpr bool operator >= (Register o) const {
+    return _index >= o._index;
+  }
+
+  constexpr int index() const {
+    return _index;
   }
 };
 
+constexpr Register NoRegister;
+
+class RegisterMask {
+private:
+  uint64_t mask;
+
+  static constexpr unsigned maskStart(uint64_t mask, unsigned offset = 64) {
+    return mask == 0 ? (offset & 63) : maskStart(mask << 1, offset - 1);
+  }
+
+  static constexpr unsigned maskLimit(uint64_t mask, unsigned offset = 0) {
+    return mask == 0 ? offset : maskLimit(mask >> 1, offset + 1);
+  }
+public:
+  constexpr RegisterMask(uint64_t mask) : mask(mask) {}
+  constexpr RegisterMask() : mask(0) {}
+  constexpr RegisterMask(Register reg) : mask(static_cast<uint64_t>(1) << reg.index()) {}
+
+  constexpr unsigned begin() const {
+    return maskStart(mask);
+  }
+
+  constexpr unsigned end() const {
+    return maskLimit(mask);
+  }
+
+  constexpr RegisterMask operator &(RegisterMask o) const {
+    return RegisterMask(mask & o.mask);
+  }
+
+  RegisterMask operator &=(RegisterMask o) {
+    mask &= o.mask;
+    return *this;
+  }
+
+  constexpr RegisterMask operator |(RegisterMask o) const {
+    return RegisterMask(mask | o.mask);
+  }
+
+  constexpr bool contains(Register reg) const {
+    return (mask & (static_cast<uint64_t>(1) << reg.index())) != 0;
+  }
+
+  constexpr bool containsExactly(Register reg) const {
+    return mask == (mask & (static_cast<uint64_t>(1) << reg.index()));
+  }
+
+  constexpr RegisterMask excluding(Register reg) const {
+    return RegisterMask(mask & ~(static_cast<uint64_t>(1) << reg.index()));
+  }
+
+  constexpr RegisterMask including(Register reg) const {
+    return RegisterMask(mask | (static_cast<uint64_t>(1) << reg.index()));
+  }
+
+  constexpr explicit operator uint64_t() const {
+    return mask;
+  }
+
+  constexpr explicit operator bool() const {
+    return mask != 0;
+  }
+};
+
+constexpr RegisterMask AnyRegisterMask(~static_cast<uint64_t>(0));
+constexpr RegisterMask NoneRegisterMask(0);
+
+constexpr RegisterMask Register::operator | (Register o) const {
+  return RegisterMask(*this) | o;
+}
+
+class RegisterIterator;
+
+class BoundedRegisterMask : public RegisterMask {
+ public:
+  uint8_t start;
+  uint8_t limit;
+
+  BoundedRegisterMask(RegisterMask mask)
+      : RegisterMask(mask), start(mask.begin()), limit(mask.end())
+  {
+  }
+
+  RegisterIterator begin() const;
+
+  RegisterIterator end() const;
+};
+
+class RegisterIterator {
+ public:
+  int index;
+  int direction;
+  int limit;
+  const RegisterMask mask;
+
+  RegisterIterator(int index, int direction, int limit, RegisterMask mask)
+      : index(index), direction(direction), limit(limit), mask(mask)
+  {
+  }
+
+  bool operator !=(const RegisterIterator& o) const {
+    return index != o.index;
+  }
+
+  Register operator *() {
+    return Register(index);
+  }
+
+  void operator ++ () {
+    if(index != limit) {
+      index += direction;
+    }
+    while(index != limit && !mask.contains(Register(index))) {
+      index += direction;
+    }
+  }
+};
+
+inline RegisterIterator BoundedRegisterMask::begin() const {
+  // We use reverse iteration... for some reason.
+  return RegisterIterator(limit - 1, -1, start - 1, *this);
+}
+
+inline RegisterIterator BoundedRegisterMask::end() const {
+  // We use reverse iteration... for some reason.
+  return RegisterIterator(start - 1, -1, start - 1, *this);
+}
+
+inline RegisterIterator begin(BoundedRegisterMask mask) {
+  return mask.begin();
+}
+
+inline RegisterIterator end(BoundedRegisterMask mask) {
+  return mask.end();
+}
+
 class RegisterFile {
  public:
-  RegisterMask allRegisters;
-  RegisterMask generalRegisters;
-  RegisterMask floatRegisters;
+  BoundedRegisterMask allRegisters;
+  BoundedRegisterMask generalRegisters;
+  BoundedRegisterMask floatRegisters;
 
-  inline RegisterFile(uint32_t generalRegisterMask, uint32_t floatRegisterMask)
+  RegisterFile(RegisterMask generalRegisterMask, RegisterMask floatRegisterMask)
       : allRegisters(generalRegisterMask | floatRegisterMask),
         generalRegisters(generalRegisterMask),
         floatRegisters(floatRegisterMask)
@@ -45,31 +206,6 @@ class RegisterFile {
   }
 };
 
-class RegisterIterator {
- public:
-  int index;
-  const RegisterMask& mask;
-
-  inline RegisterIterator(const RegisterMask& mask)
-      : index(mask.start), mask(mask)
-  {
-  }
-
-  inline bool hasNext()
-  {
-    return index < mask.limit;
-  }
-
-  inline int next()
-  {
-    int r = index;
-    do {
-      index++;
-    } while (index < mask.limit && !(mask.mask & (1 << index)));
-    return r;
-  }
-};
-
 }  // namespace codegen
 }  // namespace avian
 
diff --git a/makefile b/makefile
index cbaa4ec42a..73efef11ca 100755
--- a/makefile
+++ b/makefile
@@ -1223,7 +1223,6 @@ compiler-sources = \
 	$(src)/codegen/compiler.cpp \
 	$(wildcard $(src)/codegen/compiler/*.cpp) \
 	$(src)/debug-util.cpp \
-	$(src)/codegen/registers.cpp \
 	$(src)/codegen/runtime.cpp \
 	$(src)/codegen/targets.cpp \
 	$(src)/util/fixed-allocator.cpp
diff --git a/src/codegen/CMakeLists.txt b/src/codegen/CMakeLists.txt
index 8a0b4baaf2..001a8681d3 100644
--- a/src/codegen/CMakeLists.txt
+++ b/src/codegen/CMakeLists.txt
@@ -1,6 +1,5 @@
 add_library (avian_codegen
   compiler.cpp
-  registers.cpp
   runtime.cpp
   targets.cpp
 
diff --git a/src/codegen/compiler.cpp b/src/codegen/compiler.cpp
index fbb64bf923..389d57c3c4 100644
--- a/src/codegen/compiler.cpp
+++ b/src/codegen/compiler.cpp
@@ -256,10 +256,10 @@ Site* pickTargetSite(Context* c,
 
   expect(c, target.cost < Target::Impossible);
 
-  if (target.type == lir::MemoryOperand) {
+  if (target.type == lir::Operand::Type::Memory) {
     return frameSite(c, target.index);
   } else {
-    return registerSite(c, target.index);
+    return registerSite(c, Register(target.index));
   }
 }
 
@@ -342,7 +342,7 @@ Site* maybeMove(Context* c,
       OperandMask src;
       OperandMask tmp;
       c->arch->planMove(
-          size, src, tmp, OperandMask(dstMask.typeMask, dstMask.registerMask));
+          size, src, tmp, OperandMask(dstMask.typeMask, dstMask.registerMask, 0));
 
       SiteMask srcMask = SiteMask::lowPart(src);
       for (SiteIterator it(c, value, true, includeNextWord); it.hasMore();) {
@@ -369,7 +369,7 @@ Site* maybeMove(Context* c,
       size,
       src,
       tmp,
-      OperandMask(1 << dstSite->type(c), dstSite->registerMask(c)));
+      OperandMask(1 << (unsigned)dstSite->type(c), dstSite->registerMask(c), 0));
 
   SiteMask srcMask = SiteMask::lowPart(src);
   unsigned cost = 0xFFFFFFFF;
@@ -514,15 +514,15 @@ void steal(Context* c, Resource* r, Value* thief)
 
 SiteMask generalRegisterMask(Context* c)
 {
-  return SiteMask(1 << lir::RegisterOperand,
-                  c->regFile->generalRegisters.mask,
+  return SiteMask(lir::Operand::RegisterPairMask,
+                  c->regFile->generalRegisters,
                   NoFrameIndex);
 }
 
 SiteMask generalRegisterOrConstantMask(Context* c)
 {
-  return SiteMask((1 << lir::RegisterOperand) | (1 << lir::ConstantOperand),
-                  c->regFile->generalRegisters.mask,
+  return SiteMask(lir::Operand::RegisterPairMask | lir::Operand::ConstantMask,
+                  c->regFile->generalRegisters,
                   NoFrameIndex);
 }
 
@@ -616,11 +616,11 @@ bool isHome(Value* v, int frameIndex)
 bool acceptForResolve(Context* c, Site* s, Read* read, const SiteMask& mask)
 {
   if (acceptMatch(c, s, read, mask) and (not s->frozen(c))) {
-    if (s->type(c) == lir::RegisterOperand) {
+    if (s->type(c) == lir::Operand::Type::RegisterPair) {
       return c->availableGeneralRegisterCount > ResolveRegisterReserveCount;
     } else {
       assertT(c,
-              s->match(c, SiteMask(1 << lir::MemoryOperand, 0, AnyFrameIndex)));
+              s->match(c, SiteMask(lir::Operand::MemoryMask, 0, AnyFrameIndex)));
 
       return isHome(read->value,
                     offsetToFrameIndex(c, static_cast<MemorySite*>(s)->offset));
@@ -698,7 +698,7 @@ void apply(Context* c,
 {
   assertT(c, s1Low->type(c) == s1High->type(c));
 
-  lir::OperandType s1Type = s1Low->type(c);
+  lir::Operand::Type s1Type = s1Low->type(c);
   OperandUnion s1Union;
   asAssemblerOperand(c, s1Low, s1High, &s1Union);
 
@@ -717,11 +717,11 @@ void apply(Context* c,
   assertT(c, s1Low->type(c) == s1High->type(c));
   assertT(c, s2Low->type(c) == s2High->type(c));
 
-  lir::OperandType s1Type = s1Low->type(c);
+  lir::Operand::Type s1Type = s1Low->type(c);
   OperandUnion s1Union;
   asAssemblerOperand(c, s1Low, s1High, &s1Union);
 
-  lir::OperandType s2Type = s2Low->type(c);
+  lir::Operand::Type s2Type = s2Low->type(c);
   OperandUnion s2Union;
   asAssemblerOperand(c, s2Low, s2High, &s2Union);
 
@@ -746,15 +746,15 @@ void apply(Context* c,
   assertT(c, s2Low->type(c) == s2High->type(c));
   assertT(c, s3Low->type(c) == s3High->type(c));
 
-  lir::OperandType s1Type = s1Low->type(c);
+  lir::Operand::Type s1Type = s1Low->type(c);
   OperandUnion s1Union;
   asAssemblerOperand(c, s1Low, s1High, &s1Union);
 
-  lir::OperandType s2Type = s2Low->type(c);
+  lir::Operand::Type s2Type = s2Low->type(c);
   OperandUnion s2Union;
   asAssemblerOperand(c, s2Low, s2High, &s2Union);
 
-  lir::OperandType s3Type = s3Low->type(c);
+  lir::Operand::Type s3Type = s3Low->type(c);
   OperandUnion s3Union;
   asAssemblerOperand(c, s3Low, s3High, &s3Union);
 
@@ -782,7 +782,7 @@ void saveLocals(Context* c, Event* e)
       e->addRead(
           c,
           local->value,
-          SiteMask(1 << lir::MemoryOperand, 0, compiler::frameIndex(c, li)));
+          SiteMask(lir::Operand::MemoryMask, 0, compiler::frameIndex(c, li)));
     }
   }
 }
@@ -815,10 +815,10 @@ void maybeMove(Context* c,
 
   if (cost) {
     // todo: let c->arch->planMove decide this:
-    bool useTemporary = ((target->type(c) == lir::MemoryOperand
-                          and srcValue->source->type(c) == lir::MemoryOperand)
+    bool useTemporary = ((target->type(c) == lir::Operand::Type::Memory
+                          and srcValue->source->type(c) == lir::Operand::Type::Memory)
                          or (srcSelectSize < dstSize
-                             and target->type(c) != lir::RegisterOperand));
+                             and target->type(c) != lir::Operand::Type::RegisterPair));
 
     srcValue->source->freeze(c, srcValue);
 
@@ -827,7 +827,7 @@ void maybeMove(Context* c,
     srcValue->source->thaw(c, srcValue);
 
     bool addOffset = srcSize != srcSelectSize and c->arch->bigEndian()
-                     and srcValue->source->type(c) == lir::MemoryOperand;
+                     and srcValue->source->type(c) == lir::Operand::Type::Memory;
 
     if (addOffset) {
       static_cast<MemorySite*>(srcValue->source)->offset
@@ -874,14 +874,14 @@ void maybeMove(Context* c,
       c->arch->planSource(op, dstSize, src, dstSize, &thunk);
 
       if (isGeneralValue(srcValue)) {
-        src.registerMask &= c->regFile->generalRegisters.mask;
+        src.lowRegisterMask &= c->regFile->generalRegisters;
       }
 
       assertT(c, thunk == 0);
-      assertT(c, dstMask.typeMask & src.typeMask & (1 << lir::RegisterOperand));
+      assertT(c, dstMask.typeMask & src.typeMask & lir::Operand::RegisterPairMask);
 
       Site* tmpTarget
-          = freeRegisterSite(c, dstMask.registerMask & src.registerMask);
+          = freeRegisterSite(c, dstMask.registerMask & src.lowRegisterMask);
 
       srcValue->source->freeze(c, srcValue);
 
@@ -1635,8 +1635,8 @@ bool resolveSourceSites(Context* c,
     Read* r = live(c, v);
 
     if (r and sites[el.localIndex] == 0) {
-      SiteMask mask((1 << lir::RegisterOperand) | (1 << lir::MemoryOperand),
-                    c->regFile->generalRegisters.mask,
+      SiteMask mask(lir::Operand::RegisterPairMask | lir::Operand::MemoryMask,
+                    c->regFile->generalRegisters,
                     AnyFrameIndex);
 
       Site* s = pickSourceSite(
@@ -1677,8 +1677,8 @@ void resolveTargetSites(Context* c,
     Read* r = live(c, v);
 
     if (r and sites[el.localIndex] == 0) {
-      SiteMask mask((1 << lir::RegisterOperand) | (1 << lir::MemoryOperand),
-                    c->regFile->generalRegisters.mask,
+      SiteMask mask(lir::Operand::RegisterPairMask | lir::Operand::MemoryMask,
+                    c->regFile->generalRegisters,
                     AnyFrameIndex);
 
       Site* s = pickSourceSite(
@@ -2210,24 +2210,24 @@ class Client : public Assembler::Client {
   {
   }
 
-  virtual int acquireTemporary(uint32_t mask)
+  virtual Register acquireTemporary(RegisterMask mask)
   {
     unsigned cost;
-    int r = pickRegisterTarget(c, 0, mask, &cost);
+    Register r = pickRegisterTarget(c, 0, mask, &cost);
     expect(c, cost < Target::Impossible);
     save(r);
-    c->registerResources[r].increment(c);
+    c->registerResources[r.index()].increment(c);
     return r;
   }
 
-  virtual void releaseTemporary(int r)
+  virtual void releaseTemporary(Register r)
   {
-    c->registerResources[r].decrement(c);
+    c->registerResources[r.index()].decrement(c);
   }
 
-  virtual void save(int r)
+  virtual void save(Register r)
   {
-    RegisterResource* reg = c->registerResources + r;
+    RegisterResource* reg = c->registerResources + r.index();
 
     assertT(c, reg->referenceCount == 0);
     assertT(c, reg->freezeCount == 0);
diff --git a/src/codegen/compiler/context.cpp b/src/codegen/compiler/context.cpp
index c7b7a51741..d285f0c77f 100644
--- a/src/codegen/compiler/context.cpp
+++ b/src/codegen/compiler/context.cpp
@@ -53,19 +53,15 @@ Context::Context(vm::System* system,
                                     - regFile->generalRegisters.start),
       targetInfo(arch->targetInfo())
 {
-  for (unsigned i = regFile->generalRegisters.start;
-       i < regFile->generalRegisters.limit;
-       ++i) {
-    new (registerResources + i) RegisterResource(arch->reserved(i));
+  for (Register i : regFile->generalRegisters) {
+    new (registerResources + i.index()) RegisterResource(arch->reserved(i));
 
-    if (registerResources[i].reserved) {
+    if (registerResources[i.index()].reserved) {
       --availableGeneralRegisterCount;
     }
   }
-  for (unsigned i = regFile->floatRegisters.start;
-       i < regFile->floatRegisters.limit;
-       ++i) {
-    new (registerResources + i) RegisterResource(arch->reserved(i));
+  for (Register i : regFile->floatRegisters) {
+    new (registerResources + i.index()) RegisterResource(arch->reserved(i));
   }
 }
 
diff --git a/src/codegen/compiler/event.cpp b/src/codegen/compiler/event.cpp
index 7bb0257f3f..936c787e30 100644
--- a/src/codegen/compiler/event.cpp
+++ b/src/codegen/compiler/event.cpp
@@ -372,7 +372,7 @@ class CallEvent : public Event {
                                    ? arguments.count
                                    : 0)
   {
-    uint32_t registerMask = c->regFile->generalRegisters.mask;
+    RegisterMask registerMask = c->regFile->generalRegisters;
 
     if (callingConvention == ir::CallingConvention::Native) {
       assertT(c, (flags & Compiler::TailJump) == 0);
@@ -396,14 +396,14 @@ class CallEvent : public Event {
         SiteMask targetMask;
         if (index + (c->arch->argumentRegisterAlignment() ? footprint : 1)
             <= c->arch->argumentRegisterCount()) {
-          int number = c->arch->argumentRegister(index);
+          Register number = c->arch->argumentRegister(index);
 
           if (DebugReads) {
-            fprintf(stderr, "reg %d arg read %p\n", number, v);
+            fprintf(stderr, "reg %d arg read %p\n", number.index(), v);
           }
 
           targetMask = SiteMask::fixedRegisterMask(number);
-          registerMask &= ~(1 << number);
+          registerMask = registerMask.excluding(number);
         } else {
           if (index < c->arch->argumentRegisterCount()) {
             index = c->arch->argumentRegisterCount();
@@ -415,7 +415,7 @@ class CallEvent : public Event {
             fprintf(stderr, "stack %d arg read %p\n", frameIndex, v);
           }
 
-          targetMask = SiteMask(1 << lir::MemoryOperand, 0, frameIndex);
+          targetMask = SiteMask(lir::Operand::MemoryMask, 0, frameIndex);
         }
 
         this->addRead(c, v, targetMask);
@@ -445,7 +445,7 @@ class CallEvent : public Event {
       this->addRead(
           c,
           address,
-          SiteMask(op.typeMask, registerMask & op.registerMask, AnyFrameIndex));
+          SiteMask(op.typeMask, registerMask & op.lowRegisterMask, AnyFrameIndex));
     }
 
     Stack* stack = stackBefore;
@@ -512,7 +512,7 @@ class CallEvent : public Event {
             this->addRead(c, v, generalRegisterMask(c));
           } else {
             this->addRead(
-                c, v, SiteMask(1 << lir::MemoryOperand, 0, frameIndex));
+                c, v, SiteMask(lir::Operand::MemoryMask, 0, frameIndex));
           }
         }
       }
@@ -544,7 +544,7 @@ class CallEvent : public Event {
 
           this->addRead(c,
                         stack->value,
-                        SiteMask(1 << lir::MemoryOperand, 0, logicalIndex));
+                        SiteMask(lir::Operand::MemoryMask, 0, logicalIndex));
         }
 
         stack = stack->next;
@@ -581,29 +581,29 @@ class CallEvent : public Event {
       assertT(
           c,
           returnAddressSurrogate == 0
-          or returnAddressSurrogate->source->type(c) == lir::RegisterOperand);
+          or returnAddressSurrogate->source->type(c) == lir::Operand::Type::RegisterPair);
       assertT(
           c,
           framePointerSurrogate == 0
-          or framePointerSurrogate->source->type(c) == lir::RegisterOperand);
+          or framePointerSurrogate->source->type(c) == lir::Operand::Type::RegisterPair);
 
-      int ras;
+      Register ras;
       if (returnAddressSurrogate) {
         returnAddressSurrogate->source->freeze(c, returnAddressSurrogate);
 
         ras = static_cast<RegisterSite*>(returnAddressSurrogate->source)
                   ->number;
       } else {
-        ras = lir::NoRegister;
+        ras = NoRegister;
       }
 
-      int fps;
+      Register fps;
       if (framePointerSurrogate) {
         framePointerSurrogate->source->freeze(c, framePointerSurrogate);
 
         fps = static_cast<RegisterSite*>(framePointerSurrogate->source)->number;
       } else {
-        fps = lir::NoRegister;
+        fps = NoRegister;
       }
 
       int offset = static_cast<int>(footprint)
@@ -783,9 +783,9 @@ class MoveEvent : public Event {
         op,
         srcSelectSize,
         OperandMask(
-            1 << srcValue->source->type(c),
-            (static_cast<uint64_t>(srcValue->nextWord->source->registerMask(c))
-             << 32) | static_cast<uint64_t>(srcValue->source->registerMask(c))),
+            1 << (unsigned)srcValue->source->type(c),
+            srcValue->source->registerMask(c),
+            srcValue->nextWord->source->registerMask(c)),
         dstSize,
         dst);
 
@@ -866,7 +866,7 @@ class MoveEvent : public Event {
       assertT(c, srcSelectSize == c->targetInfo.pointerSize);
 
       if (dstValue->nextWord->target or live(c, dstValue->nextWord)) {
-        assertT(c, dstLowMask.typeMask & (1 << lir::RegisterOperand));
+        assertT(c, dstLowMask.typeMask & lir::Operand::RegisterPairMask);
 
         Site* low = freeRegisterSite(c, dstLowMask.registerMask);
 
@@ -897,7 +897,7 @@ class MoveEvent : public Event {
 
         srcValue->source->thaw(c, srcValue);
 
-        assertT(c, dstHighMask.typeMask & (1 << lir::RegisterOperand));
+        assertT(c, dstHighMask.typeMask & lir::Operand::RegisterPairMask);
 
         Site* high = freeRegisterSite(c, dstHighMask.registerMask);
 
@@ -1126,18 +1126,14 @@ class CombineEvent : public Event {
         op,
         firstValue->type.size(c->targetInfo),
         OperandMask(
-            1 << firstValue->source->type(c),
-            (static_cast<uint64_t>(
-                 firstValue->nextWord->source->registerMask(c))
-             << 32)
-            | static_cast<uint64_t>(firstValue->source->registerMask(c))),
+            1 << (unsigned)firstValue->source->type(c),
+            firstValue->source->registerMask(c),
+            firstValue->nextWord->source->registerMask(c)),
         secondValue->type.size(c->targetInfo),
         OperandMask(
-            1 << secondValue->source->type(c),
-            (static_cast<uint64_t>(
-                 secondValue->nextWord->source->registerMask(c))
-             << 32)
-            | static_cast<uint64_t>(secondValue->source->registerMask(c))),
+            1 << (unsigned)secondValue->source->type(c),
+            secondValue->source->registerMask(c),
+            secondValue->nextWord->source->registerMask(c)),
         resultValue->type.size(c->targetInfo),
         cMask);
 
@@ -1318,11 +1314,9 @@ class TranslateEvent : public Event {
         op,
         firstValue->type.size(c->targetInfo),
         OperandMask(
-            1 << firstValue->source->type(c),
-            (static_cast<uint64_t>(
-                 firstValue->nextWord->source->registerMask(c))
-             << 32)
-            | static_cast<uint64_t>(firstValue->source->registerMask(c))),
+            1 << (unsigned)firstValue->source->type(c),
+            firstValue->source->registerMask(c),
+            firstValue->nextWord->source->registerMask(c)),
         resultValue->type.size(c->targetInfo),
         bMask);
 
@@ -1457,7 +1451,7 @@ ConstantSite* findConstantSite(Context* c, Value* v)
 {
   for (SiteIterator it(c, v); it.hasMore();) {
     Site* s = it.next();
-    if (s->type(c) == lir::ConstantOperand) {
+    if (s->type(c) == lir::Operand::Type::Constant) {
       return static_cast<ConstantSite*>(s);
     }
   }
@@ -1467,7 +1461,7 @@ ConstantSite* findConstantSite(Context* c, Value* v)
 void moveIfConflict(Context* c, Value* v, MemorySite* s)
 {
   if (v->reads) {
-    SiteMask mask(1 << lir::RegisterOperand, ~0, AnyFrameIndex);
+    SiteMask mask(lir::Operand::RegisterPairMask, ~0, AnyFrameIndex);
     v->reads->intersect(&mask);
     if (s->conflicts(mask)) {
       maybeMove(c, v->reads, true, false);
@@ -1504,29 +1498,29 @@ class MemoryEvent : public Event {
 
   virtual void compile(Context* c)
   {
-    int indexRegister;
+    Register indexRegister;
     int displacement = this->displacement;
     unsigned scale = this->scale;
     if (index) {
       ConstantSite* constant = findConstantSite(c, index);
 
       if (constant) {
-        indexRegister = lir::NoRegister;
+        indexRegister = NoRegister;
         displacement += (constant->value->value() * scale);
         scale = 1;
       } else {
-        assertT(c, index->source->type(c) == lir::RegisterOperand);
+        assertT(c, index->source->type(c) == lir::Operand::Type::RegisterPair);
         indexRegister = static_cast<RegisterSite*>(index->source)->number;
       }
     } else {
-      indexRegister = lir::NoRegister;
+      indexRegister = NoRegister;
     }
-    assertT(c, base->source->type(c) == lir::RegisterOperand);
-    int baseRegister = static_cast<RegisterSite*>(base->source)->number;
+    assertT(c, base->source->type(c) == lir::Operand::Type::RegisterPair);
+    Register baseRegister = static_cast<RegisterSite*>(base->source)->number;
 
     popRead(c, this, base);
     if (index) {
-      if (c->targetInfo.pointerSize == 8 and indexRegister != lir::NoRegister) {
+      if (c->targetInfo.pointerSize == 8 and indexRegister != NoRegister) {
         apply(c,
               lir::Move,
               4,
@@ -1718,9 +1712,9 @@ class BranchEvent : public Event {
     OperandMask dstMask;
     c->arch->planDestination(op,
                              firstValue->type.size(c->targetInfo),
-                             OperandMask(0, 0),
+                             OperandMask(0, 0, 0),
                              firstValue->type.size(c->targetInfo),
-                             OperandMask(0, 0),
+                             OperandMask(0, 0, 0),
                              c->targetInfo.pointerSize,
                              dstMask);
 
@@ -1879,12 +1873,12 @@ void clean(Context* c, Value* v, unsigned popIndex)
 {
   for (SiteIterator it(c, v); it.hasMore();) {
     Site* s = it.next();
-    if (not(s->match(c, SiteMask(1 << lir::MemoryOperand, 0, AnyFrameIndex))
+    if (not(s->match(c, SiteMask(lir::Operand::MemoryMask, 0, AnyFrameIndex))
             and offsetToFrameIndex(c, static_cast<MemorySite*>(s)->offset)
                 >= popIndex)) {
       if (false
           and s->match(c,
-                       SiteMask(1 << lir::MemoryOperand, 0, AnyFrameIndex))) {
+                       SiteMask(lir::Operand::MemoryMask, 0, AnyFrameIndex))) {
         char buffer[256];
         s->toString(c, buffer, 256);
         fprintf(stderr,
@@ -2016,7 +2010,7 @@ class BoundsCheckEvent : public Event {
         lir::Constant handlerConstant(resolvedPromise(c, handler));
         a->apply(lir::Call,
                  OperandInfo(c->targetInfo.pointerSize,
-                             lir::ConstantOperand,
+                             lir::Operand::Type::Constant,
                              &handlerConstant));
       }
     } else {
@@ -2038,10 +2032,10 @@ class BoundsCheckEvent : public Event {
     }
 
     if (constant == 0 or constant->value->value() >= 0) {
-      assertT(c, object->source->type(c) == lir::RegisterOperand);
+      assertT(c, object->source->type(c) == lir::Operand::Type::RegisterPair);
       MemorySite length(static_cast<RegisterSite*>(object->source)->number,
                         lengthOffset,
-                        lir::NoRegister,
+                        NoRegister,
                         1);
       length.acquired = true;
 
@@ -2072,7 +2066,7 @@ class BoundsCheckEvent : public Event {
       lir::Constant handlerConstant(resolvedPromise(c, handler));
       a->apply(lir::Call,
                OperandInfo(c->targetInfo.pointerSize,
-                           lir::ConstantOperand,
+                           lir::Operand::Type::Constant,
                            &handlerConstant));
 
       nextPromise->offset = a->offset();
diff --git a/src/codegen/compiler/read.cpp b/src/codegen/compiler/read.cpp
index 0c7d09fd76..d734caf358 100644
--- a/src/codegen/compiler/read.cpp
+++ b/src/codegen/compiler/read.cpp
@@ -205,7 +205,7 @@ Read* StubRead::next(Context*)
 SingleRead* read(Context* c, const SiteMask& mask, Value* successor)
 {
   assertT(c,
-          (mask.typeMask != 1 << lir::MemoryOperand) or mask.frameIndex >= 0);
+          (mask.typeMask != lir::Operand::MemoryMask) or mask.frameIndex >= 0);
 
   return new (c->zone) SingleRead(mask, successor);
 }
diff --git a/src/codegen/compiler/regalloc.cpp b/src/codegen/compiler/regalloc.cpp
index 6f716df770..1aed5fda70 100644
--- a/src/codegen/compiler/regalloc.cpp
+++ b/src/codegen/compiler/regalloc.cpp
@@ -57,24 +57,24 @@ unsigned resourceCost(Context* c,
 }
 
 bool pickRegisterTarget(Context* c,
-                        int i,
+                        Register i,
                         Value* v,
-                        uint32_t mask,
-                        int* target,
+                        RegisterMask mask,
+                        Register* target,
                         unsigned* cost,
                         CostCalculator* costCalculator)
 {
-  if ((1 << i) & mask) {
-    RegisterResource* r = c->registerResources + i;
+  if (mask.contains(i)) {
+    RegisterResource* r = c->registerResources + i.index();
     unsigned myCost
         = resourceCost(
               c,
               v,
               r,
-              SiteMask(1 << lir::RegisterOperand, 1 << i, NoFrameIndex),
+              SiteMask(lir::Operand::RegisterPairMask, RegisterMask(i), NoFrameIndex),
               costCalculator) + Target::MinimumRegisterCost;
 
-    if ((static_cast<uint32_t>(1) << i) == mask) {
+    if (mask.containsExactly(i)) {
       *cost = myCost;
       return true;
     } else if (myCost < *cost) {
@@ -85,29 +85,25 @@ bool pickRegisterTarget(Context* c,
   return false;
 }
 
-int pickRegisterTarget(Context* c,
+Register pickRegisterTarget(Context* c,
                        Value* v,
-                       uint32_t mask,
+                       RegisterMask mask,
                        unsigned* cost,
                        CostCalculator* costCalculator)
 {
-  int target = lir::NoRegister;
+  Register target = NoRegister;
   *cost = Target::Impossible;
 
-  if (mask & c->regFile->generalRegisters.mask) {
-    for (int i = c->regFile->generalRegisters.limit - 1;
-         i >= c->regFile->generalRegisters.start;
-         --i) {
+  if (mask & c->regFile->generalRegisters) {
+    for (Register i : c->regFile->generalRegisters) {
       if (pickRegisterTarget(c, i, v, mask, &target, cost, costCalculator)) {
         return i;
       }
     }
   }
 
-  if (mask & c->regFile->floatRegisters.mask) {
-    for (int i = c->regFile->floatRegisters.start;
-         i < static_cast<int>(c->regFile->floatRegisters.limit);
-         ++i) {
+  if (mask & c->regFile->floatRegisters) {
+    for (Register i : c->regFile->floatRegisters) {
       if (pickRegisterTarget(c, i, v, mask, &target, cost, costCalculator)) {
         return i;
       }
@@ -119,12 +115,12 @@ int pickRegisterTarget(Context* c,
 
 Target pickRegisterTarget(Context* c,
                           Value* v,
-                          uint32_t mask,
+                          RegisterMask mask,
                           CostCalculator* costCalculator)
 {
   unsigned cost;
-  int number = pickRegisterTarget(c, v, mask, &cost, costCalculator);
-  return Target(number, lir::RegisterOperand, cost);
+  Register number = pickRegisterTarget(c, v, mask, &cost, costCalculator);
+  return Target(number, cost);
 }
 
 unsigned frameCost(Context* c,
@@ -135,7 +131,7 @@ unsigned frameCost(Context* c,
   return resourceCost(c,
                       v,
                       c->frameResources + frameIndex,
-                      SiteMask(1 << lir::MemoryOperand, 0, frameIndex),
+                      SiteMask(lir::Operand::MemoryMask, 0, frameIndex),
                       costCalculator) + Target::MinimumFrameCost;
 }
 
@@ -147,7 +143,7 @@ Target pickFrameTarget(Context* c, Value* v, CostCalculator* costCalculator)
   do {
     if (p->home >= 0) {
       Target mine(p->home,
-                  lir::MemoryOperand,
+                  lir::Operand::Type::Memory,
                   frameCost(c, v, p->home, costCalculator));
 
       if (mine.cost == Target::MinimumFrameCost) {
@@ -168,7 +164,7 @@ Target pickAnyFrameTarget(Context* c, Value* v, CostCalculator* costCalculator)
 
   unsigned count = totalFrameSize(c);
   for (unsigned i = 0; i < count; ++i) {
-    Target mine(i, lir::MemoryOperand, frameCost(c, v, i, costCalculator));
+    Target mine(i, lir::Operand::Type::Memory, frameCost(c, v, i, costCalculator));
     if (mine.cost == Target::MinimumFrameCost) {
       return mine;
     } else if (mine.cost < best.cost) {
@@ -186,7 +182,7 @@ Target pickTarget(Context* c,
                   Target best,
                   CostCalculator* costCalculator)
 {
-  if (mask.typeMask & (1 << lir::RegisterOperand)) {
+  if (mask.typeMask & lir::Operand::RegisterPairMask) {
     Target mine
         = pickRegisterTarget(c, value, mask.registerMask, costCalculator);
 
@@ -198,10 +194,10 @@ Target pickTarget(Context* c,
     }
   }
 
-  if (mask.typeMask & (1 << lir::MemoryOperand)) {
+  if (mask.typeMask & lir::Operand::MemoryMask) {
     if (mask.frameIndex >= 0) {
       Target mine(mask.frameIndex,
-                  lir::MemoryOperand,
+                  lir::Operand::Type::Memory,
                   frameCost(c, value, mask.frameIndex, costCalculator));
       if (mine.cost == Target::MinimumFrameCost) {
         return mine;
@@ -234,14 +230,14 @@ Target pickTarget(Context* c,
 
   Value* value = read->value;
 
-  uint32_t registerMask
-      = (isFloatValue(value) ? ~0 : c->regFile->generalRegisters.mask);
+  RegisterMask registerMask
+      = (isFloatValue(value) ? AnyRegisterMask : (RegisterMask)c->regFile->generalRegisters);
 
   SiteMask mask(~0, registerMask, AnyFrameIndex);
   read->intersect(&mask);
 
   if (isFloatValue(value)) {
-    uint32_t floatMask = mask.registerMask & c->regFile->floatRegisters.mask;
+    RegisterMask floatMask = mask.registerMask & c->regFile->floatRegisters;
     if (floatMask) {
       mask.registerMask = floatMask;
     }
@@ -273,9 +269,9 @@ Target pickTarget(Context* c,
   if (intersectRead) {
     if (best.cost == Target::Impossible) {
       fprintf(stderr,
-              "mask type %d reg %d frame %d\n",
+              "mask type %d reg %" LLD " frame %d\n",
               mask.typeMask,
-              mask.registerMask,
+              (uint64_t)mask.registerMask,
               mask.frameIndex);
       abort(c);
     }
diff --git a/src/codegen/compiler/regalloc.h b/src/codegen/compiler/regalloc.h
index 3c87b20c00..b9b76fac8d 100644
--- a/src/codegen/compiler/regalloc.h
+++ b/src/codegen/compiler/regalloc.h
@@ -55,13 +55,18 @@ class Target {
   {
   }
 
-  Target(int index, lir::OperandType type, unsigned cost)
+  Target(int16_t index, lir::Operand::Type type, unsigned cost)
       : index(index), type(type), cost(cost)
   {
   }
 
+  Target(Register reg, unsigned cost)
+      : index(reg.index()), type(lir::Operand::Type::RegisterPair), cost(cost)
+  {
+  }
+
   int16_t index;
-  lir::OperandType type;
+  lir::Operand::Type type;
   uint8_t cost;
 };
 
@@ -77,22 +82,22 @@ unsigned resourceCost(Context* c,
                       CostCalculator* costCalculator);
 
 bool pickRegisterTarget(Context* c,
-                        int i,
+                        Register i,
                         Value* v,
-                        uint32_t mask,
-                        int* target,
+                        RegisterMask mask,
+                        Register* target,
                         unsigned* cost,
                         CostCalculator* costCalculator = 0);
 
-int pickRegisterTarget(Context* c,
+Register pickRegisterTarget(Context* c,
                        Value* v,
-                       uint32_t mask,
+                       RegisterMask mask,
                        unsigned* cost,
                        CostCalculator* costCalculator = 0);
 
 Target pickRegisterTarget(Context* c,
                           Value* v,
-                          uint32_t mask,
+                          RegisterMask mask,
                           CostCalculator* costCalculator = 0);
 
 unsigned frameCost(Context* c,
diff --git a/src/codegen/compiler/resource.cpp b/src/codegen/compiler/resource.cpp
index f9a307c1dc..915da9ccff 100644
--- a/src/codegen/compiler/resource.cpp
+++ b/src/codegen/compiler/resource.cpp
@@ -88,7 +88,7 @@ void RegisterResource::freeze(Context* c, Value* v)
     freezeResource(c, this, v);
 
     if (freezeCount == 1
-        and ((1 << index(c)) & c->regFile->generalRegisters.mask)) {
+        and c->regFile->generalRegisters.contains(index(c))) {
       decrementAvailableGeneralRegisterCount(c);
     }
   }
@@ -100,7 +100,7 @@ void RegisterResource::thaw(Context* c, Value* v)
     thawResource(c, this, v);
 
     if (freezeCount == 0
-        and ((1 << index(c)) & c->regFile->generalRegisters.mask)) {
+        and c->regFile->generalRegisters.contains(index(c))) {
       incrementAvailableGeneralRegisterCount(c);
     }
   }
@@ -113,9 +113,9 @@ unsigned RegisterResource::toString(Context* c,
   return vm::snprintf(buffer, bufferSize, "register %d", index(c));
 }
 
-unsigned RegisterResource::index(Context* c)
+Register RegisterResource::index(Context* c)
 {
-  return this - c->registerResources;
+  return Register(this - c->registerResources);
 }
 
 void RegisterResource::increment(Context* c)
@@ -130,7 +130,7 @@ void RegisterResource::increment(Context* c)
     ++this->referenceCount;
 
     if (this->referenceCount == 1
-        and ((1 << this->index(c)) & c->regFile->generalRegisters.mask)) {
+        and c->regFile->generalRegisters.contains(this->index(c))) {
       decrementAvailableGeneralRegisterCount(c);
     }
   }
@@ -150,7 +150,7 @@ void RegisterResource::decrement(Context* c)
     --this->referenceCount;
 
     if (this->referenceCount == 0
-        and ((1 << this->index(c)) & c->regFile->generalRegisters.mask)) {
+        and c->regFile->generalRegisters.contains(this->index(c))) {
       incrementAvailableGeneralRegisterCount(c);
     }
   }
diff --git a/src/codegen/compiler/resource.h b/src/codegen/compiler/resource.h
index 1712871079..5c98c5ec46 100644
--- a/src/codegen/compiler/resource.h
+++ b/src/codegen/compiler/resource.h
@@ -48,7 +48,7 @@ class RegisterResource : public Resource {
 
   virtual unsigned toString(Context* c, char* buffer, unsigned bufferSize);
 
-  virtual unsigned index(Context*);
+  virtual Register index(Context*);
 
   void increment(Context*);
 
diff --git a/src/codegen/compiler/site.cpp b/src/codegen/compiler/site.cpp
index ea762a851f..745804fd65 100644
--- a/src/codegen/compiler/site.cpp
+++ b/src/codegen/compiler/site.cpp
@@ -152,7 +152,7 @@ class AddressSite : public Site {
 
   virtual bool match(Context*, const SiteMask& mask)
   {
-    return mask.typeMask & (1 << lir::AddressOperand);
+    return mask.typeMask & lir::Operand::AddressMask;
   }
 
   virtual bool loneMatch(Context*, const SiteMask&)
@@ -165,9 +165,9 @@ class AddressSite : public Site {
     abort(c);
   }
 
-  virtual lir::OperandType type(Context*)
+  virtual lir::Operand::Type type(Context*)
   {
-    return lir::AddressOperand;
+    return lir::Operand::Type::Address;
   }
 
   virtual void asAssemblerOperand(Context* c UNUSED,
@@ -201,7 +201,7 @@ class AddressSite : public Site {
 
   virtual SiteMask mask(Context*)
   {
-    return SiteMask(1 << lir::AddressOperand, 0, NoFrameIndex);
+    return SiteMask(lir::Operand::AddressMask, 0, NoFrameIndex);
   }
 
   virtual SiteMask nextWordMask(Context* c, unsigned)
@@ -217,14 +217,14 @@ Site* addressSite(Context* c, Promise* address)
   return new (c->zone) AddressSite(address);
 }
 
-RegisterSite::RegisterSite(uint32_t mask, int number)
+RegisterSite::RegisterSite(RegisterMask mask, Register number)
     : mask_(mask), number(number)
 {
 }
 
 unsigned RegisterSite::toString(Context*, char* buffer, unsigned bufferSize)
 {
-  if (number != lir::NoRegister) {
+  if (number != NoRegister) {
     return vm::snprintf(buffer, bufferSize, "%p register %d", this, number);
   } else {
     return vm::snprintf(
@@ -234,11 +234,11 @@ unsigned RegisterSite::toString(Context*, char* buffer, unsigned bufferSize)
 
 unsigned RegisterSite::copyCost(Context* c, Site* s)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
   if (s and (this == s
-             or (s->type(c) == lir::RegisterOperand
-                 and (static_cast<RegisterSite*>(s)->mask_ & (1 << number))))) {
+             or (s->type(c) == lir::Operand::Type::RegisterPair
+                 and (static_cast<RegisterSite*>(s)->mask_.contains(number))))) {
     return 0;
   } else {
     return RegisterCopyCost;
@@ -247,10 +247,10 @@ unsigned RegisterSite::copyCost(Context* c, Site* s)
 
 bool RegisterSite::match(Context* c UNUSED, const SiteMask& mask)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  if ((mask.typeMask & (1 << lir::RegisterOperand))) {
-    return ((static_cast<uint64_t>(1) << number) & mask.registerMask);
+  if ((mask.typeMask & lir::Operand::RegisterPairMask)) {
+    return mask.registerMask.contains(number);
   } else {
     return false;
   }
@@ -258,10 +258,10 @@ bool RegisterSite::match(Context* c UNUSED, const SiteMask& mask)
 
 bool RegisterSite::loneMatch(Context* c UNUSED, const SiteMask& mask)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  if ((mask.typeMask & (1 << lir::RegisterOperand))) {
-    return ((static_cast<uint64_t>(1) << number) == mask.registerMask);
+  if ((mask.typeMask & lir::Operand::RegisterPairMask)) {
+    return mask.registerMask.containsExactly(number);
   } else {
     return false;
   }
@@ -269,28 +269,28 @@ bool RegisterSite::loneMatch(Context* c UNUSED, const SiteMask& mask)
 
 bool RegisterSite::matchNextWord(Context* c, Site* s, unsigned)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  if (s->type(c) != lir::RegisterOperand) {
+  if (s->type(c) != lir::Operand::Type::RegisterPair) {
     return false;
   }
 
   RegisterSite* rs = static_cast<RegisterSite*>(s);
   unsigned size = rs->registerSize(c);
   if (size > c->targetInfo.pointerSize) {
-    assertT(c, number != lir::NoRegister);
+    assertT(c, number != NoRegister);
     return number == rs->number;
   } else {
-    uint32_t mask = c->regFile->generalRegisters.mask;
-    return ((1 << number) & mask) and ((1 << rs->number) & mask);
+    RegisterMask mask = c->regFile->generalRegisters;
+    return mask.contains(number) and mask.contains(rs->number);
   }
 }
 
 void RegisterSite::acquire(Context* c, Value* v)
 {
   Target target;
-  if (number != lir::NoRegister) {
-    target = Target(number, lir::RegisterOperand, 0);
+  if (number != NoRegister) {
+    target = Target(number, 0);
   } else {
     target = pickRegisterTarget(c, v, mask_);
     expect(c, target.cost < Target::Impossible);
@@ -299,65 +299,65 @@ void RegisterSite::acquire(Context* c, Value* v)
   RegisterResource* resource = c->registerResources + target.index;
   compiler::acquire(c, resource, v, this);
 
-  number = target.index;
+  number = Register(target.index);
 }
 
 void RegisterSite::release(Context* c, Value* v)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  compiler::release(c, c->registerResources + number, v, this);
+  compiler::release(c, c->registerResources + number.index(), v, this);
 }
 
 void RegisterSite::freeze(Context* c, Value* v)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  c->registerResources[number].freeze(c, v);
+  c->registerResources[number.index()].freeze(c, v);
 }
 
 void RegisterSite::thaw(Context* c, Value* v)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  c->registerResources[number].thaw(c, v);
+  c->registerResources[number.index()].thaw(c, v);
 }
 
 bool RegisterSite::frozen(Context* c UNUSED)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  return c->registerResources[number].freezeCount != 0;
+  return c->registerResources[number.index()].freezeCount != 0;
 }
 
-lir::OperandType RegisterSite::type(Context*)
+lir::Operand::Type RegisterSite::type(Context*)
 {
-  return lir::RegisterOperand;
+  return lir::Operand::Type::RegisterPair;
 }
 
 void RegisterSite::asAssemblerOperand(Context* c UNUSED,
                                       Site* high,
                                       lir::Operand* result)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  int highNumber;
+  Register highNumber;
   if (high != this) {
     highNumber = static_cast<RegisterSite*>(high)->number;
-    assertT(c, highNumber != lir::NoRegister);
+    assertT(c, highNumber != NoRegister);
   } else {
-    highNumber = lir::NoRegister;
+    highNumber = NoRegister;
   }
 
-  new (result) lir::Register(number, highNumber);
+  new (result) lir::RegisterPair(number, highNumber);
 }
 
 Site* RegisterSite::copy(Context* c)
 {
-  uint32_t mask;
+  RegisterMask mask;
 
-  if (number != lir::NoRegister) {
-    mask = 1 << number;
+  if (number != NoRegister) {
+    mask = RegisterMask(number);
   } else {
     mask = mask_;
   }
@@ -377,64 +377,64 @@ Site* RegisterSite::copyHigh(Context* c)
 
 Site* RegisterSite::makeNextWord(Context* c, unsigned)
 {
-  assertT(c, number != lir::NoRegister);
-  assertT(c, ((1 << number) & c->regFile->generalRegisters.mask));
+  assertT(c, number != NoRegister);
+  assertT(c, c->regFile->generalRegisters.contains(number));
 
-  return freeRegisterSite(c, c->regFile->generalRegisters.mask);
+  return freeRegisterSite(c, c->regFile->generalRegisters);
 }
 
 SiteMask RegisterSite::mask(Context* c UNUSED)
 {
-  return SiteMask(1 << lir::RegisterOperand, mask_, NoFrameIndex);
+  return SiteMask(lir::Operand::RegisterPairMask, mask_, NoFrameIndex);
 }
 
 SiteMask RegisterSite::nextWordMask(Context* c, unsigned)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
   if (registerSize(c) > c->targetInfo.pointerSize) {
-    return SiteMask(1 << lir::RegisterOperand, number, NoFrameIndex);
+    return SiteMask(lir::Operand::RegisterPairMask, number, NoFrameIndex);
   } else {
-    return SiteMask(1 << lir::RegisterOperand,
-                    c->regFile->generalRegisters.mask,
+    return SiteMask(lir::Operand::RegisterPairMask,
+                    c->regFile->generalRegisters,
                     NoFrameIndex);
   }
 }
 
 unsigned RegisterSite::registerSize(Context* c)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  if ((1 << number) & c->regFile->floatRegisters.mask) {
+  if (c->regFile->floatRegisters.contains(number)) {
     return c->arch->floatRegisterSize();
   } else {
     return c->targetInfo.pointerSize;
   }
 }
 
-unsigned RegisterSite::registerMask(Context* c UNUSED)
+RegisterMask RegisterSite::registerMask(Context* c UNUSED)
 {
-  assertT(c, number != lir::NoRegister);
+  assertT(c, number != NoRegister);
 
-  return 1 << number;
+  return RegisterMask(number);
 }
 
-Site* registerSite(Context* c, int number)
+Site* registerSite(Context* c, Register number)
 {
-  assertT(c, number >= 0);
+  assertT(c, number != NoRegister);
   assertT(c,
-          (1 << number) & (c->regFile->generalRegisters.mask
-                           | c->regFile->floatRegisters.mask));
+          (c->regFile->generalRegisters
+                           | c->regFile->floatRegisters).contains(number));
 
-  return new (c->zone) RegisterSite(1 << number, number);
+  return new (c->zone) RegisterSite(RegisterMask(number), number);
 }
 
-Site* freeRegisterSite(Context* c, uint32_t mask)
+Site* freeRegisterSite(Context* c, RegisterMask mask)
 {
-  return new (c->zone) RegisterSite(mask, lir::NoRegister);
+  return new (c->zone) RegisterSite(mask, NoRegister);
 }
 
-MemorySite::MemorySite(int base, int offset, int index, unsigned scale)
+MemorySite::MemorySite(Register base, int offset, Register index, unsigned scale)
     : acquired(false), base(base), offset(offset), index(index), scale(scale)
 {
 }
@@ -453,7 +453,7 @@ unsigned MemorySite::copyCost(Context* c, Site* s)
 {
   assertT(c, acquired);
 
-  if (s and (this == s or (s->type(c) == lir::MemoryOperand
+  if (s and (this == s or (s->type(c) == lir::Operand::Type::Memory
                            and static_cast<MemorySite*>(s)->base == base
                            and static_cast<MemorySite*>(s)->offset == offset
                            and static_cast<MemorySite*>(s)->index == index
@@ -466,20 +466,20 @@ unsigned MemorySite::copyCost(Context* c, Site* s)
 
 bool MemorySite::conflicts(const SiteMask& mask)
 {
-  return (mask.typeMask & (1 << lir::RegisterOperand)) != 0
-         and (((1 << base) & mask.registerMask) == 0
-              or (index != lir::NoRegister
-                  and ((1 << index) & mask.registerMask) == 0));
+  return (mask.typeMask & lir::Operand::RegisterPairMask) != 0
+         and (!mask.registerMask.contains(base)
+              or (index != NoRegister
+                  and !mask.registerMask.contains(index)));
 }
 
 bool MemorySite::match(Context* c, const SiteMask& mask)
 {
   assertT(c, acquired);
 
-  if (mask.typeMask & (1 << lir::MemoryOperand)) {
+  if (mask.typeMask & lir::Operand::MemoryMask) {
     if (mask.frameIndex >= 0) {
       if (base == c->arch->stack()) {
-        assertT(c, index == lir::NoRegister);
+        assertT(c, index == NoRegister);
         return static_cast<int>(frameIndexToOffset(c, mask.frameIndex))
                == offset;
       } else {
@@ -497,9 +497,9 @@ bool MemorySite::loneMatch(Context* c, const SiteMask& mask)
 {
   assertT(c, acquired);
 
-  if (mask.typeMask & (1 << lir::MemoryOperand)) {
+  if (mask.typeMask & lir::Operand::MemoryMask) {
     if (base == c->arch->stack()) {
-      assertT(c, index == lir::NoRegister);
+      assertT(c, index == NoRegister);
 
       if (mask.frameIndex == AnyFrameIndex) {
         return false;
@@ -513,7 +513,7 @@ bool MemorySite::loneMatch(Context* c, const SiteMask& mask)
 
 bool MemorySite::matchNextWord(Context* c, Site* s, unsigned index)
 {
-  if (s->type(c) == lir::MemoryOperand) {
+  if (s->type(c) == lir::Operand::Type::Memory) {
     MemorySite* ms = static_cast<MemorySite*>(s);
     return ms->base == this->base
            and ((index == 1
@@ -532,13 +532,13 @@ bool MemorySite::matchNextWord(Context* c, Site* s, unsigned index)
 
 void MemorySite::acquire(Context* c, Value* v)
 {
-  c->registerResources[base].increment(c);
-  if (index != lir::NoRegister) {
-    c->registerResources[index].increment(c);
+  c->registerResources[base.index()].increment(c);
+  if (index != NoRegister) {
+    c->registerResources[index.index()].increment(c);
   }
 
   if (base == c->arch->stack()) {
-    assertT(c, index == lir::NoRegister);
+    assertT(c, index == NoRegister);
     assertT(c, not c->frameResources[offsetToFrameIndex(c, offset)].reserved);
 
     compiler::acquire(
@@ -551,16 +551,16 @@ void MemorySite::acquire(Context* c, Value* v)
 void MemorySite::release(Context* c, Value* v)
 {
   if (base == c->arch->stack()) {
-    assertT(c, index == lir::NoRegister);
+    assertT(c, index == NoRegister);
     assertT(c, not c->frameResources[offsetToFrameIndex(c, offset)].reserved);
 
     compiler::release(
         c, c->frameResources + offsetToFrameIndex(c, offset), v, this);
   }
 
-  c->registerResources[base].decrement(c);
-  if (index != lir::NoRegister) {
-    c->registerResources[index].decrement(c);
+  c->registerResources[base.index()].decrement(c);
+  if (index != NoRegister) {
+    c->registerResources[index.index()].decrement(c);
   }
 
   acquired = false;
@@ -571,9 +571,9 @@ void MemorySite::freeze(Context* c, Value* v)
   if (base == c->arch->stack()) {
     c->frameResources[offsetToFrameIndex(c, offset)].freeze(c, v);
   } else {
-    c->registerResources[base].increment(c);
-    if (index != lir::NoRegister) {
-      c->registerResources[index].increment(c);
+    c->registerResources[base.index()].increment(c);
+    if (index != NoRegister) {
+      c->registerResources[index.index()].increment(c);
     }
   }
 }
@@ -583,9 +583,9 @@ void MemorySite::thaw(Context* c, Value* v)
   if (base == c->arch->stack()) {
     c->frameResources[offsetToFrameIndex(c, offset)].thaw(c, v);
   } else {
-    c->registerResources[base].decrement(c);
-    if (index != lir::NoRegister) {
-      c->registerResources[index].decrement(c);
+    c->registerResources[base.index()].decrement(c);
+    if (index != NoRegister) {
+      c->registerResources[index.index()].decrement(c);
     }
   }
 }
@@ -596,9 +596,9 @@ bool MemorySite::frozen(Context* c)
          and c->frameResources[offsetToFrameIndex(c, offset)].freezeCount != 0;
 }
 
-lir::OperandType MemorySite::type(Context*)
+lir::Operand::Type MemorySite::type(Context*)
 {
-  return lir::MemoryOperand;
+  return lir::Operand::Type::Memory;
 }
 
 void MemorySite::asAssemblerOperand(Context* c UNUSED,
@@ -657,7 +657,7 @@ Site* MemorySite::makeNextWord(Context* c, unsigned index)
 
 SiteMask MemorySite::mask(Context* c)
 {
-  return SiteMask(1 << lir::MemoryOperand,
+  return SiteMask(lir::Operand::MemoryMask,
                   0,
                   (base == c->arch->stack())
                       ? static_cast<int>(offsetToFrameIndex(c, offset))
@@ -668,13 +668,13 @@ SiteMask MemorySite::nextWordMask(Context* c, unsigned index)
 {
   int frameIndex;
   if (base == c->arch->stack()) {
-    assertT(c, this->index == lir::NoRegister);
+    assertT(c, this->index == NoRegister);
     frameIndex = static_cast<int>(offsetToFrameIndex(c, offset))
                  + ((index == 1) xor c->arch->bigEndian() ? 1 : -1);
   } else {
     frameIndex = NoFrameIndex;
   }
-  return SiteMask(1 << lir::MemoryOperand, 0, frameIndex);
+  return SiteMask(lir::Operand::MemoryMask, 0, frameIndex);
 }
 
 bool MemorySite::isVolatile(Context* c)
@@ -683,9 +683,9 @@ bool MemorySite::isVolatile(Context* c)
 }
 
 MemorySite* memorySite(Context* c,
-                       int base,
+                       Register base,
                        int offset,
-                       int index,
+                       Register index,
                        unsigned scale)
 {
   return new (c->zone) MemorySite(base, offset, index, scale);
@@ -697,7 +697,7 @@ MemorySite* frameSite(Context* c, int frameIndex)
   return memorySite(c,
                     c->arch->stack(),
                     frameIndexToOffset(c, frameIndex),
-                    lir::NoRegister,
+                    NoRegister,
                     0);
 }
 
diff --git a/src/codegen/compiler/site.h b/src/codegen/compiler/site.h
index 3f93d1f5dd..b2c10ddc39 100644
--- a/src/codegen/compiler/site.h
+++ b/src/codegen/compiler/site.h
@@ -34,30 +34,30 @@ class SiteMask {
   {
   }
 
-  SiteMask(uint8_t typeMask, uint32_t registerMask, int frameIndex)
+  SiteMask(uint8_t typeMask, RegisterMask registerMask, int frameIndex)
       : typeMask(typeMask), registerMask(registerMask), frameIndex(frameIndex)
   {
   }
 
   SiteMask intersectionWith(const SiteMask& b);
 
-  static SiteMask fixedRegisterMask(int number)
+  static SiteMask fixedRegisterMask(Register number)
   {
-    return SiteMask(1 << lir::RegisterOperand, 1 << number, NoFrameIndex);
+    return SiteMask(lir::Operand::RegisterPairMask, 1 << number.index(), NoFrameIndex);
   }
 
   static SiteMask lowPart(const OperandMask& mask)
   {
-    return SiteMask(mask.typeMask, mask.registerMask, AnyFrameIndex);
+    return SiteMask(mask.typeMask, mask.lowRegisterMask, AnyFrameIndex);
   }
 
   static SiteMask highPart(const OperandMask& mask)
   {
-    return SiteMask(mask.typeMask, mask.registerMask >> 32, AnyFrameIndex);
+    return SiteMask(mask.typeMask, mask.highRegisterMask, AnyFrameIndex);
   }
 
   uint8_t typeMask;
-  uint32_t registerMask;
+  RegisterMask registerMask;
   int frameIndex;
 };
 
@@ -103,7 +103,7 @@ class Site {
     return false;
   }
 
-  virtual lir::OperandType type(Context*) = 0;
+  virtual lir::Operand::Type type(Context*) = 0;
 
   virtual void asAssemblerOperand(Context*, Site*, lir::Operand*) = 0;
 
@@ -121,7 +121,7 @@ class Site {
 
   virtual unsigned registerSize(Context*);
 
-  virtual unsigned registerMask(Context*)
+  virtual RegisterMask registerMask(Context*)
   {
     return 0;
   }
@@ -187,7 +187,7 @@ class ConstantSite : public Site {
 
   virtual bool match(Context*, const SiteMask& mask)
   {
-    return mask.typeMask & (1 << lir::ConstantOperand);
+    return mask.typeMask & lir::Operand::ConstantMask;
   }
 
   virtual bool loneMatch(Context*, const SiteMask&)
@@ -197,12 +197,12 @@ class ConstantSite : public Site {
 
   virtual bool matchNextWord(Context* c, Site* s, unsigned)
   {
-    return s->type(c) == lir::ConstantOperand;
+    return s->type(c) == lir::Operand::Type::Constant;
   }
 
-  virtual lir::OperandType type(Context*)
+  virtual lir::Operand::Type type(Context*)
   {
-    return lir::ConstantOperand;
+    return lir::Operand::Type::Constant;
   }
 
   virtual void asAssemblerOperand(Context* c, Site* high, lir::Operand* result)
@@ -236,12 +236,12 @@ class ConstantSite : public Site {
 
   virtual SiteMask mask(Context*)
   {
-    return SiteMask(1 << lir::ConstantOperand, 0, NoFrameIndex);
+    return SiteMask(lir::Operand::ConstantMask, 0, NoFrameIndex);
   }
 
   virtual SiteMask nextWordMask(Context*, unsigned)
   {
-    return SiteMask(1 << lir::ConstantOperand, 0, NoFrameIndex);
+    return SiteMask(lir::Operand::ConstantMask, 0, NoFrameIndex);
   }
 
   Promise* value;
@@ -251,7 +251,7 @@ Site* addressSite(Context* c, Promise* address);
 
 class RegisterSite : public Site {
  public:
-  RegisterSite(uint32_t mask, int number);
+  RegisterSite(RegisterMask mask, Register number);
 
   virtual unsigned toString(Context*, char* buffer, unsigned bufferSize);
 
@@ -273,7 +273,7 @@ class RegisterSite : public Site {
 
   virtual bool frozen(Context* c UNUSED);
 
-  virtual lir::OperandType type(Context*);
+  virtual lir::Operand::Type type(Context*);
 
   virtual void asAssemblerOperand(Context* c UNUSED,
                                   Site* high,
@@ -293,18 +293,18 @@ class RegisterSite : public Site {
 
   virtual unsigned registerSize(Context* c);
 
-  virtual unsigned registerMask(Context* c UNUSED);
+  virtual RegisterMask registerMask(Context* c UNUSED);
 
-  uint32_t mask_;
-  int number;
+  RegisterMask mask_;
+  Register number;
 };
 
-Site* registerSite(Context* c, int number);
-Site* freeRegisterSite(Context* c, uint32_t mask);
+Site* registerSite(Context* c, Register number);
+Site* freeRegisterSite(Context* c, RegisterMask mask);
 
 class MemorySite : public Site {
  public:
-  MemorySite(int base, int offset, int index, unsigned scale);
+  MemorySite(Register base, int offset, Register index, unsigned scale);
 
   virtual unsigned toString(Context*, char* buffer, unsigned bufferSize);
 
@@ -328,7 +328,7 @@ class MemorySite : public Site {
 
   virtual bool frozen(Context* c);
 
-  virtual lir::OperandType type(Context*);
+  virtual lir::Operand::Type type(Context*);
 
   virtual void asAssemblerOperand(Context* c UNUSED,
                                   Site* high UNUSED,
@@ -351,16 +351,16 @@ class MemorySite : public Site {
   virtual bool isVolatile(Context* c);
 
   bool acquired;
-  int base;
+  Register base;
   int offset;
-  int index;
+  Register index;
   unsigned scale;
 };
 
 MemorySite* memorySite(Context* c,
-                       int base,
+                       Register base,
                        int offset = 0,
-                       int index = lir::NoRegister,
+                       Register index = NoRegister,
                        unsigned scale = 1);
 MemorySite* frameSite(Context* c, int frameIndex);
 
diff --git a/src/codegen/registers.cpp b/src/codegen/registers.cpp
deleted file mode 100644
index 0f13f56244..0000000000
--- a/src/codegen/registers.cpp
+++ /dev/null
@@ -1,35 +0,0 @@
-/* Copyright (c) 2008-2014, Avian Contributors
-
-   Permission to use, copy, modify, and/or distribute this software
-   for any purpose with or without fee is hereby granted, provided
-   that the above copyright notice and this permission notice appear
-   in all copies.
-
-   There is NO WARRANTY for this software.  See license.txt for
-   details. */
-
-#include <avian/codegen/registers.h>
-
-namespace avian {
-namespace codegen {
-
-unsigned RegisterMask::maskStart(uint32_t mask)
-{
-  for (int i = 0; i <= 31; ++i) {
-    if (mask & (1 << i))
-      return i;
-  }
-  return 32;
-}
-
-unsigned RegisterMask::maskLimit(uint32_t mask)
-{
-  for (int i = 31; i >= 0; --i) {
-    if (mask & (1 << i))
-      return i + 1;
-  }
-  return 0;
-}
-
-}  // namespace codegen
-}  // namespace avian
diff --git a/src/codegen/target/arm/assembler.cpp b/src/codegen/target/arm/assembler.cpp
index 7de408deba..a6c7491279 100644
--- a/src/codegen/target/arm/assembler.cpp
+++ b/src/codegen/target/arm/assembler.cpp
@@ -52,11 +52,6 @@ bool vfpSupported()
 }
 }  // namespace isa
 
-inline unsigned lo8(int64_t i)
-{
-  return (unsigned)(i & MASK_LO8);
-}
-
 const RegisterFile MyRegisterFileWithoutFloats(GPR_MASK, 0);
 const RegisterFile MyRegisterFileWithFloats(GPR_MASK, FPR_MASK);
 
@@ -169,39 +164,39 @@ class MyArchitecture : public Architecture {
                           : &MyRegisterFileWithoutFloats;
   }
 
-  virtual int scratch()
+  virtual Register scratch()
   {
-    return 5;
+    return Register(5);
   }
 
-  virtual int stack()
+  virtual Register stack()
   {
     return StackRegister;
   }
 
-  virtual int thread()
+  virtual Register thread()
   {
     return ThreadRegister;
   }
 
-  virtual int returnLow()
+  virtual Register returnLow()
   {
-    return 0;
+    return Register(0);
   }
 
-  virtual int returnHigh()
+  virtual Register returnHigh()
   {
-    return 1;
+    return Register(1);
   }
 
-  virtual int virtualCallTarget()
+  virtual Register virtualCallTarget()
   {
-    return 4;
+    return Register(4);
   }
 
-  virtual int virtualCallIndex()
+  virtual Register virtualCallIndex()
   {
-    return 3;
+    return Register(3);
   }
 
   virtual ir::TargetInfo targetInfo()
@@ -219,13 +214,13 @@ class MyArchitecture : public Architecture {
     return 0x1FFFFFF;
   }
 
-  virtual bool reserved(int register_)
+  virtual bool reserved(Register register_)
   {
-    switch (register_) {
-    case LinkRegister:
-    case StackRegister:
-    case ThreadRegister:
-    case ProgramCounter:
+    switch (register_.index()) {
+    case LinkRegister.index():
+    case StackRegister.index():
+    case ThreadRegister.index():
+    case ProgramCounter.index():
       return true;
 
     default:
@@ -266,11 +261,11 @@ class MyArchitecture : public Architecture {
     return 4;
   }
 
-  virtual int argumentRegister(unsigned index)
+  virtual Register argumentRegister(unsigned index)
   {
     assertT(&con, index < argumentRegisterCount());
 
-    return index;
+    return Register(index);
   }
 
   virtual bool hasLinkRegister()
@@ -401,8 +396,8 @@ class MyArchitecture : public Architecture {
                     OperandMask& aMask,
                     bool* thunk)
   {
-    aMask.typeMask = (1 << lir::RegisterOperand) | (1 << lir::ConstantOperand);
-    aMask.registerMask = ~static_cast<uint64_t>(0);
+    aMask.typeMask = lir::Operand::RegisterPairMask | lir::Operand::ConstantMask;
+    aMask.setLowHighRegisterMasks(AnyRegisterMask, AnyRegisterMask);
     *thunk = false;
   }
 
@@ -414,12 +409,12 @@ class MyArchitecture : public Architecture {
   {
     *thunk = false;
     aMask.typeMask = ~0;
-    aMask.registerMask = GPR_MASK64;
+    aMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
 
     switch (op) {
     case lir::Negate:
-      aMask.typeMask = (1 << lir::RegisterOperand);
-      aMask.registerMask = GPR_MASK64;
+      aMask.typeMask = lir::Operand::RegisterPairMask;
+      aMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
       break;
 
     case lir::Absolute:
@@ -431,8 +426,8 @@ class MyArchitecture : public Architecture {
     case lir::FloatNegate:
     case lir::Float2Float:
       if (vfpSupported()) {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = FPR_MASK64;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(FPR_MASK, FPR_MASK);
       } else {
         *thunk = true;
       }
@@ -444,8 +439,8 @@ class MyArchitecture : public Architecture {
       // thunks or produce inline machine code which handles edge
       // cases properly.
       if (false && vfpSupported() && bSize == 4) {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = FPR_MASK64;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(FPR_MASK, FPR_MASK);
       } else {
         *thunk = true;
       }
@@ -453,8 +448,8 @@ class MyArchitecture : public Architecture {
 
     case lir::Int2Float:
       if (vfpSupported() && aSize == 4) {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = GPR_MASK64;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
       } else {
         *thunk = true;
       }
@@ -471,13 +466,13 @@ class MyArchitecture : public Architecture {
                                unsigned,
                                OperandMask& bMask)
   {
-    bMask.typeMask = (1 << lir::RegisterOperand) | (1 << lir::MemoryOperand);
-    bMask.registerMask = GPR_MASK64;
+    bMask.typeMask = lir::Operand::RegisterPairMask | lir::Operand::MemoryMask;
+    bMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
 
     switch (op) {
     case lir::Negate:
-      bMask.typeMask = (1 << lir::RegisterOperand);
-      bMask.registerMask = GPR_MASK64;
+      bMask.typeMask = lir::Operand::RegisterPairMask;
+      bMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
       break;
 
     case lir::FloatAbsolute:
@@ -485,18 +480,18 @@ class MyArchitecture : public Architecture {
     case lir::FloatNegate:
     case lir::Float2Float:
     case lir::Int2Float:
-      bMask.typeMask = (1 << lir::RegisterOperand);
-      bMask.registerMask = FPR_MASK64;
+      bMask.typeMask = lir::Operand::RegisterPairMask;
+      bMask.setLowHighRegisterMasks(FPR_MASK, FPR_MASK);
       break;
 
     case lir::Float2Int:
-      bMask.typeMask = (1 << lir::RegisterOperand);
-      bMask.registerMask = GPR_MASK64;
+      bMask.typeMask = lir::Operand::RegisterPairMask;
+      bMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
       break;
 
     case lir::Move:
-      if (!(aMask.typeMask & 1 << lir::RegisterOperand)) {
-        bMask.typeMask = 1 << lir::RegisterOperand;
+      if (!(aMask.typeMask & lir::Operand::RegisterPairMask)) {
+        bMask.typeMask = lir::Operand::RegisterPairMask;
       }
       break;
 
@@ -511,21 +506,21 @@ class MyArchitecture : public Architecture {
                         const OperandMask& dstMask)
   {
     srcMask.typeMask = ~0;
-    srcMask.registerMask = ~static_cast<uint64_t>(0);
+    srcMask.setLowHighRegisterMasks(AnyRegisterMask, AnyRegisterMask);
 
     tmpMask.typeMask = 0;
-    tmpMask.registerMask = 0;
+    tmpMask.setLowHighRegisterMasks(0, 0);
 
-    if (dstMask.typeMask & (1 << lir::MemoryOperand)) {
+    if (dstMask.typeMask & lir::Operand::MemoryMask) {
       // can't move directly from memory or constant to memory
-      srcMask.typeMask = 1 << lir::RegisterOperand;
-      tmpMask.typeMask = 1 << lir::RegisterOperand;
-      tmpMask.registerMask = GPR_MASK64;
-    } else if (vfpSupported() && dstMask.typeMask & 1 << lir::RegisterOperand
-               && dstMask.registerMask & FPR_MASK) {
-      srcMask.typeMask = tmpMask.typeMask = 1 << lir::RegisterOperand
-                                            | 1 << lir::MemoryOperand;
-      tmpMask.registerMask = ~static_cast<uint64_t>(0);
+      srcMask.typeMask = lir::Operand::RegisterPairMask;
+      tmpMask.typeMask = lir::Operand::RegisterPairMask;
+      tmpMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
+    } else if (vfpSupported() && dstMask.typeMask & lir::Operand::RegisterPairMask
+               && dstMask.lowRegisterMask & FPR_MASK) {
+      srcMask.typeMask = tmpMask.typeMask = lir::Operand::RegisterPairMask
+                                            | lir::Operand::MemoryMask;
+      tmpMask.setLowHighRegisterMasks(AnyRegisterMask, AnyRegisterMask);
     }
   }
 
@@ -537,11 +532,11 @@ class MyArchitecture : public Architecture {
                           unsigned,
                           bool* thunk)
   {
-    aMask.typeMask = (1 << lir::RegisterOperand) | (1 << lir::ConstantOperand);
-    aMask.registerMask = GPR_MASK64;
+    aMask.typeMask = lir::Operand::RegisterPairMask | lir::Operand::ConstantMask;
+    aMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
 
-    bMask.typeMask = (1 << lir::RegisterOperand);
-    bMask.registerMask = GPR_MASK64;
+    bMask.typeMask = lir::Operand::RegisterPairMask;
+    bMask.setLowHighRegisterMasks(GPR_MASK, GPR_MASK);
 
     *thunk = false;
 
@@ -550,7 +545,7 @@ class MyArchitecture : public Architecture {
     case lir::ShiftRight:
     case lir::UnsignedShiftRight:
       if (bSize == 8)
-        aMask.typeMask = bMask.typeMask = (1 << lir::RegisterOperand);
+        aMask.typeMask = bMask.typeMask = lir::Operand::RegisterPairMask;
       break;
 
     case lir::Add:
@@ -558,7 +553,7 @@ class MyArchitecture : public Architecture {
     case lir::Or:
     case lir::Xor:
     case lir::Multiply:
-      aMask.typeMask = bMask.typeMask = (1 << lir::RegisterOperand);
+      aMask.typeMask = bMask.typeMask = lir::Operand::RegisterPairMask;
       break;
 
     case lir::Divide:
@@ -572,8 +567,9 @@ class MyArchitecture : public Architecture {
     case lir::FloatMultiply:
     case lir::FloatDivide:
       if (vfpSupported()) {
-        aMask.typeMask = bMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = bMask.registerMask = FPR_MASK64;
+        bMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(FPR_MASK, FPR_MASK);
+        bMask = aMask;
       } else {
         *thunk = true;
       }
@@ -590,8 +586,9 @@ class MyArchitecture : public Architecture {
     case lir::JumpIfFloatLessOrEqualOrUnordered:
     case lir::JumpIfFloatGreaterOrEqualOrUnordered:
       if (vfpSupported()) {
-        aMask.typeMask = bMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = bMask.registerMask = FPR_MASK64;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(FPR_MASK, FPR_MASK);
+        bMask = aMask;
       } else {
         *thunk = true;
       }
@@ -611,11 +608,12 @@ class MyArchitecture : public Architecture {
                                OperandMask& cMask)
   {
     if (isBranch(op)) {
-      cMask.typeMask = (1 << lir::ConstantOperand);
-      cMask.registerMask = 0;
+      cMask.typeMask = lir::Operand::ConstantMask;
+      cMask.setLowHighRegisterMasks(0, 0);
     } else {
-      cMask.typeMask = (1 << lir::RegisterOperand);
-      cMask.registerMask = bMask.registerMask;
+      cMask.typeMask = lir::Operand::RegisterPairMask;
+      cMask.lowRegisterMask = bMask.lowRegisterMask;
+      cMask.highRegisterMask = bMask.highRegisterMask;
     }
   }
 
@@ -658,7 +656,7 @@ class MyAssembler : public Assembler {
   virtual void checkStackOverflow(uintptr_t handler,
                                   unsigned stackLimitOffsetFromThread)
   {
-    lir::Register stack(StackRegister);
+    lir::RegisterPair stack(StackRegister);
     lir::Memory stackLimit(ThreadRegister, stackLimitOffsetFromThread);
     lir::Constant handlerConstant(new (con.zone) ResolvedPromise(handler));
     branchRM(&con,
@@ -671,11 +669,11 @@ class MyAssembler : public Assembler {
 
   virtual void saveFrame(unsigned stackOffset, unsigned ipOffset)
   {
-    lir::Register link(LinkRegister);
+    lir::RegisterPair link(LinkRegister);
     lir::Memory linkDst(ThreadRegister, ipOffset);
     moveRM(&con, TargetBytesPerWord, &link, TargetBytesPerWord, &linkDst);
 
-    lir::Register stack(StackRegister);
+    lir::RegisterPair stack(StackRegister);
     lir::Memory stackDst(ThreadRegister, stackOffset);
     moveRM(&con, TargetBytesPerWord, &stack, TargetBytesPerWord, &stackDst);
   }
@@ -684,7 +682,7 @@ class MyAssembler : public Assembler {
   {
     struct Argument {
       unsigned size;
-      lir::OperandType type;
+      lir::Operand::Type type;
       lir::Operand* operand;
     };
     RUNTIME_ARRAY(Argument, arguments, argumentCount);
@@ -695,7 +693,7 @@ class MyAssembler : public Assembler {
     for (unsigned i = 0; i < argumentCount; ++i) {
       RUNTIME_ARRAY_BODY(arguments)[i].size = va_arg(a, unsigned);
       RUNTIME_ARRAY_BODY(arguments)[i].type
-          = static_cast<lir::OperandType>(va_arg(a, int));
+          = static_cast<lir::Operand::Type>(va_arg(a, int));
       RUNTIME_ARRAY_BODY(arguments)[i].operand = va_arg(a, lir::Operand*);
       footprint += ceilingDivide(RUNTIME_ARRAY_BODY(arguments)[i].size,
                                  TargetBytesPerWord);
@@ -707,7 +705,7 @@ class MyAssembler : public Assembler {
     unsigned offset = 0;
     for (unsigned i = 0; i < argumentCount; ++i) {
       if (i < arch_->argumentRegisterCount()) {
-        lir::Register dst(arch_->argumentRegister(i));
+        lir::RegisterPair dst(arch_->argumentRegister(i));
 
         apply(lir::Move,
               OperandInfo(RUNTIME_ARRAY_BODY(arguments)[i].size,
@@ -715,7 +713,7 @@ class MyAssembler : public Assembler {
                           RUNTIME_ARRAY_BODY(arguments)[i].operand),
               OperandInfo(pad(RUNTIME_ARRAY_BODY(arguments)[i].size,
                               TargetBytesPerWord),
-                          lir::RegisterOperand,
+                          lir::Operand::Type::RegisterPair,
                           &dst));
 
         offset += ceilingDivide(RUNTIME_ARRAY_BODY(arguments)[i].size,
@@ -729,7 +727,7 @@ class MyAssembler : public Assembler {
                           RUNTIME_ARRAY_BODY(arguments)[i].operand),
               OperandInfo(pad(RUNTIME_ARRAY_BODY(arguments)[i].size,
                               TargetBytesPerWord),
-                          lir::MemoryOperand,
+                          lir::Operand::Type::Memory,
                           &dst));
 
         offset += ceilingDivide(RUNTIME_ARRAY_BODY(arguments)[i].size,
@@ -747,12 +745,12 @@ class MyAssembler : public Assembler {
     // how to handle them:
     assertT(&con, footprint < 256);
 
-    lir::Register stack(StackRegister);
+    lir::RegisterPair stack(StackRegister);
     ResolvedPromise footprintPromise(footprint * TargetBytesPerWord);
     lir::Constant footprintConstant(&footprintPromise);
     subC(&con, TargetBytesPerWord, &footprintConstant, &stack, &stack);
 
-    lir::Register returnAddress(LinkRegister);
+    lir::RegisterPair returnAddress(LinkRegister);
     lir::Memory returnAddressDst(StackRegister,
                                  (footprint - 1) * TargetBytesPerWord);
     moveRM(&con,
@@ -764,7 +762,7 @@ class MyAssembler : public Assembler {
 
   virtual void adjustFrame(unsigned difference)
   {
-    lir::Register stack(StackRegister);
+    lir::RegisterPair stack(StackRegister);
     ResolvedPromise differencePromise(difference * TargetBytesPerWord);
     lir::Constant differenceConstant(&differencePromise);
     subC(&con, TargetBytesPerWord, &differenceConstant, &stack, &stack);
@@ -774,7 +772,7 @@ class MyAssembler : public Assembler {
   {
     footprint += FrameHeaderSize;
 
-    lir::Register returnAddress(LinkRegister);
+    lir::RegisterPair returnAddress(LinkRegister);
     lir::Memory returnAddressSrc(StackRegister,
                                  (footprint - 1) * TargetBytesPerWord);
     moveMR(&con,
@@ -783,7 +781,7 @@ class MyAssembler : public Assembler {
            TargetBytesPerWord,
            &returnAddress);
 
-    lir::Register stack(StackRegister);
+    lir::RegisterPair stack(StackRegister);
     ResolvedPromise footprintPromise(footprint * TargetBytesPerWord);
     lir::Constant footprintConstant(&footprintPromise);
     addC(&con, TargetBytesPerWord, &footprintConstant, &stack, &stack);
@@ -791,16 +789,16 @@ class MyAssembler : public Assembler {
 
   virtual void popFrameForTailCall(unsigned footprint,
                                    int offset,
-                                   int returnAddressSurrogate,
-                                   int framePointerSurrogate UNUSED)
+                                   Register returnAddressSurrogate,
+                                   Register framePointerSurrogate UNUSED)
   {
-    assertT(&con, framePointerSurrogate == lir::NoRegister);
+    assertT(&con, framePointerSurrogate == NoRegister);
 
     if (TailCalls) {
       if (offset) {
         footprint += FrameHeaderSize;
 
-        lir::Register link(LinkRegister);
+        lir::RegisterPair link(LinkRegister);
         lir::Memory returnAddressSrc(StackRegister,
                                      (footprint - 1) * TargetBytesPerWord);
         moveMR(&con,
@@ -809,16 +807,16 @@ class MyAssembler : public Assembler {
                TargetBytesPerWord,
                &link);
 
-        lir::Register stack(StackRegister);
+        lir::RegisterPair stack(StackRegister);
         ResolvedPromise footprintPromise((footprint - offset)
                                          * TargetBytesPerWord);
         lir::Constant footprintConstant(&footprintPromise);
         addC(&con, TargetBytesPerWord, &footprintConstant, &stack, &stack);
 
-        if (returnAddressSurrogate != lir::NoRegister) {
+        if (returnAddressSurrogate != NoRegister) {
           assertT(&con, offset > 0);
 
-          lir::Register ras(returnAddressSurrogate);
+          lir::RegisterPair ras(returnAddressSurrogate);
           lir::Memory dst(StackRegister, (offset - 1) * TargetBytesPerWord);
           moveRM(&con, TargetBytesPerWord, &ras, TargetBytesPerWord, &dst);
         }
@@ -842,7 +840,7 @@ class MyAssembler : public Assembler {
     if (TailCalls and argumentFootprint > StackAlignmentInWords) {
       offset = argumentFootprint - StackAlignmentInWords;
 
-      lir::Register stack(StackRegister);
+      lir::RegisterPair stack(StackRegister);
       ResolvedPromise adjustmentPromise(offset * TargetBytesPerWord);
       lir::Constant adjustment(&adjustmentPromise);
       addC(&con, TargetBytesPerWord, &adjustment, &stack, &stack);
@@ -858,7 +856,7 @@ class MyAssembler : public Assembler {
   {
     popFrame(frameFootprint);
 
-    lir::Register stack(StackRegister);
+    lir::RegisterPair stack(StackRegister);
     lir::Memory newStackSrc(ThreadRegister, stackOffsetFromThread);
     moveMR(&con, TargetBytesPerWord, &newStackSrc, TargetBytesPerWord, &stack);
 
@@ -890,14 +888,14 @@ class MyAssembler : public Assembler {
     if (isBranch(op)) {
       assertT(&con, a.size == b.size);
       assertT(&con, c.size == TargetBytesPerWord);
-      assertT(&con, c.type == lir::ConstantOperand);
+      assertT(&con, c.type == lir::Operand::Type::Constant);
 
       arch_->con.branchOperations[branchIndex(&(arch_->con), a.type, b.type)](
           &con, op, a.size, a.operand, b.operand, c.operand);
     } else {
       assertT(&con, b.size == c.size);
-      assertT(&con, b.type == lir::RegisterOperand);
-      assertT(&con, c.type == lir::RegisterOperand);
+      assertT(&con, b.type == lir::Operand::Type::RegisterPair);
+      assertT(&con, c.type == lir::Operand::Type::RegisterPair);
 
       arch_->con.ternaryOperations[index(&(arch_->con), op, a.type)](
           &con, b.size, a.operand, b.operand, c.operand);
diff --git a/src/codegen/target/arm/context.h b/src/codegen/target/arm/context.h
index 45d937da9b..fde5a54a29 100644
--- a/src/codegen/target/arm/context.h
+++ b/src/codegen/target/arm/context.h
@@ -85,15 +85,15 @@ class ArchitectureContext {
   vm::System* s;
   OperationType operations[lir::OperationCount];
   UnaryOperationType
-      unaryOperations[lir::UnaryOperationCount * lir::OperandTypeCount];
+      unaryOperations[lir::UnaryOperationCount * lir::Operand::TypeCount];
   BinaryOperationType binaryOperations[lir::BinaryOperationCount
-                                       * lir::OperandTypeCount
-                                       * lir::OperandTypeCount];
+                                       * lir::Operand::TypeCount
+                                       * lir::Operand::TypeCount];
   TernaryOperationType ternaryOperations[lir::NonBranchTernaryOperationCount
-                                         * lir::OperandTypeCount];
+                                         * lir::Operand::TypeCount];
   BranchOperationType branchOperations[lir::BranchOperationCount
-                                       * lir::OperandTypeCount
-                                       * lir::OperandTypeCount];
+                                       * lir::Operand::TypeCount
+                                       * lir::Operand::TypeCount];
 };
 
 inline avian::util::Aborter* getAborter(Context* c)
diff --git a/src/codegen/target/arm/encode.h b/src/codegen/target/arm/encode.h
index edbeb3d071..2ac2809157 100644
--- a/src/codegen/target/arm/encode.h
+++ b/src/codegen/target/arm/encode.h
@@ -46,34 +46,34 @@ enum CONDITION {
 enum SHIFTOP { LSL, LSR, ASR, ROR };
 // INSTRUCTION FORMATS
 inline int
-    DATA(int cond, int opcode, int S, int Rn, int Rd, int shift, int Sh, int Rm)
+    DATA(int cond, int opcode, int S, Register Rn, Register Rd, int shift, int Sh, Register Rm)
 {
-  return cond << 28 | opcode << 21 | S << 20 | Rn << 16 | Rd << 12 | shift << 7
-         | Sh << 5 | Rm;
+  return cond << 28 | opcode << 21 | S << 20 | Rn.index() << 16 | Rd.index() << 12 | shift << 7
+         | Sh << 5 | Rm.index();
 }
 inline int
-    DATAS(int cond, int opcode, int S, int Rn, int Rd, int Rs, int Sh, int Rm)
+    DATAS(int cond, int opcode, int S, Register Rn, Register Rd, Register Rs, int Sh, Register Rm)
 {
-  return cond << 28 | opcode << 21 | S << 20 | Rn << 16 | Rd << 12 | Rs << 8
-         | Sh << 5 | 1 << 4 | Rm;
+  return cond << 28 | opcode << 21 | S << 20 | Rn.index() << 16 | Rd.index() << 12 | Rs.index() << 8
+         | Sh << 5 | 1 << 4 | Rm.index();
 }
-inline int DATAI(int cond, int opcode, int S, int Rn, int Rd, int rot, int imm)
+inline int DATAI(int cond, int opcode, int S, Register Rn, Register Rd, int rot, int imm)
 {
-  return cond << 28 | 1 << 25 | opcode << 21 | S << 20 | Rn << 16 | Rd << 12
+  return cond << 28 | 1 << 25 | opcode << 21 | S << 20 | Rn.index() << 16 | Rd.index() << 12
          | rot << 8 | (imm & 0xff);
 }
 inline int BRANCH(int cond, int L, int offset)
 {
   return cond << 28 | 5 << 25 | L << 24 | (offset & 0xffffff);
 }
-inline int BRANCHX(int cond, int L, int Rm)
+inline int BRANCHX(int cond, int L, Register Rm)
 {
-  return cond << 28 | 0x4bffc << 6 | L << 5 | 1 << 4 | Rm;
+  return cond << 28 | 0x4bffc << 6 | L << 5 | 1 << 4 | Rm.index();
 }
-inline int MULTIPLY(int cond, int mul, int S, int Rd, int Rn, int Rs, int Rm)
+inline int MULTIPLY(int cond, int mul, int S, Register Rd, Register Rn, Register Rs, Register Rm)
 {
-  return cond << 28 | mul << 21 | S << 20 | Rd << 16 | Rn << 12 | Rs << 8
-         | 9 << 4 | Rm;
+  return cond << 28 | mul << 21 | S << 20 | Rd.index() << 16 | Rn.index() << 12 | Rs.index() << 8
+         | 9 << 4 | Rm.index();
 }
 inline int XFER(int cond,
                 int P,
@@ -81,14 +81,14 @@ inline int XFER(int cond,
                 int B,
                 int W,
                 int L,
-                int Rn,
-                int Rd,
+                Register Rn,
+                Register Rd,
                 int shift,
                 int Sh,
-                int Rm)
+                Register Rm)
 {
   return cond << 28 | 3 << 25 | P << 24 | U << 23 | B << 22 | W << 21 | L << 20
-         | Rn << 16 | Rd << 12 | shift << 7 | Sh << 5 | Rm;
+         | Rn.index() << 16 | Rd.index() << 12 | shift << 7 | Sh << 5 | Rm.index();
 }
 inline int XFERI(int cond,
                  int P,
@@ -96,41 +96,41 @@ inline int XFERI(int cond,
                  int B,
                  int W,
                  int L,
-                 int Rn,
-                 int Rd,
+                 Register Rn,
+                 Register Rd,
                  int offset)
 {
   return cond << 28 | 2 << 25 | P << 24 | U << 23 | B << 22 | W << 21 | L << 20
-         | Rn << 16 | Rd << 12 | (offset & 0xfff);
+         | Rn.index() << 16 | Rd.index() << 12 | (offset & 0xfff);
 }
 inline int XFER2(int cond,
                  int P,
                  int U,
                  int W,
                  int L,
-                 int Rn,
-                 int Rd,
+                 Register Rn,
+                 Register Rd,
                  int S,
                  int H,
-                 int Rm)
+                 Register Rm)
 {
-  return cond << 28 | P << 24 | U << 23 | W << 21 | L << 20 | Rn << 16
-         | Rd << 12 | 1 << 7 | S << 6 | H << 5 | 1 << 4 | Rm;
+  return cond << 28 | P << 24 | U << 23 | W << 21 | L << 20 | Rn.index() << 16
+         | Rd.index() << 12 | 1 << 7 | S << 6 | H << 5 | 1 << 4 | Rm.index();
 }
 inline int XFER2I(int cond,
                   int P,
                   int U,
                   int W,
                   int L,
-                  int Rn,
-                  int Rd,
+                  Register Rn,
+                  Register Rd,
                   int offsetH,
                   int S,
                   int H,
                   int offsetL)
 {
-  return cond << 28 | P << 24 | U << 23 | 1 << 22 | W << 21 | L << 20 | Rn << 16
-         | Rd << 12 | offsetH << 8 | 1 << 7 | S << 6 | H << 5 | 1 << 4
+  return cond << 28 | P << 24 | U << 23 | 1 << 22 | W << 21 | L << 20 | Rn.index() << 16
+         | Rd.index() << 12 | offsetH << 8 | 1 << 7 | S << 6 | H << 5 | 1 << 4
          | (offsetL & 0xf);
 }
 inline int COOP(int cond,
@@ -150,30 +150,30 @@ inline int COXFER(int cond,
                   int N,
                   int W,
                   int L,
-                  int Rn,
+                  Register Rn,
                   int CRd,
                   int cp_num,
                   int offset)  // offset is in words, not bytes
 {
   return cond << 28 | 0x6 << 25 | P << 24 | U << 23 | N << 22 | W << 21
-         | L << 20 | Rn << 16 | CRd << 12 | cp_num << 8 | (offset & 0xff) >> 2;
+         | L << 20 | Rn.index() << 16 | CRd << 12 | cp_num << 8 | (offset & 0xff) >> 2;
 }
 inline int COREG(int cond,
                  int opcode_1,
                  int L,
                  int CRn,
-                 int Rd,
+                 Register Rd,
                  int cp_num,
                  int opcode_2,
                  int CRm)
 {
   return cond << 28 | 0xe << 24 | opcode_1 << 21 | L << 20 | CRn << 16
-         | Rd << 12 | cp_num << 8 | opcode_2 << 5 | 1 << 4 | CRm;
+         | Rd.index() << 12 | cp_num << 8 | opcode_2 << 5 | 1 << 4 | CRm;
 }
 inline int
-    COREG2(int cond, int L, int Rn, int Rd, int cp_num, int opcode, int CRm)
+    COREG2(int cond, int L, Register Rn, Register Rd, int cp_num, int opcode, int CRm)
 {
-  return cond << 28 | 0xc4 << 20 | L << 20 | Rn << 16 | Rd << 12 | cp_num << 8
+  return cond << 28 | 0xc4 << 20 | L << 20 | Rn.index() << 16 | Rd.index() << 12 | cp_num << 8
          | opcode << 4 | CRm;
 }
 // FIELD CALCULATORS
@@ -191,143 +191,143 @@ inline int bl(int offset)
 {
   return BRANCH(AL, 1, offset);
 }
-inline int bx(int Rm)
+inline int bx(Register Rm)
 {
   return BRANCHX(AL, 0, Rm);
 }
-inline int blx(int Rm)
+inline int blx(Register Rm)
 {
   return BRANCHX(AL, 1, Rm);
 }
-inline int and_(int Rd, int Rn, int Rm, int Sh = 0, int shift = 0)
+inline int and_(Register Rd, Register Rn, Register Rm, int Sh = 0, int shift = 0)
 {
   return DATA(AL, 0x0, 0, Rn, Rd, shift, Sh, Rm);
 }
-inline int eor(int Rd, int Rn, int Rm, int Sh = 0, int shift = 0)
+inline int eor(Register Rd, Register Rn, Register Rm, int Sh = 0, int shift = 0)
 {
   return DATA(AL, 0x1, 0, Rn, Rd, shift, Sh, Rm);
 }
-inline int rsb(int Rd, int Rn, int Rm, int Sh = 0, int shift = 0)
+inline int rsb(Register Rd, Register Rn, Register Rm, int Sh = 0, int shift = 0)
 {
   return DATA(AL, 0x3, 0, Rn, Rd, shift, Sh, Rm);
 }
-inline int add(int Rd, int Rn, int Rm, int Sh = 0, int shift = 0)
+inline int add(Register Rd, Register Rn, Register Rm, int Sh = 0, int shift = 0)
 {
   return DATA(AL, 0x4, 0, Rn, Rd, shift, Sh, Rm);
 }
-inline int adc(int Rd, int Rn, int Rm, int Sh = 0, int shift = 0)
+inline int adc(Register Rd, Register Rn, Register Rm, int Sh = 0, int shift = 0)
 {
   return DATA(AL, 0x5, 0, Rn, Rd, shift, Sh, Rm);
 }
-inline int rsc(int Rd, int Rn, int Rm, int Sh = 0, int shift = 0)
+inline int rsc(Register Rd, Register Rn, Register Rm, int Sh = 0, int shift = 0)
 {
   return DATA(AL, 0x7, 0, Rn, Rd, shift, Sh, Rm);
 }
-inline int cmp(int Rn, int Rm, int Sh = 0, int shift = 0)
+inline int cmp(Register Rn, Register Rm, int Sh = 0, int shift = 0)
 {
-  return DATA(AL, 0xa, 1, Rn, 0, shift, Sh, Rm);
+  return DATA(AL, 0xa, 1, Rn, Register(0), shift, Sh, Rm);
 }
-inline int orr(int Rd, int Rn, int Rm, int Sh = 0, int shift = 0)
+inline int orr(Register Rd, Register Rn, Register Rm, int Sh = 0, int shift = 0)
 {
   return DATA(AL, 0xc, 0, Rn, Rd, shift, Sh, Rm);
 }
-inline int mov(int Rd, int Rm, int Sh = 0, int shift = 0)
+inline int mov(Register Rd, Register Rm, int Sh = 0, int shift = 0)
 {
-  return DATA(AL, 0xd, 0, 0, Rd, shift, Sh, Rm);
+  return DATA(AL, 0xd, 0, Register(0), Rd, shift, Sh, Rm);
 }
-inline int mvn(int Rd, int Rm, int Sh = 0, int shift = 0)
+inline int mvn(Register Rd, Register Rm, int Sh = 0, int shift = 0)
 {
-  return DATA(AL, 0xf, 0, 0, Rd, shift, Sh, Rm);
+  return DATA(AL, 0xf, 0, Register(0), Rd, shift, Sh, Rm);
 }
-inline int andi(int Rd, int Rn, int imm, int rot = 0)
+inline int andi(Register Rd, Register Rn, int imm, int rot = 0)
 {
   return DATAI(AL, 0x0, 0, Rn, Rd, rot, imm);
 }
-inline int subi(int Rd, int Rn, int imm, int rot = 0)
+inline int subi(Register Rd, Register Rn, int imm, int rot = 0)
 {
   return DATAI(AL, 0x2, 0, Rn, Rd, rot, imm);
 }
-inline int rsbi(int Rd, int Rn, int imm, int rot = 0)
+inline int rsbi(Register Rd, Register Rn, int imm, int rot = 0)
 {
   return DATAI(AL, 0x3, 0, Rn, Rd, rot, imm);
 }
-inline int addi(int Rd, int Rn, int imm, int rot = 0)
+inline int addi(Register Rd, Register Rn, int imm, int rot = 0)
 {
   return DATAI(AL, 0x4, 0, Rn, Rd, rot, imm);
 }
-inline int adci(int Rd, int Rn, int imm, int rot = 0)
+inline int adci(Register Rd, Register Rn, int imm, int rot = 0)
 {
   return DATAI(AL, 0x5, 0, Rn, Rd, rot, imm);
 }
-inline int bici(int Rd, int Rn, int imm, int rot = 0)
+inline int bici(Register Rd, Register Rn, int imm, int rot = 0)
 {
   return DATAI(AL, 0xe, 0, Rn, Rd, rot, imm);
 }
-inline int cmpi(int Rn, int imm, int rot = 0)
+inline int cmpi(Register Rn, int imm, int rot = 0)
 {
-  return DATAI(AL, 0xa, 1, Rn, 0, rot, imm);
+  return DATAI(AL, 0xa, 1, Rn, Register(0), rot, imm);
 }
-inline int movi(int Rd, int imm, int rot = 0)
+inline int movi(Register Rd, int imm, int rot = 0)
 {
-  return DATAI(AL, 0xd, 0, 0, Rd, rot, imm);
+  return DATAI(AL, 0xd, 0, Register(0), Rd, rot, imm);
 }
-inline int orrsh(int Rd, int Rn, int Rm, int Rs, int Sh)
+inline int orrsh(Register Rd, Register Rn, Register Rm, Register Rs, int Sh)
 {
   return DATAS(AL, 0xc, 0, Rn, Rd, Rs, Sh, Rm);
 }
-inline int movsh(int Rd, int Rm, int Rs, int Sh)
+inline int movsh(Register Rd, Register Rm, Register Rs, int Sh)
 {
-  return DATAS(AL, 0xd, 0, 0, Rd, Rs, Sh, Rm);
+  return DATAS(AL, 0xd, 0, Register(0), Rd, Rs, Sh, Rm);
 }
-inline int mul(int Rd, int Rm, int Rs)
+inline int mul(Register Rd, Register Rm, Register Rs)
 {
-  return MULTIPLY(AL, 0, 0, Rd, 0, Rs, Rm);
+  return MULTIPLY(AL, 0, 0, Rd, Register(0), Rs, Rm);
 }
-inline int mla(int Rd, int Rm, int Rs, int Rn)
+inline int mla(Register Rd, Register Rm, Register Rs, Register Rn)
 {
   return MULTIPLY(AL, 1, 0, Rd, Rn, Rs, Rm);
 }
-inline int umull(int RdLo, int RdHi, int Rm, int Rs)
+inline int umull(Register RdLo, Register RdHi, Register Rm, Register Rs)
 {
   return MULTIPLY(AL, 4, 0, RdHi, RdLo, Rs, Rm);
 }
-inline int ldr(int Rd, int Rn, int Rm, int W = 0)
+inline int ldr(Register Rd, Register Rn, Register Rm, int W = 0)
 {
   return XFER(AL, 1, 1, 0, W, 1, Rn, Rd, 0, 0, Rm);
 }
-inline int ldri(int Rd, int Rn, int imm, int W = 0)
+inline int ldri(Register Rd, Register Rn, int imm, int W = 0)
 {
   return XFERI(AL, 1, calcU(imm), 0, W, 1, Rn, Rd, abs(imm));
 }
-inline int ldrb(int Rd, int Rn, int Rm)
+inline int ldrb(Register Rd, Register Rn, Register Rm)
 {
   return XFER(AL, 1, 1, 1, 0, 1, Rn, Rd, 0, 0, Rm);
 }
-inline int ldrbi(int Rd, int Rn, int imm)
+inline int ldrbi(Register Rd, Register Rn, int imm)
 {
   return XFERI(AL, 1, calcU(imm), 1, 0, 1, Rn, Rd, abs(imm));
 }
-inline int str(int Rd, int Rn, int Rm, int W = 0)
+inline int str(Register Rd, Register Rn, Register Rm, int W = 0)
 {
   return XFER(AL, 1, 1, 0, W, 0, Rn, Rd, 0, 0, Rm);
 }
-inline int stri(int Rd, int Rn, int imm, int W = 0)
+inline int stri(Register Rd, Register Rn, int imm, int W = 0)
 {
   return XFERI(AL, 1, calcU(imm), 0, W, 0, Rn, Rd, abs(imm));
 }
-inline int strb(int Rd, int Rn, int Rm)
+inline int strb(Register Rd, Register Rn, Register Rm)
 {
   return XFER(AL, 1, 1, 1, 0, 0, Rn, Rd, 0, 0, Rm);
 }
-inline int strbi(int Rd, int Rn, int imm)
+inline int strbi(Register Rd, Register Rn, int imm)
 {
   return XFERI(AL, 1, calcU(imm), 1, 0, 0, Rn, Rd, abs(imm));
 }
-inline int ldrh(int Rd, int Rn, int Rm)
+inline int ldrh(Register Rd, Register Rn, Register Rm)
 {
   return XFER2(AL, 1, 1, 0, 1, Rn, Rd, 0, 1, Rm);
 }
-inline int ldrhi(int Rd, int Rn, int imm)
+inline int ldrhi(Register Rd, Register Rn, int imm)
 {
   return XFER2I(AL,
                 1,
@@ -341,11 +341,11 @@ inline int ldrhi(int Rd, int Rn, int imm)
                 1,
                 abs(imm) & 0xf);
 }
-inline int strh(int Rd, int Rn, int Rm)
+inline int strh(Register Rd, Register Rn, Register Rm)
 {
   return XFER2(AL, 1, 1, 0, 0, Rn, Rd, 0, 1, Rm);
 }
-inline int strhi(int Rd, int Rn, int imm)
+inline int strhi(Register Rd, Register Rn, int imm)
 {
   return XFER2I(AL,
                 1,
@@ -359,11 +359,11 @@ inline int strhi(int Rd, int Rn, int imm)
                 1,
                 abs(imm) & 0xf);
 }
-inline int ldrsh(int Rd, int Rn, int Rm)
+inline int ldrsh(Register Rd, Register Rn, Register Rm)
 {
   return XFER2(AL, 1, 1, 0, 1, Rn, Rd, 1, 1, Rm);
 }
-inline int ldrshi(int Rd, int Rn, int imm)
+inline int ldrshi(Register Rd, Register Rn, int imm)
 {
   return XFER2I(AL,
                 1,
@@ -377,11 +377,11 @@ inline int ldrshi(int Rd, int Rn, int imm)
                 1,
                 abs(imm) & 0xf);
 }
-inline int ldrsb(int Rd, int Rn, int Rm)
+inline int ldrsb(Register Rd, Register Rn, Register Rm)
 {
   return XFER2(AL, 1, 1, 0, 1, Rn, Rd, 1, 0, Rm);
 }
-inline int ldrsbi(int Rd, int Rn, int imm)
+inline int ldrsbi(Register Rd, Register Rn, int imm)
 {
   return XFER2I(AL,
                 1,
@@ -403,27 +403,27 @@ inline int bkpt(int16_t immed)
 // COPROCESSOR INSTRUCTIONS
 inline int mcr(int coproc,
                int opcode_1,
-               int Rd,
+               Register Rd,
                int CRn,
                int CRm,
                int opcode_2 = 0)
 {
   return COREG(AL, opcode_1, 0, CRn, Rd, coproc, opcode_2, CRm);
 }
-inline int mcrr(int coproc, int opcode, int Rd, int Rn, int CRm)
+inline int mcrr(int coproc, int opcode, Register Rd, Register Rn, int CRm)
 {
   return COREG2(AL, 0, Rn, Rd, coproc, opcode, CRm);
 }
 inline int mrc(int coproc,
                int opcode_1,
-               int Rd,
+               Register Rd,
                int CRn,
                int CRm,
                int opcode_2 = 0)
 {
   return COREG(AL, opcode_1, 1, CRn, Rd, coproc, opcode_2, CRm);
 }
-inline int mrrc(int coproc, int opcode, int Rd, int Rn, int CRm)
+inline int mrrc(int coproc, int opcode, Register Rd, Register Rn, int CRm)
 {
   return COREG2(AL, 1, Rn, Rd, coproc, opcode, CRm);
 }
@@ -551,42 +551,42 @@ inline int ftosizd(int Sd, int Dm)
   return COOP(AL, 0xb | (Sd & 1) << 2, 0xd, Sd >> 1, 11, 6, Dm);
 }
 // single load/store instructions for both precision types
-inline int flds(int Sd, int Rn, int offset = 0)
+inline int flds(int Sd, Register Rn, int offset = 0)
 {
   return COXFER(AL, 1, 1, Sd & 1, 0, 1, Rn, Sd >> 1, 10, offset);
 };
-inline int fldd(int Dd, int Rn, int offset = 0)
+inline int fldd(int Dd, Register Rn, int offset = 0)
 {
   return COXFER(AL, 1, 1, 0, 0, 1, Rn, Dd, 11, offset);
 };
-inline int fsts(int Sd, int Rn, int offset = 0)
+inline int fsts(int Sd, Register Rn, int offset = 0)
 {
   return COXFER(AL, 1, 1, Sd & 1, 0, 0, Rn, Sd >> 1, 10, offset);
 };
-inline int fstd(int Dd, int Rn, int offset = 0)
+inline int fstd(int Dd, Register Rn, int offset = 0)
 {
   return COXFER(AL, 1, 1, 0, 0, 0, Rn, Dd, 11, offset);
 };
 // move between GPRs and FPRs
-inline int fmsr(int Sn, int Rd)
+inline int fmsr(int Sn, Register Rd)
 {
   return mcr(10, 0, Rd, Sn >> 1, 0, (Sn & 1) << 2);
 }
-inline int fmrs(int Rd, int Sn)
+inline int fmrs(Register Rd, int Sn)
 {
   return mrc(10, 0, Rd, Sn >> 1, 0, (Sn & 1) << 2);
 }
 // move to/from VFP system registers
-inline int fmrx(int Rd, int reg)
+inline int fmrx(Register Rd, int reg)
 {
   return mrc(10, 7, Rd, reg, 0);
 }
 // these move around pairs of single-precision registers
-inline int fmdrr(int Dm, int Rd, int Rn)
+inline int fmdrr(int Dm, Register Rd, Register Rn)
 {
   return mcrr(11, 1, Rd, Rn, Dm);
 }
-inline int fmrrd(int Rd, int Rn, int Dm)
+inline int fmrrd(Register Rd, Register Rn, int Dm)
 {
   return mrrc(11, 1, Rd, Rn, Dm);
 }
@@ -600,27 +600,27 @@ inline int SETS(int ins)
   return ins | 1 << 20;
 }
 // PSEUDO-INSTRUCTIONS
-inline int lsl(int Rd, int Rm, int Rs)
+inline int lsl(Register Rd, Register Rm, Register Rs)
 {
   return movsh(Rd, Rm, Rs, LSL);
 }
-inline int lsli(int Rd, int Rm, int imm)
+inline int lsli(Register Rd, Register Rm, int imm)
 {
   return mov(Rd, Rm, LSL, imm);
 }
-inline int lsr(int Rd, int Rm, int Rs)
+inline int lsr(Register Rd, Register Rm, Register Rs)
 {
   return movsh(Rd, Rm, Rs, LSR);
 }
-inline int lsri(int Rd, int Rm, int imm)
+inline int lsri(Register Rd, Register Rm, int imm)
 {
   return mov(Rd, Rm, LSR, imm);
 }
-inline int asr(int Rd, int Rm, int Rs)
+inline int asr(Register Rd, Register Rm, Register Rs)
 {
   return movsh(Rd, Rm, Rs, ASR);
 }
-inline int asri(int Rd, int Rm, int imm)
+inline int asri(Register Rd, Register Rm, int imm)
 {
   return mov(Rd, Rm, ASR, imm);
 }
@@ -670,7 +670,7 @@ inline int bpl(int offset)
 }
 inline int fmstat()
 {
-  return fmrx(15, FPSCR);
+  return fmrx(Register(15), FPSCR);
 }
 // todo: make this pretty:
 inline int dmb()
diff --git a/src/codegen/target/arm/multimethod.cpp b/src/codegen/target/arm/multimethod.cpp
index 0ee2a642bc..ec1d96491c 100644
--- a/src/codegen/target/arm/multimethod.cpp
+++ b/src/codegen/target/arm/multimethod.cpp
@@ -22,35 +22,35 @@ using namespace util;
 
 unsigned index(ArchitectureContext*,
                lir::BinaryOperation operation,
-               lir::OperandType operand1,
-               lir::OperandType operand2)
+               lir::Operand::Type operand1,
+               lir::Operand::Type operand2)
 {
-  return operation + (lir::BinaryOperationCount * operand1)
-         + (lir::BinaryOperationCount * lir::OperandTypeCount * operand2);
+  return operation + (lir::BinaryOperationCount * (unsigned)operand1)
+         + (lir::BinaryOperationCount * lir::Operand::TypeCount * (unsigned)operand2);
 }
 
 unsigned index(ArchitectureContext* con UNUSED,
                lir::TernaryOperation operation,
-               lir::OperandType operand1)
+               lir::Operand::Type operand1)
 {
   assertT(con, not isBranch(operation));
 
-  return operation + (lir::NonBranchTernaryOperationCount * operand1);
+  return operation + (lir::NonBranchTernaryOperationCount * (unsigned)operand1);
 }
 
 unsigned branchIndex(ArchitectureContext* con UNUSED,
-                     lir::OperandType operand1,
-                     lir::OperandType operand2)
+                     lir::Operand::Type operand1,
+                     lir::Operand::Type operand2)
 {
-  return operand1 + (lir::OperandTypeCount * operand2);
+  return (unsigned)operand1 + (lir::Operand::TypeCount * (unsigned)operand2);
 }
 
 void populateTables(ArchitectureContext* con)
 {
-  const lir::OperandType C = lir::ConstantOperand;
-  const lir::OperandType A = lir::AddressOperand;
-  const lir::OperandType R = lir::RegisterOperand;
-  const lir::OperandType M = lir::MemoryOperand;
+  const lir::Operand::Type C = lir::Operand::Type::Constant;
+  const lir::Operand::Type A = lir::Operand::Type::Address;
+  const lir::Operand::Type R = lir::Operand::Type::RegisterPair;
+  const lir::Operand::Type M = lir::Operand::Type::Memory;
 
   OperationType* zo = con->operations;
   UnaryOperationType* uo = con->unaryOperations;
diff --git a/src/codegen/target/arm/multimethod.h b/src/codegen/target/arm/multimethod.h
index 1949bf5486..f234385ee6 100644
--- a/src/codegen/target/arm/multimethod.h
+++ b/src/codegen/target/arm/multimethod.h
@@ -25,16 +25,16 @@ namespace arm {
 
 unsigned index(ArchitectureContext*,
                lir::BinaryOperation operation,
-               lir::OperandType operand1,
-               lir::OperandType operand2);
+               lir::Operand::Type operand1,
+               lir::Operand::Type operand2);
 
 unsigned index(ArchitectureContext* con UNUSED,
                lir::TernaryOperation operation,
-               lir::OperandType operand1);
+               lir::Operand::Type operand1);
 
 unsigned branchIndex(ArchitectureContext* con UNUSED,
-                     lir::OperandType operand1,
-                     lir::OperandType operand2);
+                     lir::Operand::Type operand1,
+                     lir::Operand::Type operand2);
 
 void populateTables(ArchitectureContext* con);
 
diff --git a/src/codegen/target/arm/operations.cpp b/src/codegen/target/arm/operations.cpp
index 8afe088729..87d88613fd 100644
--- a/src/codegen/target/arm/operations.cpp
+++ b/src/codegen/target/arm/operations.cpp
@@ -35,20 +35,20 @@ inline unsigned lo8(int64_t i)
 void andC(Context* con,
           unsigned size,
           lir::Constant* a,
-          lir::Register* b,
-          lir::Register* dst);
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst);
 
 void shiftLeftR(Context* con,
                 unsigned size,
-                lir::Register* a,
-                lir::Register* b,
-                lir::Register* t)
+                lir::RegisterPair* a,
+                lir::RegisterPair* b,
+                lir::RegisterPair* t)
 {
   if (size == 8) {
-    int tmp1 = newTemp(con), tmp2 = newTemp(con), tmp3 = newTemp(con);
+    Register tmp1 = newTemp(con), tmp2 = newTemp(con), tmp3 = newTemp(con);
     ResolvedPromise maskPromise(0x3F);
     lir::Constant mask(&maskPromise);
-    lir::Register dst(tmp3);
+    lir::RegisterPair dst(tmp3);
     andC(con, 4, &mask, a, &dst);
     emit(con, lsl(tmp1, b->high, tmp3));
     emit(con, rsbi(tmp2, tmp3, 32));
@@ -61,10 +61,10 @@ void shiftLeftR(Context* con,
     freeTemp(con, tmp2);
     freeTemp(con, tmp3);
   } else {
-    int tmp = newTemp(con);
+    Register tmp = newTemp(con);
     ResolvedPromise maskPromise(0x1F);
     lir::Constant mask(&maskPromise);
-    lir::Register dst(tmp);
+    lir::RegisterPair dst(tmp);
     andC(con, size, &mask, a, &dst);
     emit(con, lsl(t->low, b->low, tmp));
     freeTemp(con, tmp);
@@ -73,15 +73,15 @@ void shiftLeftR(Context* con,
 
 void moveRR(Context* con,
             unsigned srcSize,
-            lir::Register* src,
+            lir::RegisterPair* src,
             unsigned dstSize,
-            lir::Register* dst);
+            lir::RegisterPair* dst);
 
 void shiftLeftC(Context* con,
                 unsigned size UNUSED,
                 lir::Constant* a,
-                lir::Register* b,
-                lir::Register* t)
+                lir::RegisterPair* b,
+                lir::RegisterPair* t)
 {
   assertT(con, size == vm::TargetBytesPerWord);
   if (getValue(a) & 0x1F) {
@@ -93,15 +93,15 @@ void shiftLeftC(Context* con,
 
 void shiftRightR(Context* con,
                  unsigned size,
-                 lir::Register* a,
-                 lir::Register* b,
-                 lir::Register* t)
+                 lir::RegisterPair* a,
+                 lir::RegisterPair* b,
+                 lir::RegisterPair* t)
 {
   if (size == 8) {
-    int tmp1 = newTemp(con), tmp2 = newTemp(con), tmp3 = newTemp(con);
+    Register tmp1 = newTemp(con), tmp2 = newTemp(con), tmp3 = newTemp(con);
     ResolvedPromise maskPromise(0x3F);
     lir::Constant mask(&maskPromise);
-    lir::Register dst(tmp3);
+    lir::RegisterPair dst(tmp3);
     andC(con, 4, &mask, a, &dst);
     emit(con, lsr(tmp1, b->low, tmp3));
     emit(con, rsbi(tmp2, tmp3, 32));
@@ -114,10 +114,10 @@ void shiftRightR(Context* con,
     freeTemp(con, tmp2);
     freeTemp(con, tmp3);
   } else {
-    int tmp = newTemp(con);
+    Register tmp = newTemp(con);
     ResolvedPromise maskPromise(0x1F);
     lir::Constant mask(&maskPromise);
-    lir::Register dst(tmp);
+    lir::RegisterPair dst(tmp);
     andC(con, size, &mask, a, &dst);
     emit(con, asr(t->low, b->low, tmp));
     freeTemp(con, tmp);
@@ -127,8 +127,8 @@ void shiftRightR(Context* con,
 void shiftRightC(Context* con,
                  unsigned size UNUSED,
                  lir::Constant* a,
-                 lir::Register* b,
-                 lir::Register* t)
+                 lir::RegisterPair* b,
+                 lir::RegisterPair* t)
 {
   assertT(con, size == vm::TargetBytesPerWord);
   if (getValue(a) & 0x1F) {
@@ -140,18 +140,18 @@ void shiftRightC(Context* con,
 
 void unsignedShiftRightR(Context* con,
                          unsigned size,
-                         lir::Register* a,
-                         lir::Register* b,
-                         lir::Register* t)
+                         lir::RegisterPair* a,
+                         lir::RegisterPair* b,
+                         lir::RegisterPair* t)
 {
-  int tmpShift = newTemp(con);
+  Register tmpShift = newTemp(con);
   ResolvedPromise maskPromise(size == 8 ? 0x3F : 0x1F);
   lir::Constant mask(&maskPromise);
-  lir::Register dst(tmpShift);
+  lir::RegisterPair dst(tmpShift);
   andC(con, 4, &mask, a, &dst);
   emit(con, lsr(t->low, b->low, tmpShift));
   if (size == 8) {
-    int tmpHi = newTemp(con), tmpLo = newTemp(con);
+    Register tmpHi = newTemp(con), tmpLo = newTemp(con);
     emit(con, SETS(rsbi(tmpHi, tmpShift, 32)));
     emit(con, lsl(tmpLo, b->high, tmpHi));
     emit(con, orr(t->low, t->low, tmpLo));
@@ -168,8 +168,8 @@ void unsignedShiftRightR(Context* con,
 void unsignedShiftRightC(Context* con,
                          unsigned size UNUSED,
                          lir::Constant* a,
-                         lir::Register* b,
-                         lir::Register* t)
+                         lir::RegisterPair* b,
+                         lir::RegisterPair* t)
 {
   assertT(con, size == vm::TargetBytesPerWord);
   if (getValue(a) & 0x1F) {
@@ -274,7 +274,7 @@ void resolve(MyBlock* b)
   }
 }
 
-void jumpR(Context* con, unsigned size UNUSED, lir::Register* target)
+void jumpR(Context* con, unsigned size UNUSED, lir::RegisterPair* target)
 {
   assertT(con, size == vm::TargetBytesPerWord);
   emit(con, bx(target->low));
@@ -282,14 +282,14 @@ void jumpR(Context* con, unsigned size UNUSED, lir::Register* target)
 
 void swapRR(Context* con,
             unsigned aSize,
-            lir::Register* a,
+            lir::RegisterPair* a,
             unsigned bSize,
-            lir::Register* b)
+            lir::RegisterPair* b)
 {
   assertT(con, aSize == vm::TargetBytesPerWord);
   assertT(con, bSize == vm::TargetBytesPerWord);
 
-  lir::Register tmp(con->client->acquireTemporary(GPR_MASK));
+  lir::RegisterPair tmp(con->client->acquireTemporary(GPR_MASK));
   moveRR(con, aSize, a, bSize, &tmp);
   moveRR(con, bSize, b, aSize, a);
   moveRR(con, bSize, &tmp, bSize, b);
@@ -298,9 +298,9 @@ void swapRR(Context* con,
 
 void moveRR(Context* con,
             unsigned srcSize,
-            lir::Register* src,
+            lir::RegisterPair* src,
             unsigned dstSize,
-            lir::Register* dst)
+            lir::RegisterPair* dst)
 {
   bool srcIsFpr = isFpr(src);
   bool dstIsFpr = isFpr(dst);
@@ -343,8 +343,8 @@ void moveRR(Context* con,
       moveRR(con, 4, src, 4, dst);
       emit(con, asri(dst->high, src->low, 31));
     } else if (srcSize == 8 and dstSize == 8) {
-      lir::Register srcHigh(src->high);
-      lir::Register dstHigh(dst->high);
+      lir::RegisterPair srcHigh(src->high);
+      lir::RegisterPair dstHigh(dst->high);
 
       if (src->high == dst->low) {
         if (src->low == dst->high) {
@@ -369,9 +369,9 @@ void moveRR(Context* con,
 
 void moveZRR(Context* con,
              unsigned srcSize,
-             lir::Register* src,
+             lir::RegisterPair* src,
              unsigned,
-             lir::Register* dst)
+             lir::RegisterPair* dst)
 {
   switch (srcSize) {
   case 2:
@@ -388,16 +388,16 @@ void moveCR(Context* con,
             unsigned size,
             lir::Constant* src,
             unsigned,
-            lir::Register* dst);
+            lir::RegisterPair* dst);
 
 void moveCR2(Context* con,
              unsigned size,
              lir::Constant* src,
-             lir::Register* dst,
+             lir::RegisterPair* dst,
              Promise* callOffset)
 {
   if (isFpr(dst)) {  // floating-point
-    lir::Register tmp = size > 4 ? makeTemp64(con) : makeTemp(con);
+    lir::RegisterPair tmp = size > 4 ? makeTemp64(con) : makeTemp(con);
     moveCR(con, size, src, size, &tmp);
     moveRR(con, size, &tmp, size, dst);
     freeTemp(con, tmp);
@@ -407,7 +407,7 @@ void moveCR2(Context* con,
     lir::Constant srcLo(&loBits);
     ResolvedPromise hiBits(value >> 32);
     lir::Constant srcHi(&hiBits);
-    lir::Register dstHi(dst->high);
+    lir::RegisterPair dstHi(dst->high);
     moveCR(con, 4, &srcLo, 4, dst);
     moveCR(con, 4, &srcHi, 4, &dstHi);
   } else if (src->value->resolved() and isOfWidth(getValue(src), 8)) {
@@ -422,16 +422,16 @@ void moveCR(Context* con,
             unsigned size,
             lir::Constant* src,
             unsigned,
-            lir::Register* dst)
+            lir::RegisterPair* dst)
 {
   moveCR2(con, size, src, dst, 0);
 }
 
 void addR(Context* con,
           unsigned size,
-          lir::Register* a,
-          lir::Register* b,
-          lir::Register* t)
+          lir::RegisterPair* a,
+          lir::RegisterPair* b,
+          lir::RegisterPair* t)
 {
   if (size == 8) {
     emit(con, SETS(add(t->low, a->low, b->low)));
@@ -443,9 +443,9 @@ void addR(Context* con,
 
 void subR(Context* con,
           unsigned size,
-          lir::Register* a,
-          lir::Register* b,
-          lir::Register* t)
+          lir::RegisterPair* a,
+          lir::RegisterPair* b,
+          lir::RegisterPair* t)
 {
   if (size == 8) {
     emit(con, SETS(rsb(t->low, a->low, b->low)));
@@ -458,8 +458,8 @@ void subR(Context* con,
 void addC(Context* con,
           unsigned size,
           lir::Constant* a,
-          lir::Register* b,
-          lir::Register* dst)
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst)
 {
   assertT(con, size == vm::TargetBytesPerWord);
 
@@ -481,8 +481,8 @@ void addC(Context* con,
 void subC(Context* con,
           unsigned size,
           lir::Constant* a,
-          lir::Register* b,
-          lir::Register* dst)
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst)
 {
   assertT(con, size == vm::TargetBytesPerWord);
 
@@ -503,15 +503,15 @@ void subC(Context* con,
 
 void multiplyR(Context* con,
                unsigned size,
-               lir::Register* a,
-               lir::Register* b,
-               lir::Register* t)
+               lir::RegisterPair* a,
+               lir::RegisterPair* b,
+               lir::RegisterPair* t)
 {
   if (size == 8) {
     bool useTemporaries = b->low == t->low;
-    int tmpLow = useTemporaries ? con->client->acquireTemporary(GPR_MASK)
+    Register tmpLow = useTemporaries ? con->client->acquireTemporary(GPR_MASK)
                                 : t->low;
-    int tmpHigh = useTemporaries ? con->client->acquireTemporary(GPR_MASK)
+    Register tmpHigh = useTemporaries ? con->client->acquireTemporary(GPR_MASK)
                                  : t->high;
 
     emit(con, umull(tmpLow, tmpHigh, a->low, b->low));
@@ -531,9 +531,9 @@ void multiplyR(Context* con,
 
 void floatAbsoluteRR(Context* con,
                      unsigned size,
-                     lir::Register* a,
+                     lir::RegisterPair* a,
                      unsigned,
-                     lir::Register* b)
+                     lir::RegisterPair* b)
 {
   if (size == 8) {
     emit(con, fabsd(fpr64(b), fpr64(a)));
@@ -544,9 +544,9 @@ void floatAbsoluteRR(Context* con,
 
 void floatNegateRR(Context* con,
                    unsigned size,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned,
-                   lir::Register* b)
+                   lir::RegisterPair* b)
 {
   if (size == 8) {
     emit(con, fnegd(fpr64(b), fpr64(a)));
@@ -557,9 +557,9 @@ void floatNegateRR(Context* con,
 
 void float2FloatRR(Context* con,
                    unsigned size,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned,
-                   lir::Register* b)
+                   lir::RegisterPair* b)
 {
   if (size == 8) {
     emit(con, fcvtsd(fpr32(b), fpr64(a)));
@@ -570,11 +570,11 @@ void float2FloatRR(Context* con,
 
 void float2IntRR(Context* con,
                  unsigned size,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
-  int tmp = newTemp(con, FPR_MASK);
+  Register tmp = newTemp(con, FPR_MASK);
   int ftmp = fpr32(tmp);
   if (size == 8) {  // double to int
     emit(con, ftosizd(ftmp, fpr64(a)));
@@ -587,9 +587,9 @@ void float2IntRR(Context* con,
 
 void int2FloatRR(Context* con,
                  unsigned,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned size,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   emit(con, fmsr(fpr32(b), a->low));
   if (size == 8) {  // int to double
@@ -601,9 +601,9 @@ void int2FloatRR(Context* con,
 
 void floatSqrtRR(Context* con,
                  unsigned size,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   if (size == 8) {
     emit(con, fsqrtd(fpr64(b), fpr64(a)));
@@ -614,9 +614,9 @@ void floatSqrtRR(Context* con,
 
 void floatAddR(Context* con,
                unsigned size,
-               lir::Register* a,
-               lir::Register* b,
-               lir::Register* t)
+               lir::RegisterPair* a,
+               lir::RegisterPair* b,
+               lir::RegisterPair* t)
 {
   if (size == 8) {
     emit(con, faddd(fpr64(t), fpr64(a), fpr64(b)));
@@ -627,9 +627,9 @@ void floatAddR(Context* con,
 
 void floatSubtractR(Context* con,
                     unsigned size,
-                    lir::Register* a,
-                    lir::Register* b,
-                    lir::Register* t)
+                    lir::RegisterPair* a,
+                    lir::RegisterPair* b,
+                    lir::RegisterPair* t)
 {
   if (size == 8) {
     emit(con, fsubd(fpr64(t), fpr64(b), fpr64(a)));
@@ -640,9 +640,9 @@ void floatSubtractR(Context* con,
 
 void floatMultiplyR(Context* con,
                     unsigned size,
-                    lir::Register* a,
-                    lir::Register* b,
-                    lir::Register* t)
+                    lir::RegisterPair* a,
+                    lir::RegisterPair* b,
+                    lir::RegisterPair* t)
 {
   if (size == 8) {
     emit(con, fmuld(fpr64(t), fpr64(a), fpr64(b)));
@@ -653,9 +653,9 @@ void floatMultiplyR(Context* con,
 
 void floatDivideR(Context* con,
                   unsigned size,
-                  lir::Register* a,
-                  lir::Register* b,
-                  lir::Register* t)
+                  lir::RegisterPair* a,
+                  lir::RegisterPair* b,
+                  lir::RegisterPair* t)
 {
   if (size == 8) {
     emit(con, fdivd(fpr64(t), fpr64(b), fpr64(a)));
@@ -664,15 +664,15 @@ void floatDivideR(Context* con,
   }
 }
 
-int normalize(Context* con,
+Register normalize(Context* con,
               int offset,
-              int index,
+              Register index,
               unsigned scale,
               bool* preserveIndex,
               bool* release)
 {
   if (offset != 0 or scale != 1) {
-    lir::Register normalizedIndex(
+    lir::RegisterPair normalizedIndex(
         *preserveIndex ? con->client->acquireTemporary(GPR_MASK) : index);
 
     if (*preserveIndex) {
@@ -682,10 +682,10 @@ int normalize(Context* con,
       *release = false;
     }
 
-    int scaled;
+    Register scaled;
 
     if (scale != 1) {
-      lir::Register unscaledIndex(index);
+      lir::RegisterPair unscaledIndex(index);
 
       ResolvedPromise scalePromise(log(scale));
       lir::Constant scaleConstant(&scalePromise);
@@ -702,12 +702,12 @@ int normalize(Context* con,
     }
 
     if (offset != 0) {
-      lir::Register untranslatedIndex(scaled);
+      lir::RegisterPair untranslatedIndex(scaled);
 
       ResolvedPromise offsetPromise(offset);
       lir::Constant offsetConstant(&offsetPromise);
 
-      lir::Register tmp(con->client->acquireTemporary(GPR_MASK));
+      lir::RegisterPair tmp(con->client->acquireTemporary(GPR_MASK));
       moveCR(con,
              vm::TargetBytesPerWord,
              &offsetConstant,
@@ -730,16 +730,16 @@ int normalize(Context* con,
 
 void store(Context* con,
            unsigned size,
-           lir::Register* src,
-           int base,
+           lir::RegisterPair* src,
+           Register base,
            int offset,
-           int index,
+           Register index,
            unsigned scale,
            bool preserveIndex)
 {
-  if (index != lir::NoRegister) {
+  if (index != NoRegister) {
     bool release;
-    int normalized
+    Register normalized
         = normalize(con, offset, index, scale, &preserveIndex, &release);
 
     if (!isFpr(src)) {  // GPR store
@@ -757,7 +757,7 @@ void store(Context* con,
         break;
 
       case 8: {  // split into 2 32-bit stores
-        lir::Register srcHigh(src->high);
+        lir::RegisterPair srcHigh(src->high);
         store(con, 4, &srcHigh, base, 0, normalized, 1, preserveIndex);
         store(con, 4, src, base, 4, normalized, 1, preserveIndex);
       } break;
@@ -766,7 +766,7 @@ void store(Context* con,
         abort(con);
       }
     } else {  // FPR store
-      lir::Register base_(base), normalized_(normalized),
+      lir::RegisterPair base_(base), normalized_(normalized),
           absAddr = makeTemp(con);
       // FPR stores have only bases, so we must add the index
       addR(con, vm::TargetBytesPerWord, &base_, &normalized_, &absAddr);
@@ -798,9 +798,9 @@ void store(Context* con,
         break;
 
       case 8: {  // split into 2 32-bit stores
-        lir::Register srcHigh(src->high);
-        store(con, 4, &srcHigh, base, offset, lir::NoRegister, 1, false);
-        store(con, 4, src, base, offset + 4, lir::NoRegister, 1, false);
+        lir::RegisterPair srcHigh(src->high);
+        store(con, 4, &srcHigh, base, offset, NoRegister, 1, false);
+        store(con, 4, src, base, offset + 4, NoRegister, 1, false);
       } break;
 
       default:
@@ -815,7 +815,7 @@ void store(Context* con,
         emit(con, fsts(fpr32(src), base, offset));
     }
   } else {
-    lir::Register tmp(con->client->acquireTemporary(GPR_MASK));
+    lir::RegisterPair tmp(con->client->acquireTemporary(GPR_MASK));
     ResolvedPromise offsetPromise(offset);
     lir::Constant offsetConstant(&offsetPromise);
     moveCR(con,
@@ -832,7 +832,7 @@ void store(Context* con,
 
 void moveRM(Context* con,
             unsigned srcSize,
-            lir::Register* src,
+            lir::RegisterPair* src,
             unsigned dstSize UNUSED,
             lir::Memory* dst)
 {
@@ -844,18 +844,18 @@ void moveRM(Context* con,
 
 void load(Context* con,
           unsigned srcSize,
-          int base,
+          Register base,
           int offset,
-          int index,
+          Register index,
           unsigned scale,
           unsigned dstSize,
-          lir::Register* dst,
+          lir::RegisterPair* dst,
           bool preserveIndex,
           bool signExtend)
 {
-  if (index != lir::NoRegister) {
+  if (index != NoRegister) {
     bool release;
-    int normalized
+    Register normalized
         = normalize(con, offset, index, scale, &preserveIndex, &release);
 
     if (!isFpr(dst)) {  // GPR load
@@ -882,7 +882,7 @@ void load(Context* con,
           load(con, 4, base, 0, normalized, 1, 4, dst, preserveIndex, false);
           moveRR(con, 4, dst, 8, dst);
         } else if (srcSize == 8 and dstSize == 8) {
-          lir::Register dstHigh(dst->high);
+          lir::RegisterPair dstHigh(dst->high);
           load(con,
                4,
                base,
@@ -903,7 +903,7 @@ void load(Context* con,
         abort(con);
       }
     } else {  // FPR load
-      lir::Register base_(base), normalized_(normalized),
+      lir::RegisterPair base_(base), normalized_(normalized),
           absAddr = makeTemp(con);
       // VFP loads only have bases, so we must add the index
       addR(con, vm::TargetBytesPerWord, &base_, &normalized_, &absAddr);
@@ -946,12 +946,12 @@ void load(Context* con,
 
       case 8: {
         if (dstSize == 8) {
-          lir::Register dstHigh(dst->high);
+          lir::RegisterPair dstHigh(dst->high);
           load(con,
                4,
                base,
                offset,
-               lir::NoRegister,
+               NoRegister,
                1,
                4,
                &dstHigh,
@@ -961,7 +961,7 @@ void load(Context* con,
                4,
                base,
                offset + 4,
-               lir::NoRegister,
+               NoRegister,
                1,
                4,
                dst,
@@ -984,7 +984,7 @@ void load(Context* con,
         emit(con, flds(fpr32(dst), base, offset));
     }
   } else {
-    lir::Register tmp(con->client->acquireTemporary(GPR_MASK));
+    lir::RegisterPair tmp(con->client->acquireTemporary(GPR_MASK));
     ResolvedPromise offsetPromise(offset);
     lir::Constant offsetConstant(&offsetPromise);
     moveCR(con,
@@ -1003,7 +1003,7 @@ void moveMR(Context* con,
             unsigned srcSize,
             lir::Memory* src,
             unsigned dstSize,
-            lir::Register* dst)
+            lir::RegisterPair* dst)
 {
   load(con,
        srcSize,
@@ -1021,7 +1021,7 @@ void moveZMR(Context* con,
              unsigned srcSize,
              lir::Memory* src,
              unsigned dstSize,
-             lir::Register* dst)
+             lir::RegisterPair* dst)
 {
   load(con,
        srcSize,
@@ -1037,9 +1037,9 @@ void moveZMR(Context* con,
 
 void andR(Context* con,
           unsigned size,
-          lir::Register* a,
-          lir::Register* b,
-          lir::Register* dst)
+          lir::RegisterPair* a,
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst)
 {
   if (size == 8)
     emit(con, and_(dst->high, a->high, b->high));
@@ -1049,8 +1049,8 @@ void andR(Context* con,
 void andC(Context* con,
           unsigned size,
           lir::Constant* a,
-          lir::Register* b,
-          lir::Register* dst)
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst)
 {
   int64_t v = a->value->value();
 
@@ -1061,8 +1061,8 @@ void andC(Context* con,
     ResolvedPromise low(v & 0xFFFFFFFF);
     lir::Constant al(&low);
 
-    lir::Register bh(b->high);
-    lir::Register dh(dst->high);
+    lir::RegisterPair bh(b->high);
+    lir::RegisterPair dh(dst->high);
 
     andC(con, 4, &al, b, dst);
     andC(con, 4, &ah, &bh, &dh);
@@ -1078,7 +1078,7 @@ void andC(Context* con,
         // instruction
 
         bool useTemporary = b->low == dst->low;
-        lir::Register tmp(dst->low);
+        lir::RegisterPair tmp(dst->low);
         if (useTemporary) {
           tmp.low = con->client->acquireTemporary(GPR_MASK);
         }
@@ -1098,9 +1098,9 @@ void andC(Context* con,
 
 void orR(Context* con,
          unsigned size,
-         lir::Register* a,
-         lir::Register* b,
-         lir::Register* dst)
+         lir::RegisterPair* a,
+         lir::RegisterPair* b,
+         lir::RegisterPair* dst)
 {
   if (size == 8)
     emit(con, orr(dst->high, a->high, b->high));
@@ -1109,9 +1109,9 @@ void orR(Context* con,
 
 void xorR(Context* con,
           unsigned size,
-          lir::Register* a,
-          lir::Register* b,
-          lir::Register* dst)
+          lir::RegisterPair* a,
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst)
 {
   if (size == 8)
     emit(con, eor(dst->high, a->high, b->high));
@@ -1122,14 +1122,14 @@ void moveAR2(Context* con,
              unsigned srcSize,
              lir::Address* src,
              unsigned dstSize,
-             lir::Register* dst)
+             lir::RegisterPair* dst)
 {
   assertT(con, srcSize == 4 and dstSize == 4);
 
   lir::Constant constant(src->address);
   moveCR(con, srcSize, &constant, dstSize, dst);
 
-  lir::Memory memory(dst->low, 0, -1, 0);
+  lir::Memory memory(dst->low, 0, NoRegister, 0);
   moveMR(con, dstSize, &memory, dstSize, dst);
 }
 
@@ -1137,16 +1137,16 @@ void moveAR(Context* con,
             unsigned srcSize,
             lir::Address* src,
             unsigned dstSize,
-            lir::Register* dst)
+            lir::RegisterPair* dst)
 {
   moveAR2(con, srcSize, src, dstSize, dst);
 }
 
 void compareRR(Context* con,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize UNUSED,
-               lir::Register* b)
+               lir::RegisterPair* b)
 {
   assertT(con, !(isFpr(a) ^ isFpr(b)));  // regs must be of the same type
 
@@ -1168,14 +1168,14 @@ void compareCR(Context* con,
                unsigned aSize,
                lir::Constant* a,
                unsigned bSize,
-               lir::Register* b)
+               lir::RegisterPair* b)
 {
   assertT(con, aSize == 4 and bSize == 4);
 
   if (!isFpr(b) && a->value->resolved() && isOfWidth(a->value->value(), 8)) {
     emit(con, cmpi(b->low, a->value->value()));
   } else {
-    lir::Register tmp(con->client->acquireTemporary(GPR_MASK));
+    lir::RegisterPair tmp(con->client->acquireTemporary(GPR_MASK));
     moveCR(con, aSize, a, bSize, &tmp);
     compareRR(con, bSize, &tmp, bSize, b);
     con->client->releaseTemporary(tmp.low);
@@ -1190,7 +1190,7 @@ void compareCM(Context* con,
 {
   assertT(con, aSize == 4 and bSize == 4);
 
-  lir::Register tmp(con->client->acquireTemporary(GPR_MASK));
+  lir::RegisterPair tmp(con->client->acquireTemporary(GPR_MASK));
   moveMR(con, bSize, b, bSize, &tmp);
   compareCR(con, aSize, a, bSize, &tmp);
   con->client->releaseTemporary(tmp.low);
@@ -1198,13 +1198,13 @@ void compareCM(Context* con,
 
 void compareRM(Context* con,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize,
                lir::Memory* b)
 {
   assertT(con, aSize == 4 and bSize == 4);
 
-  lir::Register tmp(con->client->acquireTemporary(GPR_MASK));
+  lir::RegisterPair tmp(con->client->acquireTemporary(GPR_MASK));
   moveMR(con, bSize, b, bSize, &tmp);
   compareRR(con, aSize, a, bSize, &tmp);
   con->client->releaseTemporary(tmp.low);
@@ -1352,13 +1352,13 @@ void branchLong(Context* con,
 void branchRR(Context* con,
               lir::TernaryOperation op,
               unsigned size,
-              lir::Register* a,
-              lir::Register* b,
+              lir::RegisterPair* a,
+              lir::RegisterPair* b,
               lir::Constant* target)
 {
   if (!isFpr(a) && size > vm::TargetBytesPerWord) {
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
     branchLong(
         con, op, a, &ah, b, &bh, target, CAST2(compareRR), CAST2(compareRR));
@@ -1372,7 +1372,7 @@ void branchCR(Context* con,
               lir::TernaryOperation op,
               unsigned size,
               lir::Constant* a,
-              lir::Register* b,
+              lir::RegisterPair* b,
               lir::Constant* target)
 {
   assertT(con, !isFloatBranch(op));
@@ -1386,7 +1386,7 @@ void branchCR(Context* con,
     ResolvedPromise high((v >> 32) & ~static_cast<vm::target_uintptr_t>(0));
     lir::Constant ah(&high);
 
-    lir::Register bh(b->high);
+    lir::RegisterPair bh(b->high);
 
     branchLong(
         con, op, &al, &ah, b, &bh, target, CAST2(compareCR), CAST2(compareCR));
@@ -1399,7 +1399,7 @@ void branchCR(Context* con,
 void branchRM(Context* con,
               lir::TernaryOperation op,
               unsigned size,
-              lir::Register* a,
+              lir::RegisterPair* a,
               lir::Memory* b,
               lir::Constant* target)
 {
@@ -1450,7 +1450,7 @@ void moveCM(Context* con,
   } break;
 
   default:
-    lir::Register tmp(con->client->acquireTemporary(GPR_MASK));
+    lir::RegisterPair tmp(con->client->acquireTemporary(GPR_MASK));
     moveCR(con, srcSize, src, dstSize, &tmp);
     moveRM(con, dstSize, &tmp, dstSize, dst);
     con->client->releaseTemporary(tmp.low);
@@ -1459,9 +1459,9 @@ void moveCM(Context* con,
 
 void negateRR(Context* con,
               unsigned srcSize,
-              lir::Register* src,
+              lir::RegisterPair* src,
               unsigned dstSize UNUSED,
-              lir::Register* dst)
+              lir::RegisterPair* dst)
 {
   assertT(con, srcSize == dstSize);
 
@@ -1473,7 +1473,7 @@ void negateRR(Context* con,
   }
 }
 
-void callR(Context* con, unsigned size UNUSED, lir::Register* target)
+void callR(Context* con, unsigned size UNUSED, lir::RegisterPair* target)
 {
   assertT(con, size == vm::TargetBytesPerWord);
   emit(con, blx(target->low));
@@ -1491,7 +1491,7 @@ void longCallC(Context* con, unsigned size UNUSED, lir::Constant* target)
 {
   assertT(con, size == vm::TargetBytesPerWord);
 
-  lir::Register tmp(4);
+  lir::RegisterPair tmp(Register(4));
   moveCR2(con, vm::TargetBytesPerWord, target, &tmp, offsetPromise(con));
   callR(con, vm::TargetBytesPerWord, &tmp);
 }
@@ -1500,7 +1500,7 @@ void longJumpC(Context* con, unsigned size UNUSED, lir::Constant* target)
 {
   assertT(con, size == vm::TargetBytesPerWord);
 
-  lir::Register tmp(4);  // a non-arg reg that we don't mind clobbering
+  lir::RegisterPair tmp(Register(4));  // a non-arg reg that we don't mind clobbering
   moveCR2(con, vm::TargetBytesPerWord, target, &tmp, offsetPromise(con));
   jumpR(con, vm::TargetBytesPerWord, &tmp);
 }
diff --git a/src/codegen/target/arm/operations.h b/src/codegen/target/arm/operations.h
index 4c3f53cb56..1ff77d6198 100644
--- a/src/codegen/target/arm/operations.h
+++ b/src/codegen/target/arm/operations.h
@@ -25,17 +25,17 @@ class Context;
 
 // shortcut functions
 
-inline int newTemp(Context* con)
+inline Register newTemp(Context* con)
 {
   return con->client->acquireTemporary(GPR_MASK);
 }
 
-inline int newTemp(Context* con, unsigned mask)
+inline Register newTemp(Context* con, RegisterMask mask)
 {
   return con->client->acquireTemporary(mask);
 }
 
-inline void freeTemp(Context* con, int r)
+inline void freeTemp(Context* con, Register r)
 {
   con->client->releaseTemporary(r);
 }
@@ -45,67 +45,67 @@ inline int64_t getValue(lir::Constant* con)
   return con->value->value();
 }
 
-inline lir::Register makeTemp(Context* con)
+inline lir::RegisterPair makeTemp(Context* con)
 {
-  lir::Register tmp(newTemp(con));
+  lir::RegisterPair tmp(newTemp(con));
   return tmp;
 }
 
-inline lir::Register makeTemp64(Context* con)
+inline lir::RegisterPair makeTemp64(Context* con)
 {
-  lir::Register tmp(newTemp(con), newTemp(con));
+  lir::RegisterPair tmp(newTemp(con), newTemp(con));
   return tmp;
 }
 
-inline void freeTemp(Context* con, const lir::Register& tmp)
+inline void freeTemp(Context* con, const lir::RegisterPair& tmp)
 {
-  if (tmp.low != lir::NoRegister)
+  if (tmp.low != NoRegister)
     freeTemp(con, tmp.low);
-  if (tmp.high != lir::NoRegister)
+  if (tmp.high != NoRegister)
     freeTemp(con, tmp.high);
 }
 
 void shiftLeftR(Context* con,
                 unsigned size,
-                lir::Register* a,
-                lir::Register* b,
-                lir::Register* t);
+                lir::RegisterPair* a,
+                lir::RegisterPair* b,
+                lir::RegisterPair* t);
 
 void moveRR(Context* con,
             unsigned srcSize,
-            lir::Register* src,
+            lir::RegisterPair* src,
             unsigned dstSize,
-            lir::Register* dst);
+            lir::RegisterPair* dst);
 
 void shiftLeftC(Context* con,
                 unsigned size UNUSED,
                 lir::Constant* a,
-                lir::Register* b,
-                lir::Register* t);
+                lir::RegisterPair* b,
+                lir::RegisterPair* t);
 
 void shiftRightR(Context* con,
                  unsigned size,
-                 lir::Register* a,
-                 lir::Register* b,
-                 lir::Register* t);
+                 lir::RegisterPair* a,
+                 lir::RegisterPair* b,
+                 lir::RegisterPair* t);
 
 void shiftRightC(Context* con,
                  unsigned size UNUSED,
                  lir::Constant* a,
-                 lir::Register* b,
-                 lir::Register* t);
+                 lir::RegisterPair* b,
+                 lir::RegisterPair* t);
 
 void unsignedShiftRightR(Context* con,
                          unsigned size,
-                         lir::Register* a,
-                         lir::Register* b,
-                         lir::Register* t);
+                         lir::RegisterPair* a,
+                         lir::RegisterPair* b,
+                         lir::RegisterPair* t);
 
 void unsignedShiftRightC(Context* con,
                          unsigned size UNUSED,
                          lir::Constant* a,
-                         lir::Register* b,
-                         lir::Register* t);
+                         lir::RegisterPair* b,
+                         lir::RegisterPair* t);
 
 bool needJump(MyBlock* b);
 
@@ -113,133 +113,133 @@ unsigned padding(MyBlock* b, unsigned offset);
 
 void resolve(MyBlock* b);
 
-void jumpR(Context* con, unsigned size UNUSED, lir::Register* target);
+void jumpR(Context* con, unsigned size UNUSED, lir::RegisterPair* target);
 
 void swapRR(Context* con,
             unsigned aSize,
-            lir::Register* a,
+            lir::RegisterPair* a,
             unsigned bSize,
-            lir::Register* b);
+            lir::RegisterPair* b);
 
 void moveRR(Context* con,
             unsigned srcSize,
-            lir::Register* src,
+            lir::RegisterPair* src,
             unsigned dstSize,
-            lir::Register* dst);
+            lir::RegisterPair* dst);
 
 void moveZRR(Context* con,
              unsigned srcSize,
-             lir::Register* src,
+             lir::RegisterPair* src,
              unsigned,
-             lir::Register* dst);
+             lir::RegisterPair* dst);
 
 void moveCR(Context* con,
             unsigned size,
             lir::Constant* src,
             unsigned,
-            lir::Register* dst);
+            lir::RegisterPair* dst);
 
 void moveCR2(Context* con,
              unsigned size,
              lir::Constant* src,
-             lir::Register* dst,
+             lir::RegisterPair* dst,
              Promise* callOffset);
 
 void moveCR(Context* con,
             unsigned size,
             lir::Constant* src,
             unsigned,
-            lir::Register* dst);
+            lir::RegisterPair* dst);
 
 void addR(Context* con,
           unsigned size,
-          lir::Register* a,
-          lir::Register* b,
-          lir::Register* t);
+          lir::RegisterPair* a,
+          lir::RegisterPair* b,
+          lir::RegisterPair* t);
 
 void subR(Context* con,
           unsigned size,
-          lir::Register* a,
-          lir::Register* b,
-          lir::Register* t);
+          lir::RegisterPair* a,
+          lir::RegisterPair* b,
+          lir::RegisterPair* t);
 
 void addC(Context* con,
           unsigned size,
           lir::Constant* a,
-          lir::Register* b,
-          lir::Register* dst);
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst);
 
 void subC(Context* con,
           unsigned size,
           lir::Constant* a,
-          lir::Register* b,
-          lir::Register* dst);
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst);
 
 void multiplyR(Context* con,
                unsigned size,
-               lir::Register* a,
-               lir::Register* b,
-               lir::Register* t);
+               lir::RegisterPair* a,
+               lir::RegisterPair* b,
+               lir::RegisterPair* t);
 
 void floatAbsoluteRR(Context* con,
                      unsigned size,
-                     lir::Register* a,
+                     lir::RegisterPair* a,
                      unsigned,
-                     lir::Register* b);
+                     lir::RegisterPair* b);
 
 void floatNegateRR(Context* con,
                    unsigned size,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned,
-                   lir::Register* b);
+                   lir::RegisterPair* b);
 
 void float2FloatRR(Context* con,
                    unsigned size,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned,
-                   lir::Register* b);
+                   lir::RegisterPair* b);
 
 void float2IntRR(Context* con,
                  unsigned size,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void int2FloatRR(Context* con,
                  unsigned,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned size,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void floatSqrtRR(Context* con,
                  unsigned size,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void floatAddR(Context* con,
                unsigned size,
-               lir::Register* a,
-               lir::Register* b,
-               lir::Register* t);
+               lir::RegisterPair* a,
+               lir::RegisterPair* b,
+               lir::RegisterPair* t);
 
 void floatSubtractR(Context* con,
                     unsigned size,
-                    lir::Register* a,
-                    lir::Register* b,
-                    lir::Register* t);
+                    lir::RegisterPair* a,
+                    lir::RegisterPair* b,
+                    lir::RegisterPair* t);
 
 void floatMultiplyR(Context* con,
                     unsigned size,
-                    lir::Register* a,
-                    lir::Register* b,
-                    lir::Register* t);
+                    lir::RegisterPair* a,
+                    lir::RegisterPair* b,
+                    lir::RegisterPair* t);
 
 void floatDivideR(Context* con,
                   unsigned size,
-                  lir::Register* a,
-                  lir::Register* b,
-                  lir::Register* t);
+                  lir::RegisterPair* a,
+                  lir::RegisterPair* b,
+                  lir::RegisterPair* t);
 
 int normalize(Context* con,
               int offset,
@@ -250,7 +250,7 @@ int normalize(Context* con,
 
 void store(Context* con,
            unsigned size,
-           lir::Register* src,
+           lir::RegisterPair* src,
            int base,
            int offset,
            int index,
@@ -259,7 +259,7 @@ void store(Context* con,
 
 void moveRM(Context* con,
             unsigned srcSize,
-            lir::Register* src,
+            lir::RegisterPair* src,
             unsigned dstSize UNUSED,
             lir::Memory* dst);
 
@@ -270,7 +270,7 @@ void load(Context* con,
           int index,
           unsigned scale,
           unsigned dstSize,
-          lir::Register* dst,
+          lir::RegisterPair* dst,
           bool preserveIndex,
           bool signExtend);
 
@@ -278,61 +278,61 @@ void moveMR(Context* con,
             unsigned srcSize,
             lir::Memory* src,
             unsigned dstSize,
-            lir::Register* dst);
+            lir::RegisterPair* dst);
 
 void moveZMR(Context* con,
              unsigned srcSize,
              lir::Memory* src,
              unsigned dstSize,
-             lir::Register* dst);
+             lir::RegisterPair* dst);
 
 void andR(Context* con,
           unsigned size,
-          lir::Register* a,
-          lir::Register* b,
-          lir::Register* dst);
+          lir::RegisterPair* a,
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst);
 
 void andC(Context* con,
           unsigned size,
           lir::Constant* a,
-          lir::Register* b,
-          lir::Register* dst);
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst);
 
 void orR(Context* con,
          unsigned size,
-         lir::Register* a,
-         lir::Register* b,
-         lir::Register* dst);
+         lir::RegisterPair* a,
+         lir::RegisterPair* b,
+         lir::RegisterPair* dst);
 
 void xorR(Context* con,
           unsigned size,
-          lir::Register* a,
-          lir::Register* b,
-          lir::Register* dst);
+          lir::RegisterPair* a,
+          lir::RegisterPair* b,
+          lir::RegisterPair* dst);
 
 void moveAR2(Context* con,
              unsigned srcSize,
              lir::Address* src,
              unsigned dstSize,
-             lir::Register* dst);
+             lir::RegisterPair* dst);
 
 void moveAR(Context* con,
             unsigned srcSize,
             lir::Address* src,
             unsigned dstSize,
-            lir::Register* dst);
+            lir::RegisterPair* dst);
 
 void compareRR(Context* con,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize UNUSED,
-               lir::Register* b);
+               lir::RegisterPair* b);
 
 void compareCR(Context* con,
                unsigned aSize,
                lir::Constant* a,
                unsigned bSize,
-               lir::Register* b);
+               lir::RegisterPair* b);
 
 void compareCM(Context* con,
                unsigned aSize,
@@ -342,7 +342,7 @@ void compareCM(Context* con,
 
 void compareRM(Context* con,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize,
                lir::Memory* b);
 
@@ -365,21 +365,21 @@ void branchLong(Context* con,
 void branchRR(Context* con,
               lir::TernaryOperation op,
               unsigned size,
-              lir::Register* a,
-              lir::Register* b,
+              lir::RegisterPair* a,
+              lir::RegisterPair* b,
               lir::Constant* target);
 
 void branchCR(Context* con,
               lir::TernaryOperation op,
               unsigned size,
               lir::Constant* a,
-              lir::Register* b,
+              lir::RegisterPair* b,
               lir::Constant* target);
 
 void branchRM(Context* con,
               lir::TernaryOperation op,
               unsigned size,
-              lir::Register* a,
+              lir::RegisterPair* a,
               lir::Memory* b,
               lir::Constant* target);
 
@@ -403,11 +403,11 @@ void moveCM(Context* con,
 
 void negateRR(Context* con,
               unsigned srcSize,
-              lir::Register* src,
+              lir::RegisterPair* src,
               unsigned dstSize UNUSED,
-              lir::Register* dst);
+              lir::RegisterPair* dst);
 
-void callR(Context* con, unsigned size UNUSED, lir::Register* target);
+void callR(Context* con, unsigned size UNUSED, lir::RegisterPair* target);
 
 void callC(Context* con, unsigned size UNUSED, lir::Constant* target);
 
diff --git a/src/codegen/target/arm/registers.h b/src/codegen/target/arm/registers.h
index ed639acac6..ad13db466a 100644
--- a/src/codegen/target/arm/registers.h
+++ b/src/codegen/target/arm/registers.h
@@ -19,49 +19,45 @@ namespace codegen {
 namespace arm {
 
 const uint64_t MASK_LO32 = 0xffffffff;
-const unsigned MASK_LO16 = 0xffff;
 const unsigned MASK_LO8 = 0xff;
 
 const int N_GPRS = 16;
 const int N_FPRS = 16;
-const uint32_t GPR_MASK = 0xffff;
-const uint32_t FPR_MASK = 0xffff0000;
+const RegisterMask GPR_MASK = 0xffff;
+const RegisterMask FPR_MASK = 0xffff0000;
 
-const uint64_t GPR_MASK64 = GPR_MASK | (uint64_t)GPR_MASK << 32;
-const uint64_t FPR_MASK64 = FPR_MASK | (uint64_t)FPR_MASK << 32;
-
-inline bool isFpr(lir::Register* reg)
+inline bool isFpr(lir::RegisterPair* reg)
 {
-  return reg->low >= N_GPRS;
+  return reg->low.index() >= N_GPRS;
 }
 
-inline int fpr64(int reg)
+inline int fpr64(Register reg)
 {
-  return reg - N_GPRS;
+  return reg.index() - N_GPRS;
 }
-inline int fpr64(lir::Register* reg)
+inline int fpr64(lir::RegisterPair* reg)
 {
   return fpr64(reg->low);
 }
-inline int fpr32(int reg)
+inline int fpr32(Register reg)
 {
   return fpr64(reg) << 1;
 }
-inline int fpr32(lir::Register* reg)
+inline int fpr32(lir::RegisterPair* reg)
 {
   return fpr64(reg) << 1;
 }
 
 #ifdef ARCH_arm64
-const int ThreadRegister = 19;
-const int StackRegister = 31;
-const int LinkRegister = 30;
-const int ProgramCounter = 0xFF; // i.e. unaddressable
+constexpr Register ThreadRegister(19);
+constexpr Register StackRegister(31);
+constexpr Register LinkRegister(30);
+constexpr Register ProgramCounter(0xFE); // i.e. unaddressable
 #else
-const int ThreadRegister = 8;
-const int StackRegister = 13;
-const int LinkRegister = 14;
-const int ProgramCounter = 15;
+constexpr Register ThreadRegister(8);
+constexpr Register StackRegister(13);
+constexpr Register LinkRegister(14);
+constexpr Register ProgramCounter(15);
 #endif
 
 }  // namespace arm
diff --git a/src/codegen/target/multimethod.h b/src/codegen/target/multimethod.h
index e5de2ca23c..965d16a402 100644
--- a/src/codegen/target/multimethod.h
+++ b/src/codegen/target/multimethod.h
@@ -17,9 +17,9 @@ namespace codegen {
 class Multimethod {
  public:
   inline static unsigned index(lir::UnaryOperation operation,
-                               lir::OperandType operand)
+                               lir::Operand::Type operand)
   {
-    return operation + (lir::UnaryOperationCount * operand);
+    return operation + (lir::UnaryOperationCount * (unsigned)operand);
   }
 };
 
diff --git a/src/codegen/target/x86/assembler.cpp b/src/codegen/target/x86/assembler.cpp
index e9ada91b2e..86736faa99 100644
--- a/src/codegen/target/x86/assembler.cpp
+++ b/src/codegen/target/x86/assembler.cpp
@@ -183,37 +183,37 @@ class MyArchitecture : public Architecture {
     return &myRegisterFile;
   }
 
-  virtual int scratch()
+  virtual Register scratch()
   {
     return rax;
   }
 
-  virtual int stack()
+  virtual Register stack()
   {
     return rsp;
   }
 
-  virtual int thread()
+  virtual Register thread()
   {
     return rbx;
   }
 
-  virtual int returnLow()
+  virtual Register returnLow()
   {
     return rax;
   }
 
-  virtual int returnHigh()
+  virtual Register returnHigh()
   {
-    return (TargetBytesPerWord == 4 ? rdx : lir::NoRegister);
+    return (TargetBytesPerWord == 4 ? rdx : NoRegister);
   }
 
-  virtual int virtualCallTarget()
+  virtual Register virtualCallTarget()
   {
     return rax;
   }
 
-  virtual int virtualCallIndex()
+  virtual Register virtualCallIndex()
   {
     return rdx;
   }
@@ -233,14 +233,14 @@ class MyArchitecture : public Architecture {
     return 0x7FFFFFFF;
   }
 
-  virtual bool reserved(int register_)
+  virtual bool reserved(Register register_)
   {
-    switch (register_) {
-    case rbp:
+    switch (register_.index()) {
+    case rbp.index():
       return UseFramePointer;
 
-    case rsp:
-    case rbx:
+    case rsp.index():
+    case rbx.index():
       return true;
 
     default:
@@ -289,7 +289,7 @@ class MyArchitecture : public Architecture {
       return 0;
   }
 
-  virtual int argumentRegister(unsigned index)
+  virtual Register argumentRegister(unsigned index)
   {
     assertT(&c, TargetBytesPerWord == 8);
     switch (index) {
@@ -501,8 +501,8 @@ class MyArchitecture : public Architecture {
                     OperandMask& aMask,
                     bool* thunk)
   {
-    aMask.typeMask = (1 << lir::RegisterOperand) | (1 << lir::MemoryOperand)
-                     | (1 << lir::ConstantOperand);
+    aMask.typeMask = lir::Operand::RegisterPairMask | lir::Operand::MemoryMask
+                     | lir::Operand::ConstantMask;
     *thunk = false;
   }
 
@@ -512,22 +512,20 @@ class MyArchitecture : public Architecture {
                           unsigned bSize,
                           bool* thunk)
   {
-    aMask.registerMask = GeneralRegisterMask
-                         | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
+    aMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
 
     *thunk = false;
 
     switch (op) {
     case lir::Negate:
-      aMask.typeMask = (1 << lir::RegisterOperand);
-      aMask.registerMask = (static_cast<uint64_t>(1) << (rdx + 32))
-                           | (static_cast<uint64_t>(1) << rax);
+      aMask.typeMask = lir::Operand::RegisterPairMask;
+      aMask.setLowHighRegisterMasks(rax, rdx);
       break;
 
     case lir::Absolute:
       if (aSize <= TargetBytesPerWord) {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = (static_cast<uint64_t>(1) << rax);
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(rax, 0);
       } else {
         *thunk = true;
       }
@@ -535,9 +533,8 @@ class MyArchitecture : public Architecture {
 
     case lir::FloatAbsolute:
       if (useSSE(&c)) {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = (static_cast<uint64_t>(FloatRegisterMask) << 32)
-                             | FloatRegisterMask;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
       } else {
         *thunk = true;
       }
@@ -546,8 +543,8 @@ class MyArchitecture : public Architecture {
     case lir::FloatNegate:
       // floatNegateRR does not support doubles
       if (useSSE(&c) and aSize == 4 and bSize == 4) {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = FloatRegisterMask;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(FloatRegisterMask, 0);
       } else {
         *thunk = true;
       }
@@ -555,10 +552,9 @@ class MyArchitecture : public Architecture {
 
     case lir::FloatSquareRoot:
       if (useSSE(&c)) {
-        aMask.typeMask = (1 << lir::RegisterOperand)
-                         | (1 << lir::MemoryOperand);
-        aMask.registerMask = (static_cast<uint64_t>(FloatRegisterMask) << 32)
-                             | FloatRegisterMask;
+        aMask.typeMask = lir::Operand::RegisterPairMask
+                         | lir::Operand::MemoryMask;
+        aMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
       } else {
         *thunk = true;
       }
@@ -566,10 +562,9 @@ class MyArchitecture : public Architecture {
 
     case lir::Float2Float:
       if (useSSE(&c)) {
-        aMask.typeMask = (1 << lir::RegisterOperand)
-                         | (1 << lir::MemoryOperand);
-        aMask.registerMask = (static_cast<uint64_t>(FloatRegisterMask) << 32)
-                             | FloatRegisterMask;
+        aMask.typeMask = lir::Operand::RegisterPairMask
+                         | lir::Operand::MemoryMask;
+        aMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
       } else {
         *thunk = true;
       }
@@ -581,10 +576,9 @@ class MyArchitecture : public Architecture {
       // thunks or produce inline machine code which handles edge
       // cases properly.
       if (false and useSSE(&c) and bSize <= TargetBytesPerWord) {
-        aMask.typeMask = (1 << lir::RegisterOperand)
-                         | (1 << lir::MemoryOperand);
-        aMask.registerMask = (static_cast<uint64_t>(FloatRegisterMask) << 32)
-                             | FloatRegisterMask;
+        aMask.typeMask = lir::Operand::RegisterPairMask
+                         | lir::Operand::MemoryMask;
+        aMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
       } else {
         *thunk = true;
       }
@@ -592,11 +586,9 @@ class MyArchitecture : public Architecture {
 
     case lir::Int2Float:
       if (useSSE(&c) and aSize <= TargetBytesPerWord) {
-        aMask.typeMask = (1 << lir::RegisterOperand)
-                         | (1 << lir::MemoryOperand);
-        aMask.registerMask
-            = GeneralRegisterMask
-              | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
+        aMask.typeMask = lir::Operand::RegisterPairMask
+                         | lir::Operand::MemoryMask;
+        aMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
       } else {
         *thunk = true;
       }
@@ -604,21 +596,20 @@ class MyArchitecture : public Architecture {
 
     case lir::Move:
       aMask.typeMask = ~0;
-      aMask.registerMask = ~static_cast<uint64_t>(0);
+      aMask.setLowHighRegisterMasks(AnyRegisterMask, AnyRegisterMask);
 
       if (TargetBytesPerWord == 4) {
         if (aSize == 4 and bSize == 8) {
-          aMask.typeMask = (1 << lir::RegisterOperand)
-                           | (1 << lir::MemoryOperand);
-          const uint32_t mask = GeneralRegisterMask
-                                & ~((1 << rax) | (1 << rdx));
-          aMask.registerMask = (static_cast<uint64_t>(mask) << 32) | mask;
+          aMask.typeMask = lir::Operand::RegisterPairMask
+                           | lir::Operand::MemoryMask;
+          const RegisterMask mask = GeneralRegisterMask
+                                .excluding(rax).excluding(rdx);
+          aMask.setLowHighRegisterMasks(mask, mask);
         } else if (aSize == 1 or bSize == 1) {
-          aMask.typeMask = (1 << lir::RegisterOperand)
-                           | (1 << lir::MemoryOperand);
-          const uint32_t mask = (1 << rax) | (1 << rcx) | (1 << rdx)
-                                | (1 << rbx);
-          aMask.registerMask = (static_cast<uint64_t>(mask) << 32) | mask;
+          aMask.typeMask = lir::Operand::RegisterPairMask
+                           | lir::Operand::MemoryMask;
+          const RegisterMask mask = rax | rcx | rdx | rbx;
+          aMask.setLowHighRegisterMasks(mask, mask);
         }
       }
       break;
@@ -635,68 +626,62 @@ class MyArchitecture : public Architecture {
                                OperandMask& bMask)
   {
     bMask.typeMask = ~0;
-    bMask.registerMask = GeneralRegisterMask
-                         | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
+    bMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
 
     switch (op) {
     case lir::Absolute:
-      bMask.typeMask = (1 << lir::RegisterOperand);
-      bMask.registerMask = (static_cast<uint64_t>(1) << rax);
+      bMask.typeMask = lir::Operand::RegisterPairMask;
+      bMask.setLowHighRegisterMasks(rax, 0);
       break;
 
     case lir::FloatAbsolute:
-      bMask.typeMask = (1 << lir::RegisterOperand);
-      bMask.registerMask = aMask.registerMask;
+      bMask.typeMask = lir::Operand::RegisterPairMask;
+      bMask.lowRegisterMask = aMask.lowRegisterMask;
+      bMask.highRegisterMask = aMask.highRegisterMask;
       break;
 
     case lir::Negate:
-      bMask.typeMask = (1 << lir::RegisterOperand);
-      bMask.registerMask = aMask.registerMask;
+      bMask.typeMask = lir::Operand::RegisterPairMask;
+      bMask.lowRegisterMask = aMask.lowRegisterMask;
+      bMask.highRegisterMask = aMask.highRegisterMask;
       break;
 
     case lir::FloatNegate:
     case lir::FloatSquareRoot:
     case lir::Float2Float:
     case lir::Int2Float:
-      bMask.typeMask = (1 << lir::RegisterOperand);
-      bMask.registerMask = (static_cast<uint64_t>(FloatRegisterMask) << 32)
-                           | FloatRegisterMask;
+      bMask.typeMask = lir::Operand::RegisterPairMask;
+      bMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
       break;
 
     case lir::Float2Int:
-      bMask.typeMask = (1 << lir::RegisterOperand);
+      bMask.typeMask = lir::Operand::RegisterPairMask;
       break;
 
     case lir::Move:
       if (aMask.typeMask
-          & ((1 << lir::MemoryOperand) | 1 << lir::AddressOperand)) {
-        bMask.typeMask = (1 << lir::RegisterOperand);
-        bMask.registerMask = GeneralRegisterMask
-                             | (static_cast<uint64_t>(GeneralRegisterMask)
-                                << 32) | FloatRegisterMask;
-      } else if (aMask.typeMask & (1 << lir::RegisterOperand)) {
-        bMask.typeMask = (1 << lir::RegisterOperand)
-                         | (1 << lir::MemoryOperand);
-        if (aMask.registerMask & FloatRegisterMask) {
-          bMask.registerMask = FloatRegisterMask;
+          & (lir::Operand::MemoryMask | lir::Operand::AddressMask)) {
+        bMask.typeMask = lir::Operand::RegisterPairMask;
+        bMask.setLowHighRegisterMasks(GeneralRegisterMask | FloatRegisterMask, GeneralRegisterMask);
+      } else if (aMask.typeMask & lir::Operand::RegisterPairMask) {
+        bMask.typeMask = lir::Operand::RegisterPairMask
+                         | lir::Operand::MemoryMask;
+        if (aMask.lowRegisterMask & FloatRegisterMask) {
+          bMask.setLowHighRegisterMasks(FloatRegisterMask, 0);
         } else {
-          bMask.registerMask
-              = GeneralRegisterMask
-                | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
+          bMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
         }
       } else {
-        bMask.typeMask = (1 << lir::RegisterOperand)
-                         | (1 << lir::MemoryOperand);
+        bMask.typeMask = lir::Operand::RegisterPairMask
+                         | lir::Operand::MemoryMask;
       }
 
       if (TargetBytesPerWord == 4) {
         if (aSize == 4 and bSize == 8) {
-          bMask.registerMask = (static_cast<uint64_t>(1) << (rdx + 32))
-                               | (static_cast<uint64_t>(1) << rax);
+          bMask.setLowHighRegisterMasks(rax, rdx);
         } else if (aSize == 1 or bSize == 1) {
-          const uint32_t mask = (1 << rax) | (1 << rcx) | (1 << rdx)
-                                | (1 << rbx);
-          bMask.registerMask = (static_cast<uint64_t>(mask) << 32) | mask;
+          const RegisterMask mask = rax | rcx | rdx | rbx;
+          bMask.setLowHighRegisterMasks(mask, mask);
         }
       }
       break;
@@ -712,46 +697,38 @@ class MyArchitecture : public Architecture {
                         const OperandMask& dstMask)
   {
     srcMask.typeMask = ~0;
-    srcMask.registerMask = ~static_cast<uint64_t>(0);
+    srcMask.setLowHighRegisterMasks(AnyRegisterMask, AnyRegisterMask);
 
     tmpMask.typeMask = 0;
-    tmpMask.registerMask = 0;
+    tmpMask.setLowHighRegisterMasks(0, 0);
 
-    if (dstMask.typeMask & (1 << lir::MemoryOperand)) {
+    if (dstMask.typeMask & lir::Operand::MemoryMask) {
       // can't move directly from memory to memory
-      srcMask.typeMask = (1 << lir::RegisterOperand)
-                         | (1 << lir::ConstantOperand);
-      tmpMask.typeMask = 1 << lir::RegisterOperand;
-      tmpMask.registerMask
-          = GeneralRegisterMask
-            | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
-    } else if (dstMask.typeMask & (1 << lir::RegisterOperand)) {
+      srcMask.typeMask = lir::Operand::RegisterPairMask
+                         | lir::Operand::ConstantMask;
+      tmpMask.typeMask = lir::Operand::RegisterPairMask;
+      tmpMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
+    } else if (dstMask.typeMask & lir::Operand::RegisterPairMask) {
       if (size > TargetBytesPerWord) {
         // can't move directly from FPR to GPR or vice-versa for
         // values larger than the GPR size
-        if (dstMask.registerMask & FloatRegisterMask) {
-          srcMask.registerMask
-              = FloatRegisterMask
-                | (static_cast<uint64_t>(FloatRegisterMask) << 32);
-          tmpMask.typeMask = 1 << lir::MemoryOperand;
-        } else if (dstMask.registerMask & GeneralRegisterMask) {
-          srcMask.registerMask
-              = GeneralRegisterMask
-                | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
-          tmpMask.typeMask = 1 << lir::MemoryOperand;
+        if (dstMask.lowRegisterMask & FloatRegisterMask) {
+          srcMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
+          tmpMask.typeMask = lir::Operand::MemoryMask;
+        } else if (dstMask.lowRegisterMask & GeneralRegisterMask) {
+          srcMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
+          tmpMask.typeMask = lir::Operand::MemoryMask;
         }
       }
-      if (dstMask.registerMask & FloatRegisterMask) {
+      if (dstMask.lowRegisterMask & FloatRegisterMask) {
         // can't move directly from constant to FPR
-        srcMask.typeMask &= ~(1 << lir::ConstantOperand);
+        srcMask.typeMask &= ~lir::Operand::ConstantMask;
         if (size > TargetBytesPerWord) {
-          tmpMask.typeMask = 1 << lir::MemoryOperand;
+          tmpMask.typeMask = lir::Operand::MemoryMask;
         } else {
-          tmpMask.typeMask = (1 << lir::RegisterOperand)
-                             | (1 << lir::MemoryOperand);
-          tmpMask.registerMask
-              = GeneralRegisterMask
-                | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
+          tmpMask.typeMask = lir::Operand::RegisterPairMask
+                             | lir::Operand::MemoryMask;
+          tmpMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
         }
       }
     }
@@ -765,13 +742,11 @@ class MyArchitecture : public Architecture {
                           unsigned,
                           bool* thunk)
   {
-    aMask.typeMask = (1 << lir::RegisterOperand) | (1 << lir::ConstantOperand);
-    aMask.registerMask = GeneralRegisterMask
-                         | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
+    aMask.typeMask = lir::Operand::RegisterPairMask | lir::Operand::ConstantMask;
+    aMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
 
-    bMask.typeMask = (1 << lir::RegisterOperand);
-    bMask.registerMask = GeneralRegisterMask
-                         | (static_cast<uint64_t>(GeneralRegisterMask) << 32);
+    bMask.typeMask = lir::Operand::RegisterPairMask;
+    bMask.setLowHighRegisterMasks(GeneralRegisterMask, GeneralRegisterMask);
 
     *thunk = false;
 
@@ -781,14 +756,12 @@ class MyArchitecture : public Architecture {
     case lir::FloatMultiply:
     case lir::FloatDivide:
       if (useSSE(&c)) {
-        aMask.typeMask = (1 << lir::RegisterOperand)
-                         | (1 << lir::MemoryOperand);
-        bMask.typeMask = (1 << lir::RegisterOperand);
+        aMask.typeMask = lir::Operand::RegisterPairMask
+                         | lir::Operand::MemoryMask;
+        bMask.typeMask = lir::Operand::RegisterPairMask;
 
-        const uint64_t mask = (static_cast<uint64_t>(FloatRegisterMask) << 32)
-                              | FloatRegisterMask;
-        aMask.registerMask = mask;
-        bMask.registerMask = mask;
+        aMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
+        bMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
       } else {
         *thunk = true;
       }
@@ -800,12 +773,12 @@ class MyArchitecture : public Architecture {
 
     case lir::Multiply:
       if (TargetBytesPerWord == 4 and aSize == 8) {
-        const uint32_t mask = GeneralRegisterMask & ~((1 << rax) | (1 << rdx));
-        aMask.registerMask = (static_cast<uint64_t>(mask) << 32) | mask;
-        bMask.registerMask = (static_cast<uint64_t>(1) << (rdx + 32)) | mask;
+        const RegisterMask mask = GeneralRegisterMask .excluding(rax).excluding(rdx);
+        aMask.setLowHighRegisterMasks(mask, mask);
+        bMask.setLowHighRegisterMasks(mask, rdx);
       } else {
-        aMask.registerMask = GeneralRegisterMask;
-        bMask.registerMask = GeneralRegisterMask;
+        aMask.setLowHighRegisterMasks(GeneralRegisterMask, 0);
+        bMask.setLowHighRegisterMasks(GeneralRegisterMask, 0);
       }
       break;
 
@@ -813,9 +786,9 @@ class MyArchitecture : public Architecture {
       if (TargetBytesPerWord == 4 and aSize == 8) {
         *thunk = true;
       } else {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = GeneralRegisterMask & ~((1 << rax) | (1 << rdx));
-        bMask.registerMask = 1 << rax;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(GeneralRegisterMask .excluding(rax).excluding(rdx), 0);
+        bMask.setLowHighRegisterMasks(rax, 0);
       }
       break;
 
@@ -823,9 +796,9 @@ class MyArchitecture : public Architecture {
       if (TargetBytesPerWord == 4 and aSize == 8) {
         *thunk = true;
       } else {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = GeneralRegisterMask & ~((1 << rax) | (1 << rdx));
-        bMask.registerMask = 1 << rax;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(GeneralRegisterMask .excluding(rax).excluding(rdx), 0);
+        bMask.setLowHighRegisterMasks(rax, 0);
       }
       break;
 
@@ -833,14 +806,13 @@ class MyArchitecture : public Architecture {
     case lir::ShiftRight:
     case lir::UnsignedShiftRight: {
       if (TargetBytesPerWord == 4 and bSize == 8) {
-        const uint32_t mask = GeneralRegisterMask & ~(1 << rcx);
-        aMask.registerMask = (static_cast<uint64_t>(mask) << 32) | mask;
-        bMask.registerMask = (static_cast<uint64_t>(mask) << 32) | mask;
+        const RegisterMask mask = GeneralRegisterMask.excluding(rcx);
+        aMask.setLowHighRegisterMasks(mask, mask);
+        bMask.setLowHighRegisterMasks(mask, mask);
       } else {
-        aMask.registerMask = (static_cast<uint64_t>(GeneralRegisterMask) << 32)
-                             | (static_cast<uint64_t>(1) << rcx);
-        const uint32_t mask = GeneralRegisterMask & ~(1 << rcx);
-        bMask.registerMask = (static_cast<uint64_t>(mask) << 32) | mask;
+        aMask.setLowHighRegisterMasks(rcx, GeneralRegisterMask);
+        const RegisterMask mask = GeneralRegisterMask.excluding(rcx);
+        bMask.setLowHighRegisterMasks(mask, mask);
       }
     } break;
 
@@ -855,11 +827,11 @@ class MyArchitecture : public Architecture {
     case lir::JumpIfFloatLessOrEqualOrUnordered:
     case lir::JumpIfFloatGreaterOrEqualOrUnordered:
       if (useSSE(&c)) {
-        aMask.typeMask = (1 << lir::RegisterOperand);
-        aMask.registerMask = (static_cast<uint64_t>(FloatRegisterMask) << 32)
-                             | FloatRegisterMask;
+        aMask.typeMask = lir::Operand::RegisterPairMask;
+        aMask.setLowHighRegisterMasks(FloatRegisterMask, FloatRegisterMask);
         bMask.typeMask = aMask.typeMask;
-        bMask.registerMask = aMask.registerMask;
+        bMask.lowRegisterMask = aMask.lowRegisterMask;
+        bMask.highRegisterMask = aMask.highRegisterMask;
       } else {
         *thunk = true;
       }
@@ -879,11 +851,12 @@ class MyArchitecture : public Architecture {
                                OperandMask& cMask)
   {
     if (isBranch(op)) {
-      cMask.typeMask = (1 << lir::ConstantOperand);
-      cMask.registerMask = 0;
+      cMask.typeMask = lir::Operand::ConstantMask;
+      cMask.setLowHighRegisterMasks(0, 0);
     } else {
-      cMask.typeMask = (1 << lir::RegisterOperand);
-      cMask.registerMask = bMask.registerMask;
+      cMask.typeMask = lir::Operand::RegisterPairMask;
+      cMask.lowRegisterMask = bMask.lowRegisterMask;
+      cMask.highRegisterMask = bMask.highRegisterMask;
     }
   }
 
@@ -927,7 +900,7 @@ class MyAssembler : public Assembler {
   virtual void checkStackOverflow(uintptr_t handler,
                                   unsigned stackLimitOffsetFromThread)
   {
-    lir::Register stack(rsp);
+    lir::RegisterPair stack(rsp);
     lir::Memory stackLimit(rbx, stackLimitOffsetFromThread);
     lir::Constant handlerConstant(resolvedPromise(&c, handler));
     branchRM(&c,
@@ -940,11 +913,11 @@ class MyAssembler : public Assembler {
 
   virtual void saveFrame(unsigned stackOffset, unsigned)
   {
-    lir::Register stack(rsp);
+    lir::RegisterPair stack(rsp);
     lir::Memory stackDst(rbx, stackOffset);
     apply(lir::Move,
-          OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack),
-          OperandInfo(TargetBytesPerWord, lir::MemoryOperand, &stackDst));
+          OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack),
+          OperandInfo(TargetBytesPerWord, lir::Operand::Type::Memory, &stackDst));
   }
 
   virtual void pushFrame(unsigned argumentCount, ...)
@@ -952,7 +925,7 @@ class MyAssembler : public Assembler {
     // TODO: Argument should be replaced by OperandInfo...
     struct Argument {
       unsigned size;
-      lir::OperandType type;
+      lir::Operand::Type type;
       lir::Operand* operand;
     };
     RUNTIME_ARRAY(Argument, arguments, argumentCount);
@@ -962,7 +935,7 @@ class MyAssembler : public Assembler {
     for (unsigned i = 0; i < argumentCount; ++i) {
       RUNTIME_ARRAY_BODY(arguments)[i].size = va_arg(a, unsigned);
       RUNTIME_ARRAY_BODY(arguments)[i].type
-          = static_cast<lir::OperandType>(va_arg(a, int));
+          = static_cast<lir::Operand::Type>(va_arg(a, int));
       RUNTIME_ARRAY_BODY(arguments)[i].operand = va_arg(a, lir::Operand*);
       footprint += ceilingDivide(RUNTIME_ARRAY_BODY(arguments)[i].size,
                                  TargetBytesPerWord);
@@ -974,14 +947,14 @@ class MyAssembler : public Assembler {
     unsigned offset = 0;
     for (unsigned i = 0; i < argumentCount; ++i) {
       if (i < arch_->argumentRegisterCount()) {
-        lir::Register dst(arch_->argumentRegister(i));
+        lir::RegisterPair dst(arch_->argumentRegister(i));
         apply(lir::Move,
               OperandInfo(RUNTIME_ARRAY_BODY(arguments)[i].size,
                           RUNTIME_ARRAY_BODY(arguments)[i].type,
                           RUNTIME_ARRAY_BODY(arguments)[i].operand),
               OperandInfo(pad(RUNTIME_ARRAY_BODY(arguments)[i].size,
                               TargetBytesPerWord),
-                          lir::RegisterOperand,
+                          lir::Operand::Type::RegisterPair,
                           &dst));
       } else {
         lir::Memory dst(rsp, offset * TargetBytesPerWord);
@@ -991,7 +964,7 @@ class MyAssembler : public Assembler {
                           RUNTIME_ARRAY_BODY(arguments)[i].operand),
               OperandInfo(pad(RUNTIME_ARRAY_BODY(arguments)[i].size,
                               TargetBytesPerWord),
-                          lir::MemoryOperand,
+                          lir::Operand::Type::Memory,
                           &dst));
         offset += ceilingDivide(RUNTIME_ARRAY_BODY(arguments)[i].size,
                                 TargetBytesPerWord);
@@ -1001,67 +974,67 @@ class MyAssembler : public Assembler {
 
   virtual void allocateFrame(unsigned footprint)
   {
-    lir::Register stack(rsp);
+    lir::RegisterPair stack(rsp);
 
     if (UseFramePointer) {
-      lir::Register base(rbp);
+      lir::RegisterPair base(rbp);
       pushR(&c, TargetBytesPerWord, &base);
 
       apply(lir::Move,
-            OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack),
-            OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &base));
+            OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack),
+            OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &base));
     }
 
     lir::Constant footprintConstant(
         resolvedPromise(&c, footprint * TargetBytesPerWord));
     apply(lir::Subtract,
           OperandInfo(
-              TargetBytesPerWord, lir::ConstantOperand, &footprintConstant),
-          OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack),
-          OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack));
+              TargetBytesPerWord, lir::Operand::Type::Constant, &footprintConstant),
+          OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack),
+          OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack));
   }
 
   virtual void adjustFrame(unsigned difference)
   {
-    lir::Register stack(rsp);
+    lir::RegisterPair stack(rsp);
     lir::Constant differenceConstant(
         resolvedPromise(&c, difference * TargetBytesPerWord));
     apply(lir::Subtract,
           OperandInfo(
-              TargetBytesPerWord, lir::ConstantOperand, &differenceConstant),
-          OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack),
-          OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack));
+              TargetBytesPerWord, lir::Operand::Type::Constant, &differenceConstant),
+          OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack),
+          OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack));
   }
 
   virtual void popFrame(unsigned frameFootprint)
   {
     if (UseFramePointer) {
-      lir::Register base(rbp);
-      lir::Register stack(rsp);
+      lir::RegisterPair base(rbp);
+      lir::RegisterPair stack(rsp);
       apply(lir::Move,
-            OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &base),
-            OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack));
+            OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &base),
+            OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack));
 
       popR(&c, TargetBytesPerWord, &base);
     } else {
-      lir::Register stack(rsp);
+      lir::RegisterPair stack(rsp);
       lir::Constant footprint(
           resolvedPromise(&c, frameFootprint * TargetBytesPerWord));
       apply(lir::Add,
-            OperandInfo(TargetBytesPerWord, lir::ConstantOperand, &footprint),
-            OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack),
-            OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &stack));
+            OperandInfo(TargetBytesPerWord, lir::Operand::Type::Constant, &footprint),
+            OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack),
+            OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &stack));
     }
   }
 
   virtual void popFrameForTailCall(unsigned frameFootprint,
                                    int offset,
-                                   int returnAddressSurrogate,
-                                   int framePointerSurrogate)
+                                   Register returnAddressSurrogate,
+                                   Register framePointerSurrogate)
   {
     if (TailCalls) {
       if (offset) {
-        lir::Register tmp(c.client->acquireTemporary());
+        lir::RegisterPair tmp(c.client->acquireTemporary());
 
         unsigned baseSize = UseFramePointer ? 1 : 0;
 
@@ -1085,28 +1058,28 @@ class MyAssembler : public Assembler {
 
         if (UseFramePointer) {
           lir::Memory baseSrc(rsp, frameFootprint * TargetBytesPerWord);
-          lir::Register base(rbp);
+          lir::RegisterPair base(rbp);
           moveMR(&c, TargetBytesPerWord, &baseSrc, TargetBytesPerWord, &base);
         }
 
-        lir::Register stack(rsp);
+        lir::RegisterPair stack(rsp);
         lir::Constant footprint(resolvedPromise(
             &c, (frameFootprint - offset + baseSize) * TargetBytesPerWord));
 
         addCR(&c, TargetBytesPerWord, &footprint, TargetBytesPerWord, &stack);
 
-        if (returnAddressSurrogate != lir::NoRegister) {
+        if (returnAddressSurrogate != NoRegister) {
           assertT(&c, offset > 0);
 
-          lir::Register ras(returnAddressSurrogate);
+          lir::RegisterPair ras(returnAddressSurrogate);
           lir::Memory dst(rsp, offset * TargetBytesPerWord);
           moveRM(&c, TargetBytesPerWord, &ras, TargetBytesPerWord, &dst);
         }
 
-        if (framePointerSurrogate != lir::NoRegister) {
+        if (framePointerSurrogate != NoRegister) {
           assertT(&c, offset > 0);
 
-          lir::Register fps(framePointerSurrogate);
+          lir::RegisterPair fps(framePointerSurrogate);
           lir::Memory dst(rsp, (offset - 1) * TargetBytesPerWord);
           moveRM(&c, TargetBytesPerWord, &fps, TargetBytesPerWord, &dst);
         }
@@ -1127,10 +1100,10 @@ class MyAssembler : public Assembler {
     assertT(&c, (argumentFootprint % StackAlignmentInWords) == 0);
 
     if (TailCalls and argumentFootprint > StackAlignmentInWords) {
-      lir::Register returnAddress(rcx);
+      lir::RegisterPair returnAddress(rcx);
       popR(&c, TargetBytesPerWord, &returnAddress);
 
-      lir::Register stack(rsp);
+      lir::RegisterPair stack(rsp);
       lir::Constant adjustment(resolvedPromise(
           &c,
           (argumentFootprint - StackAlignmentInWords) * TargetBytesPerWord));
@@ -1147,10 +1120,10 @@ class MyAssembler : public Assembler {
   {
     popFrame(frameFootprint);
 
-    lir::Register returnAddress(rcx);
+    lir::RegisterPair returnAddress(rcx);
     popR(&c, TargetBytesPerWord, &returnAddress);
 
-    lir::Register stack(rsp);
+    lir::RegisterPair stack(rsp);
     lir::Memory stackSrc(rbx, stackOffsetFromThread);
     moveMR(&c, TargetBytesPerWord, &stackSrc, TargetBytesPerWord, &stack);
 
@@ -1182,7 +1155,7 @@ class MyAssembler : public Assembler {
     if (isBranch(op)) {
       assertT(&this->c, a.size == b.size);
       assertT(&this->c, c.size == TargetBytesPerWord);
-      assertT(&this->c, c.type == lir::ConstantOperand);
+      assertT(&this->c, c.type == lir::Operand::Type::Constant);
 
       arch_->c.branchOperations[branchIndex(&(arch_->c), a.type, b.type)](
           &this->c, op, a.size, a.operand, b.operand, c.operand);
diff --git a/src/codegen/target/x86/context.h b/src/codegen/target/x86/context.h
index 93a6f1cc5b..89f877ce1a 100644
--- a/src/codegen/target/x86/context.h
+++ b/src/codegen/target/x86/context.h
@@ -68,13 +68,13 @@ class ArchitectureContext {
   bool useNativeFeatures;
   OperationType operations[lir::OperationCount];
   UnaryOperationType
-      unaryOperations[lir::UnaryOperationCount * lir::OperandTypeCount];
+      unaryOperations[lir::UnaryOperationCount * lir::Operand::TypeCount];
   BinaryOperationType binaryOperations
       [(lir::BinaryOperationCount + lir::NonBranchTernaryOperationCount)
-       * lir::OperandTypeCount * lir::OperandTypeCount];
+       * lir::Operand::TypeCount * lir::Operand::TypeCount];
   BranchOperationType branchOperations[lir::BranchOperationCount
-                                       * lir::OperandTypeCount
-                                       * lir::OperandTypeCount];
+                                       * lir::Operand::TypeCount
+                                       * lir::Operand::TypeCount];
 };
 
 class Context {
diff --git a/src/codegen/target/x86/encode.cpp b/src/codegen/target/x86/encode.cpp
index f64d91a2c9..35525f5e1c 100644
--- a/src/codegen/target/x86/encode.cpp
+++ b/src/codegen/target/x86/encode.cpp
@@ -53,9 +53,9 @@ namespace x86 {
 
 void maybeRex(Context* c,
               unsigned size,
-              int a,
-              int index,
-              int base,
+              Register a,
+              Register index,
+              Register base,
               bool always)
 {
   if (vm::TargetBytesPerWord == 8) {
@@ -65,63 +65,63 @@ void maybeRex(Context* c,
     } else {
       byte = REX_NONE;
     }
-    if (a != lir::NoRegister and (a & 8))
+    if (a != NoRegister and (a.index() & 8))
       byte |= REX_R;
-    if (index != lir::NoRegister and (index & 8))
+    if (index != NoRegister and (index.index() & 8))
       byte |= REX_X;
-    if (base != lir::NoRegister and (base & 8))
+    if (base != NoRegister and (base.index() & 8))
       byte |= REX_B;
     if (always or byte != REX_NONE)
       c->code.append(byte);
   }
 }
 
-void maybeRex(Context* c, unsigned size, lir::Register* a, lir::Register* b)
+void maybeRex(Context* c, unsigned size, lir::RegisterPair* a, lir::RegisterPair* b)
 {
-  maybeRex(c, size, a->low, lir::NoRegister, b->low, false);
+  maybeRex(c, size, a->low, NoRegister, b->low, false);
 }
 
-void alwaysRex(Context* c, unsigned size, lir::Register* a, lir::Register* b)
+void alwaysRex(Context* c, unsigned size, lir::RegisterPair* a, lir::RegisterPair* b)
 {
-  maybeRex(c, size, a->low, lir::NoRegister, b->low, true);
+  maybeRex(c, size, a->low, NoRegister, b->low, true);
 }
 
-void maybeRex(Context* c, unsigned size, lir::Register* a)
+void maybeRex(Context* c, unsigned size, lir::RegisterPair* a)
 {
-  maybeRex(c, size, lir::NoRegister, lir::NoRegister, a->low, false);
+  maybeRex(c, size, NoRegister, NoRegister, a->low, false);
 }
 
-void maybeRex(Context* c, unsigned size, lir::Register* a, lir::Memory* b)
+void maybeRex(Context* c, unsigned size, lir::RegisterPair* a, lir::Memory* b)
 {
-  maybeRex(c, size, a->low, b->index, b->base, size == 1 and (a->low & 4));
+  maybeRex(c, size, a->low, b->index, b->base, size == 1 and (a->low.index() & 4));
 }
 
 void maybeRex(Context* c, unsigned size, lir::Memory* a)
 {
-  maybeRex(c, size, lir::NoRegister, a->index, a->base, false);
+  maybeRex(c, size, NoRegister, a->index, a->base, false);
 }
 
-void modrm(Context* c, uint8_t mod, int a, int b)
+void modrm(Context* c, uint8_t mod, Register a, Register b)
 {
   c->code.append(mod | (regCode(b) << 3) | regCode(a));
 }
 
-void modrm(Context* c, uint8_t mod, lir::Register* a, lir::Register* b)
+void modrm(Context* c, uint8_t mod, lir::RegisterPair* a, lir::RegisterPair* b)
 {
   modrm(c, mod, a->low, b->low);
 }
 
-void sib(Context* c, unsigned scale, int index, int base)
+void sib(Context* c, unsigned scale, Register index, Register base)
 {
   c->code.append((util::log(scale) << 6) | (regCode(index) << 3)
                  | regCode(base));
 }
 
-void modrmSib(Context* c, int width, int a, int scale, int index, int base)
+void modrmSib(Context* c, int width, Register a, int scale, Register index, Register base)
 {
-  if (index == lir::NoRegister) {
+  if (index == NoRegister) {
     modrm(c, width, base, a);
-    if (regCode(base) == rsp) {
+    if (regCode(base) == rsp.index()) {
       sib(c, 0x00, rsp, rsp);
     }
   } else {
@@ -130,9 +130,9 @@ void modrmSib(Context* c, int width, int a, int scale, int index, int base)
   }
 }
 
-void modrmSibImm(Context* c, int a, int scale, int index, int base, int offset)
+void modrmSibImm(Context* c, Register a, int scale, Register index, Register base, int offset)
 {
-  if (offset == 0 and regCode(base) != rbp) {
+  if (offset == 0 and regCode(base) != rbp.index()) {
     modrmSib(c, 0x00, a, scale, index, base);
   } else if (vm::fitsInInt8(offset)) {
     modrmSib(c, 0x40, a, scale, index, base);
@@ -143,7 +143,7 @@ void modrmSibImm(Context* c, int a, int scale, int index, int base, int offset)
   }
 }
 
-void modrmSibImm(Context* c, lir::Register* a, lir::Memory* b)
+void modrmSibImm(Context* c, lir::RegisterPair* a, lir::Memory* b)
 {
   modrmSibImm(c, a->low, b->scale, b->index, b->base, b->offset);
 }
@@ -177,9 +177,9 @@ void conditional(Context* c, unsigned condition, lir::Constant* a)
 
 void sseMoveRR(Context* c,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize UNUSED,
-               lir::Register* b)
+               lir::RegisterPair* b)
 {
   assertT(c, aSize >= 4);
   assertT(c, aSize == bSize);
@@ -213,10 +213,10 @@ void sseMoveCR(Context* c,
                unsigned aSize,
                lir::Constant* a,
                unsigned bSize,
-               lir::Register* b)
+               lir::RegisterPair* b)
 {
   assertT(c, aSize <= vm::TargetBytesPerWord);
-  lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+  lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
   moveCR2(c, aSize, a, aSize, &tmp, 0);
   sseMoveRR(c, aSize, &tmp, bSize, b);
   c->client->releaseTemporary(tmp.low);
@@ -226,7 +226,7 @@ void sseMoveMR(Context* c,
                unsigned aSize,
                lir::Memory* a,
                unsigned bSize UNUSED,
-               lir::Register* b)
+               lir::RegisterPair* b)
 {
   assertT(c, aSize >= 4);
 
@@ -244,7 +244,7 @@ void sseMoveMR(Context* c,
 
 void sseMoveRM(Context* c,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                UNUSED unsigned bSize,
                lir::Memory* b)
 {
@@ -353,9 +353,9 @@ void branchFloat(Context* c, lir::TernaryOperation op, lir::Constant* target)
 
 void floatRegOp(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize,
-                lir::Register* b,
+                lir::RegisterPair* b,
                 uint8_t op,
                 uint8_t mod)
 {
@@ -373,7 +373,7 @@ void floatMemOp(Context* c,
                 unsigned aSize,
                 lir::Memory* a,
                 unsigned bSize,
-                lir::Register* b,
+                lir::RegisterPair* b,
                 uint8_t op)
 {
   if (aSize == 4) {
@@ -390,13 +390,13 @@ void moveCR(Context* c,
             unsigned aSize,
             lir::Constant* a,
             unsigned bSize,
-            lir::Register* b);
+            lir::RegisterPair* b);
 
 void moveCR2(Context* c,
              UNUSED unsigned aSize,
              lir::Constant* a,
              UNUSED unsigned bSize,
-             lir::Register* b,
+             lir::RegisterPair* b,
              unsigned promiseOffset)
 {
   if (vm::TargetBytesPerWord == 4 and bSize == 8) {
@@ -408,7 +408,7 @@ void moveCR2(Context* c,
     ResolvedPromise low(v & 0xFFFFFFFF);
     lir::Constant al(&low);
 
-    lir::Register bh(b->high);
+    lir::RegisterPair bh(b->high);
 
     moveCR(c, 4, &al, 4, b);
     moveCR(c, 4, &ah, 4, &bh);
diff --git a/src/codegen/target/x86/encode.h b/src/codegen/target/x86/encode.h
index 564b84f6e9..c89af512cf 100644
--- a/src/codegen/target/x86/encode.h
+++ b/src/codegen/target/x86/encode.h
@@ -32,42 +32,42 @@ void maybeRex(Context* c,
               int base,
               bool always);
 
-void maybeRex(Context* c, unsigned size, lir::Register* a, lir::Register* b);
+void maybeRex(Context* c, unsigned size, lir::RegisterPair* a, lir::RegisterPair* b);
 
-void alwaysRex(Context* c, unsigned size, lir::Register* a, lir::Register* b);
+void alwaysRex(Context* c, unsigned size, lir::RegisterPair* a, lir::RegisterPair* b);
 
-void maybeRex(Context* c, unsigned size, lir::Register* a);
+void maybeRex(Context* c, unsigned size, lir::RegisterPair* a);
 
-void maybeRex(Context* c, unsigned size, lir::Register* a, lir::Memory* b);
+void maybeRex(Context* c, unsigned size, lir::RegisterPair* a, lir::Memory* b);
 
 void maybeRex(Context* c, unsigned size, lir::Memory* a);
 
-inline int regCode(int a)
+inline int regCode(Register a)
 {
-  return a & 7;
+  return a.index() & 7;
 }
 
-inline int regCode(lir::Register* a)
+inline int regCode(lir::RegisterPair* a)
 {
   return regCode(a->low);
 }
 
-inline bool isFloatReg(lir::Register* a)
+inline bool isFloatReg(lir::RegisterPair* a)
 {
   return a->low >= xmm0;
 }
 
-void modrm(Context* c, uint8_t mod, int a, int b);
+void modrm(Context* c, uint8_t mod, Register a, Register b);
 
-void modrm(Context* c, uint8_t mod, lir::Register* a, lir::Register* b);
+void modrm(Context* c, uint8_t mod, lir::RegisterPair* a, lir::RegisterPair* b);
 
-void sib(Context* c, unsigned scale, int index, int base);
+void sib(Context* c, unsigned scale, Register index, Register base);
 
-void modrmSib(Context* c, int width, int a, int scale, int index, int base);
+void modrmSib(Context* c, int width, Register a, int scale, Register index, Register base);
 
-void modrmSibImm(Context* c, int a, int scale, int index, int base, int offset);
+void modrmSibImm(Context* c, Register a, int scale, Register index, Register base, int offset);
 
-void modrmSibImm(Context* c, lir::Register* a, lir::Memory* b);
+void modrmSibImm(Context* c, lir::RegisterPair* a, lir::Memory* b);
 
 void opcode(Context* c, uint8_t op);
 
@@ -79,25 +79,25 @@ void conditional(Context* c, unsigned condition, lir::Constant* a);
 
 void sseMoveRR(Context* c,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize UNUSED,
-               lir::Register* b);
+               lir::RegisterPair* b);
 
 void sseMoveCR(Context* c,
                unsigned aSize,
                lir::Constant* a,
                unsigned bSize,
-               lir::Register* b);
+               lir::RegisterPair* b);
 
 void sseMoveMR(Context* c,
                unsigned aSize,
                lir::Memory* a,
                unsigned bSize UNUSED,
-               lir::Register* b);
+               lir::RegisterPair* b);
 
 void sseMoveRM(Context* c,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                UNUSED unsigned bSize,
                lir::Memory* b);
 
@@ -107,9 +107,9 @@ void branchFloat(Context* c, lir::TernaryOperation op, lir::Constant* target);
 
 void floatRegOp(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize,
-                lir::Register* b,
+                lir::RegisterPair* b,
                 uint8_t op,
                 uint8_t mod = 0xc0);
 
@@ -117,14 +117,14 @@ void floatMemOp(Context* c,
                 unsigned aSize,
                 lir::Memory* a,
                 unsigned bSize,
-                lir::Register* b,
+                lir::RegisterPair* b,
                 uint8_t op);
 
 void moveCR2(Context* c,
              UNUSED unsigned aSize,
              lir::Constant* a,
              UNUSED unsigned bSize,
-             lir::Register* b,
+             lir::RegisterPair* b,
              unsigned promiseOffset);
 
 }  // namespace x86
diff --git a/src/codegen/target/x86/multimethod.cpp b/src/codegen/target/x86/multimethod.cpp
index 19e6135e1a..17bc3d91cc 100644
--- a/src/codegen/target/x86/multimethod.cpp
+++ b/src/codegen/target/x86/multimethod.cpp
@@ -28,42 +28,42 @@ using namespace util;
 
 unsigned index(ArchitectureContext*,
                lir::BinaryOperation operation,
-               lir::OperandType operand1,
-               lir::OperandType operand2)
+               lir::Operand::Type operand1,
+               lir::Operand::Type operand2)
 {
   return operation + ((lir::BinaryOperationCount
-                       + lir::NonBranchTernaryOperationCount) * operand1)
+                       + lir::NonBranchTernaryOperationCount) * (unsigned)operand1)
          + ((lir::BinaryOperationCount + lir::NonBranchTernaryOperationCount)
-            * lir::OperandTypeCount * operand2);
+            * lir::Operand::TypeCount * (unsigned)operand2);
 }
 
 unsigned index(ArchitectureContext* c UNUSED,
                lir::TernaryOperation operation,
-               lir::OperandType operand1,
-               lir::OperandType operand2)
+               lir::Operand::Type operand1,
+               lir::Operand::Type operand2)
 {
   assertT(c, not isBranch(operation));
 
   return lir::BinaryOperationCount + operation
          + ((lir::BinaryOperationCount + lir::NonBranchTernaryOperationCount)
-            * operand1)
+            * (unsigned)operand1)
          + ((lir::BinaryOperationCount + lir::NonBranchTernaryOperationCount)
-            * lir::OperandTypeCount * operand2);
+            * lir::Operand::TypeCount * (unsigned)operand2);
 }
 
 unsigned branchIndex(ArchitectureContext* c UNUSED,
-                     lir::OperandType operand1,
-                     lir::OperandType operand2)
+                     lir::Operand::Type operand1,
+                     lir::Operand::Type operand2)
 {
-  return operand1 + (lir::OperandTypeCount * operand2);
+  return (unsigned)operand1 + (lir::Operand::TypeCount * (unsigned)operand2);
 }
 
 void populateTables(ArchitectureContext* c)
 {
-  const lir::OperandType C = lir::ConstantOperand;
-  const lir::OperandType A = lir::AddressOperand;
-  const lir::OperandType R = lir::RegisterOperand;
-  const lir::OperandType M = lir::MemoryOperand;
+  const lir::Operand::Type C = lir::Operand::Type::Constant;
+  const lir::Operand::Type A = lir::Operand::Type::Address;
+  const lir::Operand::Type R = lir::Operand::Type::RegisterPair;
+  const lir::Operand::Type M = lir::Operand::Type::Memory;
 
   OperationType* zo = c->operations;
   UnaryOperationType* uo = c->unaryOperations;
diff --git a/src/codegen/target/x86/multimethod.h b/src/codegen/target/x86/multimethod.h
index e9a4b36de6..5816ccb543 100644
--- a/src/codegen/target/x86/multimethod.h
+++ b/src/codegen/target/x86/multimethod.h
@@ -23,17 +23,17 @@ class ArchitectureContext;
 
 unsigned index(ArchitectureContext*,
                lir::BinaryOperation operation,
-               lir::OperandType operand1,
-               lir::OperandType operand2);
+               lir::Operand::Type operand1,
+               lir::Operand::Type operand2);
 
 unsigned index(ArchitectureContext* c UNUSED,
                lir::TernaryOperation operation,
-               lir::OperandType operand1,
-               lir::OperandType operand2);
+               lir::Operand::Type operand1,
+               lir::Operand::Type operand2);
 
 unsigned branchIndex(ArchitectureContext* c UNUSED,
-                     lir::OperandType operand1,
-                     lir::OperandType operand2);
+                     lir::Operand::Type operand1,
+                     lir::Operand::Type operand2);
 
 void populateTables(ArchitectureContext* c);
 
diff --git a/src/codegen/target/x86/operations.cpp b/src/codegen/target/x86/operations.cpp
index 0301cda1eb..f936be8d3d 100644
--- a/src/codegen/target/x86/operations.cpp
+++ b/src/codegen/target/x86/operations.cpp
@@ -78,7 +78,7 @@ void longCallC(Context* c, unsigned size, lir::Constant* a)
   assertT(c, size == vm::TargetBytesPerWord);
 
   if (vm::TargetBytesPerWord == 8) {
-    lir::Register r(LongJumpRegister);
+    lir::RegisterPair r(LongJumpRegister);
     moveCR2(c, size, a, size, &r, 11);
     callR(c, size, &r);
   } else {
@@ -86,7 +86,7 @@ void longCallC(Context* c, unsigned size, lir::Constant* a)
   }
 }
 
-void jumpR(Context* c, unsigned size UNUSED, lir::Register* a)
+void jumpR(Context* c, unsigned size UNUSED, lir::RegisterPair* a)
 {
   assertT(c, size == vm::TargetBytesPerWord);
 
@@ -115,7 +115,7 @@ void longJumpC(Context* c, unsigned size, lir::Constant* a)
   assertT(c, size == vm::TargetBytesPerWord);
 
   if (vm::TargetBytesPerWord == 8) {
-    lir::Register r(LongJumpRegister);
+    lir::RegisterPair r(LongJumpRegister);
     moveCR2(c, size, a, size, &r, 11);
     jumpR(c, size, &r);
   } else {
@@ -123,7 +123,7 @@ void longJumpC(Context* c, unsigned size, lir::Constant* a)
   }
 }
 
-void callR(Context* c, unsigned size UNUSED, lir::Register* a)
+void callR(Context* c, unsigned size UNUSED, lir::RegisterPair* a)
 {
   assertT(c, size == vm::TargetBytesPerWord);
 
@@ -177,10 +177,10 @@ void alignedLongJumpC(Context* c, unsigned size, lir::Constant* a)
   }
 }
 
-void pushR(Context* c, unsigned size, lir::Register* a)
+void pushR(Context* c, unsigned size, lir::RegisterPair* a)
 {
   if (vm::TargetBytesPerWord == 4 and size == 8) {
-    lir::Register ah(a->high);
+    lir::RegisterPair ah(a->high);
 
     pushR(c, 4, &ah);
     pushR(c, 4, a);
@@ -190,10 +190,10 @@ void pushR(Context* c, unsigned size, lir::Register* a)
   }
 }
 
-void popR(Context* c, unsigned size, lir::Register* a)
+void popR(Context* c, unsigned size, lir::RegisterPair* a)
 {
   if (vm::TargetBytesPerWord == 4 and size == 8) {
-    lir::Register ah(a->high);
+    lir::RegisterPair ah(a->high);
 
     popR(c, 4, a);
     popR(c, 4, &ah);
@@ -206,7 +206,7 @@ void popR(Context* c, unsigned size, lir::Register* a)
   }
 }
 
-void negateR(Context* c, unsigned size, lir::Register* a)
+void negateR(Context* c, unsigned size, lir::RegisterPair* a)
 {
   if (vm::TargetBytesPerWord == 4 and size == 8) {
     assertT(c, a->low == rax and a->high == rdx);
@@ -214,7 +214,7 @@ void negateR(Context* c, unsigned size, lir::Register* a)
     ResolvedPromise zeroPromise(0);
     lir::Constant zero(&zeroPromise);
 
-    lir::Register ah(a->high);
+    lir::RegisterPair ah(a->high);
 
     negateR(c, 4, a);
     addCarryCR(c, 4, &zero, &ah);
@@ -227,9 +227,9 @@ void negateR(Context* c, unsigned size, lir::Register* a)
 
 void negateRR(Context* c,
               unsigned aSize,
-              lir::Register* a,
+              lir::RegisterPair* a,
               unsigned bSize UNUSED,
-              lir::Register* b UNUSED)
+              lir::RegisterPair* b UNUSED)
 {
   assertT(c, aSize == bSize);
 
@@ -240,7 +240,7 @@ void moveCR(Context* c,
             unsigned aSize,
             lir::Constant* a,
             unsigned bSize,
-            lir::Register* b)
+            lir::RegisterPair* b)
 {
   if (isFloatReg(b)) {
     sseMoveCR(c, aSize, a, bSize, b);
@@ -253,7 +253,7 @@ void moveZCR(Context* c,
              unsigned aSize UNUSED,
              lir::Constant* a,
              unsigned bSize UNUSED,
-             lir::Register* b)
+             lir::RegisterPair* b)
 {
   assertT(c, not isFloatReg(b));
   assertT(c, aSize == 2);
@@ -267,9 +267,9 @@ void moveZCR(Context* c,
 
 void swapRR(Context* c,
             unsigned aSize UNUSED,
-            lir::Register* a,
+            lir::RegisterPair* a,
             unsigned bSize UNUSED,
-            lir::Register* b)
+            lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
   assertT(c, aSize == vm::TargetBytesPerWord);
@@ -281,9 +281,9 @@ void swapRR(Context* c,
 
 void moveRR(Context* c,
             unsigned aSize,
-            lir::Register* a,
+            lir::RegisterPair* a,
             UNUSED unsigned bSize,
-            lir::Register* b)
+            lir::RegisterPair* b)
 {
   if (isFloatReg(a) or isFloatReg(b)) {
     sseMoveRR(c, aSize, a, bSize, b);
@@ -291,8 +291,8 @@ void moveRR(Context* c,
   }
 
   if (vm::TargetBytesPerWord == 4 and aSize == 8 and bSize == 8) {
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
     if (a->high == b->low) {
       if (a->low == b->high) {
@@ -366,7 +366,7 @@ void moveMR(Context* c,
             unsigned aSize,
             lir::Memory* a,
             unsigned bSize,
-            lir::Register* b)
+            lir::RegisterPair* b)
 {
   if (isFloatReg(b)) {
     sseMoveMR(c, aSize, a, bSize, b);
@@ -408,7 +408,7 @@ void moveMR(Context* c,
   case 8:
     if (vm::TargetBytesPerWord == 4 and bSize == 8) {
       lir::Memory ah(a->base, a->offset + 4, a->index, a->scale);
-      lir::Register bh(b->high);
+      lir::RegisterPair bh(b->high);
 
       moveMR(c, 4, a, 4, b);
       moveMR(c, 4, &ah, 4, &bh);
@@ -426,7 +426,7 @@ void moveMR(Context* c,
 
 void moveRM(Context* c,
             unsigned aSize,
-            lir::Register* a,
+            lir::RegisterPair* a,
             unsigned bSize UNUSED,
             lir::Memory* b)
 {
@@ -469,7 +469,7 @@ void moveRM(Context* c,
       opcode(c, 0x89);
       modrmSibImm(c, a, b);
     } else {
-      lir::Register ah(a->high);
+      lir::RegisterPair ah(a->high);
       lir::Memory bh(b->base, b->offset + 4, b->index, b->scale);
 
       moveRM(c, 4, a, 4, b);
@@ -486,12 +486,12 @@ void moveAR(Context* c,
             unsigned aSize,
             lir::Address* a,
             unsigned bSize,
-            lir::Register* b)
+            lir::RegisterPair* b)
 {
   assertT(c, vm::TargetBytesPerWord == 8 or (aSize == 4 and bSize == 4));
 
   lir::Constant constant(a->address);
-  lir::Memory memory(b->low, 0, -1, 0);
+  lir::Memory memory(b->low, 0, NoRegister, 0);
 
   moveCR(c, aSize, &constant, bSize, b);
   moveMR(c, bSize, &memory, bSize, b);
@@ -507,7 +507,7 @@ void moveCM(Context* c,
   case 1:
     maybeRex(c, bSize, b);
     opcode(c, 0xc6);
-    modrmSibImm(c, 0, b->scale, b->index, b->base, b->offset);
+    modrmSibImm(c, rax, b->scale, b->index, b->base, b->offset);
     c->code.append(a->value->value());
     break;
 
@@ -515,14 +515,14 @@ void moveCM(Context* c,
     opcode(c, 0x66);
     maybeRex(c, bSize, b);
     opcode(c, 0xc7);
-    modrmSibImm(c, 0, b->scale, b->index, b->base, b->offset);
+    modrmSibImm(c, rax, b->scale, b->index, b->base, b->offset);
     c->code.append2(a->value->value());
     break;
 
   case 4:
     maybeRex(c, bSize, b);
     opcode(c, 0xc7);
-    modrmSibImm(c, 0, b->scale, b->index, b->base, b->offset);
+    modrmSibImm(c, rax, b->scale, b->index, b->base, b->offset);
     if (a->value->resolved()) {
       c->code.append4(a->value->value());
     } else {
@@ -536,10 +536,10 @@ void moveCM(Context* c,
       if (a->value->resolved() and vm::fitsInInt32(a->value->value())) {
         maybeRex(c, bSize, b);
         opcode(c, 0xc7);
-        modrmSibImm(c, 0, b->scale, b->index, b->base, b->offset);
+        modrmSibImm(c, rax, b->scale, b->index, b->base, b->offset);
         c->code.append4(a->value->value());
       } else {
-        lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+        lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
         moveCR(c, 8, a, 8, &tmp);
         moveRM(c, 8, &tmp, 8, b);
         c->client->releaseTemporary(tmp.low);
@@ -562,9 +562,9 @@ void moveCM(Context* c,
 
 void moveZRR(Context* c,
              unsigned aSize,
-             lir::Register* a,
+             lir::RegisterPair* a,
              unsigned bSize UNUSED,
-             lir::Register* b)
+             lir::RegisterPair* b)
 {
   switch (aSize) {
   case 2:
@@ -582,7 +582,7 @@ void moveZMR(Context* c,
              unsigned aSize UNUSED,
              lir::Memory* a,
              unsigned bSize UNUSED,
-             lir::Register* b)
+             lir::RegisterPair* b)
 {
   assertT(c, bSize == vm::TargetBytesPerWord);
   assertT(c, aSize == 2);
@@ -592,7 +592,7 @@ void moveZMR(Context* c,
   modrmSibImm(c, b->low, a->scale, a->index, a->base, a->offset);
 }
 
-void addCarryRR(Context* c, unsigned size, lir::Register* a, lir::Register* b)
+void addCarryRR(Context* c, unsigned size, lir::RegisterPair* a, lir::RegisterPair* b)
 {
   assertT(c, vm::TargetBytesPerWord == 8 or size == 4);
 
@@ -603,15 +603,15 @@ void addCarryRR(Context* c, unsigned size, lir::Register* a, lir::Register* b)
 
 void addRR(Context* c,
            unsigned aSize,
-           lir::Register* a,
+           lir::RegisterPair* a,
            unsigned bSize UNUSED,
-           lir::Register* b)
+           lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
   if (vm::TargetBytesPerWord == 4 and aSize == 8) {
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
     addRR(c, 4, a, 4, b);
     addCarryRR(c, 4, &ah, &bh);
@@ -622,7 +622,7 @@ void addRR(Context* c,
   }
 }
 
-void addCarryCR(Context* c, unsigned size, lir::Constant* a, lir::Register* b)
+void addCarryCR(Context* c, unsigned size, lir::Constant* a, lir::RegisterPair* b)
 {
   int64_t v = a->value->value();
   maybeRex(c, size, b);
@@ -639,7 +639,7 @@ void addCR(Context* c,
            unsigned aSize,
            lir::Constant* a,
            unsigned bSize,
-           lir::Register* b)
+           lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
@@ -652,7 +652,7 @@ void addCR(Context* c,
       ResolvedPromise low(v & 0xFFFFFFFF);
       lir::Constant al(&low);
 
-      lir::Register bh(b->high);
+      lir::RegisterPair bh(b->high);
 
       addCR(c, 4, &al, 4, b);
       addCarryCR(c, 4, &ah, &bh);
@@ -667,7 +667,7 @@ void addCR(Context* c,
           c->code.append4(v);
         }
       } else {
-        lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+        lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
         moveCR(c, aSize, a, aSize, &tmp);
         addRR(c, aSize, &tmp, bSize, b);
         c->client->releaseTemporary(tmp.low);
@@ -679,7 +679,7 @@ void addCR(Context* c,
 void subtractBorrowCR(Context* c,
                       unsigned size UNUSED,
                       lir::Constant* a,
-                      lir::Register* b)
+                      lir::RegisterPair* b)
 {
   assertT(c, vm::TargetBytesPerWord == 8 or size == 4);
 
@@ -697,7 +697,7 @@ void subtractCR(Context* c,
                 unsigned aSize,
                 lir::Constant* a,
                 unsigned bSize,
-                lir::Register* b)
+                lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
@@ -710,7 +710,7 @@ void subtractCR(Context* c,
       ResolvedPromise low(v & 0xFFFFFFFF);
       lir::Constant al(&low);
 
-      lir::Register bh(b->high);
+      lir::RegisterPair bh(b->high);
 
       subtractCR(c, 4, &al, 4, b);
       subtractBorrowCR(c, 4, &ah, &bh);
@@ -725,7 +725,7 @@ void subtractCR(Context* c,
           c->code.append4(v);
         }
       } else {
-        lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+        lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
         moveCR(c, aSize, a, aSize, &tmp);
         subtractRR(c, aSize, &tmp, bSize, b);
         c->client->releaseTemporary(tmp.low);
@@ -736,8 +736,8 @@ void subtractCR(Context* c,
 
 void subtractBorrowRR(Context* c,
                       unsigned size,
-                      lir::Register* a,
-                      lir::Register* b)
+                      lir::RegisterPair* a,
+                      lir::RegisterPair* b)
 {
   assertT(c, vm::TargetBytesPerWord == 8 or size == 4);
 
@@ -748,15 +748,15 @@ void subtractBorrowRR(Context* c,
 
 void subtractRR(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize UNUSED,
-                lir::Register* b)
+                lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
   if (vm::TargetBytesPerWord == 4 and aSize == 8) {
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
     subtractRR(c, 4, a, 4, b);
     subtractBorrowRR(c, 4, &ah, &bh);
@@ -769,15 +769,15 @@ void subtractRR(Context* c,
 
 void andRR(Context* c,
            unsigned aSize,
-           lir::Register* a,
+           lir::RegisterPair* a,
            unsigned bSize UNUSED,
-           lir::Register* b)
+           lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
   if (vm::TargetBytesPerWord == 4 and aSize == 8) {
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
     andRR(c, 4, a, 4, b);
     andRR(c, 4, &ah, 4, &bh);
@@ -792,7 +792,7 @@ void andCR(Context* c,
            unsigned aSize,
            lir::Constant* a,
            unsigned bSize,
-           lir::Register* b)
+           lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
@@ -805,7 +805,7 @@ void andCR(Context* c,
     ResolvedPromise low(v & 0xFFFFFFFF);
     lir::Constant al(&low);
 
-    lir::Register bh(b->high);
+    lir::RegisterPair bh(b->high);
 
     andCR(c, 4, &al, 4, b);
     andCR(c, 4, &ah, 4, &bh);
@@ -820,7 +820,7 @@ void andCR(Context* c,
         c->code.append4(v);
       }
     } else {
-      lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+      lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
       moveCR(c, aSize, a, aSize, &tmp);
       andRR(c, aSize, &tmp, bSize, b);
       c->client->releaseTemporary(tmp.low);
@@ -830,15 +830,15 @@ void andCR(Context* c,
 
 void orRR(Context* c,
           unsigned aSize,
-          lir::Register* a,
+          lir::RegisterPair* a,
           unsigned bSize UNUSED,
-          lir::Register* b)
+          lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
   if (vm::TargetBytesPerWord == 4 and aSize == 8) {
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
     orRR(c, 4, a, 4, b);
     orRR(c, 4, &ah, 4, &bh);
@@ -853,7 +853,7 @@ void orCR(Context* c,
           unsigned aSize,
           lir::Constant* a,
           unsigned bSize,
-          lir::Register* b)
+          lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
@@ -866,7 +866,7 @@ void orCR(Context* c,
       ResolvedPromise low(v & 0xFFFFFFFF);
       lir::Constant al(&low);
 
-      lir::Register bh(b->high);
+      lir::RegisterPair bh(b->high);
 
       orCR(c, 4, &al, 4, b);
       orCR(c, 4, &ah, 4, &bh);
@@ -881,7 +881,7 @@ void orCR(Context* c,
           c->code.append4(v);
         }
       } else {
-        lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+        lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
         moveCR(c, aSize, a, aSize, &tmp);
         orRR(c, aSize, &tmp, bSize, b);
         c->client->releaseTemporary(tmp.low);
@@ -892,13 +892,13 @@ void orCR(Context* c,
 
 void xorRR(Context* c,
            unsigned aSize,
-           lir::Register* a,
+           lir::RegisterPair* a,
            unsigned bSize UNUSED,
-           lir::Register* b)
+           lir::RegisterPair* b)
 {
   if (vm::TargetBytesPerWord == 4 and aSize == 8) {
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
     xorRR(c, 4, a, 4, b);
     xorRR(c, 4, &ah, 4, &bh);
@@ -913,7 +913,7 @@ void xorCR(Context* c,
            unsigned aSize,
            lir::Constant* a,
            unsigned bSize,
-           lir::Register* b)
+           lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
@@ -926,7 +926,7 @@ void xorCR(Context* c,
       ResolvedPromise low(v & 0xFFFFFFFF);
       lir::Constant al(&low);
 
-      lir::Register bh(b->high);
+      lir::RegisterPair bh(b->high);
 
       xorCR(c, 4, &al, 4, b);
       xorCR(c, 4, &ah, 4, &bh);
@@ -941,7 +941,7 @@ void xorCR(Context* c,
           c->code.append4(v);
         }
       } else {
-        lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+        lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
         moveCR(c, aSize, a, aSize, &tmp);
         xorRR(c, aSize, &tmp, bSize, b);
         c->client->releaseTemporary(tmp.low);
@@ -952,9 +952,9 @@ void xorCR(Context* c,
 
 void multiplyRR(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize UNUSED,
-                lir::Register* b)
+                lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
@@ -966,14 +966,14 @@ void multiplyRR(Context* c,
 
     c->client->save(rax);
 
-    lir::Register axdx(rax, rdx);
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair axdx(rax, rdx);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
-    lir::Register tmp(-1);
-    lir::Register* scratch;
+    lir::RegisterPair tmp(NoRegister);
+    lir::RegisterPair* scratch;
     if (a->low == b->low) {
-      tmp.low = c->client->acquireTemporary(GeneralRegisterMask & ~(1 << rax));
+      tmp.low = c->client->acquireTemporary(GeneralRegisterMask.excluding(rax));
       scratch = &tmp;
       moveRR(c, 4, b, 4, scratch);
     } else {
@@ -986,12 +986,12 @@ void multiplyRR(Context* c,
     addRR(c, 4, &bh, 4, scratch);
 
     // mul a->low,%eax%edx
-    opcode(c, 0xf7, 0xe0 + a->low);
+    opcode(c, 0xf7, 0xe0 + a->low.index());
 
     addRR(c, 4, scratch, 4, &bh);
     moveRR(c, 4, &axdx, 4, b);
 
-    if (tmp.low != -1) {
+    if (tmp.low != NoRegister) {
       c->client->releaseTemporary(tmp.low);
     }
   } else {
@@ -1003,9 +1003,9 @@ void multiplyRR(Context* c,
 
 void compareRR(Context* c,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize UNUSED,
-               lir::Register* b)
+               lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
   assertT(c, aSize <= vm::TargetBytesPerWord);
@@ -1019,7 +1019,7 @@ void compareCR(Context* c,
                unsigned aSize,
                lir::Constant* a,
                unsigned bSize,
-               lir::Register* b)
+               lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
   assertT(c, vm::TargetBytesPerWord == 8 or aSize == 4);
@@ -1035,7 +1035,7 @@ void compareCR(Context* c,
       c->code.append4(v);
     }
   } else {
-    lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+    lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
     moveCR(c, aSize, a, aSize, &tmp);
     compareRR(c, aSize, &tmp, bSize, b);
     c->client->releaseTemporary(tmp.low);
@@ -1044,7 +1044,7 @@ void compareCR(Context* c,
 
 void compareRM(Context* c,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize UNUSED,
                lir::Memory* b)
 {
@@ -1082,7 +1082,7 @@ void compareCM(Context* c,
       abort(c);
     }
   } else {
-    lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+    lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
     moveCR(c, aSize, a, bSize, &tmp);
     compareRM(c, bSize, &tmp, bSize, b);
     c->client->releaseTemporary(tmp.low);
@@ -1091,9 +1091,9 @@ void compareCM(Context* c,
 
 void compareFloatRR(Context* c,
                     unsigned aSize,
-                    lir::Register* a,
+                    lir::RegisterPair* a,
                     unsigned bSize UNUSED,
-                    lir::Register* b)
+                    lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
@@ -1192,16 +1192,16 @@ void branchLong(Context* c,
 void branchRR(Context* c,
               lir::TernaryOperation op,
               unsigned size,
-              lir::Register* a,
-              lir::Register* b,
+              lir::RegisterPair* a,
+              lir::RegisterPair* b,
               lir::Constant* target)
 {
   if (isFloatBranch(op)) {
     compareFloatRR(c, size, a, size, b);
     branchFloat(c, op, target);
   } else if (size > vm::TargetBytesPerWord) {
-    lir::Register ah(a->high);
-    lir::Register bh(b->high);
+    lir::RegisterPair ah(a->high);
+    lir::RegisterPair bh(b->high);
 
     branchLong(c, op, a, &ah, b, &bh, target, CAST2(compareRR));
   } else {
@@ -1214,7 +1214,7 @@ void branchCR(Context* c,
               lir::TernaryOperation op,
               unsigned size,
               lir::Constant* a,
-              lir::Register* b,
+              lir::RegisterPair* b,
               lir::Constant* target)
 {
   assertT(c, not isFloatBranch(op));
@@ -1228,7 +1228,7 @@ void branchCR(Context* c,
     ResolvedPromise high((v >> 32) & ~static_cast<uintptr_t>(0));
     lir::Constant ah(&high);
 
-    lir::Register bh(b->high);
+    lir::RegisterPair bh(b->high);
 
     branchLong(c, op, &al, &ah, b, &bh, target, CAST2(compareCR));
   } else {
@@ -1240,7 +1240,7 @@ void branchCR(Context* c,
 void branchRM(Context* c,
               lir::TernaryOperation op,
               unsigned size,
-              lir::Register* a,
+              lir::RegisterPair* a,
               lir::Memory* b,
               lir::Constant* target)
 {
@@ -1269,13 +1269,13 @@ void multiplyCR(Context* c,
                 unsigned aSize,
                 lir::Constant* a,
                 unsigned bSize,
-                lir::Register* b)
+                lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
   if (vm::TargetBytesPerWord == 4 and aSize == 8) {
-    const uint32_t mask = GeneralRegisterMask & ~((1 << rax) | (1 << rdx));
-    lir::Register tmp(c->client->acquireTemporary(mask),
+    const RegisterMask mask = GeneralRegisterMask.excluding(rax).excluding(rdx);
+    lir::RegisterPair tmp(c->client->acquireTemporary(mask),
                       c->client->acquireTemporary(mask));
 
     moveCR(c, aSize, a, aSize, &tmp);
@@ -1297,7 +1297,7 @@ void multiplyCR(Context* c,
           c->code.append4(v);
         }
       } else {
-        lir::Register tmp(c->client->acquireTemporary(GeneralRegisterMask));
+        lir::RegisterPair tmp(c->client->acquireTemporary(GeneralRegisterMask));
         moveCR(c, aSize, a, aSize, &tmp);
         multiplyRR(c, aSize, &tmp, bSize, b);
         c->client->releaseTemporary(tmp.low);
@@ -1308,9 +1308,9 @@ void multiplyCR(Context* c,
 
 void divideRR(Context* c,
               unsigned aSize,
-              lir::Register* a,
+              lir::RegisterPair* a,
               unsigned bSize UNUSED,
-              lir::Register* b UNUSED)
+              lir::RegisterPair* b UNUSED)
 {
   assertT(c, aSize == bSize);
 
@@ -1327,9 +1327,9 @@ void divideRR(Context* c,
 
 void remainderRR(Context* c,
                  unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize UNUSED,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   assertT(c, aSize == bSize);
 
@@ -1343,28 +1343,28 @@ void remainderRR(Context* c,
   maybeRex(c, aSize, b, a);
   opcode(c, 0xf7, 0xf8 + regCode(a));
 
-  lir::Register dx(rdx);
+  lir::RegisterPair dx(rdx);
   moveRR(c, vm::TargetBytesPerWord, &dx, vm::TargetBytesPerWord, b);
 }
 
 void doShift(Context* c,
              UNUSED void (*shift)(Context*,
                                   unsigned,
-                                  lir::Register*,
+                                  lir::RegisterPair*,
                                   unsigned,
-                                  lir::Register*),
+                                  lir::RegisterPair*),
              int type,
              UNUSED unsigned aSize,
              lir::Constant* a,
              unsigned bSize,
-             lir::Register* b)
+             lir::RegisterPair* b)
 {
   int64_t v = a->value->value();
 
   if (vm::TargetBytesPerWord == 4 and bSize == 8) {
     c->client->save(rcx);
 
-    lir::Register cx(rcx);
+    lir::RegisterPair cx(rcx);
     ResolvedPromise promise(v & 0x3F);
     lir::Constant masked(&promise);
     moveCR(c, 4, &masked, 4, &cx);
@@ -1384,12 +1384,12 @@ void doShift(Context* c,
 
 void shiftLeftRR(Context* c,
                  UNUSED unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   if (vm::TargetBytesPerWord == 4 and bSize == 8) {
-    lir::Register cx(rcx);
+    lir::RegisterPair cx(rcx);
     if (a->low != rcx) {
       c->client->save(rcx);
       ResolvedPromise promise(0x3F);
@@ -1403,7 +1403,7 @@ void shiftLeftRR(Context* c,
     modrm(c, 0xc0, b->high, b->low);
 
     // shl
-    opcode(c, 0xd3, 0xe0 + b->low);
+    opcode(c, 0xd3, 0xe0 + b->low.index());
 
     ResolvedPromise promise(32);
     lir::Constant constant(&promise);
@@ -1413,7 +1413,7 @@ void shiftLeftRR(Context* c,
     opcode(c, 0x7c);  // jl
     c->code.append(2 + 2);
 
-    lir::Register bh(b->high);
+    lir::RegisterPair bh(b->high);
     moveRR(c, 4, b, 4, &bh);  // 2 bytes
     xorRR(c, 4, b, 4, b);     // 2 bytes
   } else {
@@ -1428,19 +1428,19 @@ void shiftLeftCR(Context* c,
                  unsigned aSize,
                  lir::Constant* a,
                  unsigned bSize,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   doShift(c, shiftLeftRR, 0xe0, aSize, a, bSize, b);
 }
 
 void shiftRightRR(Context* c,
                   UNUSED unsigned aSize,
-                  lir::Register* a,
+                  lir::RegisterPair* a,
                   unsigned bSize,
-                  lir::Register* b)
+                  lir::RegisterPair* b)
 {
   if (vm::TargetBytesPerWord == 4 and bSize == 8) {
-    lir::Register cx(rcx);
+    lir::RegisterPair cx(rcx);
     if (a->low != rcx) {
       c->client->save(rcx);
       ResolvedPromise promise(0x3F);
@@ -1454,7 +1454,7 @@ void shiftRightRR(Context* c,
     modrm(c, 0xc0, b->low, b->high);
 
     // sar
-    opcode(c, 0xd3, 0xf8 + b->high);
+    opcode(c, 0xd3, 0xf8 + b->high.index());
 
     ResolvedPromise promise(32);
     lir::Constant constant(&promise);
@@ -1464,11 +1464,11 @@ void shiftRightRR(Context* c,
     opcode(c, 0x7c);  // jl
     c->code.append(2 + 3);
 
-    lir::Register bh(b->high);
+    lir::RegisterPair bh(b->high);
     moveRR(c, 4, &bh, 4, b);  // 2 bytes
 
     // sar 31,high
-    opcode(c, 0xc1, 0xf8 + b->high);
+    opcode(c, 0xc1, 0xf8 + b->high.index());
     c->code.append(31);
   } else {
     assertT(c, a->low == rcx);
@@ -1482,19 +1482,19 @@ void shiftRightCR(Context* c,
                   unsigned aSize,
                   lir::Constant* a,
                   unsigned bSize,
-                  lir::Register* b)
+                  lir::RegisterPair* b)
 {
   doShift(c, shiftRightRR, 0xf8, aSize, a, bSize, b);
 }
 
 void unsignedShiftRightRR(Context* c,
                           UNUSED unsigned aSize,
-                          lir::Register* a,
+                          lir::RegisterPair* a,
                           unsigned bSize,
-                          lir::Register* b)
+                          lir::RegisterPair* b)
 {
   if (vm::TargetBytesPerWord == 4 and bSize == 8) {
-    lir::Register cx(rcx);
+    lir::RegisterPair cx(rcx);
     if (a->low != rcx) {
       c->client->save(rcx);
       ResolvedPromise promise(0x3F);
@@ -1508,7 +1508,7 @@ void unsignedShiftRightRR(Context* c,
     modrm(c, 0xc0, b->low, b->high);
 
     // shr
-    opcode(c, 0xd3, 0xe8 + b->high);
+    opcode(c, 0xd3, 0xe8 + b->high.index());
 
     ResolvedPromise promise(32);
     lir::Constant constant(&promise);
@@ -1518,7 +1518,7 @@ void unsignedShiftRightRR(Context* c,
     opcode(c, 0x7c);  // jl
     c->code.append(2 + 2);
 
-    lir::Register bh(b->high);
+    lir::RegisterPair bh(b->high);
     moveRR(c, 4, &bh, 4, b);   // 2 bytes
     xorRR(c, 4, &bh, 4, &bh);  // 2 bytes
   } else {
@@ -1533,16 +1533,16 @@ void unsignedShiftRightCR(Context* c,
                           unsigned aSize UNUSED,
                           lir::Constant* a,
                           unsigned bSize,
-                          lir::Register* b)
+                          lir::RegisterPair* b)
 {
   doShift(c, unsignedShiftRightRR, 0xe8, aSize, a, bSize, b);
 }
 
 void floatSqrtRR(Context* c,
                  unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize UNUSED,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   floatRegOp(c, aSize, a, 4, b, 0x51);
 }
@@ -1551,16 +1551,16 @@ void floatSqrtMR(Context* c,
                  unsigned aSize,
                  lir::Memory* a,
                  unsigned bSize UNUSED,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   floatMemOp(c, aSize, a, 4, b, 0x51);
 }
 
 void floatAddRR(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize UNUSED,
-                lir::Register* b)
+                lir::RegisterPair* b)
 {
   floatRegOp(c, aSize, a, 4, b, 0x58);
 }
@@ -1569,16 +1569,16 @@ void floatAddMR(Context* c,
                 unsigned aSize,
                 lir::Memory* a,
                 unsigned bSize UNUSED,
-                lir::Register* b)
+                lir::RegisterPair* b)
 {
   floatMemOp(c, aSize, a, 4, b, 0x58);
 }
 
 void floatSubtractRR(Context* c,
                      unsigned aSize,
-                     lir::Register* a,
+                     lir::RegisterPair* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b)
+                     lir::RegisterPair* b)
 {
   floatRegOp(c, aSize, a, 4, b, 0x5c);
 }
@@ -1587,16 +1587,16 @@ void floatSubtractMR(Context* c,
                      unsigned aSize,
                      lir::Memory* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b)
+                     lir::RegisterPair* b)
 {
   floatMemOp(c, aSize, a, 4, b, 0x5c);
 }
 
 void floatMultiplyRR(Context* c,
                      unsigned aSize,
-                     lir::Register* a,
+                     lir::RegisterPair* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b)
+                     lir::RegisterPair* b)
 {
   floatRegOp(c, aSize, a, 4, b, 0x59);
 }
@@ -1605,16 +1605,16 @@ void floatMultiplyMR(Context* c,
                      unsigned aSize,
                      lir::Memory* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b)
+                     lir::RegisterPair* b)
 {
   floatMemOp(c, aSize, a, 4, b, 0x59);
 }
 
 void floatDivideRR(Context* c,
                    unsigned aSize,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b)
+                   lir::RegisterPair* b)
 {
   floatRegOp(c, aSize, a, 4, b, 0x5e);
 }
@@ -1623,16 +1623,16 @@ void floatDivideMR(Context* c,
                    unsigned aSize,
                    lir::Memory* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b)
+                   lir::RegisterPair* b)
 {
   floatMemOp(c, aSize, a, 4, b, 0x5e);
 }
 
 void float2FloatRR(Context* c,
                    unsigned aSize,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b)
+                   lir::RegisterPair* b)
 {
   floatRegOp(c, aSize, a, 4, b, 0x5a);
 }
@@ -1641,16 +1641,16 @@ void float2FloatMR(Context* c,
                    unsigned aSize,
                    lir::Memory* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b)
+                   lir::RegisterPair* b)
 {
   floatMemOp(c, aSize, a, 4, b, 0x5a);
 }
 
 void float2IntRR(Context* c,
                  unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   assertT(c, not isFloatReg(b));
   floatRegOp(c, aSize, a, bSize, b, 0x2c);
@@ -1660,16 +1660,16 @@ void float2IntMR(Context* c,
                  unsigned aSize,
                  lir::Memory* a,
                  unsigned bSize,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   floatMemOp(c, aSize, a, bSize, b, 0x2c);
 }
 
 void int2FloatRR(Context* c,
                  unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   floatRegOp(c, bSize, a, aSize, b, 0x2a);
 }
@@ -1678,16 +1678,16 @@ void int2FloatMR(Context* c,
                  unsigned aSize,
                  lir::Memory* a,
                  unsigned bSize,
-                 lir::Register* b)
+                 lir::RegisterPair* b)
 {
   floatMemOp(c, bSize, a, aSize, b, 0x2a);
 }
 
 void floatNegateRR(Context* c,
                    unsigned aSize,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b)
+                   lir::RegisterPair* b)
 {
   assertT(c, isFloatReg(a) and isFloatReg(b));
   // unlike most of the other floating point code, this does NOT
@@ -1696,7 +1696,7 @@ void floatNegateRR(Context* c,
   ResolvedPromise pcon(0x80000000);
   lir::Constant con(&pcon);
   if (a->low == b->low) {
-    lir::Register tmp(c->client->acquireTemporary(FloatRegisterMask));
+    lir::RegisterPair tmp(c->client->acquireTemporary(FloatRegisterMask));
     moveCR(c, 4, &con, 4, &tmp);
     maybeRex(c, 4, a, &tmp);
     opcode(c, 0x0f, 0x57);
@@ -1714,9 +1714,9 @@ void floatNegateRR(Context* c,
 
 void floatAbsoluteRR(Context* c,
                      unsigned aSize UNUSED,
-                     lir::Register* a,
+                     lir::RegisterPair* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b)
+                     lir::RegisterPair* b)
 {
   assertT(c, isFloatReg(a) and isFloatReg(b));
   // unlike most of the other floating point code, this does NOT
@@ -1725,7 +1725,7 @@ void floatAbsoluteRR(Context* c,
   ResolvedPromise pcon(0x7fffffff);
   lir::Constant con(&pcon);
   if (a->low == b->low) {
-    lir::Register tmp(c->client->acquireTemporary(FloatRegisterMask));
+    lir::RegisterPair tmp(c->client->acquireTemporary(FloatRegisterMask));
     moveCR(c, 4, &con, 4, &tmp);
     maybeRex(c, 4, a, &tmp);
     opcode(c, 0x0f, 0x54);
@@ -1741,12 +1741,12 @@ void floatAbsoluteRR(Context* c,
 
 void absoluteRR(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize UNUSED,
-                lir::Register* b UNUSED)
+                lir::RegisterPair* b UNUSED)
 {
   assertT(c, aSize == bSize and a->low == rax and b->low == rax);
-  lir::Register d(c->client->acquireTemporary(static_cast<uint64_t>(1) << rdx));
+  lir::RegisterPair d(c->client->acquireTemporary(rdx));
   maybeRex(c, aSize, a, b);
   opcode(c, 0x99);
   xorRR(c, aSize, &d, aSize, a);
diff --git a/src/codegen/target/x86/operations.h b/src/codegen/target/x86/operations.h
index 80f985f042..1b322788e8 100644
--- a/src/codegen/target/x86/operations.h
+++ b/src/codegen/target/x86/operations.h
@@ -33,7 +33,7 @@ void callC(Context* c, unsigned size UNUSED, lir::Constant* a);
 
 void longCallC(Context* c, unsigned size, lir::Constant* a);
 
-void jumpR(Context* c, unsigned size UNUSED, lir::Register* a);
+void jumpR(Context* c, unsigned size UNUSED, lir::RegisterPair* a);
 
 void jumpC(Context* c, unsigned size UNUSED, lir::Constant* a);
 
@@ -41,7 +41,7 @@ void jumpM(Context* c, unsigned size UNUSED, lir::Memory* a);
 
 void longJumpC(Context* c, unsigned size, lir::Constant* a);
 
-void callR(Context* c, unsigned size UNUSED, lir::Register* a);
+void callR(Context* c, unsigned size UNUSED, lir::RegisterPair* a);
 
 void callM(Context* c, unsigned size UNUSED, lir::Memory* a);
 
@@ -53,51 +53,51 @@ void alignedJumpC(Context* c, unsigned size, lir::Constant* a);
 
 void alignedLongJumpC(Context* c, unsigned size, lir::Constant* a);
 
-void pushR(Context* c, unsigned size, lir::Register* a);
+void pushR(Context* c, unsigned size, lir::RegisterPair* a);
 
-void popR(Context* c, unsigned size, lir::Register* a);
+void popR(Context* c, unsigned size, lir::RegisterPair* a);
 
-void negateR(Context* c, unsigned size, lir::Register* a);
+void negateR(Context* c, unsigned size, lir::RegisterPair* a);
 
 void negateRR(Context* c,
               unsigned aSize,
-              lir::Register* a,
+              lir::RegisterPair* a,
               unsigned bSize UNUSED,
-              lir::Register* b UNUSED);
+              lir::RegisterPair* b UNUSED);
 
 void moveCR(Context* c,
             unsigned aSize,
             lir::Constant* a,
             unsigned bSize,
-            lir::Register* b);
+            lir::RegisterPair* b);
 
 void moveZCR(Context* c,
              unsigned aSize,
              lir::Constant* a,
              unsigned bSize,
-             lir::Register* b);
+             lir::RegisterPair* b);
 
 void swapRR(Context* c,
             unsigned aSize UNUSED,
-            lir::Register* a,
+            lir::RegisterPair* a,
             unsigned bSize UNUSED,
-            lir::Register* b);
+            lir::RegisterPair* b);
 
 void moveRR(Context* c,
             unsigned aSize,
-            lir::Register* a,
+            lir::RegisterPair* a,
             UNUSED unsigned bSize,
-            lir::Register* b);
+            lir::RegisterPair* b);
 
 void moveMR(Context* c,
             unsigned aSize,
             lir::Memory* a,
             unsigned bSize,
-            lir::Register* b);
+            lir::RegisterPair* b);
 
 void moveRM(Context* c,
             unsigned aSize,
-            lir::Register* a,
+            lir::RegisterPair* a,
             unsigned bSize UNUSED,
             lir::Memory* b);
 
@@ -105,7 +105,7 @@ void moveAR(Context* c,
             unsigned aSize,
             lir::Address* a,
             unsigned bSize,
-            lir::Register* b);
+            lir::RegisterPair* b);
 
 void moveCM(Context* c,
             unsigned aSize UNUSED,
@@ -115,111 +115,111 @@ void moveCM(Context* c,
 
 void moveZRR(Context* c,
              unsigned aSize,
-             lir::Register* a,
+             lir::RegisterPair* a,
              unsigned bSize UNUSED,
-             lir::Register* b);
+             lir::RegisterPair* b);
 
 void moveZMR(Context* c,
              unsigned aSize UNUSED,
              lir::Memory* a,
              unsigned bSize UNUSED,
-             lir::Register* b);
+             lir::RegisterPair* b);
 
-void addCarryRR(Context* c, unsigned size, lir::Register* a, lir::Register* b);
+void addCarryRR(Context* c, unsigned size, lir::RegisterPair* a, lir::RegisterPair* b);
 
 void addRR(Context* c,
            unsigned aSize,
-           lir::Register* a,
+           lir::RegisterPair* a,
            unsigned bSize UNUSED,
-           lir::Register* b);
+           lir::RegisterPair* b);
 
-void addCarryCR(Context* c, unsigned size, lir::Constant* a, lir::Register* b);
+void addCarryCR(Context* c, unsigned size, lir::Constant* a, lir::RegisterPair* b);
 
 void addCR(Context* c,
            unsigned aSize,
            lir::Constant* a,
            unsigned bSize,
-           lir::Register* b);
+           lir::RegisterPair* b);
 
 void subtractBorrowCR(Context* c,
                       unsigned size UNUSED,
                       lir::Constant* a,
-                      lir::Register* b);
+                      lir::RegisterPair* b);
 
 void subtractCR(Context* c,
                 unsigned aSize,
                 lir::Constant* a,
                 unsigned bSize,
-                lir::Register* b);
+                lir::RegisterPair* b);
 
 void subtractBorrowRR(Context* c,
                       unsigned size,
-                      lir::Register* a,
-                      lir::Register* b);
+                      lir::RegisterPair* a,
+                      lir::RegisterPair* b);
 
 void subtractRR(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize UNUSED,
-                lir::Register* b);
+                lir::RegisterPair* b);
 
 void andRR(Context* c,
            unsigned aSize,
-           lir::Register* a,
+           lir::RegisterPair* a,
            unsigned bSize UNUSED,
-           lir::Register* b);
+           lir::RegisterPair* b);
 
 void andCR(Context* c,
            unsigned aSize,
            lir::Constant* a,
            unsigned bSize,
-           lir::Register* b);
+           lir::RegisterPair* b);
 
 void orRR(Context* c,
           unsigned aSize,
-          lir::Register* a,
+          lir::RegisterPair* a,
           unsigned bSize UNUSED,
-          lir::Register* b);
+          lir::RegisterPair* b);
 
 void orCR(Context* c,
           unsigned aSize,
           lir::Constant* a,
           unsigned bSize,
-          lir::Register* b);
+          lir::RegisterPair* b);
 
 void xorRR(Context* c,
            unsigned aSize,
-           lir::Register* a,
+           lir::RegisterPair* a,
            unsigned bSize UNUSED,
-           lir::Register* b);
+           lir::RegisterPair* b);
 
 void xorCR(Context* c,
            unsigned aSize,
            lir::Constant* a,
            unsigned bSize,
-           lir::Register* b);
+           lir::RegisterPair* b);
 
 void multiplyRR(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize UNUSED,
-                lir::Register* b);
+                lir::RegisterPair* b);
 
 void compareRR(Context* c,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize UNUSED,
-               lir::Register* b);
+               lir::RegisterPair* b);
 
 void compareCR(Context* c,
                unsigned aSize,
                lir::Constant* a,
                unsigned bSize,
-               lir::Register* b);
+               lir::RegisterPair* b);
 
 void compareRM(Context* c,
                unsigned aSize,
-               lir::Register* a,
+               lir::RegisterPair* a,
                unsigned bSize UNUSED,
                lir::Memory* b);
 
@@ -231,9 +231,9 @@ void compareCM(Context* c,
 
 void compareFloatRR(Context* c,
                     unsigned aSize,
-                    lir::Register* a,
+                    lir::RegisterPair* a,
                     unsigned bSize UNUSED,
-                    lir::Register* b);
+                    lir::RegisterPair* b);
 
 void branchLong(Context* c,
                 lir::TernaryOperation op,
@@ -247,21 +247,21 @@ void branchLong(Context* c,
 void branchRR(Context* c,
               lir::TernaryOperation op,
               unsigned size,
-              lir::Register* a,
-              lir::Register* b,
+              lir::RegisterPair* a,
+              lir::RegisterPair* b,
               lir::Constant* target);
 
 void branchCR(Context* c,
               lir::TernaryOperation op,
               unsigned size,
               lir::Constant* a,
-              lir::Register* b,
+              lir::RegisterPair* b,
               lir::Constant* target);
 
 void branchRM(Context* c,
               lir::TernaryOperation op,
               unsigned size,
-              lir::Register* a,
+              lir::RegisterPair* a,
               lir::Memory* b,
               lir::Constant* target);
 
@@ -276,181 +276,181 @@ void multiplyCR(Context* c,
                 unsigned aSize,
                 lir::Constant* a,
                 unsigned bSize,
-                lir::Register* b);
+                lir::RegisterPair* b);
 
 void divideRR(Context* c,
               unsigned aSize,
-              lir::Register* a,
+              lir::RegisterPair* a,
               unsigned bSize UNUSED,
-              lir::Register* b UNUSED);
+              lir::RegisterPair* b UNUSED);
 
 void remainderRR(Context* c,
                  unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize UNUSED,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void doShift(Context* c,
              UNUSED void (*shift)(Context*,
                                   unsigned,
-                                  lir::Register*,
+                                  lir::RegisterPair*,
                                   unsigned,
-                                  lir::Register*),
+                                  lir::RegisterPair*),
              int type,
              UNUSED unsigned aSize,
              lir::Constant* a,
              unsigned bSize,
-             lir::Register* b);
+             lir::RegisterPair* b);
 
 void shiftLeftRR(Context* c,
                  UNUSED unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void shiftLeftCR(Context* c,
                  unsigned aSize,
                  lir::Constant* a,
                  unsigned bSize,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void shiftRightRR(Context* c,
                   UNUSED unsigned aSize,
-                  lir::Register* a,
+                  lir::RegisterPair* a,
                   unsigned bSize,
-                  lir::Register* b);
+                  lir::RegisterPair* b);
 
 void shiftRightCR(Context* c,
                   unsigned aSize,
                   lir::Constant* a,
                   unsigned bSize,
-                  lir::Register* b);
+                  lir::RegisterPair* b);
 
 void unsignedShiftRightRR(Context* c,
                           UNUSED unsigned aSize,
-                          lir::Register* a,
+                          lir::RegisterPair* a,
                           unsigned bSize,
-                          lir::Register* b);
+                          lir::RegisterPair* b);
 
 void unsignedShiftRightCR(Context* c,
                           unsigned aSize UNUSED,
                           lir::Constant* a,
                           unsigned bSize,
-                          lir::Register* b);
+                          lir::RegisterPair* b);
 
 void floatSqrtRR(Context* c,
                  unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize UNUSED,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void floatSqrtMR(Context* c,
                  unsigned aSize,
                  lir::Memory* a,
                  unsigned bSize UNUSED,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void floatAddRR(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize UNUSED,
-                lir::Register* b);
+                lir::RegisterPair* b);
 
 void floatAddMR(Context* c,
                 unsigned aSize,
                 lir::Memory* a,
                 unsigned bSize UNUSED,
-                lir::Register* b);
+                lir::RegisterPair* b);
 
 void floatSubtractRR(Context* c,
                      unsigned aSize,
-                     lir::Register* a,
+                     lir::RegisterPair* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b);
+                     lir::RegisterPair* b);
 
 void floatSubtractMR(Context* c,
                      unsigned aSize,
                      lir::Memory* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b);
+                     lir::RegisterPair* b);
 
 void floatMultiplyRR(Context* c,
                      unsigned aSize,
-                     lir::Register* a,
+                     lir::RegisterPair* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b);
+                     lir::RegisterPair* b);
 
 void floatMultiplyMR(Context* c,
                      unsigned aSize,
                      lir::Memory* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b);
+                     lir::RegisterPair* b);
 
 void floatDivideRR(Context* c,
                    unsigned aSize,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b);
+                   lir::RegisterPair* b);
 
 void floatDivideMR(Context* c,
                    unsigned aSize,
                    lir::Memory* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b);
+                   lir::RegisterPair* b);
 
 void float2FloatRR(Context* c,
                    unsigned aSize,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b);
+                   lir::RegisterPair* b);
 
 void float2FloatMR(Context* c,
                    unsigned aSize,
                    lir::Memory* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b);
+                   lir::RegisterPair* b);
 
 void float2IntRR(Context* c,
                  unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void float2IntMR(Context* c,
                  unsigned aSize,
                  lir::Memory* a,
                  unsigned bSize,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void int2FloatRR(Context* c,
                  unsigned aSize,
-                 lir::Register* a,
+                 lir::RegisterPair* a,
                  unsigned bSize,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void int2FloatMR(Context* c,
                  unsigned aSize,
                  lir::Memory* a,
                  unsigned bSize,
-                 lir::Register* b);
+                 lir::RegisterPair* b);
 
 void floatNegateRR(Context* c,
                    unsigned aSize,
-                   lir::Register* a,
+                   lir::RegisterPair* a,
                    unsigned bSize UNUSED,
-                   lir::Register* b);
+                   lir::RegisterPair* b);
 
 void floatAbsoluteRR(Context* c,
                      unsigned aSize UNUSED,
-                     lir::Register* a,
+                     lir::RegisterPair* a,
                      unsigned bSize UNUSED,
-                     lir::Register* b);
+                     lir::RegisterPair* b);
 
 void absoluteRR(Context* c,
                 unsigned aSize,
-                lir::Register* a,
+                lir::RegisterPair* a,
                 unsigned bSize UNUSED,
-                lir::Register* b UNUSED);
+                lir::RegisterPair* b UNUSED);
 
 }  // namespace x86
 }  // namespace codegen
diff --git a/src/codegen/target/x86/registers.h b/src/codegen/target/x86/registers.h
index b61e521b7c..79b1218b13 100644
--- a/src/codegen/target/x86/registers.h
+++ b/src/codegen/target/x86/registers.h
@@ -15,50 +15,45 @@ namespace avian {
 namespace codegen {
 namespace x86 {
 
-enum {
-  rax = 0,
-  rcx = 1,
-  rdx = 2,
-  rbx = 3,
-  rsp = 4,
-  rbp = 5,
-  rsi = 6,
-  rdi = 7,
-  r8 = 8,
-  r9 = 9,
-  r10 = 10,
-  r11 = 11,
-  r12 = 12,
-  r13 = 13,
-  r14 = 14,
-  r15 = 15,
-};
+constexpr Register rax((int)0);
+constexpr Register rcx(1);
+constexpr Register rdx(2);
+constexpr Register rbx(3);
+constexpr Register rsp(4);
+constexpr Register rbp(5);
+constexpr Register rsi(6);
+constexpr Register rdi(7);
+constexpr Register r8(8);
+constexpr Register r9(9);
+constexpr Register r10(10);
+constexpr Register r11(11);
+constexpr Register r12(12);
+constexpr Register r13(13);
+constexpr Register r14(14);
+constexpr Register r15(15);
+constexpr Register xmm0(16);
+constexpr Register xmm1(16 + 1);
+constexpr Register xmm2(16 + 2);
+constexpr Register xmm3(16 + 3);
+constexpr Register xmm4(16 + 4);
+constexpr Register xmm5(16 + 5);
+constexpr Register xmm6(16 + 6);
+constexpr Register xmm7(16 + 7);
+constexpr Register xmm8(16 + 8);
+constexpr Register xmm9(16 + 9);
+constexpr Register xmm10(16 + 10);
+constexpr Register xmm11(16 + 11);
+constexpr Register xmm12(16 + 12);
+constexpr Register xmm13(16 + 13);
+constexpr Register xmm14(16 + 14);
+constexpr Register xmm15(16 + 15);
 
-enum {
-  xmm0 = r15 + 1,
-  xmm1,
-  xmm2,
-  xmm3,
-  xmm4,
-  xmm5,
-  xmm6,
-  xmm7,
-  xmm8,
-  xmm9,
-  xmm10,
-  xmm11,
-  xmm12,
-  xmm13,
-  xmm14,
-  xmm15,
-};
+constexpr Register LongJumpRegister = r10;
 
-const int LongJumpRegister = r10;
-
-const unsigned GeneralRegisterMask = vm::TargetBytesPerWord == 4 ? 0x000000ff
+constexpr RegisterMask GeneralRegisterMask = vm::TargetBytesPerWord == 4 ? 0x000000ff
                                                                  : 0x0000ffff;
 
-const unsigned FloatRegisterMask = vm::TargetBytesPerWord == 4 ? 0x00ff0000
+constexpr RegisterMask FloatRegisterMask = vm::TargetBytesPerWord == 4 ? 0x00ff0000
                                                                : 0xffff0000;
 
 }  // namespace x86
diff --git a/src/compile.cpp b/src/compile.cpp
index b64d06f1c5..47b55574e7 100644
--- a/src/compile.cpp
+++ b/src/compile.cpp
@@ -9783,22 +9783,22 @@ void compileCall(MyThread* t, Context* c, ThunkIndex index, bool call = true)
 
   if (processor(t)->bootImage) {
     lir::Memory table(t->arch->thread(), TARGET_THREAD_THUNKTABLE);
-    lir::Register scratch(t->arch->scratch());
+    lir::RegisterPair scratch(t->arch->scratch());
     a->apply(lir::Move,
-             OperandInfo(TargetBytesPerWord, lir::MemoryOperand, &table),
-             OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &scratch));
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::Memory, &table),
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &scratch));
     lir::Memory proc(scratch.low, index * TargetBytesPerWord);
     a->apply(lir::Move,
-             OperandInfo(TargetBytesPerWord, lir::MemoryOperand, &proc),
-             OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &scratch));
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::Memory, &proc),
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &scratch));
     a->apply(call ? lir::Call : lir::Jump,
-             OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &scratch));
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &scratch));
   } else {
     lir::Constant proc(new (&c->zone) avian::codegen::ResolvedPromise(
         reinterpret_cast<intptr_t>(t->thunkTable[index])));
 
     a->apply(call ? lir::LongCall : lir::LongJump,
-             OperandInfo(TargetBytesPerWord, lir::ConstantOperand, &proc));
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::Constant, &proc));
   }
 }
 
@@ -9814,16 +9814,16 @@ void compileThunks(MyThread* t, FixedAllocator* allocator)
 
     p->thunks.default_.frameSavedOffset = a->length();
 
-    lir::Register thread(t->arch->thread());
-    a->pushFrame(1, TargetBytesPerWord, lir::RegisterOperand, &thread);
+    lir::RegisterPair thread(t->arch->thread());
+    a->pushFrame(1, TargetBytesPerWord, lir::Operand::Type::RegisterPair, &thread);
 
     compileCall(t, &context, compileMethodIndex);
 
     a->popFrame(t->arch->alignFrameSize(1));
 
-    lir::Register result(t->arch->returnLow());
+    lir::RegisterPair result(t->arch->returnLow());
     a->apply(lir::Jump,
-             OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &result));
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &result));
 
     p->thunks.default_.length = a->endBlock(false)->resolve(0, 0);
 
@@ -9835,7 +9835,7 @@ void compileThunks(MyThread* t, FixedAllocator* allocator)
     Context context(t);
     avian::codegen::Assembler* a = context.assembler;
 
-    lir::Register class_(t->arch->virtualCallTarget());
+    lir::RegisterPair class_(t->arch->virtualCallTarget());
     lir::Memory virtualCallTargetSrc(
         t->arch->stack(),
         (t->arch->frameFooterSize() + t->arch->frameReturnAddressSize())
@@ -9843,41 +9843,41 @@ void compileThunks(MyThread* t, FixedAllocator* allocator)
 
     a->apply(lir::Move,
              OperandInfo(
-                 TargetBytesPerWord, lir::MemoryOperand, &virtualCallTargetSrc),
-             OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &class_));
+                 TargetBytesPerWord, lir::Operand::Type::Memory, &virtualCallTargetSrc),
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &class_));
 
     lir::Memory virtualCallTargetDst(t->arch->thread(),
                                      TARGET_THREAD_VIRTUALCALLTARGET);
 
     a->apply(
         lir::Move,
-        OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &class_),
+        OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &class_),
         OperandInfo(
-            TargetBytesPerWord, lir::MemoryOperand, &virtualCallTargetDst));
+            TargetBytesPerWord, lir::Operand::Type::Memory, &virtualCallTargetDst));
 
-    lir::Register index(t->arch->virtualCallIndex());
+    lir::RegisterPair index(t->arch->virtualCallIndex());
     lir::Memory virtualCallIndex(t->arch->thread(),
                                  TARGET_THREAD_VIRTUALCALLINDEX);
 
     a->apply(
         lir::Move,
-        OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &index),
-        OperandInfo(TargetBytesPerWord, lir::MemoryOperand, &virtualCallIndex));
+        OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &index),
+        OperandInfo(TargetBytesPerWord, lir::Operand::Type::Memory, &virtualCallIndex));
 
     a->saveFrame(TARGET_THREAD_STACK, TARGET_THREAD_IP);
 
     p->thunks.defaultVirtual.frameSavedOffset = a->length();
 
-    lir::Register thread(t->arch->thread());
-    a->pushFrame(1, TargetBytesPerWord, lir::RegisterOperand, &thread);
+    lir::RegisterPair thread(t->arch->thread());
+    a->pushFrame(1, TargetBytesPerWord, lir::Operand::Type::RegisterPair, &thread);
 
     compileCall(t, &context, compileVirtualMethodIndex);
 
     a->popFrame(t->arch->alignFrameSize(1));
 
-    lir::Register result(t->arch->returnLow());
+    lir::RegisterPair result(t->arch->returnLow());
     a->apply(lir::Jump,
-             OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &result));
+             OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &result));
 
     p->thunks.defaultVirtual.length = a->endBlock(false)->resolve(0, 0);
 
@@ -9893,8 +9893,8 @@ void compileThunks(MyThread* t, FixedAllocator* allocator)
 
     p->thunks.native.frameSavedOffset = a->length();
 
-    lir::Register thread(t->arch->thread());
-    a->pushFrame(1, TargetBytesPerWord, lir::RegisterOperand, &thread);
+    lir::RegisterPair thread(t->arch->thread());
+    a->pushFrame(1, TargetBytesPerWord, lir::Operand::Type::RegisterPair, &thread);
 
     compileCall(t, &context, invokeNativeIndex);
 
@@ -9915,8 +9915,8 @@ void compileThunks(MyThread* t, FixedAllocator* allocator)
 
     p->thunks.aioob.frameSavedOffset = a->length();
 
-    lir::Register thread(t->arch->thread());
-    a->pushFrame(1, TargetBytesPerWord, lir::RegisterOperand, &thread);
+    lir::RegisterPair thread(t->arch->thread());
+    a->pushFrame(1, TargetBytesPerWord, lir::Operand::Type::RegisterPair, &thread);
 
     compileCall(t, &context, throwArrayIndexOutOfBoundsIndex);
 
@@ -9934,8 +9934,8 @@ void compileThunks(MyThread* t, FixedAllocator* allocator)
 
     p->thunks.stackOverflow.frameSavedOffset = a->length();
 
-    lir::Register thread(t->arch->thread());
-    a->pushFrame(1, TargetBytesPerWord, lir::RegisterOperand, &thread);
+    lir::RegisterPair thread(t->arch->thread());
+    a->pushFrame(1, TargetBytesPerWord, lir::Operand::Type::RegisterPair, &thread);
 
     compileCall(t, &context, throwStackOverflowIndex);
 
@@ -10058,17 +10058,17 @@ uintptr_t compileVirtualThunk(MyThread* t, unsigned index, unsigned* size)
 
   avian::codegen::ResolvedPromise indexPromise(index);
   lir::Constant indexConstant(&indexPromise);
-  lir::Register indexRegister(t->arch->virtualCallIndex());
+  lir::RegisterPair indexRegister(t->arch->virtualCallIndex());
   a->apply(
       lir::Move,
-      OperandInfo(TargetBytesPerWord, lir::ConstantOperand, &indexConstant),
-      OperandInfo(TargetBytesPerWord, lir::RegisterOperand, &indexRegister));
+      OperandInfo(TargetBytesPerWord, lir::Operand::Type::Constant, &indexConstant),
+      OperandInfo(TargetBytesPerWord, lir::Operand::Type::RegisterPair, &indexRegister));
 
   avian::codegen::ResolvedPromise defaultVirtualThunkPromise(
       defaultVirtualThunk(t));
   lir::Constant thunk(&defaultVirtualThunkPromise);
   a->apply(lir::Jump,
-           OperandInfo(TargetBytesPerWord, lir::ConstantOperand, &thunk));
+           OperandInfo(TargetBytesPerWord, lir::Operand::Type::Constant, &thunk));
 
   *size = a->endBlock(false)->resolve(0, 0);
 
diff --git a/unittest/codegen/assembler-test.cpp b/unittest/codegen/assembler-test.cpp
index f463b0c5b4..70be09554d 100644
--- a/unittest/codegen/assembler-test.cpp
+++ b/unittest/codegen/assembler-test.cpp
@@ -79,6 +79,6 @@ TEST(ArchitecturePlan)
         (lir::UnaryOperation)op, vm::TargetBytesPerWord, mask, &thunk);
     assertFalse(thunk);
     assertNotEqual(static_cast<uint8_t>(0), mask.typeMask);
-    assertNotEqual(static_cast<uint64_t>(0), mask.registerMask);
+    assertNotEqual(static_cast<uint64_t>(0), (uint64_t)mask.lowRegisterMask);
   }
 }
diff --git a/unittest/codegen/registers-test.cpp b/unittest/codegen/registers-test.cpp
index f565e813d7..861a1e76c3 100644
--- a/unittest/codegen/registers-test.cpp
+++ b/unittest/codegen/registers-test.cpp
@@ -19,18 +19,29 @@ using namespace vm;
 
 TEST(RegisterIterator)
 {
-  RegisterMask regs(0x55);
+  BoundedRegisterMask regs(0x55);
   assertEqual<unsigned>(0, regs.start);
   assertEqual<unsigned>(7, regs.limit);
 
-  RegisterIterator it(regs);
-  assertTrue(it.hasNext());
-  assertEqual<unsigned>(0, it.next());
-  assertTrue(it.hasNext());
-  assertEqual<unsigned>(2, it.next());
-  assertTrue(it.hasNext());
-  assertEqual<unsigned>(4, it.next());
-  assertTrue(it.hasNext());
-  assertEqual<unsigned>(6, it.next());
-  assertFalse(it.hasNext());
+  for(int i = 0; i < 64; i++) {
+    assertEqual<unsigned>(i, BoundedRegisterMask(static_cast<uint64_t>(1) << i).start);
+    assertEqual<unsigned>(i + 1, BoundedRegisterMask(static_cast<uint64_t>(1) << i).limit);
+  }
+
+  auto it = regs.begin();
+  auto end = regs.end();
+
+  assertTrue(it != end);
+  assertEqual<unsigned>(6, (*it).index());
+  ++it;
+  assertTrue(it != end);
+  assertEqual<unsigned>(4, (*it).index());
+  ++it;
+  assertTrue(it != end);
+  assertEqual<unsigned>(2, (*it).index());
+  ++it;
+  assertTrue(it != end);
+  assertEqual<unsigned>(0, (*it).index());
+  ++it;
+  assertFalse(it != end);
 }