snapshot

src/compile.cpp

@@ -28,11 +28,13 @@ class MyThread: public Thread {
    public:
     CallTrace(MyThread* t):
       t(t),
+      ip(t->ip),
       base(t->base),
       stack(t->stack),
       next(t->trace)
     {
       t->trace = this;
+      t->ip = 0;
       t->base = 0;
       t->stack = 0;
     }
@@ -40,10 +42,12 @@ class MyThread: public Thread {
     ~CallTrace() {
       t->stack = stack;
       t->base = base;
+      t->ip = ip;
       t->trace = next;
     }
 
     MyThread* t;
+    void* ip;
     void* base;
     void* stack;
     CallTrace* next;
@@ -51,12 +55,14 @@ class MyThread: public Thread {
 
   MyThread(Machine* m, object javaThread, Thread* parent):
     Thread(m, javaThread, parent),
+    ip(0),
     base(0),
     stack(0),
     trace(0),
     reference(0)
   { }
 
+  void* ip;
   void* base;
   void* stack;
   CallTrace* trace;
@@ -116,7 +122,9 @@ class MyStackWalker: public Processor::StackWalker {
     base(t->base),
     stack(t->stack),
     trace(t->trace),
-    node(stack ? findTraceNode(t, *static_cast<void**>(stack)) : 0),
+    node(t->ip ? findTraceNode(t, t->ip) :
+         (stack ? findTraceNode(t, *static_cast<void**>(stack)) :
+          0)),
     nativeMethod(resolveNativeMethod(t, stack, node)),
     protector(this)
   { }
@@ -663,9 +671,9 @@ class Frame {
     }
   }
 
-  void trace(Promise* returnAddress, object target, bool virtualCall) {
+  void trace(Promise* address, object target, bool virtualCall) {
     TraceElement* e = new (traceLog->allocate(traceSizeInBytes(t, method)))
-      TraceElement(target, returnAddress, virtualCall);
+      TraceElement(target, address, virtualCall);
     memcpy(e->map, map, mapSizeInWords(t, method) * BytesPerWord);
   }
 
@@ -947,21 +955,38 @@ class Frame {
   MyProtector protector;
 };
 
+class MyNullHandler: public Compiler::NullHandler {
+ public:
+  MyNullHandler(Frame* frame): frame(frame) {
+    frame->c->setNullHandler(this);
+  }
+
+  virtual void handleMaybeNull(Promise* address) {
+    frame->trace(address, 0, false);
+  }
+
+  Frame* frame;
+};
+
 void NO_RETURN
 unwind(MyThread* t)
 {
+  void* ip = t->ip;
   void* base = t->base;
   void** stack = static_cast<void**>(t->stack);
+  if (ip == 0) {
+    ip = *stack;
+  }
+
   while (true) {
-    void* returnAddress = *stack;
-    object node = findTraceNode(t, returnAddress);
+    object node = findTraceNode(t, ip);
     if (node) {
       object method = traceNodeMethod(t, node);
       uint8_t* compiled = reinterpret_cast<uint8_t*>
         (&singletonValue(t, methodCompiled(t, method), 0));
 
       ExceptionHandler* handler = findExceptionHandler
-        (t, method, difference(returnAddress, compiled));
+        (t, method, difference(ip, compiled));
 
       if (handler) {
         unsigned parameterFootprint = methodParameterFootprint(t, method);
@@ -987,20 +1012,26 @@ unwind(MyThread* t)
         }
 
         stack = static_cast<void**>(base) + 1;
+        ip = *stack;
         base = *static_cast<void**>(base);
       }
     } else {
-      vmJump(returnAddress, base, stack + 1, 0);
+      vmJump(ip, base, stack + 1, 0);
     }
   }
 }
 
 void*
-findInterfaceMethodFromInstance(Thread* t, object method, object instance)
+findInterfaceMethodFromInstance(MyThread* t, object method, object instance)
 {
-  return &singletonValue
-    (t, methodCompiled
-     (t, findInterfaceMethod(t, method, objectClass(t, instance))), 0);
+  if (instance) {
+    return &singletonValue
+      (t, methodCompiled
+       (t, findInterfaceMethod(t, method, objectClass(t, instance))), 0);
+  } else {
+    t->exception = makeNullPointerException(t);
+    unwind(t);
+  }
 }
 
 intptr_t
@@ -1240,15 +1271,36 @@ lookUpAddress(int32_t key, uintptr_t* start, int32_t count,
 }
 
 void
-acquireMonitorForObject(Thread* t, object o)
+setMaybeNull(MyThread* t, object o, unsigned offset, object value)
 {
-  acquire(t, o);
+  if (o) {
+    set(t, o, offset, value);
+  } else {
+    t->exception = makeNullPointerException(t);
+    unwind(t);
+  }
 }
 
 void
-releaseMonitorForObject(Thread* t, object o)
+acquireMonitorForObject(MyThread* t, object o)
 {
-  release(t, o);
+  if (o) {
+    acquire(t, o);
+  } else {
+    t->exception = makeNullPointerException(t);
+    unwind(t);
+  }
+}
+
+void
+releaseMonitorForObject(MyThread* t, object o)
+{
+  if (o) {
+    release(t, o);
+  } else {
+    t->exception = makeNullPointerException(t);
+    unwind(t);
+  }
 }
 
 object
@@ -1465,29 +1517,30 @@ compile(MyThread* t, Frame* initialFrame, unsigned ip)
 
       switch (instruction) {
       case aaload:
-        frame->pushObject(c->memory(array, ArrayBody, index, BytesPerWord));
+        frame->pushObject
+          (c->memory(array, ArrayBody, index, BytesPerWord, true));
         break;
 
       case faload:
       case iaload:
-        frame->pushInt4(c->memory(array, ArrayBody, index, 4));
+        frame->pushInt4(c->memory(array, ArrayBody, index, 4, true));
         break;
 
       case baload:
-        frame->pushInt1(c->memory(array, ArrayBody, index, 1));
+        frame->pushInt1(c->memory(array, ArrayBody, index, 1, true));
         break;
 
       case caload:
-        frame->pushInt2z(c->memory(array, ArrayBody, index, 2));
+        frame->pushInt2z(c->memory(array, ArrayBody, index, 2, true));
         break;
 
       case daload:
       case laload:
-        frame->pushLong(c->memory(array, ArrayBody, index, 8));
+        frame->pushLong(c->memory(array, ArrayBody, index, 8, true));
         break;
 
       case saload:
-        frame->pushInt2(c->memory(array, ArrayBody, index, 2));
+        frame->pushInt2(c->memory(array, ArrayBody, index, 2, true));
         break;
       }
 
@@ -1536,8 +1589,8 @@ compile(MyThread* t, Frame* initialFrame, unsigned ip)
         c->shl4(c->constant(log(BytesPerWord)), index);
         c->add4(c->constant(ArrayBody), index);
           
-        Promise* returnAddress = c->directCall
-          (c->constant(reinterpret_cast<intptr_t>(set)),
+        Promise* returnAddress = c->indirectCall
+          (c->constant(reinterpret_cast<intptr_t>(setMaybeNull)),
            4, c->thread(), array, index, value);
 
         frame->trace(returnAddress, 0, false);
@@ -1545,21 +1598,21 @@ compile(MyThread* t, Frame* initialFrame, unsigned ip)
 
       case fastore:
       case iastore:
-        c->mov4(value, c->memory(array, ArrayBody, index, 4));
+        c->mov4(value, c->memory(array, ArrayBody, index, 4, true));
         break;
 
       case bastore:
-        c->mov1(value, c->memory(array, ArrayBody, index, 1));
+        c->mov1(value, c->memory(array, ArrayBody, index, 1, true));
         break;
 
       case castore:
       case sastore:
-        c->mov2(value, c->memory(array, ArrayBody, index, 2));
+        c->mov2(value, c->memory(array, ArrayBody, index, 2, true));
         break;
 
       case dastore:
       case lastore:
-        c->mov8(value, c->memory(array, ArrayBody, index, 8));
+        c->mov8(value, c->memory(array, ArrayBody, index, 8, true));
         break;
       }
 
@@ -1638,7 +1691,7 @@ compile(MyThread* t, Frame* initialFrame, unsigned ip)
 
     case arraylength: {
       Operand* array = frame->popObject();
-      frame->pushInt4(c->memory(array, ArrayLength));
+      frame->pushInt4(c->memory(array, ArrayLength, 0, 1, true));
       c->release(array);
     } break;
 
@@ -2055,32 +2108,40 @@ compile(MyThread* t, Frame* initialFrame, unsigned ip)
         table = frame->popObject();
       }
 
+      bool maybeNull = (instruction == getfield);
+
       switch (fieldCode(t, field)) {
       case ByteField:
       case BooleanField:
-        frame->pushInt1(c->memory(table, fieldOffset(t, field)));
+        frame->pushInt1
+          (c->memory(table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       case CharField:
-        frame->pushInt2z(c->memory(table, fieldOffset(t, field)));
+        frame->pushInt2z
+          (c->memory(table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       case ShortField:
-        frame->pushInt2(c->memory(table, fieldOffset(t, field)));
+        frame->pushInt2
+          (c->memory(table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       case FloatField:
       case IntField:
-        frame->pushInt4(c->memory(table, fieldOffset(t, field)));
+        frame->pushInt4
+          (c->memory(table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       case DoubleField:
       case LongField:
-        frame->pushLong(c->memory(table, fieldOffset(t, field)));
+        frame->pushLong
+          (c->memory(table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       case ObjectField:
-        frame->pushObject(c->memory(table, fieldOffset(t, field)));
+        frame->pushObject
+          (c->memory(table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       default:
@@ -2392,7 +2453,7 @@ compile(MyThread* t, Frame* initialFrame, unsigned ip)
 
       unsigned instance = parameterFootprint - 1;
 
-      c->directCall
+      c->indirectCall
          (c->constant
           (reinterpret_cast<intptr_t>(findInterfaceMethodFromInstance)),
           3, c->thread(), frame->append(target),
@@ -2455,7 +2516,7 @@ compile(MyThread* t, Frame* initialFrame, unsigned ip)
       mov(c, c->memory(instance), class_);
       and_(c, c->constant(PointerMask), class_);
 
-      c->call(c->memory(class_, offset));
+      c->call(c->memory(class_, offset, 0, 1, true));
 
       frame->trace(c->machineIp(), target, true);
 
@@ -3022,31 +3083,43 @@ compile(MyThread* t, Frame* initialFrame, unsigned ip)
         table = frame->popObject();
       }
 
+      bool maybeNull = (instruction == getstatic);
+
       switch (fieldCode(t, field)) {
       case ByteField:
       case BooleanField:
-        c->mov1(value, c->memory(table, fieldOffset(t, field)));
+        c->mov1(value, c->memory
+                (table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       case CharField:
       case ShortField:
-        c->mov2(value, c->memory(table, fieldOffset(t, field)));
+        c->mov2(value, c->memory
+                (table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
             
       case FloatField:
       case IntField:
-        c->mov4(value, c->memory(table, fieldOffset(t, field)));
+        c->mov4(value, c->memory
+                (table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       case DoubleField:
       case LongField:
-        c->mov8(value, c->memory(table, fieldOffset(t, field)));
+        c->mov8(value, c->memory
+                (table, fieldOffset(t, field), 0, 1, maybeNull));
         break;
 
       case ObjectField:
-        c->directCall
-          (c->constant(reinterpret_cast<intptr_t>(set)),
-           4, c->thread(), table, c->constant(fieldOffset(t, field)), value);
+        if (maybeNull) {
+          c->indirectCall
+            (c->constant(reinterpret_cast<intptr_t>(setMaybeNull)),
+             4, c->thread(), table, c->constant(fieldOffset(t, field)), value);
+        } else {
+          c->directCall
+            (c->constant(reinterpret_cast<intptr_t>(set)),
+             4, c->thread(), table, c->constant(fieldOffset(t, field)), value);
+        }
         break;
 
       default: abort(t);
@@ -3348,6 +3421,7 @@ compile(MyThread* t, Compiler* c, object method)
   Vector traceLog(t->m->system, 1024);
   uintptr_t map[Frame::mapSizeInWords(t, method)];
   Frame frame(t, c, method, map, &objectPool, &traceLog);
+  MyNullHandler nullHandler(&frame);
 
   handleEntrance(t, &frame);
 
@@ -3640,12 +3714,17 @@ visitStackAndLocals(MyThread* t, Heap::Visitor* v, void* base, object node)
 void
 visitStack(MyThread* t, Heap::Visitor* v)
 {
+  void* ip = t->ip;
   void* base = t->base;
   void** stack = static_cast<void**>(t->stack);
+  if (ip == 0) {
+    ip = *stack;
+  }
+
   MyThread::CallTrace* trace = t->trace;
 
   while (stack) {
-    object node = findTraceNode(t, *stack);
+    object node = findTraceNode(t, ip);
     if (node) {
       PROTECT(t, node);
 
@@ -3661,6 +3740,7 @@ visitStack(MyThread* t, Heap::Visitor* v)
       visitStackAndLocals(t, v, base, node);
 
       stack = static_cast<void**>(base) + 1;
+      ip = *stack;
       base = *static_cast<void**>(base);
     } else if (trace) {
       base = trace->base;
@@ -3828,11 +3908,14 @@ invoke(Thread* thread, object method, ArgumentList* arguments)
   unsigned returnType = fieldType(t, returnCode);
 
   Reference* reference = t->reference;
-  MyThread::CallTrace trace(t);
+  uint64_t result;
 
-  uint64_t result = vmInvoke
-    (t, &singletonValue(t, methodCompiled(t, method), 0), arguments->array,
-     arguments->position, returnType);
+  { MyThread::CallTrace trace(t);
+
+    result = vmInvoke
+      (t, &singletonValue(t, methodCompiled(t, method), 0), arguments->array,
+       arguments->position, returnType);
+  }
 
   while (t->reference != reference) {
     dispose(t, t->reference);
@@ -3869,6 +3952,27 @@ invoke(Thread* thread, object method, ArgumentList* arguments)
   return r;
 }
 
+class SegFaultHandler: public System::SignalHandler {
+ public:
+  SegFaultHandler(): m(0) { }
+
+  virtual bool handleSignal(void* ip, void* base, void* stack) {
+    MyThread* t = static_cast<MyThread*>(m->localThread->get());
+    object node = findTraceNode(t, ip);
+    if (node) {
+      t->ip = ip;
+      t->base = base;
+      t->stack = stack;
+      t->exception = makeNullPointerException(t);
+      unwind(t);
+    } else {
+      return false;
+    }
+  }
+
+  Machine* m;
+};
+
 class MyProcessor: public Processor {
  public:
   MyProcessor(System* s):
@@ -4126,8 +4230,11 @@ class MyProcessor: public Processor {
     return 0;
   }
 
-  virtual void dispose(Thread* thread) {
-    MyThread* t = static_cast<MyThread*>(thread);
+  virtual void dispose(Thread* vmt) {
+    MyThread* t = static_cast<MyThread*>(vmt);
+
+    t->m->system->handleSegFault(0);
+
     while (t->reference) {
       vm::dispose(t, t->reference);
     }
@@ -4147,6 +4254,7 @@ class MyProcessor: public Processor {
   object addressTable;
   unsigned addressCount;
   uint8_t* indirectCaller;
+  SegFaultHandler segFaultHandler;
 };
 
 MyProcessor*
@@ -4176,6 +4284,10 @@ processor(MyThread* t)
 
       c->dispose();
     }
+
+    p->segFaultHandler.m = t->m;
+    expect(t, t->m->system->success
+           (t->m->system->handleSegFault(&(p->segFaultHandler))));
   }
   return p;
 }

src/compiler.cpp

@@ -146,7 +146,8 @@ class Context {
     indirectCaller(reinterpret_cast<intptr_t>(indirectCaller)),
     segmentTable(0),
     reserved(0),
-    codeLength(-1)
+    codeLength(-1),
+    nullHandler(0)
   {
     plan.appendAddress(new (zone.allocate(sizeof(Segment))) Segment
                        (-1, new (zone.allocate(sizeof(Event))) Event(0)));
@@ -173,6 +174,7 @@ class Context {
   Segment** segmentTable;
   unsigned reserved;
   int codeLength;
+  Compiler::NullHandler* nullHandler;
   RegisterData registers[RegisterCount];
 };
 
@@ -311,7 +313,7 @@ register_(Context* c, Register = NoRegister, Register = NoRegister);
 
 MemoryOperand*
 memory(Context* c, MyOperand* base, int displacement,
-       MyOperand* index, unsigned scale);
+       MyOperand* index, unsigned scale, bool maybeNull);
 
 class MyOperand: public Operand {
  public:
@@ -615,11 +617,12 @@ class AbsoluteOperand: public MyOperand {
 class MemoryOperand: public MyOperand {
  public:
   MemoryOperand(MyOperand* base, int displacement, MyOperand* index,
-                unsigned scale):
+                unsigned scale, bool maybeNull):
     base(base),
     displacement(displacement),
     index(index),
-    scale(scale)
+    scale(scale),
+    maybeNull(maybeNull)
   { }
 
   virtual Register asRegister(Context*);
@@ -649,6 +652,20 @@ class MemoryOperand: public MyOperand {
   int displacement;
   MyOperand* index;
   unsigned scale;
+  bool maybeNull;
+};
+
+class CodePromiseTask: public Task {
+ public:
+  CodePromiseTask(CodePromise* promise, Task* next):
+    Task(next), promise(promise)
+  { }
+
+  virtual void run(Context* c UNUSED, unsigned offset) {
+    promise->offset = offset;
+  }
+
+  CodePromise* promise;
 };
 
 AddressOperand*
@@ -687,10 +704,10 @@ register_(Context* c, Register v, Register h)
 
 MemoryOperand*
 memory(Context* c, MyOperand* base, int displacement,
-       MyOperand* index, unsigned scale)
+       MyOperand* index, unsigned scale, bool maybeNull)
 {
   return new (c->zone.allocate(sizeof(MemoryOperand)))
-    MemoryOperand(base, displacement, index, scale);
+    MemoryOperand(base, displacement, index, scale, maybeNull);
 }
 
 RegisterOperand*
@@ -721,6 +738,29 @@ currentSegment(Context* c)
   return s;
 }
 
+Promise*
+machineIp(Context* c)
+{
+  CodePromise* p = new (c->zone.allocate(sizeof(CodePromise))) CodePromise();
+  
+  Segment* s = currentSegment(c);
+  s->event->task = new (c->zone.allocate(sizeof(CodePromiseTask)))
+    CodePromiseTask(p, s->event->task);
+  
+  return p;
+}
+
+void
+handleMaybeNull(Context* c)
+{
+  if (c->codeLength >= 0) {
+    c->nullHandler->handleMaybeNull
+      (new (c->zone.allocate(sizeof(ResolvedPromise)))
+       ResolvedPromise
+       (reinterpret_cast<intptr_t>(c->code.data + c->code.length)));
+  }
+}
+
 void
 apply(Context* c, MyOperand::Operation op)
 {
@@ -990,7 +1030,8 @@ pushed(Context* c, MyStack* stack)
   int index = (stack ? stack->index + 1 : 0);
 
   MyOperand* value = memory
-    (c, register_(c, rbp), - (c->reserved + index + 1) * BytesPerWord, 0, 1);
+    (c, register_(c, rbp), - (c->reserved + index + 1) * BytesPerWord, 0, 1,
+     false);
 
   stack = new (c->zone.allocate(sizeof(MyStack))) MyStack(value, index, stack);
 
@@ -1426,6 +1467,8 @@ void
 RegisterOperand::accept(Context* c, Operation operation,
                         MemoryOperand* operand)
 {
+  assert(c, not operand->maybeNull);
+
   switch (operation) {
   case cmp4:
   case cmp8:
@@ -1442,7 +1485,7 @@ RegisterOperand::accept(Context* c, Operation operation,
       register_(c, high(c))->accept
         (c, mov, memory
          (c, operand->base, operand->displacement + BytesPerWord,
-          operand->index, operand->scale));
+          operand->index, operand->scale, false));
     } else if (BytesPerWord == 8 and operation == mov4) {
       encode(c, 0x63, value(c), operand, true);
     } else {
@@ -1500,7 +1543,7 @@ RegisterOperand::accept(Context* c, Operation operation,
 
     RegisterOperand* tmp = temporary(c);
     tmp->accept(c, mov, ::value(c, operand));
-    accept(c, cmp, memory(c, tmp, 0, 0, 1));
+    accept(c, cmp, memory(c, tmp, 0, 0, 1, false));
     tmp->release(c);
   } break;
 
@@ -1509,7 +1552,7 @@ RegisterOperand::accept(Context* c, Operation operation,
     assert(c, BytesPerWord == 8 or operation == mov4); // todo
 
     accept(c, mov, ::value(c, operand));
-    accept(c, mov, memory(c, this, 0, 0, 1));
+    accept(c, mov, memory(c, this, 0, 0, 1, false));
   } break;
 
   default: abort(c);
@@ -1682,7 +1725,7 @@ absoluteApply(Context* c, MyOperand::Operation operation,
 {
   RegisterOperand* tmp = temporary(c);
   tmp->accept(c, MyOperand::mov, value(c, operand));
-  memory(c, tmp, 0, 0, 1)->apply(c, operation);
+  memory(c, tmp, 0, 0, 1, false)->apply(c, operation);
   tmp->release(c);
 }
 
@@ -1713,22 +1756,28 @@ MemoryOperand::apply(Context* c, Operation operation)
 {
   switch (operation) {
   case call:
+    if (maybeNull) handleMaybeNull(c);
+
     encode(c, 0xff, 2, this, false);
     break;
 
   case jmp:
+    if (maybeNull) handleMaybeNull(c);
+
     encode(c, 0xff, 4, this, false);
     break;
 
   case neg4:
   case neg8:
+    assert(c, not maybeNull);
+
     if (BytesPerWord == 4 and operation == neg8) {
       RegisterOperand* ax = temporary(c, rax);
       RegisterOperand* dx = temporary(c, rdx);
 
       MemoryOperand* low = this;
       MemoryOperand* high = memory
-        (c, base, displacement + BytesPerWord, index, scale);
+        (c, base, displacement + BytesPerWord, index, scale, false);
 
       ax->accept(c, mov, low);
       dx->accept(c, mov, high);
@@ -1749,10 +1798,12 @@ MemoryOperand::apply(Context* c, Operation operation)
 
   case pop4:
   case pop8:
+    assert(c, not maybeNull);
+
     if (BytesPerWord == 4 and operation == pop8) {
       MemoryOperand* low = this;
       MemoryOperand* high = memory
-        (c, base, displacement + BytesPerWord, index, scale);
+        (c, base, displacement + BytesPerWord, index, scale, false);
 
       low->apply(c, pop);
       high->apply(c, pop);
@@ -1765,10 +1816,13 @@ MemoryOperand::apply(Context* c, Operation operation)
 
   case push4:
   case push8:
+    assert(c, not maybeNull);
+
     if (BytesPerWord == 4 and operation == push8) {
-      MemoryOperand* low = this;
+      MemoryOperand* low = memory
+        (c, base, displacement, index, scale, false);
       MemoryOperand* high = memory
-        (c, base, displacement + BytesPerWord, index, scale);
+        (c, base, displacement + BytesPerWord, index, scale, maybeNull);
         
       high->apply(c, push);
       low->apply(c, push);
@@ -1785,6 +1839,8 @@ MemoryOperand::apply(Context* c, Operation operation)
   case push1:
   case push2:
   case push2z: {
+    assert(c, not maybeNull);
+
     RegisterOperand* tmp = temporary(c);
     switch (operation) {
     case push1:
@@ -1813,6 +1869,8 @@ void
 MemoryOperand::accept(Context* c, Operation operation,
                       RegisterOperand* operand)
 {
+  assert(c, not maybeNull);
+
   switch (operation) {
   case and4:
   case and8:
@@ -1831,7 +1889,7 @@ MemoryOperand::accept(Context* c, Operation operation,
       dx->accept(c, mov, register_(c, operand->high(c)));
 
       accept(c, add, ax);
-      memory(c, base, displacement + BytesPerWord, index, scale)->accept
+      memory(c, base, displacement + BytesPerWord, index, scale, false)->accept
         (c, addc, dx);
 
       ax->release(c);
@@ -1880,7 +1938,7 @@ MemoryOperand::accept(Context* c, Operation operation,
     if (BytesPerWord == 4 and operation == mov8) {
       accept(c, mov, operand);
 
-      memory(c, base, displacement + BytesPerWord, index, scale)->accept
+      memory(c, base, displacement + BytesPerWord, index, scale, false)->accept
         (c, mov, register_(c, operand->high(c)));
     } else if (BytesPerWord == 8 and operation == mov4) {
       encode(c, 0x89, operand->value(c), this, false);
@@ -1929,7 +1987,7 @@ MemoryOperand::accept(Context* c, Operation operation,
 
       MemoryOperand* lowDst = this;
       MemoryOperand* highDst = memory
-        (c, base, displacement + BytesPerWord, index, scale);
+        (c, base, displacement + BytesPerWord, index, scale, false);
       
       tmp->accept(c, mov, highSrc);
       tmp->accept(c, mul, lowDst);
@@ -2008,9 +2066,11 @@ MemoryOperand::accept(Context* c, Operation operation,
       tmp->accept(c, mov, this);
       // shld
       encode2(c, 0x0fa5, tmp->value(c), memory
-              (c, base, displacement + BytesPerWord, index, scale), false);
+              (c, base, displacement + BytesPerWord, index, scale, maybeNull),
+              false);
       // shl
-      encode(c, 0xd3, 4, this, false);
+      encode(c, 0xd3, 4, memory(c, base, displacement, index, scale, false),
+             false);
 
       tmp->release(c);
       cx->release(c);
@@ -2034,7 +2094,8 @@ MemoryOperand::accept(Context* c, Operation operation,
       encode2(c, 0x0fad, tmp->value(c), this, false);
       // sar
       encode(c, 0xd3, 5, memory
-             (c, base, displacement + BytesPerWord, index, scale), false);
+             (c, base, displacement + BytesPerWord, index, scale, false),
+             false);
 
       tmp->release(c);
       cx->release(c);
@@ -2058,7 +2119,8 @@ MemoryOperand::accept(Context* c, Operation operation,
       encode2(c, 0x0fad, tmp->value(c), this, false);
       // shr
       encode(c, 0xd3, 7, memory
-             (c, base, displacement + BytesPerWord, index, scale), false);
+             (c, base, displacement + BytesPerWord, index, scale, false),
+             false);
 
       tmp->release(c);
       cx->release(c);
@@ -2080,7 +2142,7 @@ MemoryOperand::accept(Context* c, Operation operation,
       dx->accept(c, mov, register_(c, operand->high(c)));
 
       accept(c, sub, ax);
-      memory(c, base, displacement + BytesPerWord, index, scale)->accept
+      memory(c, base, displacement + BytesPerWord, index, scale, false)->accept
         (c, subb, dx);
 
       ax->release(c);
@@ -2109,6 +2171,8 @@ void
 MemoryOperand::accept(Context* c, Operation operation,
                       ImmediateOperand* operand)
 {
+  assert(c, not maybeNull);
+
   switch (operation) {
   case add4:
   case add8: {
@@ -2143,6 +2207,8 @@ void
 MemoryOperand::accept(Context* c, Operation operation,
                       AbsoluteOperand* operand)
 {
+  assert(c, not maybeNull);
+
   RegisterOperand* tmp = temporary(c);
     
   tmp->accept(c, mov, operand);
@@ -2155,6 +2221,9 @@ void
 MemoryOperand::accept(Context* c, Operation operation,
                       MemoryOperand* operand)
 {
+  assert(c, not maybeNull);
+  assert(c, not operand->maybeNull);
+
   switch (operation) {
   case mov1ToW:
   case mov2ToW:
@@ -2249,33 +2318,19 @@ writeCode(Context* c)
   c->codeLength = pad(c->code.length());
 }
 
-class CodePromiseTask: public Task {
- public:
-  CodePromiseTask(CodePromise* promise, Task* next):
-    Task(next), promise(promise)
-  { }
-
-  virtual void run(Context* c UNUSED, unsigned offset) {
-    promise->offset = offset;
-  }
-
-  CodePromise* promise;
-};
-
 class MyCompiler: public Compiler {
  public:
   MyCompiler(System* s, void* indirectCaller):
     c(s, indirectCaller)
   { }
 
+  virtual void setNullHandler(NullHandler* nullHandler) {
+    assert(&c, c.nullHandler == 0);
+    c.nullHandler = nullHandler;
+  }
+
   virtual Promise* machineIp() {
-    CodePromise* p = new (c.zone.allocate(sizeof(CodePromise))) CodePromise();
-
-    Segment* s = currentSegment(&c);
-    s->event->task = new (c.zone.allocate(sizeof(CodePromiseTask)))
-      CodePromiseTask(p, s->event->task);
-
-    return p;
+    return ::machineIp(&c);
   }
 
   virtual Promise* machineIp(unsigned logicalIp) {
@@ -2662,10 +2717,11 @@ class MyCompiler: public Compiler {
   }
 
   virtual Operand* memory(Operand* base, int displacement,
-                          Operand* index, unsigned scale)
+                          Operand* index, unsigned scale,
+                          bool maybeNull)
   {
     return ::memory(&c, static_cast<MyOperand*>(base), displacement,
-                    static_cast<MyOperand*>(index), scale);
+                    static_cast<MyOperand*>(index), scale, maybeNull);
   }
 
   virtual void prologue() {

src/compiler.h

@@ -20,8 +20,17 @@ class Promise {
 
 class Compiler {
  public:
+  class NullHandler {
+   public:
+    virtual ~NullHandler() { }
+
+    virtual void handleMaybeNull(Promise* address) = 0;
+  };
+
   virtual ~Compiler() { }
 
+  virtual void setNullHandler(NullHandler*) = 0;
+
   virtual Promise* machineIp() = 0;
   virtual Promise* machineIp(unsigned logicalIp) = 0;
 
@@ -31,8 +40,11 @@ class Compiler {
   virtual Operand* constant(int64_t) = 0;
   virtual Operand* promiseConstant(Promise*) = 0;
   virtual Operand* absolute(Promise*) = 0;
-  virtual Operand* memory(Operand* base, int displacement = 0,
-                          Operand* index = 0, unsigned scale = 1) = 0;
+  virtual Operand* memory(Operand* base,
+                          int displacement = 0,
+                          Operand* index = 0,
+                          unsigned scale = 1,
+                          bool maybeNull = false) = 0;
 
   virtual Operand* stack() = 0;
   virtual Operand* base() = 0;

src/posix.cpp

@@ -20,6 +20,9 @@ using namespace vm;
 
 namespace {
 
+System::SignalHandler* segFaultHandler = 0;
+struct sigaction oldSegFaultHandler;
+
 class MutexResource {
  public:
   MutexResource(pthread_mutex_t& m): m(&m) {
@@ -36,10 +39,38 @@ class MutexResource {
 
 const int InterruptSignal = SIGUSR2;
 
+#ifdef __x86_64__
+const int IpRegister = REG_RIP;
+const int BaseRegister = REG_RBP;
+const int StackRegister = REG_RSP;
+#elif defined __i386__
+const int IpRegister = REG_EIP;
+const int BaseRegister = REG_EBP;
+const int StackRegister = REG_ESP;
+#else
+#  error unsupported architecture
+#endif
+
 void
-handleSignal(int)
+handleSignal(int signal, siginfo_t* info, void* context)
 {
-  // ignore
+  if (signal == SIGSEGV) {
+    greg_t* registers
+      = static_cast<ucontext_t*>(context)->uc_mcontext.gregs;
+
+    bool handled = segFaultHandler->handleSignal
+      (reinterpret_cast<void*>(registers[IpRegister]),
+       reinterpret_cast<void*>(registers[BaseRegister]),
+       reinterpret_cast<void*>(registers[StackRegister]));
+
+    if (not handled) {
+      if (oldSegFaultHandler.sa_flags & SA_SIGINFO) {
+        oldSegFaultHandler.sa_sigaction(signal, info, context);
+      } else {
+        oldSegFaultHandler.sa_handler(signal);
+      }
+    }
+  }
 }
 
 void*
@@ -403,7 +434,8 @@ class MySystem: public System {
     struct sigaction sa;
     memset(&sa, 0, sizeof(struct sigaction));
     sigemptyset(&(sa.sa_mask));
-    sa.sa_handler = handleSignal;
+    sa.sa_flags = SA_SIGINFO;
+    sa.sa_sigaction = handleSignal;
     
     int rv UNUSED = sigaction(InterruptSignal, &sa, 0);
     expect(this, rv == 0);
@@ -480,6 +512,24 @@ class MySystem: public System {
     return 0;
   }
 
+  virtual Status handleSegFault(SignalHandler* handler) {
+    if (handler) {
+      segFaultHandler = handler;
+
+      struct sigaction sa;
+      memset(&sa, 0, sizeof(struct sigaction));
+      sigemptyset(&(sa.sa_mask));
+      sa.sa_flags = SA_SIGINFO;
+      sa.sa_sigaction = handleSignal;
+    
+      return sigaction(SIGSEGV, &sa, &oldSegFaultHandler);
+    } else if (segFaultHandler) {
+      return sigaction(SIGSEGV, &oldSegFaultHandler, 0);
+    } else {
+      return 1;
+    }
+  }
+
   virtual uint64_t call(void* function, uintptr_t* arguments, uint8_t* types,
                         unsigned count, unsigned size, unsigned returnType)
   {

src/system.h

@@ -80,6 +80,13 @@ class System: public Allocator {
     virtual void dispose() = 0;
   };
 
+  class SignalHandler {
+   public:
+    virtual ~SignalHandler() { }
+
+    virtual bool handleSignal(void* ip, void* base, void* stack) = 0;
+  };
+
   virtual ~System() { }
 
   virtual bool success(Status) = 0;
@@ -87,6 +94,7 @@ class System: public Allocator {
   virtual Status start(Runnable*) = 0;
   virtual Status make(Monitor**) = 0;
   virtual Status make(Local**) = 0;
+  virtual Status handleSegFault(SignalHandler* handler) = 0;
   virtual uint64_t call(void* function, uintptr_t* arguments, uint8_t* types,
                         unsigned count, unsigned size,
                         unsigned returnType) = 0;