fail quickly if an object allocation cannot be satisfied

Previously, we would blithely exceed the heap ceiling and force the
next allocation to deal with the problem, including a major GC and
possible OutOfMemoryError.  As of this commit, we throw an error
immediately if we find that the allocation will push us over the
ceiling.

src/heap.cpp

@@ -1895,21 +1895,43 @@ class MyHeap: public Heap {
     local::collect(&c);
   }
 
-  virtual void* allocateFixed(Allocator* allocator, unsigned sizeInWords,
-                              bool objectMask, unsigned* totalInBytes)
+  void* tryAllocateFixed(Allocator* allocator, unsigned sizeInWords,
+                         bool objectMask, unsigned* totalInBytes,
+                         Fixie** handle, bool immortal)
   {
-    *totalInBytes = Fixie::totalSize(sizeInWords, objectMask);
-    return (new (allocator->allocate(*totalInBytes))
-            Fixie(&c, sizeInWords, objectMask, &(c.fixies), false))->body();
+    *totalInBytes = 0;
+
+    if (limitExceeded()) {
+      return 0;
     }
 
-  virtual void* allocateImmortalFixed(Allocator* allocator,
+    unsigned total = Fixie::totalSize(sizeInWords, objectMask);
+    void* p = allocator->tryAllocate(total);
+    if (p == 0) {
+      return 0;
+    } else if (limitExceeded()) {
+      allocator->free(p, total);
+      return 0;
+    } else {
+      *totalInBytes = total;
+      return (new (p) Fixie(&c, sizeInWords, objectMask, handle, immortal))
+        ->body();
+    }
+  }
+
+  virtual void* tryAllocateFixed(Allocator* allocator, unsigned sizeInWords,
+                                 bool objectMask, unsigned* totalInBytes)
+  {
+    return tryAllocateFixed
+      (allocator, sizeInWords, objectMask, totalInBytes, &(c.fixies), false);
+  }
+
+  virtual void* tryAllocateImmortalFixed(Allocator* allocator,
                                          unsigned sizeInWords, bool objectMask,
                                          unsigned* totalInBytes)
   {
-    *totalInBytes = Fixie::totalSize(sizeInWords, objectMask);
-    return (new (allocator->allocate(*totalInBytes))
-            Fixie(&c, sizeInWords, objectMask, 0, true))->body();
+    return tryAllocateFixed
+      (allocator, sizeInWords, objectMask, totalInBytes, 0, true);
   }
 
   bool needsMark(void* p) {

src/heap.h

@@ -62,9 +62,9 @@ class Heap: public Allocator {
   virtual unsigned limit() = 0;
   virtual bool limitExceeded() = 0;
   virtual void collect(CollectionType type, unsigned footprint) = 0;
-  virtual void* allocateFixed(Allocator* allocator, unsigned sizeInWords,
+  virtual void* tryAllocateFixed(Allocator* allocator, unsigned sizeInWords,
                                  bool objectMask, unsigned* totalInBytes) = 0;
-  virtual void* allocateImmortalFixed(Allocator* allocator,
+  virtual void* tryAllocateImmortalFixed(Allocator* allocator,
                                          unsigned sizeInWords, bool objectMask,
                                          unsigned* totalInBytes) = 0;
   virtual void mark(void* p, unsigned offset, unsigned count) = 0;

src/machine.cpp

@@ -3540,9 +3540,10 @@ allocate3(Thread* t, Allocator* allocator, Machine::AllocationType type,
   case Machine::FixedAllocation: {
     unsigned total;
     object o = static_cast<object>
-      (t->m->heap->allocateFixed
+      (t->m->heap->tryAllocateFixed
        (allocator, ceiling(sizeInBytes, BytesPerWord), objectMask, &total));
 
+    if (o) {
       memset(o, 0, sizeInBytes);
 
       alias(o, 0) = FixedMark;
@@ -3550,19 +3551,26 @@ allocate3(Thread* t, Allocator* allocator, Machine::AllocationType type,
       t->m->fixedFootprint += total;
       
       return o;
+    } else {
+      throw_(t, root(t, Machine::OutOfMemoryError));
+    }
   }
 
   case Machine::ImmortalAllocation: {
     unsigned total;
     object o = static_cast<object>
-      (t->m->heap->allocateImmortalFixed
+      (t->m->heap->tryAllocateImmortalFixed
        (allocator, ceiling(sizeInBytes, BytesPerWord), objectMask, &total));
 
+    if (o) {
       memset(o, 0, sizeInBytes);
 
       alias(o, 0) = FixedMark;
 
       return o;
+    } else {
+      throw_(t, root(t, Machine::OutOfMemoryError));
+    }
   }
 
   default: abort(t);

src/machine.h

@@ -1784,8 +1784,8 @@ class FixedAllocator: public Allocator {
     s(s), base(base), offset(0), capacity(capacity)
   { }
 
-  virtual void* tryAllocate(unsigned) {
-    abort(s);
+  virtual void* tryAllocate(unsigned size) {
+    return allocate(size);
   }
 
   void* allocate(unsigned size, unsigned padAlignment) {