Supported folding of constant size allocation followed by dynamic size allocation.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 25ac2df12152fcfd25a32d5e064386afde483fe5..c19d7902c74c35a91079e270ab63d77384ab56e6 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -1237,6 +1237,8 @@ class HInstruction : public HValue {
     position_.set_operand_position(index, pos);
   }
 
+  bool Dominates(HInstruction* other);
+
   bool CanTruncateToInt32() const { return CheckFlag(kTruncatingToInt32); }
 
   virtual LInstruction* CompileToLithium(LChunkBuilder* builder) = 0;
@@ -5456,6 +5458,10 @@ class HAllocate V8_FINAL : public HTemplateInstruction<2> {
   HValue* context() { return OperandAt(0); }
   HValue* size() { return OperandAt(1); }
 
+  bool has_size_upper_bound() { return size_upper_bound_ != NULL; }
+  HConstant* size_upper_bound() { return size_upper_bound_; }
+  void set_size_upper_bound(HConstant* value) { size_upper_bound_ = value; }

[Hannes Payer (out of office), 2014/01/16 12:58:39]

I guess set_size_upper_bound should only be called once for an instruction. Can
we assert that?

+
   virtual Representation RequiredInputRepresentation(int index) V8_OVERRIDE {
     if (index == 0) {
       return Representation::Tagged();
@@ -5526,9 +5532,10 @@ class HAllocate V8_FINAL : public HTemplateInstruction<2> {
       : HTemplateInstruction<2>(type),
         dominating_allocate_(NULL),
         filler_free_space_size_(NULL),
+        size_upper_bound_(NULL),
         clear_next_map_word_(false) {
     SetOperandAt(0, context);
-    SetOperandAt(1, size);
+    UpdateSize(size);
     set_representation(Representation::Tagged());
     SetFlag(kTrackSideEffectDominators);
     SetGVNFlag(kChangesNewSpacePromotion);
@@ -5562,6 +5569,11 @@ class HAllocate V8_FINAL : public HTemplateInstruction<2> {
 
   void UpdateSize(HValue* size) {
     SetOperandAt(1, size);
+    if (size->IsInteger32Constant()) {
+      size_upper_bound_ = HConstant::cast(size);
+    } else {
+      size_upper_bound_ = NULL;
+    }
   }
 
   HAllocate* GetFoldableDominator(HAllocate* dominator);
@@ -5583,6 +5595,7 @@ class HAllocate V8_FINAL : public HTemplateInstruction<2> {
   Handle<Map> known_initial_map_;
   HAllocate* dominating_allocate_;
   HStoreNamedField* filler_free_space_size_;
+  HConstant* size_upper_bound_;
   bool clear_next_map_word_;
 };