Added tracing information to the case when allocation folding cannot be performed.

src/hydrogen-instructions.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3163 matching lines @@
 
 
 HType HAllocate::CalculateInferredType() {
   return type_;
 }
 
 
 void HAllocate::HandleSideEffectDominator(GVNFlag side_effect,
                                           HValue* dominator) {
   ASSERT(side_effect == kChangesNewSpacePromotion);
+  if (!FLAG_use_allocation_folding) return;
+
   // Try to fold allocations together with their dominating allocations.
-  if (!FLAG_use_allocation_folding || !dominator->IsAllocate()) {
+  if (!dominator->IsAllocate()) {
+    if (FLAG_trace_allocation_folding) {
+      PrintF("#%d (%s) cannot fold into #%d (%s)\n",
+          id(), Mnemonic(), dominator->id(), dominator->Mnemonic());
+    }
     return;
   }
+
   HAllocate* dominator_allocate_instr = HAllocate::cast(dominator);
   HValue* dominator_size = dominator_allocate_instr->size();
   HValue* current_size = size();
   // We can just fold allocations that are guaranteed in new space.
   // TODO(hpayer): Support double aligned allocations.
   // TODO(hpayer): Add support for non-constant allocation in dominator.
   if (!GuaranteedInNewSpace() || MustAllocateDoubleAligned() ||
       !current_size->IsInteger32Constant() ||
       !dominator_allocate_instr->GuaranteedInNewSpace() ||
       dominator_allocate_instr->MustAllocateDoubleAligned() ||
       !dominator_size->IsInteger32Constant()) {
+    if (FLAG_trace_allocation_folding) {
+      PrintF("#%d (%s) cannot fold into #%d (%s)\n",
+          id(), Mnemonic(), dominator->id(), dominator->Mnemonic());
+    }
     return;
   }
 
   // First update the size of the dominator allocate instruction.
   int32_t dominator_size_constant =
       HConstant::cast(dominator_size)->GetInteger32Constant();
   int32_t current_size_constant =
       HConstant::cast(current_size)->GetInteger32Constant();
   HBasicBlock* block = dominator->block();
   Zone* zone = block->zone();
@@ skipping 760 matching lines @@
     case kBackingStore:
       if (!name_.is_null()) stream->Add(*String::cast(*name_)->ToCString());
       stream->Add("[backing-store]");
       break;
   }
 
   stream->Add("@%d", offset());
 }
 
 } }  // namespace v8::internal