Allocation space decisions are precisely made in hydrogen.

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 3748 matching lines @@
           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): Add support for non-constant allocation in dominator.
-  if (!GuaranteedInNewSpace() || !current_size->IsInteger32Constant() ||
-      !dominator_allocate_instr->GuaranteedInNewSpace() ||
+  if (!IsNewSpaceAllocation() || !current_size->IsInteger32Constant() ||
+      !dominator_allocate_instr->IsNewSpaceAllocation() ||
       !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();
   int32_t new_dominator_size = dominator_size_constant + current_size_constant;
 
   if (MustAllocateDoubleAligned()) {
     if (!dominator_allocate_instr->MustAllocateDoubleAligned()) {
-      dominator_allocate_instr->SetFlags(HAllocate::ALLOCATE_DOUBLE_ALIGNED);
+      dominator_allocate_instr->MakeDoubleAligned();
     }
     if ((dominator_size_constant & kDoubleAlignmentMask) != 0) {
       dominator_size_constant += kDoubleSize / 2;
       new_dominator_size += kDoubleSize / 2;
     }
   }
 
   if (new_dominator_size > Page::kMaxNonCodeHeapObjectSize) {
     if (FLAG_trace_allocation_folding) {
       PrintF("#%d (%s) cannot fold into #%d (%s) due to size: %d\n",
           id(), Mnemonic(), dominator->id(), dominator->Mnemonic(),
           new_dominator_size);
     }
     return;
   }
   HBasicBlock* block = dominator->block();
   Zone* zone = block->zone();
   HInstruction* new_dominator_size_constant = new(zone) HConstant(
       new_dominator_size);
   new_dominator_size_constant->InsertBefore(dominator_allocate_instr);
   dominator_allocate_instr->UpdateSize(new_dominator_size_constant);
 
 #ifdef VERIFY_HEAP
   if (FLAG_verify_heap) {
-    dominator_allocate_instr->SetFlags(HAllocate::PREFILL_WITH_FILLER);
+    dominator_allocate_instr->MakePrefillWithFiller();
   }
 #endif
 
   // After that replace the dominated allocate instruction.
   HInstruction* dominated_allocate_instr =
       new(zone) HInnerAllocatedObject(dominator_allocate_instr,
                                       dominator_size_constant,
                                       type());
   dominated_allocate_instr->InsertBefore(this);
   DeleteAndReplaceWith(dominated_allocate_instr);
   if (FLAG_trace_allocation_folding) {
     PrintF("#%d (%s) folded into #%d (%s)\n",
         id(), Mnemonic(), dominator->id(), dominator->Mnemonic());
   }
 }
 
 
 void HAllocate::PrintDataTo(StringStream* stream) {
   size()->PrintNameTo(stream);
-  if (!GuaranteedInNewSpace()) stream->Add(" (pretenure)");
+  if (!IsNewSpaceAllocation()) stream->Add(" (pretenure)");
 }
 
 
 HType HRegExpLiteral::CalculateInferredType() {
   return HType::JSObject();
 }
 
 
 HType HFunctionLiteral::CalculateInferredType() {
   return HType::JSObject();
@@ skipping 726 matching lines @@
       break;
     case kExternalMemory:
       stream->Add("[external-memory]");
       break;
   }
 
   stream->Add("@%d", offset());
 }
 
 } }  // namespace v8::internal