Optimize write barrier of map-only elements transitions

src/ia32/lithium-codegen-ia32.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 3554 matching lines @@
 void LCodeGen::DoTransitionElementsKind(LTransitionElementsKind* instr) {
   Register object_reg = ToRegister(instr->object());
   Register new_map_reg = ToRegister(instr->new_map_reg());
 
   Handle<Map> from_map = instr->original_map();
   Handle<Map> to_map = instr->transitioned_map();
   ElementsKind from_kind = from_map->elements_kind();
   ElementsKind to_kind = to_map->elements_kind();
 
   Label not_applicable;
+  bool is_simple_map_transition =
+      IsSimpleMapChangeTransition(from_kind, to_kind);
+  Label::Distance branch_distance =
+      is_simple_map_transition ? Label::kNear : Label::kFar;
   __ cmp(FieldOperand(object_reg, HeapObject::kMapOffset), from_map);
-  __ j(not_equal, &not_applicable);
-  __ mov(new_map_reg, to_map);
-  if (IsSimpleMapChangeTransition(from_kind, to_kind)) {
+  __ j(not_equal, &not_applicable, branch_distance);
+  if (is_simple_map_transition) {
     Register object_reg = ToRegister(instr->object());
-    __ mov(FieldOperand(object_reg, HeapObject::kMapOffset), new_map_reg);
+    Handle<Map> map = instr->hydrogen()->transitioned_map();
+    __ mov(FieldOperand(object_reg, HeapObject::kMapOffset),
+           Immediate(map));
     // Write barrier.
     ASSERT_NE(instr->temp_reg(), NULL);
-    __ RecordWriteField(object_reg, HeapObject::kMapOffset, new_map_reg,
-                        ToRegister(instr->temp_reg()), kDontSaveFPRegs);
+    __ RecordWriteForMap(object_reg, to_map, new_map_reg,
+                         ToRegister(instr->temp_reg()),
+                         kDontSaveFPRegs);
   } else if (IsFastSmiElementsKind(from_kind) &&
              IsFastDoubleElementsKind(to_kind)) {
+    __ mov(new_map_reg, to_map);
     Register fixed_object_reg = ToRegister(instr->temp_reg());
     ASSERT(fixed_object_reg.is(edx));
     ASSERT(new_map_reg.is(ebx));
     __ mov(fixed_object_reg, object_reg);
     CallCode(isolate()->builtins()->TransitionElementsSmiToDouble(),
              RelocInfo::CODE_TARGET, instr);
   } else if (IsFastDoubleElementsKind(from_kind) &&
              IsFastObjectElementsKind(to_kind)) {
+    __ mov(new_map_reg, to_map);
     Register fixed_object_reg = ToRegister(instr->temp_reg());
     ASSERT(fixed_object_reg.is(edx));
     ASSERT(new_map_reg.is(ebx));
     __ mov(fixed_object_reg, object_reg);
     CallCode(isolate()->builtins()->TransitionElementsDoubleToObject(),
              RelocInfo::CODE_TARGET, instr);
   } else {
     UNREACHABLE();
   }
   __ bind(&not_applicable);
@@ skipping 1490 matching lines @@
                               FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_IA32