Optimize write barrier of map-only elements transitions

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
diff --git a/src/ia32/lithium-codegen-ia32.cc b/src/ia32/lithium-codegen-ia32.cc
index ca093ec6512e6d0b4337d91efc8725270cb06286..12c9fa3afb4140208eb7d9a6cc6740161e158159 100644
--- a/src/ia32/lithium-codegen-ia32.cc
+++ b/src/ia32/lithium-codegen-ia32.cc
@@ -3572,18 +3572,25 @@ void LCodeGen::DoTransitionElementsKind(LTransitionElementsKind* instr) {
   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));
@@ -3592,6 +3599,7 @@ void LCodeGen::DoTransitionElementsKind(LTransitionElementsKind* instr) {
              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));