X87: Skip write barriers in the fast case when setting up local context.

src/x87/full-codegen-x87.cc

Index: src/x87/full-codegen-x87.cc
diff --git a/src/x87/full-codegen-x87.cc b/src/x87/full-codegen-x87.cc
index 06eaa21f311b8e4ab839e56a0976ca1d1d4ac457..006383d4d1974087dbbd62f8d2cd11a2b48395ad 100644
--- a/src/x87/full-codegen-x87.cc
+++ b/src/x87/full-codegen-x87.cc
@@ -198,6 +198,7 @@ void FullCodeGenerator::Generate() {
   int heap_slots = info->scope()->num_heap_slots() - Context::MIN_CONTEXT_SLOTS;
   if (heap_slots > 0) {
     Comment cmnt(masm_, "[ Allocate context");
+    bool need_write_barrier = true;
     // Argument to NewContext is the function, which is still in edi.
     if (FLAG_harmony_scoping && info->scope()->is_global_scope()) {
       __ push(edi);
@@ -206,6 +207,8 @@ void FullCodeGenerator::Generate() {
     } else if (heap_slots <= FastNewContextStub::kMaximumSlots) {
       FastNewContextStub stub(isolate(), heap_slots);
       __ CallStub(&stub);
+      // Result of FastNewContextStub is always in new space.
+      need_write_barrier = false;
     } else {
       __ push(edi);
       __ CallRuntime(Runtime::kHiddenNewFunctionContext, 1);
@@ -229,10 +232,17 @@ void FullCodeGenerator::Generate() {
         int context_offset = Context::SlotOffset(var->index());
         __ mov(Operand(esi, context_offset), eax);
         // Update the write barrier. This clobbers eax and ebx.
-        __ RecordWriteContextSlot(esi,
-                                  context_offset,
-                                  eax,
-                                  ebx);
+        if (need_write_barrier) {
+          __ RecordWriteContextSlot(esi,
+                                    context_offset,
+                                    eax,
+                                    ebx);
+        } else if (FLAG_debug_code) {
+          Label done;
+          __ JumpIfInNewSpace(esi, eax, &done, Label::kNear);
+          __ Abort(kExpectedNewSpaceObject);
+          __ bind(&done);
+        }
       }
     }
   }