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

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 8a5f09a9b1602a33297c975a1bc51abe1def254b..ad72095cd0cbb6427c838454925903f80094fd3a 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -191,10 +191,13 @@ bool LCodeGen::GeneratePrologue() {
   int heap_slots = info_->num_heap_slots() - Context::MIN_CONTEXT_SLOTS;
   if (heap_slots > 0) {
     Comment(";;; Allocate local context");
+    bool need_write_barrier = true;
     // Argument to NewContext is the function, which is still in rdi.
     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(rdi);
       __ CallRuntime(Runtime::kHiddenNewFunctionContext, 1);
@@ -218,7 +221,14 @@ bool LCodeGen::GeneratePrologue() {
         int context_offset = Context::SlotOffset(var->index());
         __ movp(Operand(rsi, context_offset), rax);
         // Update the write barrier. This clobbers rax and rbx.
-        __ RecordWriteContextSlot(rsi, context_offset, rax, rbx, kSaveFPRegs);
+        if (need_write_barrier) {
+          __ RecordWriteContextSlot(rsi, context_offset, rax, rbx, kSaveFPRegs);
+        } else if (FLAG_debug_code) {
+          Label done;
+          __ JumpIfInNewSpace(rsi, rax, &done, Label::kNear);
+          __ Abort(kExpectedNewSpaceObject);
+          __ bind(&done);
+        }
       }
     }
     Comment(";;; End allocate local context");