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

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index d2e665e58cbb8a22218235ce1ac45594b225055b..b417c801d58f9b31cb0956cba3df95fd0ca58098 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -179,10 +179,13 @@ 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 in r1.
     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(r1);
       __ CallRuntime(Runtime::kHiddenNewFunctionContext, 1);
@@ -205,13 +208,20 @@ bool LCodeGen::GeneratePrologue() {
         MemOperand target = ContextOperand(cp, var->index());
         __ str(r0, target);
         // Update the write barrier. This clobbers r3 and r0.
-        __ RecordWriteContextSlot(
-            cp,
-            target.offset(),
-            r0,
-            r3,
-            GetLinkRegisterState(),
-            kSaveFPRegs);
+        if (need_write_barrier) {
+          __ RecordWriteContextSlot(
+              cp,
+              target.offset(),
+              r0,
+              r3,
+              GetLinkRegisterState(),
+              kSaveFPRegs);
+        } else if (FLAG_debug_code) {
+          Label done;
+          __ JumpIfInNewSpace(cp, r0, &done);
+          __ Abort(kExpectedNewSpaceObject);
+          __ bind(&done);
+        }
       }
     }
     Comment(";;; End allocate local context");