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

src/arm/full-codegen-arm.cc

Index: src/arm/full-codegen-arm.cc
diff --git a/src/arm/full-codegen-arm.cc b/src/arm/full-codegen-arm.cc
index cb7d7da89f854c9eeb88a327a19cc715e482917e..ccde8b6b227901c07fd5be6e9beb1d5537400963 100644
--- a/src/arm/full-codegen-arm.cc
+++ b/src/arm/full-codegen-arm.cc
@@ -212,6 +212,7 @@ void FullCodeGenerator::Generate() {
   if (heap_slots > 0) {
     // Argument to NewContext is the function, which is still in r1.
     Comment cmnt(masm_, "[ Allocate context");
+    bool need_write_barrier = true;
     if (FLAG_harmony_scoping && info->scope()->is_global_scope()) {
       __ push(r1);
       __ Push(info->scope()->GetScopeInfo());
@@ -219,6 +220,8 @@ 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(r1);
       __ CallRuntime(Runtime::kHiddenNewFunctionContext, 1);
@@ -242,8 +245,15 @@ void FullCodeGenerator::Generate() {
         __ str(r0, target);
 
         // Update the write barrier.
-        __ RecordWriteContextSlot(
-            cp, target.offset(), r0, r3, kLRHasBeenSaved, kDontSaveFPRegs);
+        if (need_write_barrier) {
+          __ RecordWriteContextSlot(
+              cp, target.offset(), r0, r3, kLRHasBeenSaved, kDontSaveFPRegs);
+        } else if (FLAG_debug_code) {
+          Label done;
+          __ JumpIfInNewSpace(cp, r0, &done);
+          __ Abort(kExpectedNewSpaceObject);
+          __ bind(&done);
+        }
       }
     }
   }