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

src/arm64/lithium-codegen-arm64.cc

Index: src/arm64/lithium-codegen-arm64.cc
diff --git a/src/arm64/lithium-codegen-arm64.cc b/src/arm64/lithium-codegen-arm64.cc
index 9161aaf1f2258cb0fdea8eff0fd2c2163bacccf4..36605514b7041813ee4de3d6e9796794bc731e3a 100644
--- a/src/arm64/lithium-codegen-arm64.cc
+++ b/src/arm64/lithium-codegen-arm64.cc
@@ -694,10 +694,13 @@ 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 x1.
     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(x1);
       __ CallRuntime(Runtime::kHiddenNewFunctionContext, 1);
@@ -723,8 +726,15 @@ bool LCodeGen::GeneratePrologue() {
         MemOperand target = ContextMemOperand(cp, var->index());
         __ Str(value, target);
         // Update the write barrier. This clobbers value and scratch.
-        __ RecordWriteContextSlot(cp, target.offset(), value, scratch,
-                                  GetLinkRegisterState(), kSaveFPRegs);
+        if (need_write_barrier) {
+          __ RecordWriteContextSlot(cp, target.offset(), value, scratch,
+                                    GetLinkRegisterState(), kSaveFPRegs);
+        } else if (FLAG_debug_code) {
+          Label done;
+          __ JumpIfInNewSpace(cp, &done);
+          __ Abort(kExpectedNewSpaceObject);
+          __ bind(&done);
+        }
       }
     }
     Comment(";;; End allocate local context");