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

src/mips/full-codegen-mips.cc

Index: src/mips/full-codegen-mips.cc
diff --git a/src/mips/full-codegen-mips.cc b/src/mips/full-codegen-mips.cc
index 5a47a7ce46e4f0ffe9c95d11cb8a07c16a7eb0b9..7310eee88a4e2ec7952cd6346403b109cf928447 100644
--- a/src/mips/full-codegen-mips.cc
+++ b/src/mips/full-codegen-mips.cc
@@ -219,6 +219,7 @@ void FullCodeGenerator::Generate() {
   if (heap_slots > 0) {
     Comment cmnt(masm_, "[ Allocate context");
     // Argument to NewContext is the function, which is still in a1.
+    bool need_write_barrier = true;
     if (FLAG_harmony_scoping && info->scope()->is_global_scope()) {
       __ push(a1);
       __ Push(info->scope()->GetScopeInfo());
@@ -226,6 +227,8 @@ 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(a1);
       __ CallRuntime(Runtime::kHiddenNewFunctionContext, 1);
@@ -249,8 +252,15 @@ void FullCodeGenerator::Generate() {
         __ sw(a0, target);
 
         // Update the write barrier.
-        __ RecordWriteContextSlot(
-            cp, target.offset(), a0, a3, kRAHasBeenSaved, kDontSaveFPRegs);
+        if (need_write_barrier) {
+          __ RecordWriteContextSlot(
+              cp, target.offset(), a0, a3, kRAHasBeenSaved, kDontSaveFPRegs);
+        } else if (FLAG_debug_code) {
+          Label done;
+          __ JumpIfInNewSpace(cp, a0, &done);
+          __ Abort(kExpectedNewSpaceObject);
+          __ bind(&done);
+        }
       }
     }
   }