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

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index fd30b1a8fff050f52f90ee93b03648c017f7afd1..f1deb387059d6071fc698ce172b3199cf3d2be94 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -198,10 +198,13 @@ 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 a1.
     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);
@@ -224,8 +227,15 @@ bool LCodeGen::GeneratePrologue() {
         MemOperand target = ContextOperand(cp, var->index());
         __ sw(a0, target);
         // Update the write barrier. This clobbers a3 and a0.
-        __ RecordWriteContextSlot(
-            cp, target.offset(), a0, a3, GetRAState(), kSaveFPRegs);
+        if (need_write_barrier) {
+          __ RecordWriteContextSlot(
+              cp, target.offset(), a0, a3, GetRAState(), kSaveFPRegs);
+        } else if (FLAG_debug_code) {
+          Label done;
+          __ JumpIfInNewSpace(cp, a0, &done);
+          __ Abort(kExpectedNewSpaceObject);
+          __ bind(&done);
+        }
       }
     }
     Comment(";;; End allocate local context");