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

src/x64/full-codegen-x64.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "code-stubs.h"
 #include "codegen.h"
@@ skipping 181 matching lines @@
       }
     }
   }
 
   bool function_in_register = true;
 
   // Possibly allocate a local context.
   int heap_slots = info->scope()->num_heap_slots() - Context::MIN_CONTEXT_SLOTS;
   if (heap_slots > 0) {
     Comment cmnt(masm_, "[ Allocate context");
+    bool need_write_barrier = true;
     // Argument to NewContext is the function, which is still in rdi.
     if (FLAG_harmony_scoping && info->scope()->is_global_scope()) {
       __ Push(rdi);
       __ Push(info->scope()->GetScopeInfo());
       __ CallRuntime(Runtime::kHiddenNewGlobalContext, 2);
     } 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(rdi);
       __ CallRuntime(Runtime::kHiddenNewFunctionContext, 1);
     }
     function_in_register = false;
     // Context is returned in rax.  It replaces the context passed to us.
     // It's saved in the stack and kept live in rsi.
     __ movp(rsi, rax);
     __ movp(Operand(rbp, StandardFrameConstants::kContextOffset), rax);
 
     // Copy any necessary parameters into the context.
     int num_parameters = info->scope()->num_parameters();
     for (int i = 0; i < num_parameters; i++) {
       Variable* var = scope()->parameter(i);
       if (var->IsContextSlot()) {
         int parameter_offset = StandardFrameConstants::kCallerSPOffset +
             (num_parameters - 1 - i) * kPointerSize;
         // Load parameter from stack.
         __ movp(rax, Operand(rbp, parameter_offset));
         // Store it in the context.
         int context_offset = Context::SlotOffset(var->index());
         __ movp(Operand(rsi, context_offset), rax);
         // Update the write barrier.  This clobbers rax and rbx.
-        __ RecordWriteContextSlot(
-            rsi, context_offset, rax, rbx, kDontSaveFPRegs);
+        if (need_write_barrier) {
+          __ RecordWriteContextSlot(
+              rsi, context_offset, rax, rbx, kDontSaveFPRegs);
+        } else if (FLAG_debug_code) {
+          Label done;
+          __ JumpIfInNewSpace(rsi, rax, &done, Label::kNear);
+          __ Abort(kExpectedNewSpaceObject);
+          __ bind(&done);
+        }
       }
     }
   }
 
   // Possibly allocate an arguments object.
   Variable* arguments = scope()->arguments();
   if (arguments != NULL) {
     // Arguments object must be allocated after the context object, in
     // case the "arguments" or ".arguments" variables are in the context.
     Comment cmnt(masm_, "[ Allocate arguments object");
@@ skipping 4555 matching lines @@
   ASSERT_EQ(isolate->builtins()->OsrAfterStackCheck()->entry(),
             Assembler::target_address_at(call_target_address,
                                          unoptimized_code));
   return OSR_AFTER_STACK_CHECK;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64