Turn FastNewContextStub into a HydrogenCodeStub.

src/x64/lithium-codegen-x64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 200 matching lines @@
     if (info()->saves_caller_doubles()) {
       SaveCallerDoubles();
     }
   }
 
   // Possibly allocate a local context.
   int heap_slots = info_->num_heap_slots() - Context::MIN_CONTEXT_SLOTS;
   if (heap_slots > 0) {
     Comment(";;; Allocate local context");
     // Argument to NewContext is the function, which is still in rdi.
-    __ push(rdi);
     if (heap_slots <= FastNewContextStub::kMaximumSlots) {
       FastNewContextStub stub(heap_slots);
       __ CallStub(&stub);
     } else {
+      __ push(rdi);
       __ CallRuntime(Runtime::kNewFunctionContext, 1);
     }
     RecordSafepoint(Safepoint::kNoLazyDeopt);
-    // Context is returned in both rax and rsi.  It replaces the context
-    // passed to us.  It's saved in the stack and kept live in rsi.
-    __ movp(Operand(rbp, StandardFrameConstants::kContextOffset), rsi);
+    // 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 = 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));
@@ skipping 5387 matching lines @@
                                FixedArray::kHeaderSize - kPointerSize));
   __ bind(&done);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64