Delegating yield does not re-box result objects

src/x64/full-codegen-x64.cc

 // Copyright 2012 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 2008 matching lines @@
 
       // catch (e) { receiver = iter; f = 'throw'; arg = e; goto l_call; }
       __ bind(&l_catch);
       handler_table()->set(expr->index(), Smi::FromInt(l_catch.pos()));
       __ LoadRoot(rcx, Heap::kthrow_stringRootIndex);    // "throw"
       __ push(rcx);
       __ push(Operand(rsp, 2 * kPointerSize));           // iter
       __ push(rax);                                      // exception
       __ jmp(&l_call);
 
-      // try { received = yield result.value }
+      // try { received = %yield result }
+      // Shuffle the received result above a try handler and yield it without
+      // re-boxing.
       __ bind(&l_try);
-      EmitCreateIteratorResult(false);                    // pop and box to rax
+      __ pop(rax);                                       // result
       __ PushTryHandler(StackHandler::CATCH, expr->index());
       const int handler_size = StackHandlerConstants::kSize;
       __ push(rax);                                      // result
       __ push(Operand(rsp, (0 + 1) * kPointerSize + handler_size));  // g
       __ CallRuntime(Runtime::kSuspendJSGeneratorObject, 1);
       __ movq(context_register(),
               Operand(rbp, StandardFrameConstants::kContextOffset));
       __ CompareRoot(rax, Heap::kTheHoleValueRootIndex);
       __ j(not_equal, &l_resume);
       __ pop(rax);                                       // result
       EmitReturnSequence();
       __ bind(&l_resume);                                // received in rax
       __ PopTryHandler();
 
       // receiver = iter; f = 'next'; arg = received;
       __ bind(&l_next);
       __ LoadRoot(rcx, Heap::knext_stringRootIndex);     // "next"
       __ push(rcx);
       __ push(Operand(rsp, 2 * kPointerSize));           // iter
       __ push(rax);                                      // received
 
       // result = receiver[f](arg);
       __ bind(&l_call);
       Handle<Code> ic = isolate()->stub_cache()->ComputeKeyedCallInitialize(1);
       CallIC(ic);
       __ movq(rsi, Operand(rbp, StandardFrameConstants::kContextOffset));
       __ Drop(1);  // The key is still on the stack; drop it.
 
-      // val = result.value; if (!result.done) goto l_try;
+      // if (!result.done) goto l_try;
       __ bind(&l_loop);
-      // result.value
       __ push(rax);                                      // save result
-      __ LoadRoot(rcx, Heap::kvalue_stringRootIndex);    // "value"
-      Handle<Code> value_ic = isolate()->builtins()->LoadIC_Initialize();
-      CallIC(value_ic);                                  // result.value in rax
-      __ pop(rbx);                                       // result
-      __ push(rax);                                      // result.value
-      __ movq(rax, rbx);                                 // result
       __ LoadRoot(rcx, Heap::kdone_stringRootIndex);     // "done"
       Handle<Code> done_ic = isolate()->builtins()->LoadIC_Initialize();
       CallIC(done_ic);                                   // result.done in rax
       Handle<Code> bool_ic = ToBooleanStub::GetUninitialized(isolate());
       CallIC(bool_ic);
       __ testq(result_register(), result_register());
       __ j(zero, &l_try);
 
       // result.value
-      __ pop(rax);                                       // result.value
+      __ pop(rax);                                       // result
+      __ LoadRoot(rcx, Heap::kvalue_stringRootIndex);    // "value"
+      Handle<Code> value_ic = isolate()->builtins()->LoadIC_Initialize();
+      CallIC(value_ic);                                  // result.value in rax
       context()->DropAndPlug(2, rax);                    // drop iter and g
       break;
     }
   }
 }
 
 
 void FullCodeGenerator::EmitGeneratorResume(Expression *generator,
     Expression *value,
     JSGeneratorObject::ResumeMode resume_mode) {
@@ skipping 2793 matching lines @@
   *context_length = 0;
   return previous_;
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64