[esnext] prototype runtime implementation for async functions

src/full-codegen/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.
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/ast/scopes.h"
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
 #include "src/codegen.h"
@@ skipping 1756 matching lines @@
       context()->Plug(rax);
       break;
     case KEYED_PROPERTY:
       EmitKeyedPropertyAssignment(expr);
       break;
   }
 }
 
 
 void FullCodeGenerator::VisitYield(Yield* expr) {
-  Comment cmnt(masm_, "[ Yield");
+  Comment cmnt(masm_, expr->is_async_function_start() ? "[ AsyncFunction start"
+                                                      : "[ Yield");
   SetExpressionPosition(expr);
 
+  Label eval, suspend, continuation, post_runtime, resume, exception;
+
   // Evaluate yielded value first; the initial iterator definition depends on
   // this.  It stays on the stack while we update the iterator.
-  VisitForStackValue(expr->expression());
-
-  Label suspend, continuation, post_runtime, resume, exception;
+  if (!expr->is_async_function_start()) {
+    VisitForStackValue(expr->expression());
+  }
 
   __ jmp(&suspend);
   __ bind(&continuation);
   // When we arrive here, rax holds the generator object.
   __ RecordGeneratorContinuation();
   __ movp(rbx, FieldOperand(rax, JSGeneratorObject::kResumeModeOffset));
   __ movp(rax, FieldOperand(rax, JSGeneratorObject::kInputOffset));
   STATIC_ASSERT(JSGeneratorObject::kNext < JSGeneratorObject::kReturn);
   STATIC_ASSERT(JSGeneratorObject::kThrow > JSGeneratorObject::kReturn);
   __ SmiCompare(rbx, Smi::FromInt(JSGeneratorObject::kReturn));
   __ j(less, &resume);
   __ Push(result_register());
   __ j(greater, &exception);
-  EmitCreateIteratorResult(true);
-  EmitUnwindAndReturn();
+  if (!expr->is_async_function_start()) {
+    EmitCreateIteratorResult(true);
+    EmitUnwindAndReturn();
+  } else {
+    // AsyncFunction never resumed with `return` mode, should be unreachable.
+  }
 
   __ bind(&exception);
   __ CallRuntime(Runtime::kThrow);
 
   __ bind(&suspend);
-  OperandStackDepthIncrement(1);  // Not popped on this path.
+
+  if (!expr->is_async_function_start()) {
+    OperandStackDepthIncrement(1);  // Not popped on this path.
+  }
   VisitForAccumulatorValue(expr->generator_object());
   DCHECK(continuation.pos() > 0 && Smi::IsValid(continuation.pos()));
   __ Move(FieldOperand(rax, JSGeneratorObject::kContinuationOffset),
           Smi::FromInt(continuation.pos()));
   __ movp(FieldOperand(rax, JSGeneratorObject::kContextOffset), rsi);
   __ movp(rcx, rsi);
   __ RecordWriteField(rax, JSGeneratorObject::kContextOffset, rcx, rdx,
                       kDontSaveFPRegs);
-  __ leap(rbx, Operand(rbp, StandardFrameConstants::kExpressionsOffset));
-  __ cmpp(rsp, rbx);
-  __ j(equal, &post_runtime);
-  __ Push(rax);  // generator object
-  __ CallRuntime(Runtime::kSuspendJSGeneratorObject, 1);
-  __ movp(context_register(),
-          Operand(rbp, StandardFrameConstants::kContextOffset));
-  __ bind(&post_runtime);
+  if (!expr->is_async_function_start()) {
+    // AsyncFunction start always occurs with an empty operand stack
+    __ leap(rbx, Operand(rbp, StandardFrameConstants::kExpressionsOffset));
+    __ cmpp(rsp, rbx);
+    __ j(equal, &post_runtime);
+    __ Push(rax);  // generator object
+    __ CallRuntime(Runtime::kSuspendJSGeneratorObject, 1);
+    __ movp(context_register(),
+            Operand(rbp, StandardFrameConstants::kContextOffset));
+    __ bind(&post_runtime);
 
-  PopOperand(result_register());
-  EmitReturnSequence();
+    PopOperand(result_register());
+    EmitReturnSequence();
+  } else {
+    VisitForAccumulatorValue(expr->expression());
+    DCHECK_EQ(result_register().code(), rax.code());

[caitp (gmail), 2016/04/16 18:33:03]

Not sure if this really adds anything, I guess it's nice to avoid pointless
stack pushing

+    EmitReturnSequence();
+  }
 
   __ bind(&resume);
   context()->Plug(result_register());
 }
 
 void FullCodeGenerator::PushOperand(MemOperand operand) {
   OperandStackDepthIncrement(1);
   __ Push(operand);
 }
 
@@ skipping 2045 matching lines @@
   DCHECK_EQ(
       isolate->builtins()->OnStackReplacement()->entry(),
       Assembler::target_address_at(call_target_address, unoptimized_code));
   return ON_STACK_REPLACEMENT;
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64