[esnext] prototype runtime implementation for async functions

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index f2850ceebb8b680ad3c4ebedd529c69d28d07b62..bdca0bc022fb928382874049696bfa24f951cd89 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -1774,14 +1774,17 @@ void FullCodeGenerator::VisitAssignment(Assignment* expr) {
 
 
 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);
@@ -1795,14 +1798,21 @@ void FullCodeGenerator::VisitYield(Yield* expr) {
   __ 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),
@@ -1811,17 +1821,24 @@ void FullCodeGenerator::VisitYield(Yield* expr) {
   __ 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);
-
-  PopOperand(result_register());
-  EmitReturnSequence();
+  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();
+  } else {
+    VisitForAccumulatorValue(expr->expression());
+    DCHECK_EQ(result_register().code(), rax.code());
+    EmitReturnSequence();
+  }
 
   __ bind(&resume);
   context()->Plug(result_register());