Async/await Promise dependency graph

src/parsing/parser.cc

Index: src/parsing/parser.cc
diff --git a/src/parsing/parser.cc b/src/parsing/parser.cc
index 4973857c37ea0f2cf2854c35000be559db065e3a..30da25a9eb67861579c0e7f13be640036da88e08 100644
--- a/src/parsing/parser.cc
+++ b/src/parsing/parser.cc
@@ -4379,8 +4379,10 @@ Expression* Parser::ExpressionListToExpression(ZoneList<Expression*>* args) {
 
 Expression* Parser::RewriteAwaitExpression(Expression* value, int await_pos) {
   // yield do {
+  //   promise_tmp = .promise;
   //   tmp = <operand>;
-  //   tmp = %AsyncFunctionAwait(.generator_object, tmp);
+  //   %AsyncFunctionAwait(.generator_object, tmp, promise_tmp);
+  //   promise_tmp
   // }
   Variable* generator_object_variable =
       function_state_->generator_object_variable();
@@ -4394,6 +4396,15 @@ Expression* Parser::RewriteAwaitExpression(Expression* value, int await_pos) {
   Block* do_block = factory()->NewBlock(nullptr, 2, false, nopos);
 
   // Wrap value evaluation to provide a break location.
+  Variable* promise_temp_var =

[adamk, 2016/09/15 22:39:40]

Why do we need a new var here? Is something going to clobber .promise before we
can "return" it from the do block?

[Dan Ehrenberg, 2016/09/17 00:04:31]

- To output something from a block, we need a Variable, not a VariableProxy, so
we need to read it into something (since .promise is not resolved until scope
analysis).
- Even without that part, if I put a nopos variable read into an expression
position as the third argument of AsyncFunctionAwait, it seems to affect the
line number output (putting it at the beginning of the script). Reading .promise
here seems to function as a workaround.

+      NewTemporary(ast_value_factory()->empty_string());
+  Expression* promise_assignment = factory()->NewAssignment(
+      Token::ASSIGN, factory()->NewVariableProxy(promise_temp_var),
+      BuildDotPromise(), nopos);
+  do_block->statements()->Add(
+      factory()->NewExpressionStatement(promise_assignment, value->position()),

[adamk, 2016/09/15 22:39:40]

Is it important that this uses the same position as the value assignment? That
seems odd.

[Dan Ehrenberg, 2016/09/17 00:04:31]

It does not; changed it to nopos.

+      zone());
+
   Expression* value_assignment = factory()->NewAssignment(
       Token::ASSIGN, factory()->NewVariableProxy(temp_var), value, nopos);
   do_block->statements()->Add(
@@ -4401,11 +4412,13 @@ Expression* Parser::RewriteAwaitExpression(Expression* value, int await_pos) {
       zone());
 
   ZoneList<Expression*>* async_function_await_args =
-      new (zone()) ZoneList<Expression*>(2, zone());
+      new (zone()) ZoneList<Expression*>(3, zone());
   Expression* generator_object =
       factory()->NewVariableProxy(generator_object_variable);
   async_function_await_args->Add(generator_object, zone());
   async_function_await_args->Add(factory()->NewVariableProxy(temp_var), zone());
+  async_function_await_args->Add(factory()->NewVariableProxy(promise_temp_var),
+                                 zone());
 
   // The parser emits calls to AsyncFunctionAwaitCaught, but the
   // AstNumberingVisitor will rewrite this to AsyncFunctionAwaitUncaught
@@ -4413,14 +4426,13 @@ Expression* Parser::RewriteAwaitExpression(Expression* value, int await_pos) {
   Expression* async_function_await =
       factory()->NewCallRuntime(Context::ASYNC_FUNCTION_AWAIT_CAUGHT_INDEX,
                                 async_function_await_args, nopos);
+  do_block->statements()->Add(
+      factory()->NewExpressionStatement(async_function_await, await_pos),
+      zone());
 
   // Wrap await to provide a break location between value evaluation and yield.
-  Expression* await_assignment = factory()->NewAssignment(
-      Token::ASSIGN, factory()->NewVariableProxy(temp_var),
-      async_function_await, nopos);
-  do_block->statements()->Add(
-      factory()->NewExpressionStatement(await_assignment, await_pos), zone());
-  Expression* do_expr = factory()->NewDoExpression(do_block, temp_var, nopos);
+  Expression* do_expr =
+      factory()->NewDoExpression(do_block, promise_temp_var, nopos);
 
   generator_object = factory()->NewVariableProxy(generator_object_variable);
   return factory()->NewYield(generator_object, do_expr, nopos,