Add a synthetic return for async generator functions

Currently implicit returns do not correctly resolve the async generator
objects. This is observable via AsyncGenerator#throw as the implicit
return won't override the rejection.

Bug: v8:10238
Change-Id: I012fc3507d1e4106e7f35b21275be180a6e274c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2065343Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66413}

src/parsing/parser.cc

@@ -1788,6 +1788,11 @@ void Parser::ParseAndRewriteAsyncGeneratorFunctionBody(
     statements.Add(
         factory()->NewExpressionStatement(initial_yield, kNoSourcePosition));
     ParseStatementList(&statements, Token::RBRACE);
+    // Since the whole body is wrapped in a try-catch, make the implicit
+    // end-of-function return explicit to ensure BytecodeGenerator's special
+    // handling for ReturnStatements in async generators applies.
+    statements.Add(factory()->NewSyntheticAsyncReturnStatement(
+        factory()->NewUndefinedLiteral(kNoSourcePosition), kNoSourcePosition));
 
     // Don't create iterator result for async generators, as the resume methods
     // will create it.

test/cctest/interpreter/bytecode_expectations/AsyncGenerators.golden

@@ -14,7 +14,7 @@ snippet: "
 "
 frame size: 8
 parameter count: 1
-bytecode array length: 147
+bytecode array length: 144
 bytecodes: [
                 B(SwitchOnGeneratorState), R(0), U8(0), U8(1),
                 B(Mov), R(closure), R(1),
@@ -34,9 +34,12 @@ bytecodes: [
                 B(LdaSmi), I8(1),
                 B(Star), R(1),
                 B(Mov), R(5), R(2),
-                B(Jump), U8(53),
-                B(Ldar), R(5),
-                B(Jump), U8(36),
+                B(Jump), U8(50),
+                B(LdaUndefined),
+                B(Star), R(2),
+                B(LdaSmi), I8(1),
+                B(Star), R(1),
+                B(Jump), U8(41),
                 B(Star), R(5),
                 B(CreateCatchContext), R(5), U8(3),
                 B(Star), R(4),
@@ -52,10 +55,6 @@ bytecodes: [
                 B(Star), R(2),
                 B(LdaSmi), I8(2),
                 B(Star), R(1),
-                B(Jump), U8(15),
-                B(LdaSmi), I8(-1),
-                B(Star), R(2),
-                B(Star), R(1),
                 B(Jump), U8(7),
                 B(Star), R(2),
                 B(LdaZero),
@@ -92,8 +91,8 @@ constant pool: [
   Smi [23],
 ]
 handlers: [
-  [19, 93, 101],
-  [22, 55, 59],
+  [19, 98, 98],
+  [22, 64, 64],
 ]
 
 ---
@@ -103,7 +102,7 @@ snippet: "
 "
 frame size: 8
 parameter count: 1
-bytecode array length: 192
+bytecode array length: 189
 bytecodes: [
                 B(SwitchOnGeneratorState), R(0), U8(0), U8(2),
                 B(Mov), R(closure), R(1),
@@ -123,7 +122,7 @@ bytecodes: [
                 B(LdaSmi), I8(1),
                 B(Star), R(1),
                 B(Mov), R(5), R(2),
-                B(Jump), U8(98),
+                B(Jump), U8(95),
   /*   22 S> */ B(LdaSmi), I8(42),
                 B(Star), R(6),
                 B(LdaFalse),
@@ -140,9 +139,12 @@ bytecodes: [
                 B(LdaSmi), I8(1),
                 B(Star), R(1),
                 B(Mov), R(5), R(2),
-                B(Jump), U8(53),
-                B(Ldar), R(5),
-                B(Jump), U8(36),
+                B(Jump), U8(50),
+                B(LdaUndefined),
+                B(Star), R(2),
+                B(LdaSmi), I8(1),
+                B(Star), R(1),
+                B(Jump), U8(41),
                 B(Star), R(5),
                 B(CreateCatchContext), R(5), U8(6),
                 B(Star), R(4),
@@ -158,10 +160,6 @@ bytecodes: [
                 B(Star), R(2),
                 B(LdaSmi), I8(2),
                 B(Star), R(1),
-                B(Jump), U8(15),
-                B(LdaSmi), I8(-1),
-                B(Star), R(2),
-                B(Star), R(1),
                 B(Jump), U8(7),
                 B(Star), R(2),
                 B(LdaZero),
@@ -201,8 +199,8 @@ constant pool: [
   Smi [23],
 ]
 handlers: [
-  [19, 138, 146],
-  [22, 100, 104],
+  [19, 143, 143],
+  [22, 109, 109],
 ]
 
 ---
@@ -212,7 +210,7 @@ snippet: "
 "
 frame size: 19
 parameter count: 1
-bytecode array length: 363
+bytecode array length: 362
 bytecodes: [
                 B(SwitchOnGeneratorState), R(0), U8(0), U8(2),
                 B(Mov), R(closure), R(4),
@@ -322,8 +320,12 @@ bytecodes: [
                 B(LdaSmi), I8(1),
                 B(Star), R(4),
                 B(Mov), R(12), R(5),
-                B(Jump), U8(51),
-                B(Jump), U8(36),
+                B(Jump), U8(50),
+                B(LdaUndefined),
+                B(Star), R(5),
+                B(LdaSmi), I8(1),
+                B(Star), R(4),
+                B(Jump), U8(41),
                 B(Star), R(8),
                 B(CreateCatchContext), R(8), U8(14),
                 B(Star), R(7),
@@ -339,10 +341,6 @@ bytecodes: [
                 B(Star), R(5),
                 B(LdaSmi), I8(2),
                 B(Star), R(4),
-                B(Jump), U8(15),
-                B(LdaSmi), I8(-1),
-                B(Star), R(5),
-                B(Star), R(4),
                 B(Jump), U8(7),
                 B(Star), R(5),
                 B(LdaZero),
@@ -385,14 +383,14 @@ constant pool: [
   Smi [6],
   Smi [9],
   SCOPE_INFO_TYPE,
-  Smi [269],
+  Smi [268],
   Smi [6],
   Smi [9],
   Smi [23],
 ]
 handlers: [
-  [19, 309, 317],
-  [22, 273, 275],
+  [19, 316, 316],
+  [22, 282, 282],
   [86, 173, 181],
   [205, 238, 240],
 ]
@@ -405,7 +403,7 @@ snippet: "
 "
 frame size: 17
 parameter count: 1
-bytecode array length: 466
+bytecode array length: 463
 bytecodes: [
                 B(SwitchOnGeneratorState), R(0), U8(0), U8(5),
                 B(Mov), R(closure), R(1),
@@ -473,7 +471,7 @@ bytecodes: [
                 B(LdaSmi), I8(1),
                 B(Star), R(1),
                 B(Mov), R(10), R(2),
-                B(Jump), U8(241),
+                B(Jump), U8(238),
                 B(LdaNamedProperty), R(7), U8(14), U8(20),
                 B(JumpIfUndefinedOrNull), U8(11),
                 B(Star), R(12),
@@ -541,9 +539,12 @@ bytecodes: [
                 B(LdaSmi), I8(1),
                 B(Star), R(1),
                 B(Mov), R(7), R(2),
-                B(Jump), U8(53),
-                B(Ldar), R(7),
-                B(Jump), U8(36),
+                B(Jump), U8(50),
+                B(LdaUndefined),
+                B(Star), R(2),
+                B(LdaSmi), I8(1),
+                B(Star), R(1),
+                B(Jump), U8(41),
                 B(Star), R(5),
                 B(CreateCatchContext), R(5), U8(17),
                 B(Star), R(4),
@@ -559,10 +560,6 @@ bytecodes: [
                 B(Star), R(2),
                 B(LdaSmi), I8(2),
                 B(Star), R(1),
-                B(Jump), U8(15),
-                B(LdaSmi), I8(-1),
-                B(Star), R(2),
-                B(Star), R(1),
                 B(Jump), U8(7),
                 B(Star), R(2),
                 B(LdaZero),
@@ -608,13 +605,13 @@ constant pool: [
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["done"],
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["value"],
   SCOPE_INFO_TYPE,
-  Smi [372],
+  Smi [369],
   Smi [6],
   Smi [9],
   Smi [23],
 ]
 handlers: [
-  [19, 412, 420],
-  [22, 374, 378],
+  [19, 417, 417],
+  [22, 383, 383],
 ]
 

test/mjsunit/harmony/async-generators-throw-caught.js

+// Copyright 2020 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.
+
+let caught_in_gen = false;
+async function* catch_gen() {
+  try {
+    yield 42;
+  } catch (e) {
+    caught_in_gen = true;
+  }
+}
+
+(async () => {
+  const g = catch_gen();
+  await g.next();
+  try {
+    await g.throw(new Error()); // Should be caught in catch_gen, then catch_gen
+                                // completes normally.
+  } catch (e) {
+    assertUnreachable();
+  }
+  assertTrue(caught_in_gen);
+})();