[dart2wasm] Complete async fun futures directly, instead of via completer

In `async` functions, instead of completing the function's `Future` via
`Completer`, do it directly.

This should be slightly more efficient as we eliminate a layer of
indirection when completing.

Move reading the `_future` field of `_AsyncSuspendState` to the
completion functions, to avoid adding a `struct.get`s at each call site.

In ACX demo, makes the final binary 0.1% smaller (13,689 bytes).

Fixes #60719.

Issue: #60719
CoreLibraryReviewExempt: Wasm-specific change.
Change-Id: I27b376eb2fb9c3705ee930fb33b06d9accfd14b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429000
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

pkg/dart2wasm/lib/async.dart

@@ -41,9 +41,9 @@ mixin AsyncCodeGeneratorMixin on StateMachineEntryAstCodeGenerator {
       b.ref_null(w.HeapType.struct);
     }
 
-    // _AsyncCompleter _completer
+    // _Future _future
     types.makeType(this, functionNode.emittedValueType!);
-    call(translator.makeAsyncCompleter.reference);
+    call(translator.makeFuture.reference);
 
     // Allocate `_AsyncSuspendState`
     call(translator.newAsyncSuspendState.reference);
@@ -61,18 +61,11 @@ mixin AsyncCodeGeneratorMixin on StateMachineEntryAstCodeGenerator {
     translator.callFunction(resumeFun, b);
     b.drop(); // drop null
 
-    // (3) Return the completer's future.
+    // (3) Return the future.
 
     b.local_get(asyncStateLocal);
-    final completerFutureGetterType = translator
-        .signatureForDirectCall(translator.completerFuture.getterReference);
     b.struct_get(
-        asyncSuspendStateInfo.struct, FieldIndex.asyncSuspendStateCompleter);
-    translator.convertType(
-        b,
-        asyncSuspendStateInfo.struct.fields[5].type.unpacked,
-        completerFutureGetterType.inputs[0]);
-    call(translator.completerFuture.getterReference);
+        asyncSuspendStateInfo.struct, FieldIndex.asyncSuspendStateFuture);
     b.return_();
     b.end();
 
@@ -179,10 +172,9 @@ class AsyncStateMachineCodeGenerator extends StateMachineCodeGenerator {
   @override
   void emitReturn(void Function() emitValue) {
     b.local_get(_suspendStateLocal);
-    b.struct_get(
-        asyncSuspendStateInfo.struct, FieldIndex.asyncSuspendStateCompleter);
     emitValue();
-    call(translator.getFunctionEntry(translator.completerComplete.reference,
+    call(translator.getFunctionEntry(
+        translator.asyncSuspendStateComplete.reference,
         uncheckedEntry: true));
     b.return_();
   }
@@ -262,10 +254,9 @@ class AsyncStateMachineCodeGenerator extends StateMachineCodeGenerator {
     // Final state: return.
     emitTargetLabel(targets.last);
     b.local_get(_suspendStateLocal);
-    b.struct_get(
-        asyncSuspendStateInfo.struct, FieldIndex.asyncSuspendStateCompleter);
     b.ref_null(translator.topInfo.struct);
-    call(translator.getFunctionEntry(translator.completerComplete.reference,
+    call(translator.getFunctionEntry(
+        translator.asyncSuspendStateComplete.reference,
         uncheckedEntry: true));
     b.return_();
     b.end(); // masterLoop
@@ -277,20 +268,16 @@ class AsyncStateMachineCodeGenerator extends StateMachineCodeGenerator {
 
     void callCompleteError() {
       b.local_get(_suspendStateLocal);
-      b.struct_get(
-          asyncSuspendStateInfo.struct, FieldIndex.asyncSuspendStateCompleter);
       b.local_get(exceptionLocal);
       b.local_get(stackTraceLocal);
-      call(translator.completerCompleteError.reference);
+      call(translator.asyncSuspendStateCompleteError.reference);
       b.return_();
     }
 
     void callCompleteErrorWithCurrentStack() {
       b.local_get(_suspendStateLocal);
-      b.struct_get(
-          asyncSuspendStateInfo.struct, FieldIndex.asyncSuspendStateCompleter);
       b.local_get(exceptionLocal);
-      call(translator.completerCompleteErrorWithCurrentStack.reference);
+      call(translator.asyncSuspendStateCompleteErrorWithCurrentStack.reference);
       b.return_();
     }
 

pkg/dart2wasm/lib/class_info.dart

@@ -21,7 +21,7 @@ class FieldIndex {
   static const asyncSuspendStateResume = 2;
   static const asyncSuspendStateContext = 3;
   static const asyncSuspendStateTargetIndex = 4;
-  static const asyncSuspendStateCompleter = 5;
+  static const asyncSuspendStateFuture = 5;
   static const asyncSuspendStateCurrentException = 6;
   static const asyncSuspendStateCurrentExceptionStackTrace = 7;
   static const asyncSuspendStateCurrentReturnValue = 8;
@@ -95,8 +95,8 @@ class FieldIndex {
         FieldIndex.asyncSuspendStateContext);
     check(translator.asyncSuspendStateClass, "_targetIndex",
         FieldIndex.asyncSuspendStateTargetIndex);
-    check(translator.asyncSuspendStateClass, "_completer",
-        FieldIndex.asyncSuspendStateCompleter);
+    check(translator.asyncSuspendStateClass, "_future",
+        FieldIndex.asyncSuspendStateFuture);
     check(translator.asyncSuspendStateClass, "_currentException",
         FieldIndex.asyncSuspendStateCurrentException);
     check(translator.asyncSuspendStateClass, "_currentExceptionStackTrace",

pkg/dart2wasm/lib/kernel_nodes.dart

@@ -141,24 +141,23 @@ mixin KernelNodes {
   // async support classes
   late final Class asyncSuspendStateClass =
       index.getClass("dart:async", "_AsyncSuspendState");
-  late final Procedure makeAsyncCompleter =
-      index.getTopLevelProcedure("dart:async", "_makeAsyncCompleter");
-  late final Field completerFuture =
-      index.getField("dart:async", "_Completer", "future");
-  late final Procedure completerComplete =
-      index.getProcedure("dart:async", "_AsyncCompleter", "complete");
-  late final Procedure completerCompleteErrorWithCurrentStack =
-      index.getProcedure(
-          "dart:async", "_AsyncCompleter", "_completeErrorWithCurrentStack");
-  late final Procedure completerCompleteError =
-      index.getProcedure("dart:async", "_Completer", "completeError");
   late final Procedure awaitHelper =
       index.getTopLevelProcedure("dart:async", "_awaitHelper");
   late final Procedure awaitHelperWithTypeCheck =
       index.getTopLevelProcedure("dart:async", "_awaitHelperWithTypeCheck");
   late final Procedure newAsyncSuspendState =
       index.getTopLevelProcedure("dart:async", "_newAsyncSuspendState");
 
+  late final Procedure asyncSuspendStateComplete =
+      index.getProcedure("dart:async", "_AsyncSuspendState", "_complete");
+  late final Procedure asyncSuspendStateCompleteError =
+      index.getProcedure("dart:async", "_AsyncSuspendState", "_completeError");
+  late final Procedure asyncSuspendStateCompleteErrorWithCurrentStack =
+      index.getProcedure(
+          "dart:async", "_AsyncSuspendState", "_completeErrorWithCurrentStack");
+  late final Procedure makeFuture =
+      index.getTopLevelProcedure("dart:async", "_makeFuture");
+
   // dart:ffi classes
   late final Class ffiPointerClass = index.getClass("dart:ffi", "Pointer");
 

sdk/lib/_internal/wasm/lib/async_patch.dart

@@ -41,10 +41,9 @@ class _AsyncSuspendState {
   @pragma("wasm:entry-point")
   WasmI32 _targetIndex;
 
-  // The completer. The inner function calls `_completer.complete` or
-  // `_completer.onError` on completion.
+  // The future that will be completed.
   @pragma("wasm:entry-point")
-  final _AsyncCompleter _completer;
+  final _Future _future;
 
   // When a called function throws this stores the thrown exception. Used when
   // performing type tests in catch blocks.
@@ -63,11 +62,26 @@ class _AsyncSuspendState {
   Object? _currentReturnValue;
 
   @pragma("wasm:entry-point")
-  _AsyncSuspendState(this._resume, this._context, this._completer)
+  _AsyncSuspendState(this._resume, this._context, this._future)
     : _targetIndex = WasmI32.fromInt(0),
       _currentException = null,
       _currentExceptionStackTrace = null,
       _currentReturnValue = null;
+
+  @pragma("wasm:entry-point")
+  void _complete(FutureOr value) {
+    _future._asyncComplete(value == null ? value as dynamic : value);
+  }
+
+  @pragma("wasm:entry-point")
+  void _completeError(Object error, StackTrace stackTrace) {
+    _future._asyncCompleteError(error, stackTrace);
+  }
+
+  @pragma("wasm:entry-point")
+  void _completeErrorWithCurrentStack(Object error) {
+    _future._asyncCompleteError(error, StackTrace.current);
+  }
 }
 
 // Note: [_AsyncCompleter] is taken as an argument to be able to pass the type
@@ -78,20 +92,11 @@ class _AsyncSuspendState {
 _AsyncSuspendState _newAsyncSuspendState(
   _AsyncResumeFun resume,
   WasmStructRef? context,
-  _AsyncCompleter completer,
-) => _AsyncSuspendState(resume, context, completer);
-
-@pragma("wasm:entry-point")
-_AsyncCompleter<T> _makeAsyncCompleter<T>() => _AsyncCompleter<T>();
+  _Future future,
+) => _AsyncSuspendState(resume, context, future);
 
-@patch
 @pragma("wasm:entry-point")
-class _AsyncCompleter<T> extends _Completer<T> {
-  @pragma("wasm:entry-point")
-  void _completeErrorWithCurrentStack(Object error) {
-    completeError(error, StackTrace.current);
-  }
-}
+_Future<T> _makeFuture<T>() => _Future<T>();
 
 @pragma("wasm:entry-point")
 void _awaitHelper(_AsyncSuspendState suspendState, Future operand) {

sdk/lib/_internal/wasm/lib/errors_patch.dart

@@ -24,6 +24,7 @@ class Error {
   @pragma("wasm:entry-point")
   StackTrace? _stackTrace;
 
+  @pragma("wasm:entry-point")
   static void _trySetStackTrace(Object object, StackTrace stackTrace) {
     // Guard against implementors of [Error] that do not have the stack trace
     // field by ensuring the error object is a direct/indirect subclass.

sdk/lib/async/future_impl.dart

@@ -71,14 +71,12 @@ AsyncError _interceptUserError(Object error, StackTrace? stackTrace) {
 }
 
 abstract class _Completer<T> implements Completer<T> {
-  @pragma("wasm:entry-point")
   @pragma("vm:entry-point")
   final _Future<T> future = _Future<T>();
 
   // Overridden by either a synchronous or asynchronous implementation.
   void complete([FutureOr<T>? value]);
 
-  @pragma("wasm:entry-point")
   void completeError(Object error, [StackTrace? stackTrace]) {
     if (!future._mayComplete) throw StateError("Future already completed");
     _completeErrorObject(_interceptUserError(error, stackTrace));
@@ -95,7 +93,6 @@ abstract class _Completer<T> implements Completer<T> {
 /// Completer which completes future asynchronously.
 @pragma("vm:entry-point")
 class _AsyncCompleter<T> extends _Completer<T> {
-  @pragma("wasm:entry-point")
   void complete([FutureOr<T>? value]) {
     if (!future._mayComplete) throw StateError("Future already completed");
     future._asyncComplete(value == null ? value as dynamic : value);
@@ -280,7 +277,6 @@ class _FutureListener<S, T> {
   bool shouldChain(Future<dynamic> value) => value is Future<T> || value is! T;
 }
 
-@pragma("wasm:entry-point")
 class _Future<T> implements Future<T> {
   /// Initial state, waiting for a result. In this state, the
   /// [_resultOrListeners] field holds a single-linked list of