[dart2wasm] Convert simple async function bodies to sync Future.value calls

This transforms functions like

    async foo() => const ...;

to

    foo() => Future.value(const ...);

The transformation is done when the async function body is a `const`
expression or a basic literal (string, int, double, bool).

These expressions don't have side effects and they cannot throw, so it's
safe to convert them to `Future.value`s.

This makes the generated code in the ACX demo 2.5% smaller: (`-O4` with
symbol names removed)

- Before: 9,040,020 bytes
- After: 8,805,471 bytes
- Diff: -234,549 bytes, -2.59%

With this we also remove the same special case in the backend to avoid
generating a state machine for these functions, as the special case
handled before the backend now and backend never sees this kind of
functions.

(Technically with inlining or other backend optimizations it could still
see these cases, but the pattern it matches is too strict, and currently
the special case in the backend doesn't do anything on the ACX demo.)

Note: I tried implementing the same in dart2js's await lowering pass in
https://dart-review.googlesource.com/c/sdk/+/422061 and reusing that
pass in https://dart-review.googlesource.com/c/sdk/+/420140.

However while that transformed simple programs as expected, in ACX demo
it still introduced a lot of `Future.sync` calls and made the overall
binary larger. I think we never want to introduce `Future.sync` calls
(at least until we improve code size for closures, see relevant issue
#60458), so for now we don't
reuse dart2js's pass.

Issue: #60433
Change-Id: I206ac8c6081201041f67e7fc91776077e52180a0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/422120
Reviewed-by: Nate Biggs <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

pkg/dart2wasm/lib/async.dart

@@ -191,21 +191,7 @@ class AsyncStateMachineCodeGenerator extends StateMachineCodeGenerator {
   void generateInner(FunctionNode functionNode, Context? context) {
     // void Function(_AsyncSuspendState, Object?, Object?, StackTrace?)
 
-    // Special cases for function bodies with that directly return a constant
-    // expression or basic literal. These functions can't throw an exception,
-    // and they will have only one state so the `br_table` loop below can also
-    // be omitted.
-    //
-    // Note: these functions currently have two states instead of one, as we
-    // always generate a state for implicitly returning `null`. With #55939 we
-    // will be able to remove the implicit return state. After that we can check
-    // the function bodies to check whether we need exception handling, and
-    // special-case single-state state machines to omit the `br_table` loop.
     final functionBody = functionNode.body!;
-    final simpleReturn = _getSimpleReturn(functionBody);
-    if (simpleReturn != null && _handleSimpleReturn(simpleReturn)) {
-      return;
-    }
 
     // Set up locals for contexts and `this`.
     thisLocal = null;
@@ -434,48 +420,4 @@ class AsyncStateMachineCodeGenerator extends StateMachineCodeGenerator {
           b, _awaitValueLocal.type, translateType(awaitValueVar.type));
     });
   }
-
-  bool _handleSimpleReturn(Expression returnedExpression) {
-    if (returnedExpression is! BasicLiteral &&
-        returnedExpression is! ConstantExpression) {
-      return false;
-    }
-
-    b.local_get(_suspendStateLocal);
-    b.struct_get(
-        asyncSuspendStateInfo.struct, FieldIndex.asyncSuspendStateCompleter);
-    returnedExpression.accept1(
-      this,
-      asyncSuspendStateInfo.struct
-          .fields[FieldIndex.asyncSuspendStateCurrentReturnValue].type.unpacked,
-    );
-    call(translator.getFunctionEntry(translator.completerComplete.reference,
-        uncheckedEntry: true));
-    b.return_();
-    b.end(); // inner function
-    return true;
-  }
-}
-
-Expression? _getSimpleReturn(Statement functionBody) {
-  if (functionBody is ReturnStatement) {
-    if (functionBody.expression == null) {
-      return NullLiteral();
-    }
-    return functionBody.expression;
-  }
-
-  if (functionBody is Block) {
-    if (functionBody.statements.isEmpty) {
-      return NullLiteral();
-    }
-    if (functionBody.statements.length == 1) {
-      final statement = functionBody.statements.single;
-      if (statement is ReturnStatement) {
-        return statement.expression;
-      }
-    }
-  }
-
-  return null;
 }

pkg/dart2wasm/lib/transformers.dart

@@ -619,6 +619,34 @@ class _WasmTransformer extends Transformer {
         dartAsyncMarker: AsyncMarker.Sync);
   }
 
+  void _lowerAsync(FunctionNode functionNode) {
+    /*
+    Convert `async` functions with "simple" bodies to `sync` functions, using
+    `Future.value`.
+
+    "Simple" means: constant or basic literal. In general, this transformation
+    can be done on any function body that doesn't `await` and doesn't throw.
+
+    Example:
+
+    foo() async { return const ...; }
+    ==>
+    foo() { return Future.value(const ...); }
+    */
+    final functionBody = functionNode.body!;
+    final simpleReturn = _getSimpleReturn(functionBody);
+    if (simpleReturn is BasicLiteral || simpleReturn is ConstantExpression) {
+      final futureValueType = functionNode.emittedValueType!;
+      final newBody = ReturnStatement(StaticInvocation(
+        coreTypes.futureValueFactory,
+        Arguments([simpleReturn!], types: [futureValueType]),
+      ));
+      newBody.parent = functionNode;
+      functionNode.body = newBody;
+      functionNode.asyncMarker = AsyncMarker.Sync;
+    }
+  }
+
   @override
   TreeNode visitYieldStatement(YieldStatement yield) {
     // We currently ignore yields in 'sync*'.
@@ -716,17 +744,24 @@ class _WasmTransformer extends Transformer {
   @override
   TreeNode visitFunctionNode(FunctionNode functionNode) {
     final previousEnclosing = _enclosingIsAsyncStar;
-    if (functionNode.dartAsyncMarker == AsyncMarker.AsyncStar) {
+    final FunctionNode transformed;
+
+    if (functionNode.asyncMarker == AsyncMarker.AsyncStar) {
       _enclosingIsAsyncStar = true;
       functionNode = _lowerAsyncStar(functionNode) as FunctionNode;
       _enclosingIsAsyncStar = previousEnclosing;
-      return super.visitFunctionNode(functionNode);
+      transformed = super.visitFunctionNode(functionNode) as FunctionNode;
     } else {
       _enclosingIsAsyncStar = false;
-      TreeNode result = super.visitFunctionNode(functionNode);
+      transformed = super.visitFunctionNode(functionNode) as FunctionNode;
       _enclosingIsAsyncStar = previousEnclosing;
-      return result;
     }
+
+    if (transformed.asyncMarker == AsyncMarker.Async) {
+      _lowerAsync(transformed);
+    }
+
+    return transformed;
   }
 
   @override
@@ -1051,3 +1086,26 @@ class _VariableCollector extends RecursiveVisitor {
     variables.add(node.variable);
   }
 }
+
+Expression? _getSimpleReturn(Statement functionBody) {
+  if (functionBody is ReturnStatement) {
+    if (functionBody.expression == null) {
+      return NullLiteral();
+    }
+    return functionBody.expression;
+  }
+
+  if (functionBody is Block) {
+    if (functionBody.statements.isEmpty) {
+      return NullLiteral();
+    }
+    if (functionBody.statements.length == 1) {
+      final statement = functionBody.statements.single;
+      if (statement is ReturnStatement) {
+        return statement.expression;
+      }
+    }
+  }
+
+  return null;
+}