Compile AOT. (#4250)

* Compile AOT.

* Address review comments.

Author: davidmorgan

build_runner/CHANGELOG.md

@@ -1,11 +1,17 @@
 ## 2.9.1-wip
 
-- Require `analyzer` 8.0.0. Remove use of deprecated `analyzer` members, use
-  their recommended and compatible replacements.
-- Internal changes for `build_test`.
+- Add AOT compilation of builders. A future release will AOT compile builders
+  automatically, for this release it's behind a flag. AOT compiled builders
+  start up faster and have higher throughput, for faster builds overall.
+  Builders that use `dart:mirrors` cannot be AOT compiled.
+- Add `force-aot` flag to AOT compile builders.
+- Add `force-jit` flag to force the current default of JIT compiling builders.
 - Add the `--dart-jit-vm-arg` option. Its values are passed to `dart run` when
   a build script is started in JIT mode. This allows specifying options to
   attach a debugger to builders.
+- Require `analyzer` 8.0.0. Remove use of deprecated `analyzer` members, use
+  their recommended and compatible replacements.
+- Internal changes for `build_test`.
 
 ## 2.9.0
 

build_runner/lib/src/bootstrap/aot_compiler.dart

@@ -0,0 +1,74 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+
+import '../constants.dart';
+import 'compiler.dart';
+import 'depfile.dart';
+import 'processes.dart';
+
+const entrypointAotPath = '$entrypointScriptPath.aot';
+const entrypointAotDepfilePath = '$entrypointScriptPath.aot.deps';
+const entrypointAotDigestPath = '$entrypointScriptPath.aot.digest';
+
+/// Compiles the build script to an AOT snapshot.
+class AotCompiler implements Compiler {
+  final Depfile _outputDepfile = Depfile(
+    outputPath: entrypointAotPath,
+    depfilePath: entrypointAotDepfilePath,
+    digestPath: entrypointAotDigestPath,
+  );
+
+  @override
+  FreshnessResult checkFreshness({required bool digestsAreFresh}) =>
+      _outputDepfile.checkFreshness(digestsAreFresh: digestsAreFresh);
+
+  @override
+  bool isDependency(String path) => _outputDepfile.isDependency(path);
+
+  @override
+  Future<CompileResult> compile({Iterable<String>? experiments}) async {
+    final dart = Platform.resolvedExecutable;
+    final result = await ParentProcess.run(dart, [
+      'compile',
+      'aot-snapshot',
+      entrypointScriptPath,
+      '--output',
+      entrypointAotPath,
+      '--depfile',
+      entrypointAotDepfilePath,
+      if (experiments != null)
+        for (final experiment in experiments) '--enable-experiment=$experiment',
+    ]);
+
+    if (result.exitCode == 0) {
+      final stdout = result.stdout as String;
+
+      // Convert "unknown experiment" warnings to errors.
+      if (stdout.contains('Unknown experiment:')) {
+        if (File(entrypointAotPath).existsSync()) {
+          File(entrypointAotPath).deleteSync();
+        }
+        final messages = stdout
+            .split('\n')
+            .where((e) => e.startsWith('Unknown experiment'))
+            .join('\n');
+        return CompileResult(messages: messages);
+      }
+
+      // Update depfile digest on successful compile.
+      _outputDepfile.writeDigest();
+    }
+
+    var stderr = result.stderr as String;
+    // Tidy up the compiler output to leave just the failure.
+    stderr =
+        stderr
+            .replaceAll('Error: AOT compilation failed', '')
+            .replaceAll('Bad state: Generating AOT snapshot failed!', '')
+            .trim();
+    return CompileResult(messages: result.exitCode == 0 ? null : stderr);
+  }
+}

build_runner/lib/src/bootstrap/bootstrapper.dart

@@ -2,13 +2,16 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:async';
 import 'dart:io';
 
 import 'package:built_collection/built_collection.dart';
 import 'package:io/io.dart';
 
 import '../exceptions.dart';
 import '../internal.dart';
+import 'aot_compiler.dart';
+import 'compiler.dart';
 import 'depfile.dart';
 import 'kernel_compiler.dart';
 import 'processes.dart';
@@ -24,9 +27,14 @@ import 'processes.dart';
 /// contents of all these files so they can be checked for freshness later.
 ///
 /// The entrypoint script is launched using [ParentProcess.runAndSend]
-/// which passes initial state to it and received updated state when it exits.
+/// or [ParentProcess.runAotSnapshotAndSend] which passes initial state to it
+/// and receives updated state when it exits.
 class Bootstrapper {
-  final KernelCompiler _compiler = KernelCompiler();
+  final bool compileAot;
+  final Compiler _compiler;
+
+  Bootstrapper({required this.compileAot})
+    : _compiler = compileAot ? AotCompiler() : KernelCompiler();
 
   /// Generates the entrypoint script, compiles it and runs it with [arguments].
   ///
@@ -52,24 +60,46 @@ class Bootstrapper {
       if (!_compiler.checkFreshness(digestsAreFresh: false).outputIsFresh) {
         final result = await _compiler.compile(experiments: experiments);
         if (!result.succeeded) {
-          if (result.messages != null) {
+          final bool failedDueToMirrors;
+          if (result.messages == null) {
+            failedDueToMirrors = false;
+          } else {
             buildLog.error(result.messages!);
+            failedDueToMirrors =
+                compileAot && result.messages!.contains('dart:mirrors');
+          }
+          if (failedDueToMirrors) {
+            // TODO(davidmorgan): when build_runner manages use of AOT compile
+            // this will be an automatic fallback to JIT instead of a message.
+            buildLog.error(
+              'Failed to compile build script. A configured builder '
+              'uses `dart:mirrors` and cannot be compiled AOT. Try again '
+              'without --force-aot to use a JIT compile.',
+            );
+          } else {
+            buildLog.error(
+              'Failed to compile build script. '
+              'Check builder definitions and generated script '
+              '$entrypointScriptPath.',
+            );
           }
-          buildLog.error(
-            'Failed to compile build script. '
-            'Check builder definitions and generated script '
-            '$entrypointScriptPath.',
-          );
           throw const CannotBuildException();
         }
       }
 
-      final result = await ParentProcess.runAndSend(
-        script: entrypointDillPath,
-        arguments: arguments,
-        message: buildProcessState.serialize(),
-        jitVmArgs: jitVmArgs,
-      );
+      final result =
+          compileAot
+              ? await ParentProcess.runAotSnapshotAndSend(
+                aotSnapshot: entrypointAotPath,
+                arguments: arguments,
+                message: buildProcessState.serialize(),
+              )
+              : await ParentProcess.runAndSend(
+                script: entrypointDillPath,
+                arguments: arguments,
+                message: buildProcessState.serialize(),
+                jitVmArgs: jitVmArgs,
+              );
       buildProcessState.deserializeAndSet(result.message);
       final exitCode = result.exitCode;
 
@@ -94,11 +124,11 @@ class Bootstrapper {
     }
   }
 
-  /// Checks freshness of the entrypoint script compiled to kernel.
+  /// Checks freshness of the compiled entrypoint script.
   ///
   /// Set [digestsAreFresh] if digests were very recently updated. Then, they
   /// will be re-used from disk if possible instead of recomputed.
-  Future<FreshnessResult> checkKernelFreshness({
+  Future<FreshnessResult> checkCompileFreshness({
     required bool digestsAreFresh,
   }) async {
     if (!ChildProcess.isRunning) {
@@ -116,9 +146,8 @@ class Bootstrapper {
     return _compiler.checkFreshness(digestsAreFresh: false);
   }
 
-  /// Whether [path] is a dependency of the entrypoint script compiled to
-  /// kernel.
-  bool isKernelDependency(String path) {
+  /// Whether [path] is a dependency of the compiled entrypoint script.
+  bool isCompileDependency(String path) {
     if (!ChildProcess.isRunning) {
       // Any real use or realistic test has a child process; so this is only hit
       // in small tests. Return "not a dependency" so nothing related to

build_runner/lib/src/bootstrap/build_process_state.dart

@@ -45,6 +45,11 @@ class BuildProcessState {
   set elapsedMillis(int elapsedMillis) =>
       _state['elapsedMillis'] = elapsedMillis;
 
+  /// The package config URI.
+  String get packageConfigUri =>
+      (_state['packageConfigUri'] ??= Isolate.packageConfigSync!.toString())
+          as String;
+
   void resetForTests() {
     _state.clear();
   }
@@ -55,6 +60,10 @@ class BuildProcessState {
   }
 
   String serialize() {
+    // Set any unset values that should be set by the parent process.
+    stdio;
+    packageConfigUri;
+
     for (final beforeSend in _beforeSends) {
       beforeSend();
     }

build_runner/lib/src/bootstrap/compiler.dart

@@ -0,0 +1,34 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'depfile.dart';
+
+/// Compiles the build script.
+abstract class Compiler {
+  /// Checks freshness of the build script compile output.
+  ///
+  /// Set [digestsAreFresh] if digests were very recently updated. Then, they
+  /// will be re-used from disk if possible instead of recomputed.
+  FreshnessResult checkFreshness({required bool digestsAreFresh});
+
+  /// Checks whether [path] in a dependency of the build script compile.
+  ///
+  /// Call [checkFreshness] first to load the depfile.
+  bool isDependency(String path);
+
+  /// Compiles the entrypoint script.
+  Future<CompileResult> compile({Iterable<String>? experiments});
+}
+
+class CompileResult {
+  final String? messages;
+
+  CompileResult({required this.messages});
+
+  bool get succeeded => messages == null;
+
+  @override
+  String toString() =>
+      'CompileResult(succeeded: $succeeded, messages: $messages)';
+}

build_runner/lib/src/bootstrap/kernel_compiler.dart

@@ -5,6 +5,7 @@
 import 'dart:io';
 
 import '../constants.dart';
+import 'compiler.dart';
 import 'depfile.dart';
 import 'processes.dart';
 
@@ -13,26 +14,21 @@ const entrypointDillDepfilePath = '$entrypointScriptPath.dill.deps';
 const entrypointDillDigestPath = '$entrypointScriptPath.dill.digest';
 
 /// Compiles the build script to kernel.
-class KernelCompiler {
+class KernelCompiler implements Compiler {
   final Depfile _outputDepfile = Depfile(
     outputPath: entrypointDillPath,
     depfilePath: entrypointDillDepfilePath,
     digestPath: entrypointDillDigestPath,
   );
 
-  /// Checks freshness of the build script compiled kernel.
-  ///
-  /// Set [digestsAreFresh] if digests were very recently updated. Then, they
-  /// will be re-used from disk if possible instead of recomputed.
+  @override
   FreshnessResult checkFreshness({required bool digestsAreFresh}) =>
       _outputDepfile.checkFreshness(digestsAreFresh: digestsAreFresh);
 
-  /// Checks whether [path] in a dependency of the build script compiled kernel.
-  ///
-  /// Call [checkFreshness] first to load the depfile.
+  @override
   bool isDependency(String path) => _outputDepfile.isDependency(path);
 
-  /// Compiles the entrypoint script to kernel.
+  @override
   Future<CompileResult> compile({Iterable<String>? experiments}) async {
     final dart = Platform.resolvedExecutable;
     final result = await ParentProcess.run(dart, [
@@ -58,8 +54,7 @@ class KernelCompiler {
         final messages = stdout
             .split('\n')
             .where((e) => e.startsWith('Unknown experiment'))
-            .map((l) => '$l\n')
-            .join('');
+            .join('\n');
         return CompileResult(messages: messages);
       }
 
@@ -74,15 +69,3 @@ class KernelCompiler {
     return CompileResult(messages: result.exitCode == 0 ? null : stderr);
   }
 }
-
-class CompileResult {
-  final String? messages;
-
-  CompileResult({required this.messages});
-
-  bool get succeeded => messages == null;
-
-  @override
-  String toString() =>
-      'CompileResult(succeeded: $succeeded, messages: $messages)';
-}