Add benchmark_base_perf.dart override of benchmark_harness that start…

…s and stops perf tool.

lib/benchmark_harness.dart

@@ -3,5 +3,6 @@
 // BSD-style license that can be found in the LICENSE file.
 
 export 'src/async_benchmark_base.dart';
-export 'src/benchmark_base.dart';
+export 'src/benchmark_base.dart'
+    if (dart.library.io) 'src/benchmark_base_perf.dart';
 export 'src/score_emitter.dart';

lib/src/benchmark_base.dart

@@ -38,6 +38,13 @@ class BenchmarkBase {
   /// Not measured teardown code executed after the benchmark runs.
   void teardown() {}
 
+  /// Not measured code run just before starting the timed runs
+  void beforeTimedRuns() {}
+
+  /// Not measured code run just after the timed runs finish.
+  /// Receives the total number of iterations run.
+  void afterTimedRuns(int totalIterations) {}
+
   /// Measures the score for this benchmark by executing it enough times
   /// to reach [minimumMillis].
   static _Measurement _measureForImpl(void Function() f, int minimumMillis) {
@@ -47,20 +54,22 @@ class BenchmarkBase {
     final allowedJitter =
         minimumMillis < 1000 ? 0 : (minimumMicros * 0.1).floor();
     var iter = 2;
+    var totalIterations = iter;
     final watch = Stopwatch()..start();
     while (true) {
       watch.reset();
       for (var i = 0; i < iter; i++) {
         f();
       }
       final elapsed = watch.elapsedMicroseconds;
-      final measurement = _Measurement(elapsed, iter);
+      final measurement = _Measurement(elapsed, iter, totalIterations);
       if (measurement.elapsedMicros >= (minimumMicros - allowedJitter)) {
         return measurement;
       }
 
       iter = measurement.estimateIterationsNeededToReach(
           minimumMicros: minimumMicros);
+      totalIterations += iter;
     }
   }
 
@@ -74,8 +83,10 @@ class BenchmarkBase {
     setup();
     // Warmup for at least 100ms. Discard result.
     _measureForImpl(warmup, 100);
+    beforeTimedRuns();
     // Run the benchmark for at least 2000ms.
     var result = _measureForImpl(exercise, _minimumMeasureDurationMillis);
+    afterTimedRuns(result.totalIterations);
     teardown();
     return result.score;
   }
@@ -88,8 +99,9 @@ class BenchmarkBase {
 class _Measurement {
   final int elapsedMicros;
   final int iterations;
+  final int totalIterations;
 
-  _Measurement(this.elapsedMicros, this.iterations);
+  _Measurement(this.elapsedMicros, this.iterations, this.totalIterations);
 
   double get score => elapsedMicros / iterations;
 

lib/src/benchmark_base_perf.dart

@@ -0,0 +1,37 @@
+// Copyright (c) 2023, 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 'benchmark_base.dart' as base;
+import 'score_emitter.dart';
+
+const perfControlFifoVariable = 'PERF_CONTROL_FIFO';
+
+class BenchmarkBase extends base.BenchmarkBase {
+  BenchmarkBase(super.name, {super.emitter = const PrintEmitter()});
+
+  String? perfControlFifo;
+  late RandomAccessFile openedFifo;
+
+  @override
+  void beforeTimedRuns() {
+    perfControlFifo = Platform.environment[perfControlFifoVariable];
+    if (perfControlFifo != null) {
+      openedFifo = File(perfControlFifo!).openSync();
+      openedFifo.writeStringSync('enable\n');
+      // TODO: read 'ack\n' from second ack fifo, before proceeding.
+    }
+  }
+
+  @override
+  void afterTimedRuns(int totalIterations) {
+    if (perfControlFifo != null) {
+      openedFifo.writeStringSync('disable\n');
+    }
+    // TODO: await ack.
+    openedFifo.closeSync();
+    emitter.emit('$name.totalIterations', totalIterations.toDouble());
+  }
+}