Strip out debug fields to reduce memory footprint

[goderbauer]

The Flutter framework defines many debug* fields that are only used during debug builds. They are still present in release builds, but mostly unused there. While they don't take up any significant space in the snapshot (8B for hello_world, 6.4KB for flutter_gallery) they may significantly increase our memory footprint when the app runs on device.

We should measure what the impact on memory usage is and if it is significant strip those fields out in debug mode - either with a kernel transformer or by extending Dart's treeshakeing-algorithm to remove those unused fields.

/cc @jacob314

[mraleph]

@xster split-debug-info is for splitting debug information like information used to format stack traces. This bug was about shaking away debug only fields. FWIW @alexmarkov has implemented an optimization that shakes fields which are never read (even if they are written to) - and we know that it shakes away debug only fields in widget hierarchy.

[xster]

ah I misunderstood. These are for the actual Dart functions in the framework implementation like debugFillProperties, debugPaintSize etc.

Sorry, I totally misunderstood the OP.

[a-siva]

/cc @alexmarkov

[alexmarkov]

Indeed, Dart AOT compiler can tree shake fields if they are never read.

In order to verify how well this optimization works for Flutter debug fields, I looked at debug fields retained on Flutter Gallery (https://github.com/flutter/gallery @ flutter/gallery@c858388), flutter @ 6cb4357.

The following simple program dumps all fields which start with debug or _debug in the kernel file:

import 'dart:io';

import 'package:kernel/ast.dart';
import 'package:kernel/binary/ast_from_binary.dart' show BinaryBuilder;

class ListDebugFields extends RecursiveVisitor {
  @override
  visitField(Field node) {
    if (node.name.text.startsWith('debug') ||
        node.name.text.startsWith('_debug')) {
      final cls = node.enclosingClass;
      if (cls != null) {
        print('${cls.name}.${node.name.text}');
      } else {
        print('${node.enclosingLibrary.importUri} :: ${node.name.text}');
      }
    }
  }
}

main(List<String> args) async {
  final component = new Component();
  final List<int> bytes = new File(args[0]).readAsBytesSync();
  new BinaryBuilder(bytes).readComponent(component);

  ListDebugFields().visitComponent(component);
}

In debug mode, Flutter Gallery app.dill contains 230 debug fields.
In release mode, tree-shaken Flutter Gallery app.dill contains only 35 debug fields. So most of the debug fields are actually removed.

I grepped retained debug fields and found that 32 of those fields are actually used in release mode:

_Channel.debugEnableDiscardWarnings
  // Used in _Channel.push

AnimationController.debugLabel
  // Used in AnimationController.toStringDetails

CupertinoDynamicColor._debugLabel
  // Used in CupertinoDynamicColor.resolveFrom

CupertinoDynamicColor._debugResolveContext
  // Used in CupertinoDynamicColor.toString

_IsolateConfiguration.debugLabel
  // Used in _IsolateConfiguration._spawn

MaterialApp.debugShowCheckedModeBanner
  // Used in _MaterialAppState._buildWidgetApp

ImageInfo.debugLabel
  // Used in ImageInfo.clone

ImageStreamCompleter.debugLabel
  // Used in MultiFrameImageStreamCompleter._handleAppFrame

TextStyle.debugLabel
  // Used in StrutStyle.fromTextStyle

RenderImage.debugImageLabel
  // Used in RenderImage.paint

RenderObject._debugDisposed
  // Used in RenderObject.toStringShort

RenderObject.debugCreator
  // Used in RenderObject._debugReportException

_FrameCallbackEntry.debugStack
  // Used in SchedulerBinding.handleBeginFrame

RestorationBucket._debugOwner
  // Used in RestorationBucket.debugOwner

RawImage.debugImageLabel
  // Used in RawImage.createRenderObject

RenderObjectToWidgetAdapter.debugShortDescription
  // Used in RenderObjectToWidgetAdapter.toStringShort

EditableText.debugDeterministicCursor
  // Used in EditableTextState._startCursorTimer

FocusNode._debugLabel
  // Used in FocusNode.debugLabel

Focus.debugLabel
  // Used in _FocusState._createNode

LabeledGlobalKey._debugLabel
  // Used in LabeledGlobalKey.toString

ScrollController.debugLabel
  // Used in ScrollController.createScrollPosition

ScrollPosition.debugLabel
  // Used in ScrollPosition.debugFillDescription

TextSelectionOverlay.debugRequiredFor
  // Used in TextSelectionOverlay.showHandles

HighlightFocus.debugLabel
  // Used in _HighlightFocusState.build

package:flutter/src/foundation/debug.dart :: debugDoublePrecision
  // Used in debugFormatDouble

package:flutter/src/foundation/platform.dart :: debugDefaultTargetPlatformOverride
  // Used in defaultTargetPlatform

package:flutter/src/foundation/print.dart :: debugPrint
  // Used in debugPrintStack

package:flutter/src/foundation/print.dart :: _debugPrintedCharacters
  // Used in _debugPrintTask

package:flutter/src/foundation/print.dart :: _debugPrintBuffer
  // Used in debugPrintThrottled

package:flutter/src/foundation/print.dart :: _debugPrintStopwatch
  // Used in _debugPrintTask

package:flutter/src/foundation/print.dart :: _debugPrintCompleter
  // Used in _debugPrintTask

package:flutter/src/foundation/print.dart :: _debugPrintScheduled
  // Used in debugPrintThrottled

It is possible to change Flutter code to avoid these uses (for example, their uses can be guarded with kDebugMode flag).

The following 3 retained debug fields don't have uses in release mode:

BuildOwner._debugIllFatedElements
BuildOwner._debugGlobalKeyReservations
Element._debugForgottenChildrenWithGlobalKey

These fields are retained because they have initializers and constructors of their classes are retained.
Filed dart-lang/sdk#46402 to improve tree shaking to be able to remove these fields.

[alexmarkov]

AOT tree shaker was improved to handle instance fields with initializers at dart-lang/sdk@9c190c5, and tree shaker now removes 3 debug fields which are not used in release mode.

The remaining 32 debug fields are not eliminated because they are still used in release mode.

[jacob314]

Fyi @kenzieschmoll. This would be a good use case for the code size tooling. None of the remaining 32 debug fields should be included in release mode but likely there are bugs in Flutter that result in the fields surviving in release mode.

[mraleph]

Fyi @kenzieschmoll. This would be a good use case for the code size tooling.

FWIW we don't really write enough information into either snapshot profile or precompiler trace to connect a retained field back to its uses.

[mraleph]

@gintominto5329 anything specific you are asking about?

[mraleph]

Based on @alexmarkov comments above I think there is nothing planned on the Dart side and all treeshakable fields are now actually treeshaken.

[github-actions]

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