add gn flag to optimize builds for size #176835

planetmarshall · 2025-10-10T15:27:30Z

Adds a gn flag to optimize builds for size over speed when optimization is requested. The existing behaviour is that Windows, Android and iOS builds are always optimized for size. This has not changed.

The flag allows other builds to prefer size over speed.

Closes #176685

Pre-launch Checklist

I read the Contributor Guide and followed the process outlined there for submitting PRs.
I read the Tree Hygiene wiki page, which explains my responsibilities.
I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
I signed the CLA.
I listed at least one issue that this PR fixes in the description above.
I updated/added relevant documentation (doc comments with ///).

flutter-dashboard · 2025-10-10T15:27:33Z

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gemini-code-assist

Code Review

This pull request introduces a new gn flag, optimize_for_size, to allow builds to be optimized for size instead of speed. This behavior is already the default for Windows, iOS, and Android, and this flag makes it available for other platforms. My review focuses on improving the clarity of comments and help messages related to this new flag, and using a more specific exception type for argument validation.

gemini-code-assist · 2025-10-10T15:29:03Z

engine/src/build/config/BUILDCONFIG.gn

+  # Optimize for size over speed in optimized builds. This is always true for
+  # Windows, ios, and Android.


This comment could be misinterpreted. It might suggest that this optimize_for_size variable is always true for Windows, iOS, and Android, which isn't the case. The size optimization for those platforms is handled separately. To improve clarity, I suggest rephrasing the comment to explain that this flag allows other platforms to opt into size optimization, which is the default behavior for Windows, iOS, and Android.

Suggested change

# Optimize for size over speed in optimized builds. This is always true for

# Windows, ios, and Android.

# When true, optimize for size over speed in optimized builds. This is the

# default behavior for Windows, iOS, and Android builds.

gemini-code-assist · 2025-10-10T15:29:03Z

engine/src/flutter/tools/gn

  gn_args['buildtools_path'] = '//flutter/buildtools'

+  if args.unoptimized and args.optimize_for_size:
+    raise Exception('--unoptimized and --optimize-for-size are mutually exclusive')


For better error specificity, it's preferable to raise a ValueError here instead of a generic Exception. ValueError is more idiomatic for indicating that an argument has an inappropriate value, which is the case when mutually exclusive flags are used together.¹

Suggested change

raise Exception('--unoptimized and --optimize-for-size are mutually exclusive')

raise ValueError('--unoptimized and --optimize-for-size are mutually exclusive')

Style Guide References

Footnotes

↩

Exception is the existing convention in the source. I have chosen not to depart from it.

gemini-code-assist · 2025-10-10T15:29:03Z

engine/src/flutter/tools/gn

+      '--optimize-for-size',
+      default=False,
+      action='store_true',
+      help="Optimize for code size rather than speed. This is always true for windows, ios, and android."


The current help text could be a bit confusing. It might imply that this flag is always on for Windows, iOS, and Android, which isn't the case. The optimization strategy is for size on those platforms, but this flag is for other platforms. A clearer message would be helpful to avoid misunderstanding.

Suggested change

help="Optimize for code size rather than speed. This is always true for windows, ios, and android."

help="Optimize for code size rather than speed. Builds for Windows, iOS, and Android are always optimized for size."

chinmaygarde · 2025-10-13T17:34:09Z

How much are the size savings?

chinmaygarde

Not fundamentally opposed to this. But I am curious about the actual size savings.

planetmarshall · 2025-10-13T18:21:44Z

Not fundamentally opposed to this. But I am curious about the actual size savings.

Typically about 20% on libflutter_engine.so versus the current defaults for a Linux ARM build (O2). More concretely -

Using the following configuration

./engine/src/flutter/tools/gn \
--target-os=linux \
--linux-cpu=arm64 \
--disable-desktop-embeddings \
--embedder-for-target \
--no-enable-unittests \
--runtime-mode=release \
--stripped

libflutter_engine.so is 14.9Mb

With the above and --optimize-for-size -

libflutter_engine.so is 12.4Mb.

I haven't done any performance benchmarks.

auto-submit · 2025-10-20T18:19:21Z

autosubmit label was removed for flutter/flutter/176835, because The base commit of the PR is older than 7 days and can not be merged. Please merge the latest changes from the main into this branch and resubmit the PR.

auto-submit · 2025-10-27T18:38:03Z

autosubmit label was removed for flutter/flutter/176835, because The base commit of the PR is older than 7 days and can not be merged. Please merge the latest changes from the main into this branch and resubmit the PR.

auto-submit · 2025-10-27T20:35:11Z

autosubmit label was removed for flutter/flutter/176835, because - The status or check suite Linux web_long_running_tests_5_5 has failed. Please fix the issues identified (or deflake) before re-applying this label.

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planetmarshall added 2 commits October 27, 2025 12:23

add gn flag to optimize builds for size

bb0026a

clarify docs for --optimize-for-size

2a92a98

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e0ff78989 add gn flag to optimize builds for size (flutter/flutter#17…

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…6835)

		# Optimize for size over speed in optimized builds. This is always true for
		# Windows, ios, and Android.

	raise Exception('--unoptimized and --optimize-for-size are mutually exclusive')
	raise ValueError('--unoptimized and --optimize-for-size are mutually exclusive')

	help="Optimize for code size rather than speed. This is always true for windows, ios, and android."
	help="Optimize for code size rather than speed. Builds for Windows, iOS, and Android are always optimized for size."

add gn flag to optimize builds for size #176835

add gn flag to optimize builds for size #176835

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planetmarshall commented Oct 10, 2025 •

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