add gn flag to optimize builds for size (#176835)

planetmarshall · web-flow · commit e0ff789894c4 · 2025-10-27T21:47:02.000Z
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 - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`).  [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md
diff --git a/engine/src/build/config/BUILDCONFIG.gn b/engine/src/build/config/BUILDCONFIG.gn
@@ -132,6 +132,11 @@ declare_args() {
   # Debug build.
   is_debug = true
 
+  # Optimize for code size rather than speed.
+  # Optimized builds for Windows, iOS, and Android are
+  # always optimized for size regardless of the value of this flag.
+  optimize_for_size = false
+
   # Whether this is a minimal linux instance without x11, etc.
   is_minimal_linux = false
 
diff --git a/engine/src/build/config/compiler/BUILD.gn b/engine/src/build/config/compiler/BUILD.gn
@@ -1015,7 +1015,11 @@ config("optimize") {
       cflags = [ "-O3" ]
     }
   } else {
-    cflags = [ "-O2" ] + common_optimize_on_cflags
+    if (optimize_for_size) {
+      cflags = [ "-Os" ] + common_optimize_on_cflags
+    } else {
+      cflags = [ "-O2" ] + common_optimize_on_cflags
+    }
   }
 
   lto_flags = []
diff --git a/engine/src/flutter/tools/gn b/engine/src/flutter/tools/gn
@@ -439,8 +439,13 @@ def to_gn_args(args):
 
   gn_args['buildtools_path'] = '//flutter/buildtools'
 
+  if args.unoptimized and args.optimize_for_size:
+    raise Exception('--unoptimized and --optimize-for-size are mutually exclusive')
+
   gn_args['is_debug'] = args.unoptimized
 
+  gn_args['optimize_for_size'] = args.optimize_for_size
+
   gn_args.update(setup_git_versions())
 
   gn_args.update(setup_rbe(args))
@@ -843,6 +848,7 @@ def to_gn_wasm_args(args, gn_args):
   gn_args['target_os'] = 'wasm'
   gn_args['target_cpu'] = 'wasm'
   gn_args['wasm_use_dwarf'] = args.wasm_use_dwarf
+  gn_args['wasm_prioritize_size'] = args.optimize_for_size
   gn_args['skia_use_angle'] = False
   gn_args['skia_use_dng_sdk'] = False
   gn_args['skia_use_expat'] = False
@@ -899,7 +905,14 @@ def parse_args(args):
   parser = argparse.ArgumentParser(description='A script to run `gn gen`.')
 
   parser.add_argument('--unoptimized', default=False, action='store_true')
-
+  parser.add_argument(
+      '--optimize-for-size',
+      default=False,
+      action='store_true',
+      help='Optimize for code size rather than speed. '
+      'Optimized builds for Windows, iOS, and Android are '
+      'always optimized for size regardless of the value of this flag.'
+  )
   parser.add_argument(
       '--enable-unittests',
       action='store_true',

Original file line number	Diff line number	Diff line change
`@@ -1015,7 +1015,11 @@ config("optimize") {`
`1015`	`1015`	`cflags = [ "-O3" ]`
`1016`	`1016`	`}`
`1017`	`1017`	`} else {`
`1018`		`- cflags = [ "-O2" ] + common_optimize_on_cflags`
	`1018`	`+ if (optimize_for_size) {`
	`1019`	`+ cflags = [ "-Os" ] + common_optimize_on_cflags`
	`1020`	`+ } else {`
	`1021`	`+ cflags = [ "-O2" ] + common_optimize_on_cflags`
	`1022`	`+ }`
`1019`	`1023`	`}`
`1020`	`1024`
`1021`	`1025`	`lto_flags = []`