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Moves linux to ThinLTO. - #196314
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Moves linux to ThinLTO.#196314
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This pull request introduces support for ThinLTO by adding --thin-lto and --no-thin-lto command-line arguments to the GN build tool, configuring compiler build settings to apply -flto=thin when enabled, and updating CI builder configurations to use --thin-lto. It also adds corresponding unit tests in gn_test.py. Feedback suggests expanding the unit tests to explicitly assert the behavior of the --no-thin-lto flag.
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| @unittest.skipIf(sys.platform.startswith('win'), 'LTO GN args are not set on Windows') | ||
| def test_thin_lto(self): | ||
| release = ['--runtime-mode', 'release'] | ||
| self.assertFalse(self._gn_args(release)['use_thin_lto']) | ||
| self.assertTrue(self._gn_args(release + ['--thin-lto'])['use_thin_lto']) | ||
| self.assertTrue(self._gn_args(release + ['--thin-lto'])['enable_lto']) | ||
| # ThinLTO has no effect when LTO is disabled. | ||
| no_lto_args = self._gn_args(release + ['--thin-lto', '--no-lto']) | ||
| self.assertFalse(no_lto_args['enable_lto']) | ||
| self.assertNotIn('use_thin_lto', no_lto_args) | ||
| unopt_args = self._gn_args(release + ['--thin-lto', '--unoptimized']) | ||
| self.assertFalse(unopt_args['enable_lto']) | ||
| self.assertNotIn('use_thin_lto', unopt_args) |
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To ensure comprehensive test coverage for the new CLI options, consider adding an assertion that verifies the behavior of the --no-thin-lto flag. This ensures that explicitly disabling ThinLTO works as expected when combined with other flags.
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| @unittest.skipIf(sys.platform.startswith('win'), 'LTO GN args are not set on Windows') | |
| def test_thin_lto(self): | |
| release = ['--runtime-mode', 'release'] | |
| self.assertFalse(self._gn_args(release)['use_thin_lto']) | |
| self.assertTrue(self._gn_args(release + ['--thin-lto'])['use_thin_lto']) | |
| self.assertTrue(self._gn_args(release + ['--thin-lto'])['enable_lto']) | |
| # ThinLTO has no effect when LTO is disabled. | |
| no_lto_args = self._gn_args(release + ['--thin-lto', '--no-lto']) | |
| self.assertFalse(no_lto_args['enable_lto']) | |
| self.assertNotIn('use_thin_lto', no_lto_args) | |
| unopt_args = self._gn_args(release + ['--thin-lto', '--unoptimized']) | |
| self.assertFalse(unopt_args['enable_lto']) | |
| self.assertNotIn('use_thin_lto', unopt_args) | |
| @unittest.skipIf(sys.platform.startswith('win'), 'LTO GN args are not set on Windows') | |
| def test_thin_lto(self): | |
| release = ['--runtime-mode', 'release'] | |
| self.assertFalse(self._gn_args(release)['use_thin_lto']) | |
| self.assertTrue(self._gn_args(release + ['--thin-lto'])['use_thin_lto']) | |
| self.assertTrue(self._gn_args(release + ['--thin-lto'])['enable_lto']) | |
| # ThinLTO can be explicitly disabled. | |
| self.assertFalse(self._gn_args(release + ['--thin-lto', '--no-thin-lto'])['use_thin_lto']) | |
| # ThinLTO has no effect when LTO is disabled. | |
| no_lto_args = self._gn_args(release + ['--thin-lto', '--no-lto']) | |
| self.assertFalse(no_lto_args['enable_lto']) | |
| self.assertNotIn('use_thin_lto', no_lto_args) | |
| unopt_args = self._gn_args(release + ['--thin-lto', '--unoptimized']) | |
| self.assertFalse(unopt_args['enable_lto']) | |
| self.assertNotIn('use_thin_lto', unopt_args) |
jtmcdole
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Nov 10, 2026
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autosubmit label was removed for flutter/flutter/196314, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label. |
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Thanks, this matches what we measured on our side: on a dedicated Linux x86-64 machine, Four things to consider, in order of importance:
macOS desktop stays on |
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flutter/flutter@b98afb5...9c427b5 2026-11-12 [email protected] Roll Skia from d67d0f22e3ec to 9aa2513a4ac9 (2 revisions) (flutter/flutter#196429) 2026-11-12 [email protected] [flutter_tools] Move CpuArch to flutter_tools_core and define HostPlatform by osName and cpuArch (flutter/flutter#195841) 2026-11-12 [email protected] Roll Skia from 23e7e63789b8 to d67d0f22e3ec (2 revisions) (flutter/flutter#196424) 2026-11-12 [email protected] Uber compactify (flutter/flutter#196392) 2026-11-12 [email protected] Roll Skia from 3d7e335da165 to 23e7e63789b8 (12 revisions) (flutter/flutter#196422) 2026-11-12 [email protected] Roll Dart SDK from 13af329ea4f2 to e5add7a25b32 (1 revision) (flutter/flutter#196413) 2026-11-12 [email protected] Don't terminate the shared EGL display when an engine is disposed (flutter/flutter#194520) 2026-11-11 [email protected] Adds manifest flag to turn on SDFs for Android. (flutter/flutter#196056) 2026-11-11 [email protected] Remove bringup from Linux_pixel_7pro integration_ui_keyboard_inset_jump (flutter/flutter#196334) 2026-11-11 [email protected] Uses system default locale for text rendering when preferred locale isn't supported (flutter/flutter#195997) 2026-11-11 [email protected] [android]: prewarms shaders when the context is created (flutter/flutter#196069) 2026-11-11 [email protected] iOS: Recomposite platform views only if their parameters change (flutter/flutter#196287) 2026-11-11 [email protected] [flutter_tools] Forward encoding in ErrorHandlingFile.readAsStringSync (flutter/flutter#196136) 2026-11-11 [email protected] Roll Skia from b89c24bbab8c to 3d7e335da165 (8 revisions) (flutter/flutter#196390) 2026-11-11 [email protected] Add Flutter Gardener instructions on how to disable a test (flutter/flutter#196312) 2026-11-11 [email protected] [Impeller] Enable malioc analysis for framebuffer_blend shaders (flutter/flutter#196343) 2026-11-11 [email protected] [flutter_tools] Migrate general.shard Android, asset, and build tests off globals (flutter/flutter#196125) 2026-11-11 [email protected] Roll Dart SDK from 8f15c4ca2d41 to 13af329ea4f2 (1 revision) (flutter/flutter#196386) 2026-11-11 [email protected] [Impeller] Do not clamp the local coverage limit to the root render target (flutter/flutter#195860) 2026-11-11 [email protected] [Impeller] Use separate uniform buffers on VMware SVGA3D (flutter/flutter#196022) 2026-11-11 [email protected] [flutter_tools] Introduce AndroidSdkProvider for lazy native_assets SDK lookup (flutter/flutter#196122) 2026-11-11 [email protected] Moves linux to ThinLTO. (flutter/flutter#196314) 2026-11-11 [email protected] [flutter_tools] Migrate bin/fuchsia_tester.dart off globals and runInContext (flutter/flutter#196074) 2026-11-11 [email protected] Changes the tolerances for frame_rate_main_test.dart. (flutter/flutter#196304) 2026-11-11 [email protected] Roll Packages from 08faf64 to 653628e (16 revisions) (flutter/flutter#196381) 2026-11-11 [email protected] Allow SliverCrossAxisGroup children to lay out with zero cross-axis extent instead of asserting (flutter/flutter#191287) 2026-11-11 [email protected] Roll Skia from cc4c54f752ba to b89c24bbab8c (4 revisions) (flutter/flutter#196375) 2026-11-11 [email protected] [Linux] Fix layer offset when compositing with glBlitFramebuffer (flutter/flutter#196189) 2026-11-11 [email protected] [flutter_tools] Inject CrashReportSender into SwiftPM migration (flutter/flutter#196315) If this roll has caused a breakage, revert this CL and set the roller to dry run mode using the controls here: https://autoroll.skia.org/r/flutter-packages Please CC [email protected],[email protected] on the revert to ensure that a human is aware of the problem. To file a bug in Packages: https://github.com/flutter/flutter/issues/new/choose To report a problem with the AutoRoller itself, please file a bug: https://issues.skia.org/issues/new?component=1389291&template=1850622 Documentation for the AutoRoller is here: https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md
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Upstream moved the Linux profile and release builds to ThinLTO (flutter#196314): tools/gn --thin-lto passes GN's use_thin_lto under enable_lto, and //build/config/compiler:optimize then has -flto=thin where full LTO has -flto. The merge left the seven of those builds that run under Bazel on --no-lto. This takes upstream's flag in them and gives the Bazel build the setting. - //engine/src/flutter/build/bazel:thin_lto, a flag beside `lto`, and the toolchain configuration's `thin_lto` attribute: under it the `lto` feature has -flto=thin on compiles and links (the Linux, Android, macOS and iOS toolchains share the configuration). It does nothing without `lto`, as in GN. - The converter maps --thin-lto and --no-thin-lto to the setting in a build with LTO and to nothing otherwise; to nothing for the web too, for which tools/gn reads no LTO flag. Seven configurations gain `lto` and `thin_lto`: ci_host_release, host_profile_desktop, host_release_desktop, linux_profile_arm64_desktop, linux_release_arm64_desktop, host_profile_test, host_release_test. - in_configuration carries the setting, and the tests' Android artifacts say `thin_lto = "false"`: the Android builders build with full LTO whatever the build around them has. - The recipe's --no-lto, in a release builder's presubmit, turns LTO off in such a build as in any other (bazel_ci.py's match is unchanged; a test). ThinLTO therefore runs in the three release builders' postsubmit builds, and in every build of linux_host_engine_test. Not done: the wrapped Dart SDK build still compiles the VM without LTO in every build. GN compiles it with -flto=thin in these builds (gn desc of libdart_vm_aotruntime_product), as with -flto in the Android AOT and iOS device builds. The README's known gaps now say so. Checked against GN (tools/gn --runtime-mode release --thin-lto, gn desc): //flutter/fml:fml's cflags and flutter_tester's ldflags have -flto=thin once, as Bazel's 46 compiles of fml and the link of flutter_tester under --config=host_release_desktop do; the host tools (the execution configuration) have it too, as GN's one toolchain does; with --//engine/src/flutter/build/bazel:lto=false after the configuration fml's compiles have no LTO flag. Built and run locally: `bazelisk test --config=host_release_test //engine/src/flutter/fml:fml_unittests //engine/src/flutter/shell/common:shell_unittests` passes (fml's 46 objects are ThinLTO bitcode), and `bazelisk build --config=host_release_desktop //engine/src/flutter/ci/builders:host_release_desktop` builds; its libflutter_linux_gtk.so exports the same 310 symbols as the debug build's, which has no LTO (that one has four more, the debug mode's snapshot data). Also: gn_to_bazel's tests (284) and --check, check_build_configs.sh, check_generated_bazel_files.sh, bazel_ci_test.py (23), engine_build_configs (84), bazel_tools (46), buildifier, yapf; Bazel analysis of the seven builds' install targets, of ci_host_debug and ci_android_release_arm64 (whose compiles keep the bare -flto), and of //:tool_tests //:dart_tests. Not built: the arm64 cross builds, and the profile mode. Not measured: the test builders' ThinLTO links of every unit test on RBE workers.
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The actual measurements are in #196308, but the tradeoffs are roughly:
This is a far cry from the 100% build time with full LTO that caused us to turn off LTO for linux. I think this is a fair tradeoff, especially for a platform like linux with is more accepting off deviations on binary size.
issue #196308
cc @Rusino
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