Configure LTO during development

WARNING: In most cases full LTO is recommended for production builds. This should only be modified when the trade-offs are fully understood.

Building with link-time optimization (LTO) may take a very long time that brings little benefit during development. You may disable LTO to shorten the build time for development purposes.

Confirming the value of --lto

The default value for LTO is set per branch (i.e. mainline's default might differ from the android14-5.15 one, etc).

You may build the following to confirm the value of LTO setting:

$ tools/bazel build [flags] //build/kernel/kleaf:print_flags

Note: print_flags shows values specified in the command line. For LTO, the above command may show default or none. However, if --kasan is specified, --lto is coerced into none, no matter if --lto is specified in the command line.

Note: default in this context means: do not do any additional re-configuration.

Option 1: One-time build without LTO

For example:

$ tools/bazel build --lto=none //private/path/to/sources:tuna_dist

The --lto option is applied to the build, not the copy_to_dist_dir step. Hence, put it before the -- delimiter when running a *_dist target. For example:

$ tools/bazel run --lto=none //private/path/to/sources:tuna_dist -- --dist_dir=out/dist

NOTE: If you are using --lto with --config=fast, --lto must be specified after --config=fast because flags specified later take precedence. If unsure, use --config=local instead. For example:

$ tools/bazel run --config=fast --lto=none //common:kernel_dist

$ tools/bazel run --config=local --lto=none //common:kernel_dist

# WRONG: --lto is set to thin
# tools/bazel run --lto=none --config=fast //common:kernel_dist

Option 2: Disable LTO for this workspace

You only need to do this once per workspace.

# Do this at workspace root next to the file WORKSPACE
$ test -f WORKSPACE && echo 'build --lto=none' >> user.bazelrc
# Future builds in this workspace always disables LTO.
$ tools/bazel build //private/path/to/sources:tuna_dist

If you are using --config=fast, you need to add build:fast --lto=none as well, because --config=fast implies thin LTO. See

See also


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