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.
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.
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:
# CORRECT: $ tools/bazel run --config=fast --lto=none //common:kernel_dist # CORRECT: $ 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
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 fast.md.