Re: [PATCH 06/22] kbuild: lto: limit inlining

Peter Zijlstra
2020-06-24T21:21:22Z

On Wed, Jun 24, 2020 at 01:31:44PM -0700, Sami Tolvanen wrote:
> This change limits function inlining across translation unit
> boundaries in order to reduce the binary size with LTO.
>
> The -import-instr-limit flag defines a size limit, as the number
> of LLVM IR instructions, for importing functions from other TUs.
> The default value is 100, and decreasing it to 5 reduces the size
> of a stripped arm64 defconfig vmlinux by 11%.

Is that also the right number for x86? What about the effect on
performance? What did 6 do? or 4?

> Suggested-by: George Burgess IV <gbiv@google.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
> Makefile | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 3a7e5e5c17b9..ee66513a5b66 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -894,6 +894,10 @@ else
> CC_FLAGS_LTO_CLANG := -flto
> endif
> CC_FLAGS_LTO_CLANG += -fvisibility=default
> +
> +# Limit inlining across translation units to reduce binary size
> +LD_FLAGS_LTO_CLANG := -mllvm -import-instr-limit=5
> +KBUILD_LDFLAGS += $(LD_FLAGS_LTO_CLANG)
> endif
>
> ifdef CONFIG_LTO
> --
> 2.27.0.212.ge8ba1cc988-goog
>