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Message-ID: <20200624212055.GU4817@hirez.programming.kicks-ass.net>
Date: Wed, 24 Jun 2020 23:20:55 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Will Deacon <will@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Kees Cook <keescook@...omium.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
clang-built-linux@...glegroups.com,
kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
x86@...nel.org, George Burgess IV <gbiv@...gle.com>
Subject: Re: [PATCH 06/22] kbuild: lto: limit inlining
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@...gle.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.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
>
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