linux-kernel - Re: [PATCH 3/3] MIPS: Add support for LTO

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Date:   Tue, 14 Dec 2021 11:09:58 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        list@...ndingux.net, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 3/3] MIPS: Add support for LTO

On Mon, Dec 13, 2021 at 10:49:14PM +0000, Paul Cercueil wrote:
> Allow CONFIG_LTO_CLANG to be enabled. The ThinLTO variant is not yet
> supported.
> 
> While this option allows to build a LTO'd kernel, the result kernel file
> ends up being *bigger* than the non-LTO variant (about 3.6 MiB with LTO
> vs. 3.1 MiB without with a ZSTD-compressed kernel).

I believe this happens because the linker is able to inline more, which
should make the code faster but makes it larger. I believe we limit the
inlining threshold so that it is not too bad but maybe that could be
further tuned.

> Signed-off-by: Paul Cercueil <paul@...pouillou.net>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  arch/mips/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 0215dc1529e9..6987db8d5f64 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -22,6 +22,7 @@ config MIPS
>  	select ARCH_USE_QUEUED_RWLOCKS
>  	select ARCH_USE_QUEUED_SPINLOCKS
>  	select ARCH_SUPPORTS_HUGETLBFS if CPU_SUPPORTS_HUGEPAGES
> +	select ARCH_SUPPORTS_LTO_CLANG if CPU_LITTLE_ENDIAN
>  	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
>  	select ARCH_WANT_IPC_PARSE_VERSION
>  	select ARCH_WANT_LD_ORPHAN_WARN
> -- 
> 2.33.0
> 
>