linux-kernel - Re: [PATCH] Revert "kbuild: use -Oz instead of -Os when using clang"

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Date:   Mon, 18 Mar 2019 14:47:13 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Behan Webster <behanw@...verseincode.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Manoj Gupta <manojgupta@...omium.org>,
        Tiancong Wang <tcwang@...omium.org>,
        Stephen Hines <srhines@...gle.com>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] Revert "kbuild: use -Oz instead of -Os when using clang"

On Mon, Mar 18, 2019 at 2:10 PM Matthias Kaehlcke <mka@...omium.org> wrote:
>
> The clang option -Oz enables *aggressive* optimization for size,
> which doesn't necessarily result in smaller images, but can have
> negative impact on performance. Switch back to the less aggressive
> -Os.
>
> This reverts commit 6748cb3c299de1ffbe56733647b01dbcc398c419.

Does scripts/checkpatch.pl complain about this format (full sha, no
title)?  Not sure that necessitates a v2, but if so would be good to
mention that Clang generates excessive calls into libgcc/compiler-rt
at -Oz.  Thanks for the patch and the discussion/sanity check.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
>  Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 9ef547fc7ffe..191f3ce3cb5e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -667,7 +667,7 @@ KBUILD_CFLAGS       += $(call cc-disable-warning, format-overflow)
>  KBUILD_CFLAGS  += $(call cc-disable-warning, int-in-bool-context)
>
>  ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
> -KBUILD_CFLAGS  += $(call cc-option,-Oz,-Os)
> +KBUILD_CFLAGS  += -Os
>  else
>  KBUILD_CFLAGS   += -O2
>  endif
> --
> 2.21.0.225.g810b269d1ac-goog



-- 
Thanks,
~Nick Desaulniers