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

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Date:   Mon, 18 Mar 2019 14:56:05 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
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 02:47:13PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> 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)?

Nope, for reverts checkpatch is happy with it, I suppose because by
default the subject of the reverted patch is part of the revert's
subject.

> 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.

I can send a v2 if 'needed'.

> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Thanks!

Matthias

> >
> > 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
> 
> 
>