linux-kernel - Re: [PATCH 2/4] kbuild: remove cc-option test of -fno-merge-all-constants

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Message-ID: <CAK7LNAQbjsKpbSKRNEzFTXsD1+nj-gkKon3c1u9J=x8wL4H-DA@mail.gmail.com>
Date:   Fri, 11 Sep 2020 01:41:29 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] kbuild: remove cc-option test of -fno-merge-all-constants

On Fri, Sep 11, 2020 at 1:33 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Thu, Sep 10, 2020 at 10:51:18PM +0900, Masahiro Yamada wrote:
> > The minimal compiler versions, GCC 4.9 and Clang 10 support this flag.
> >
> > Here is the godbolt:
> > https://godbolt.org/z/8T4177
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
>
> For what it's worth, the commit that introduced this block is going to
> be reverted:
>
> https://lore.kernel.org/mm-commits/20200903201518.JKrRS%25akpm@linux-foundation.org/


My bad - you are right.
This hunk has already gone by Nick's patch.

Thanks.



> Regardless, it was introduced in LLVM 2.7.0 in commit
> f9d41df0930986c52e198292cf875eb5d1761ece.
>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
>
> > ---
> >
> >  Makefile | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 059b36f2ea53..1b6abecc5cab 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -927,7 +927,7 @@ KBUILD_CFLAGS     += -fno-strict-overflow
> >  # clang sets -fmerge-all-constants by default as optimization, but this
> >  # is non-conforming behavior for C and in fact breaks the kernel, so we
> >  # need to disable it here generally.
> > -KBUILD_CFLAGS        += $(call cc-option,-fno-merge-all-constants)
> > +KBUILD_CFLAGS        += -fno-merge-all-constants
> >
> >  # for gcc -fno-merge-all-constants disables everything, but it is fine
> >  # to have actual conforming behavior enabled.
> > --
> > 2.25.1
> >
>
> --
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-- 
Best Regards
Masahiro Yamada