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

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Date:   Thu, 10 Sep 2020 09:33:11 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] kbuild: remove cc-option test of
 -fno-merge-all-constants

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/

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
>