linux-kernel - Re: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang

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Message-ID: <CAKwvOd=eXk6dJ3xodkaqA6h_npHENe8x2NLD53+Hnij1nytj5A@mail.gmail.com>
Date:   Wed, 25 Aug 2021 15:32:30 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Masahiro Yamada <masahiroy@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com,
        llvm@...ts.linux.dev, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/2] x86: Do not add -falign flags unconditionally for clang

On Mon, Aug 23, 2021 at 7:27 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> clang does not support -falign-jumps and only recently gained support
> for -falign-loops. When one of the configuration options that adds these
> flags is enabled, clang warns and all cc-{disable-warning,option} that
> follow fail because -Werror gets added to test for the presence of this
> warning:
>
> clang-14: warning: optimization flag '-falign-jumps=0' is not supported
> [-Wignored-optimization-argument]
>
> To resolve this, add a couple of cc-option calls when building with
> clang; gcc has supported these options since 3.2 so there is no point in
> testing for their support. -falign-functions was implemented in clang-7,
> -falign-loops was implemented in clang-14, and -falign-jumps has not
> been implemented yet.
>
> Link: https://lore.kernel.org/r/YSQE2f5teuvKLkON@Ryzen-9-3900X.localdomain/
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

Thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> ---
>  arch/x86/Makefile_32.cpu | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu
> index cd3056759880..e8c65f990afd 100644
> --- a/arch/x86/Makefile_32.cpu
> +++ b/arch/x86/Makefile_32.cpu
> @@ -10,6 +10,12 @@ else
>  tune           = $(call cc-option,-mcpu=$(1),$(2))
>  endif
>
> +ifdef CONFIG_CC_IS_CLANG
> +align          := -falign-functions=0 $(call cc-option,-falign-jumps=0) $(call cc-option,-falign-loops=0)
> +else
> +align          := -falign-functions=0 -falign-jumps=0 -falign-loops=0
> +endif
> +
>  cflags-$(CONFIG_M486SX)                += -march=i486
>  cflags-$(CONFIG_M486)          += -march=i486
>  cflags-$(CONFIG_M586)          += -march=i586
> @@ -25,11 +31,11 @@ cflags-$(CONFIG_MK6)                += -march=k6
>  # They make zero difference whatsosever to performance at this time.
>  cflags-$(CONFIG_MK7)           += -march=athlon
>  cflags-$(CONFIG_MK8)           += $(call cc-option,-march=k8,-march=athlon)
> -cflags-$(CONFIG_MCRUSOE)       += -march=i686 -falign-functions=0 -falign-jumps=0 -falign-loops=0
> -cflags-$(CONFIG_MEFFICEON)     += -march=i686 $(call tune,pentium3) -falign-functions=0 -falign-jumps=0 -falign-loops=0
> +cflags-$(CONFIG_MCRUSOE)       += -march=i686 $(align)
> +cflags-$(CONFIG_MEFFICEON)     += -march=i686 $(call tune,pentium3) $(align)
>  cflags-$(CONFIG_MWINCHIPC6)    += $(call cc-option,-march=winchip-c6,-march=i586)
>  cflags-$(CONFIG_MWINCHIP3D)    += $(call cc-option,-march=winchip2,-march=i586)
> -cflags-$(CONFIG_MCYRIXIII)     += $(call cc-option,-march=c3,-march=i486) -falign-functions=0 -falign-jumps=0 -falign-loops=0
> +cflags-$(CONFIG_MCYRIXIII)     += $(call cc-option,-march=c3,-march=i486) $(align)
>  cflags-$(CONFIG_MVIAC3_2)      += $(call cc-option,-march=c3-2,-march=i686)
>  cflags-$(CONFIG_MVIAC7)                += -march=i686
>  cflags-$(CONFIG_MCORE2)                += -march=i686 $(call tune,core2)
> --
> 2.33.0
>


-- 
Thanks,
~Nick Desaulniers