linux-kernel - Re: [PATCH 2/2] MIPS: Loongson64: Wrap -mno-branch-likely with cc-option

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Date:   Mon, 24 Jan 2022 12:40:58 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 2/2] MIPS: Loongson64: Wrap -mno-branch-likely with cc-option

On Thu, Jan 20, 2022 at 1:40 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> This flag is not supported by clang, which results in a warning:
>
>   clang-14: warning: argument unused during compilation: '-mno-branch-likely' [-Wunused-command-line-argument]
>
> This breaks cc-option, which adds -Werror to make this warning fatal and
> catch flags that are not supported. Wrap the flag in cc-option so that
> it does not cause cc-option to fail, which can cause randconfigs to be
> really noisy.
>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

We should convert more of these tests to Kconfig checks that run once
per configuration, rather than multiple times for a build. IIRC Linus
mentioned this somewhere...yeah, the -Wimplicit-fallthrough patches.
See
dee2b702bcf06 ("kconfig: Add support for -Wimplicit-fallthrough")

I wonder if we can check ARCH or SUBARCH in Kconfig to limit invoking
the tool under test for certain arch specific command line flags?

I'll take this patch over such a larger change, but I think towards
the goal of speeding up already configured builds, we eventually want
to be migrating cc-option and ld-option checks to Kconfig.

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

> ---
>  arch/mips/loongson64/Platform | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/mips/loongson64/Platform b/arch/mips/loongson64/Platform
> index 981d3abc150e..acf9edc9b15d 100644
> --- a/arch/mips/loongson64/Platform
> +++ b/arch/mips/loongson64/Platform
> @@ -26,5 +26,6 @@ cflags-y += $(call cc-option,-mno-loongson-mmi)
>  # Loongson Machines' Support
>  #
>
> -cflags-$(CONFIG_MACH_LOONGSON64) += -I$(srctree)/arch/mips/include/asm/mach-loongson64 -mno-branch-likely
> +cflags-$(CONFIG_MACH_LOONGSON64) += -I$(srctree)/arch/mips/include/asm/mach-loongson64
> +cflags-$(CONFIG_MACH_LOONGSON64) += $(call cc-option,-mno-branch-likely)
>  load-$(CONFIG_CPU_LOONGSON64) += 0xffffffff80200000
> --
> 2.34.1
>
>


-- 
Thanks,
~Nick Desaulniers