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

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Date:   Mon, 24 Jan 2022 14:49:29 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        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 Mon, Jan 24, 2022 at 2:37 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Mon, Jan 24, 2022 at 12:40:58PM -0800, Nick Desaulniers wrote:
> > 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>
>
> Something like this appears to work, if that is more preferrable?
>
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 058446f01487..a27a3ade810e 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -3232,3 +3232,9 @@ endmenu
>  source "arch/mips/kvm/Kconfig"
>
>  source "arch/mips/vdso/Kconfig"
> +
> +config CC_MNO_BRANCH_LIKELY
> +       string
> +       default "-mno-branch-likely"
> +       depends on MACH_LOONGSON64 || MACH_LOONGSON2EF
> +       depends on $(cc-option,-mno-branch-likely)
> diff --git a/arch/mips/loongson2ef/Platform b/arch/mips/loongson2ef/Platform
> index 50e659aca543..66ed09581417 100644
> --- a/arch/mips/loongson2ef/Platform
> +++ b/arch/mips/loongson2ef/Platform
> @@ -41,6 +41,6 @@ cflags-y += $(call cc-option,-mno-loongson-mmi)
>  # Loongson Machines' Support
>  #
>
> -cflags-$(CONFIG_MACH_LOONGSON2EF) += -I$(srctree)/arch/mips/include/asm/mach-loongson2ef -mno-branch-likely
> +cflags-$(CONFIG_MACH_LOONGSON2EF) += -I$(srctree)/arch/mips/include/asm/mach-loongson2ef $(CONFIG_CC_MNO_BRANCH_LIKELY)

Does that allow someone to modify the value for CC_MNO_BRANCH_LIKELY
in menuconfig?

If so, I'd rather the Makefiles have:

cflags-$(CONFIG_CC_MNO_BRANCH_LIKELY) += -mno-branch-likely

and CONFIG_CC_MNO_BRANCH_LIKELY be a bool rather than an editable
string.  I think that makes the Makefile more readable; you don't have
to see what CONFIG_CC_MNO_BRANCH_LIKELY expands to in a different
file.

See also CC_HAS_ASM_GOTO and CC_HAS_ASM_GOTO_OUTPUT in init/Kconfig.

>  load-$(CONFIG_LEMOTE_FULOONG2E) += 0xffffffff80100000
>  load-$(CONFIG_LEMOTE_MACH2F) += 0xffffffff80200000
> diff --git a/arch/mips/loongson64/Platform b/arch/mips/loongson64/Platform
> index 3e660d6d3c2b..88fbdfe9ffcc 100644
> --- a/arch/mips/loongson64/Platform
> +++ b/arch/mips/loongson64/Platform
> @@ -33,5 +33,5 @@ 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 $(CONFIG_CC_MNO_BRANCH_LIKELY)
>  load-$(CONFIG_CPU_LOONGSON64) += 0xffffffff80200000
>
> > > ---
> > >  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
>


-- 
Thanks,
~Nick Desaulniers