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

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Date:   Mon, 24 Jan 2022 15:37:46 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
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 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 $(CONFIG_CC_MNO_BRANCH_LIKELY)
 load-$(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