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Subject: Re: [PATCH] x86/build: remove stale cc-option checks
To:     Nick Desaulniers <ndesaulniers@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>
Cc:     Masahiro Yamada <masahiroy@kernel.org>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
References: <20210811175647.3851629-1-ndesaulniers@google.com>
From:   Nathan Chancellor <nathan@kernel.org>
Message-ID: <cf568688-01a3-849e-2bcc-1cbe6845c9f8@kernel.org>
Date:   Wed, 11 Aug 2021 12:22:53 -0700
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On 8/11/2021 10:56 AM, Nick Desaulniers wrote:
> cc-option, __cc-option, cc-option-yn, and cc-disable-warning all invoke
> the compiler during build time, and can slow down the build when these
> checks become stale for our supported compilers, whose minimally
> supported versions increases over time. See
> Documentation/process/changes.rst for the current supported minimal
> versions (GCC 4.9+, clang 10.0.1+). Compiler version support for these
> flags may be verified on godbolt.org.
> 
> The following flags are supported by all supported versions of GCC and
> Clang. Remove their cc-option, __cc-option, and cc-option-yn tests.
> * -Wno-address-of-packed-member
> * -mno-avx
> * -m32
> * -mno-80387
> * -march=k8
> * -march=nocona
> * -march=core2
> * -march=atom
> * -mtune=generic
> * -mfentry
> 
> -mpreferred-stack-boundary= is specific to GCC, while
> -mstack-alignment= is specific to Clang. Rather than test for this three
> times via cc-option and __cc-option, rely on CONFIG_CC_IS_* from
> Kconfig.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1436
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

A few comments below.

> ---
>   arch/x86/Makefile | 51 +++++++++++++++++------------------------------
>   1 file changed, 18 insertions(+), 33 deletions(-)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 0fa7dc73b5d8..6de2985ba1a5 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -14,10 +14,13 @@ endif
>   
>   # For gcc stack alignment is specified with -mpreferred-stack-boundary,
>   # clang has the option -mstack-alignment for that purpose.
> +ifdef CONFIG_CC_IS_GCC
>   ifneq ($(call cc-option, -mpreferred-stack-boundary=4),)

Is this cc-option call still needed? It looks like it was added by 
commit d77698df39a5 ("x86/build: Specify stack alignment for clang"), 
which at the time claimed to support back to gcc 3.2, but that flag 
appears to be supported at least back to gcc 4.1.2 according to godbolt.

>         cc_stack_align4 := -mpreferred-stack-boundary=2
>         cc_stack_align8 := -mpreferred-stack-boundary=3
> -else ifneq ($(call cc-option, -mstack-alignment=16),)
> +endif
> +endif
> +ifdef CONFIG_CC_IS_CLANG
>         cc_stack_align4 := -mstack-alignment=4
>         cc_stack_align8 := -mstack-alignment=8
>   endif
> @@ -31,8 +34,8 @@ REALMODE_CFLAGS	:= -m16 -g -Os -DDISABLE_BRANCH_PROFILING \
>   
>   REALMODE_CFLAGS += -ffreestanding
>   REALMODE_CFLAGS += -fno-stack-protector
> -REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -Wno-address-of-packed-member)
> -REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align4))
> +REALMODE_CFLAGS += -Wno-address-of-packed-member
> +REALMODE_CFLAGS += $(cc_stack_align4)
>   REALMODE_CFLAGS += $(CLANG_FLAGS)
>   export REALMODE_CFLAGS
>   
> @@ -48,8 +51,7 @@ export BITS
>   #
>   #    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
>   #
> -KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow
> -KBUILD_CFLAGS += $(call cc-option,-mno-avx,)
> +KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
>   
>   # Intel CET isn't enabled in the kernel
>   KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
> @@ -59,9 +61,8 @@ ifeq ($(CONFIG_X86_32),y)
>           UTS_MACHINE := i386
>           CHECKFLAGS += -D__i386__
>   
> -        biarch := $(call cc-option,-m32)
> -        KBUILD_AFLAGS += $(biarch)
> -        KBUILD_CFLAGS += $(biarch)
> +        KBUILD_AFLAGS += -m32
> +        KBUILD_CFLAGS += -m32
>   
>           KBUILD_CFLAGS += -msoft-float -mregparm=3 -freg-struct-return
>   
> @@ -72,7 +73,7 @@ ifeq ($(CONFIG_X86_32),y)
>           # Align the stack to the register width instead of using the default
>           # alignment of 16 bytes. This reduces stack usage and the number of
>           # alignment instructions.
> -        KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align4))
> +        KBUILD_CFLAGS += $(cc_stack_align4)
>   
>           # CPU-specific tuning. Anything which can be shared with UML should go here.
>           include $(srctree)/arch/x86/Makefile_32.cpu

What tree were you intending to target with this? If it is -tip, this is 
going to cause it to fail to apply because the '$(srctree)' comes from 
Masahiro:

https://git.kernel.org/masahiroy/linux-kbuild/c/d8285639550578a1bf2d102391d1a9e08e0586ca

> @@ -93,7 +94,6 @@ else
>           UTS_MACHINE := x86_64
>           CHECKFLAGS += -D__x86_64__
>   
> -        biarch := -m64
>           KBUILD_AFLAGS += -m64
>           KBUILD_CFLAGS += -m64
>   
> @@ -104,7 +104,7 @@ else
>           KBUILD_CFLAGS += $(call cc-option,-falign-loops=1)
>   
>           # Don't autogenerate traditional x87 instructions
> -        KBUILD_CFLAGS += $(call cc-option,-mno-80387)
> +        KBUILD_CFLAGS += -mno-80387
>           KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)
>   
>           # By default gcc and clang use a stack alignment of 16 bytes for x86.
> @@ -114,20 +114,17 @@ else
>           # default alignment which keep the stack *mis*aligned.
>           # Furthermore an alignment to the register width reduces stack usage
>           # and the number of alignment instructions.
> -        KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align8))
> +        KBUILD_CFLAGS += $(cc_stack_align8)
>   
>   	# Use -mskip-rax-setup if supported.
>   	KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
>   
>           # FIXME - should be integrated in Makefile.cpu (Makefile_32.cpu)
> -        cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8)
> -        cflags-$(CONFIG_MPSC) += $(call cc-option,-march=nocona)
> -
> -        cflags-$(CONFIG_MCORE2) += \
> -                $(call cc-option,-march=core2,$(call cc-option,-mtune=generic))
> -	cflags-$(CONFIG_MATOM) += $(call cc-option,-march=atom) \
> -		$(call cc-option,-mtune=atom,$(call cc-option,-mtune=generic))
> -        cflags-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=generic)
> +        cflags-$(CONFIG_MK8)		+= -march=k8
> +        cflags-$(CONFIG_MPSC)		+= -march=nocona
> +        cflags-$(CONFIG_MCORE2)		+= -march=core2
> +        cflags-$(CONFIG_MATOM)		+= -march=atom
> +        cflags-$(CONFIG_GENERIC_CPU)	+= -mtune=generic
>           KBUILD_CFLAGS += $(cflags-y)
>   
>           KBUILD_CFLAGS += -mno-red-zone
> @@ -158,18 +155,6 @@ export CONFIG_X86_X32_ABI
>   ifdef CONFIG_FUNCTION_GRAPH_TRACER
>     ifndef CONFIG_HAVE_FENTRY
>   	ACCUMULATE_OUTGOING_ARGS := 1
> -  else
> -    ifeq ($(call cc-option-yn, -mfentry), n)
> -	ACCUMULATE_OUTGOING_ARGS := 1
> -
> -	# GCC ignores '-maccumulate-outgoing-args' when used with '-Os'.
> -	# If '-Os' is enabled, disable it and print a warning.
> -        ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
> -          undefine CONFIG_CC_OPTIMIZE_FOR_SIZE
> -          $(warning Disabling CONFIG_CC_OPTIMIZE_FOR_SIZE.  Your compiler does not have -mfentry so you cannot optimize for size with CONFIG_FUNCTION_GRAPH_TRACER.)
> -        endif
> -
> -    endif
>     endif
>   endif
>   
> @@ -193,7 +178,7 @@ ifdef CONFIG_RETPOLINE
>     # only been fixed starting from gcc stable version 8.4.0 and
>     # onwards, but not for older ones. See gcc bug #86952.
>     ifndef CONFIG_CC_IS_CLANG
> -    KBUILD_CFLAGS += $(call cc-option,-fno-jump-tables)
> +    KBUILD_CFLAGS += -fno-jump-tables
>     endif
>   endif
>   
>