linux-kernel - Re: [PATCH v3 4/4] ARM: pass -march= only to compiler

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Message-ID: <CAKwvOd=Fmb5NTFPuc8EhELoysj9zz0pthccOUmTvx_G6zdUB7g@mail.gmail.com>
Date:   Mon, 16 May 2022 14:16:16 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>, Ard Biesheuvel <ardb@...nel.org>
Cc:     Russell King <rmk+kernel@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v3 4/4] ARM: pass -march= only to compiler

+ Ard (I messed up my command line invocation of git send-email, sorry
for more noise)
https://lore.kernel.org/llvm/20220516210954.1660716-1-ndesaulniers@google.com/

On Mon, May 16, 2022 at 2:10 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> When both -march= and -Wa,-march= are specified for assembler or
> assembler-with-cpp sources, GCC and Clang will prefer the -Wa,-march=
> value but Clang will warn that -march= is unused.
>
> warning: argument unused during compilation: '-march=armv6k'
> [-Wunused-command-line-argument]
>
> This is the top group of warnings we observe when using clang to
> assemble the kernel via `ARCH=arm make LLVM=1`.
>
> Split the arch-y make variable into two, so that -march= flags only get
> passed to the compiler, not the assembler. -D flags are added to
> KBUILD_CPPFLAGS which is used for both C and assembler-with-cpp sources.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1315
> Link: https://github.com/ClangBuiltLinux/linux/issues/1587
> Link: https://lore.kernel.org/llvm/628249e8.1c69fb81.d20fd.02ea@mx.google.com/
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  arch/arm/Makefile | 34 ++++++++++++++++++++++++----------
>  1 file changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index 1029c2503aef..47a300f6f99c 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -57,21 +57,34 @@ endif
>  KBUILD_CFLAGS  += $(call cc-option,-fno-ipa-sra)
>
>  # This selects which instruction set is used.
> +arch-$(CONFIG_CPU_32v7M)       :=-march=armv7-m
> +arch-$(CONFIG_CPU_32v7)                :=-march=armv7-a
> +arch-$(CONFIG_CPU_32v6)                :=-march=armv6
> +# Only override the compiler option if ARMv6. The ARMv6K extensions are
> +# always available in ARMv7
> +ifeq ($(CONFIG_CPU_32v6),y)
> +arch-$(CONFIG_CPU_32v6K)       :=-march=armv6k
> +endif
> +arch-$(CONFIG_CPU_32v5)                :=-march=armv5te
> +arch-$(CONFIG_CPU_32v4T)       :=-march=armv4t
> +arch-$(CONFIG_CPU_32v4)                :=-march=armv4
> +arch-$(CONFIG_CPU_32v3)                :=-march=armv3m
> +
>  # Note that GCC does not numerically define an architecture version
>  # macro, but instead defines a whole series of macros which makes
>  # testing for a specific architecture or later rather impossible.
> -arch-$(CONFIG_CPU_32v7M)       :=-D__LINUX_ARM_ARCH__=7 -march=armv7-m
> -arch-$(CONFIG_CPU_32v7)                :=-D__LINUX_ARM_ARCH__=7 -march=armv7-a
> -arch-$(CONFIG_CPU_32v6)                :=-D__LINUX_ARM_ARCH__=6 -march=armv6
> -# Only override the compiler opt:ion if ARMv6. The ARMv6K extensions are
> +cpp-$(CONFIG_CPU_32v7M)                :=-D__LINUX_ARM_ARCH__=7
> +cpp-$(CONFIG_CPU_32v7)         :=-D__LINUX_ARM_ARCH__=7
> +cpp-$(CONFIG_CPU_32v6)         :=-D__LINUX_ARM_ARCH__=6
> +# Only override the compiler option if ARMv6. The ARMv6K extensions are
>  # always available in ARMv7
>  ifeq ($(CONFIG_CPU_32v6),y)
> -arch-$(CONFIG_CPU_32v6K)       :=-D__LINUX_ARM_ARCH__=6 -march=armv6k
> +cpp-$(CONFIG_CPU_32v6K)                :=-D__LINUX_ARM_ARCH__=6
>  endif
> -arch-$(CONFIG_CPU_32v5)                :=-D__LINUX_ARM_ARCH__=5 -march=armv5te
> -arch-$(CONFIG_CPU_32v4T)       :=-D__LINUX_ARM_ARCH__=4 -march=armv4t
> -arch-$(CONFIG_CPU_32v4)                :=-D__LINUX_ARM_ARCH__=4 -march=armv4
> -arch-$(CONFIG_CPU_32v3)                :=-D__LINUX_ARM_ARCH__=3 -march=armv3m
> +cpp-$(CONFIG_CPU_32v5)         :=-D__LINUX_ARM_ARCH__=5
> +cpp-$(CONFIG_CPU_32v4T)                :=-D__LINUX_ARM_ARCH__=4
> +cpp-$(CONFIG_CPU_32v4)         :=-D__LINUX_ARM_ARCH__=4
> +cpp-$(CONFIG_CPU_32v3)         :=-D__LINUX_ARM_ARCH__=3
>
>  # This selects how we optimise for the processor.
>  tune-$(CONFIG_CPU_ARM7TDMI)    :=-mtune=arm7tdmi
> @@ -123,8 +136,9 @@ AFLAGS_ISA  :=$(CFLAGS_ISA)
>  endif
>
>  # Need -Uarm for gcc < 3.x
> +KBUILD_CPPFLAGS        +=$(cpp-y)
>  KBUILD_CFLAGS  +=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
> -KBUILD_AFLAGS  +=$(CFLAGS_ABI) $(AFLAGS_ISA) $(arch-y) $(tune-y) -include asm/unified.h -msoft-float
> +KBUILD_AFLAGS  +=$(CFLAGS_ABI) $(AFLAGS_ISA) -Wa,$(arch-y) $(tune-y) -include asm/unified.h -msoft-float
>
>  CHECKFLAGS     += -D__arm__
>
> --
> 2.36.0.550.gb090851708-goog
>


-- 
Thanks,
~Nick Desaulniers