Re: [PATCH v4 4/4] ARM: pass -march= only to compiler

Nathan Chancellor
2022-10-14T21:22:03Z

On Fri, Oct 14, 2022 at 01:13:54PM -0700, Nick Desaulniers 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.
>
> Clang is trying to warn that it doesn't support different values for
> -march= and -Wa,-march= (like GCC does, but the kernel doesn't need this)
> though the value of the preprocessor define __thumb2__ is based on
> -march=. Make sure to re-set __thumb2__ via -D flag for assembler
> sources now that we're no longer passing -march= to the assembler. Set
> it to a different value than the preprocessor would for -march= in case
> -march= gets accidentally re-added to KBUILD_AFLAGS in the future.
> Thanks to Ard and Nathan for this suggestion.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1315
> Link: https://github.com/ClangBuiltLinux/linux/issues/1587
> Link: https://github.com/llvm/llvm-project/issues/55656
> Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> Suggested-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

This passed through my build matrix on top of commit dca45efbe3c8
("Merge tag 'sound-fix-6.1-rc1' of
git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound") and I saw no
additional warnings/errors:

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

> ---
> Changes v3 -> v4:
> * Add -D__thumb2__=2 to KBUILD_AFLAGS as per in-person discussion with
> Ard and Nathan, and their SB tags.
> * Reword commit message.
>
> arch/arm/Makefile | 36 +++++++++++++++++++++++++-----------
> 1 file changed, 25 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index ee888070b2ff..b58998749ead 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -60,21 +60,34 @@ 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
> @@ -119,15 +132,16 @@ AFLAGS_NOWARN :=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
>
> ifeq ($(CONFIG_THUMB2_KERNEL),y)
> CFLAGS_ISA :=-mthumb -Wa,-mimplicit-it=always $(AFLAGS_NOWARN)
> -AFLAGS_ISA :=$(CFLAGS_ISA) -Wa$(comma)-mthumb
> +AFLAGS_ISA :=$(CFLAGS_ISA) -Wa$(comma)-mthumb -D__thumb2__=2
> else
> CFLAGS_ISA :=$(call cc-option,-marm,) $(AFLAGS_NOWARN)
> AFLAGS_ISA :=$(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__
>
> --
> 2.38.0.413.g74048e4d9e-goog