Re: [PATCH 7/9] LoongArch: Tweak CFLAGS for Clang compatibility

[Nick Desaulniers]

On Fri, Jun 23, 2023 at 6:44 AM WANG Xuerui <kernel@xxxxxxxxxx> wrote:
>
> From: WANG Xuerui <git@xxxxxxxxxx>
>
> Now the arch code is mostly ready for LLVM/Clang consumption, it is time
> to re-organize the CFLAGS a little to actually enable the LLVM build.
>
> A build with !RELOCATABLE && !MODULE is confirmed working within a QEMU
> environment; support for the two features are currently blocked by
> LLVM/Clang, and will come later.
>
> Signed-off-by: WANG Xuerui <git@xxxxxxxxxx>
> ---
>  arch/loongarch/Makefile      | 14 +++++++++++---
>  arch/loongarch/vdso/Makefile |  6 +++++-
>  2 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index a27e264bdaa5..efe9b50bd829 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -46,12 +46,18 @@ ld-emul                     = $(64bit-emul)
>  cflags-y               += -mabi=lp64s
>  endif
>
> -cflags-y                       += -G0 -pipe -msoft-float

This seems to drop -msoft-float for GCC. Intentional?

> -LDFLAGS_vmlinux                        += -G0 -static -n -nostdlib
> +ifndef CONFIG_CC_IS_CLANG
> +cflags-y                       += -G0
> +LDFLAGS_vmlinux                        += -G0

Thanks for the patch!

I can understand not passing -G0 to clang if clang doesn't understand
it, but should you be using CONFIG_LD_IS_LLD for LDFLAGS?

What does -G0 do?

Is there a plan to support it in clang and lld?

If so, please file a bug in LLVM's issue tracker
https://github.com/llvm/llvm-project/issues
then link to it in a comment in this Makefile above the relevant condition.

> +endif
> +cflags-y                       += -pipe
> +LDFLAGS_vmlinux                        += -static -n -nostdlib
>
>  # When the assembler supports explicit relocation hint, we must use it.
>  # GCC may have -mexplicit-relocs off by default if it was built with an old
> -# assembler, so we force it via an option.
> +# assembler, so we force it via an option. For LLVM/Clang the desired behavior
> +# is the default, and the flag is not supported, so don't pass it if Clang is
> +# being used.
>  #
>  # When the assembler does not supports explicit relocation hint, we can't use
>  # it.  Disable it if the compiler supports it.
> @@ -61,8 +67,10 @@ LDFLAGS_vmlinux                      += -G0 -static -n -nostdlib
>  # combination of a "new" assembler and "old" compiler is not supported.  Either
>  # upgrade the compiler or downgrade the assembler.
>  ifdef CONFIG_AS_HAS_EXPLICIT_RELOCS
> +ifndef CONFIG_CC_IS_CLANG
>  cflags-y                       += -mexplicit-relocs
>  KBUILD_CFLAGS_KERNEL           += -mdirect-extern-access
> +endif

Why would AS_HAS_EXPLICIT_RELOCS be set if -mexplicit-relocs isn't
supported? Is the kconfig for that broken?

Does AS_HAS_EXPLICIT_RELOCS also need to test for the support for
-mdirect-extern-access or should there be a new config for that?
CC_SUPPORTS_DIRECT_EXTERN_ACCESS

>  else
>  cflags-y                       += $(call cc-option,-mno-explicit-relocs)
>  KBUILD_AFLAGS_KERNEL           += -Wa,-mla-global-with-pcrel
> diff --git a/arch/loongarch/vdso/Makefile b/arch/loongarch/vdso/Makefile
> index 4c859a0e4754..19f6c75a1106 100644
> --- a/arch/loongarch/vdso/Makefile
> +++ b/arch/loongarch/vdso/Makefile
> @@ -25,13 +25,17 @@ endif
>  cflags-vdso := $(ccflags-vdso) \
>         -isystem $(shell $(CC) -print-file-name=include) \
>         $(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
> -       -O2 -g -fno-strict-aliasing -fno-common -fno-builtin -G0 \
> +       -O2 -g -fno-strict-aliasing -fno-common -fno-builtin \
>         -fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \
>         $(call cc-option, -fno-asynchronous-unwind-tables) \
>         $(call cc-option, -fno-stack-protector)
>  aflags-vdso := $(ccflags-vdso) \
>         -D__ASSEMBLY__ -Wa,-gdwarf-2
>
> +ifndef CONFIG_CC_IS_CLANG
> +cflags-vdso += -G0
> +endif
> +
>  ifneq ($(c-gettimeofday-y),)
>    CFLAGS_vgettimeofday.o += -include $(c-gettimeofday-y)
>  endif
> --
> 2.40.0
>
>


-- 
Thanks,
~Nick Desaulniers