linux-kernel - Re: [PATCH v2 7/9] LoongArch: Tweak CFLAGS for Clang compatibility

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Date:   Sun, 25 Jun 2023 10:13:21 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     WANG Xuerui <kernel@...0n.name>
Cc:     WANG Rui <wangrui@...ngson.cn>, Xi Ruoyao <xry111@...111.site>,
        loongarch@...ts.linux.dev, linux-kbuild@...r.kernel.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        WANG Xuerui <git@...0n.name>
Subject: Re: [PATCH v2 7/9] LoongArch: Tweak CFLAGS for Clang compatibility

Hi, Ruoyao,

On Sun, Jun 25, 2023 at 2:42 AM WANG Xuerui <kernel@...0n.name> wrote:
>
> From: WANG Xuerui <git@...0n.name>
>
> 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.
>
> In particular, -mexplicit-relocs and -mdirect-extern-access are not
> necessary nor supported on Clang; feature detection via cc-option would
> not work, because that way the broken combo of "new GNU as + old GCC"
> would seem to get "fixed", but actually produce broken kernels.
> Explicitly depending on CONFIG_CC_IS_CLANG is thus necessary to not
> regress UX for those building their own kernels.
>
> A build with !RELOCATABLE && !MODULE is confirmed working within a QEMU
> environment; support for the two features are currently blocked on
> LLVM/Clang, and will come later.
>
> Signed-off-by: WANG Xuerui <git@...0n.name>
> ---
>  arch/loongarch/Makefile | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index 366771016b99..82c619791a63 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -51,7 +51,9 @@ 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 +63,10 @@ LDFLAGS_vmlinux                      += -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
I prefer to drop CONFIG_CC_IS_CLANG and use
cflags-y                       += $(call cc-option,-mexplicit-relocs)
KBUILD_CFLAGS_KERNEL           += $(call cc-option,-mdirect-extern-access)

Then Patch-6 can be merged in this.

What's your opinion?

Huacai
>  else
>  cflags-y                       += $(call cc-option,-mno-explicit-relocs)
>  KBUILD_AFLAGS_KERNEL           += -Wa,-mla-global-with-pcrel
> --
> 2.40.0
>