linux-kernel - Re: [PATCH] kbuild: check CONFIG_AS_IS_LLVM instead of LLVM

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Date:   Thu, 5 Aug 2021 11:46:10 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        linux-kbuild@...r.kernel.org
Cc:     Albert Ou <aou@...s.berkeley.edu>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH] kbuild: check CONFIG_AS_IS_LLVM instead of LLVM_IAS

On 8/5/2021 8:01 AM, Masahiro Yamada wrote:
> LLVM_IAS is the user interface to set the -(no-)integrated-as flag,
> and it should be used only for that purpose.
> 
> LLVM_IAS is checked in some places to determine the assembler type,
> but it is not precise.
> 
>   $ make CC=gcc LLVM_IAS=1
> 
> ... will use the GNU assembler since LLVM_IAS=1 is effective only when
> $(CC) is clang.
> 
> Of course, the combination of CC=gcc and LLVM_IAS=1 does not make
> sense, but the build system can be more robust against such insane
> input.
> 
> Commit ba64beb17493a ("kbuild: check the minimum assembler version in
> Kconfig") introduced CONFIG_AS_IS_GNU/LLVM, which is more precise
> because Kconfig checks the version string from the assembler in use.
> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>

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

> ---
> 
>   Makefile            | 2 +-
>   arch/riscv/Makefile | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 4ab151060931..8b56c9692c3c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -844,7 +844,7 @@ else
>   DEBUG_CFLAGS	+= -g
>   endif
>   
> -ifneq ($(LLVM_IAS),1)
> +ifndef CONFIG_AS_IS_LLVM
>   KBUILD_AFLAGS	+= -Wa,-gdwarf-2
>   endif
>   
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index bc74afdbf31e..dcfbd2a87d41 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -41,7 +41,7 @@ endif
>   ifeq ($(CONFIG_LD_IS_LLD),y)
>   	KBUILD_CFLAGS += -mno-relax
>   	KBUILD_AFLAGS += -mno-relax
> -ifneq ($(LLVM_IAS),1)
> +ifndef CONFIG_AS_IS_LLVM
>   	KBUILD_CFLAGS += -Wa,-mno-relax
>   	KBUILD_AFLAGS += -Wa,-mno-relax
>   endif
>