linux-kernel - Re: [PATCH 1/4] kbuild: remove LLVM=1 test from HAS_LTO

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Date:   Wed, 3 Mar 2021 16:37:51 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] kbuild: remove LLVM=1 test from HAS_LTO_CLANG

On Thu, Mar 04, 2021 at 03:33:30AM +0900, Masahiro Yamada wrote:
> This guarding is wrong. As Documentation/kbuild/llvm.rst notes, LLVM=1
> switches the default of tools, but you can still override CC, LD, etc.
> individually.
> 
> BTW, LLVM is not 1/0 flag. If LLVM is not passed in, it is empty.
> 
> Non-zero return code is all treated as failure anyway.
> 
> So, $(success,test $(LLVM) -eq 1) and $(success,test "$(LLVM)" = 1)
> works equivalently in the sense that both are expanded to 'n' if LLVM
> is not given. The difference is that the former internally fails due
> to syntax error.
> 
>   $ test ${LLVM} -eq 1
>   bash: test: -eq: unary operator expected
>   $ echo $?
>   2
> 
>   $ test "${LLVM}" -eq 1
>   bash: test: : integer expression expected
>   $ echo $?
>   2
> 
>   $ test "${LLVM}" = 1
>   echo $?
>   1
> 
>   $ test -n "${LLVM}"
>   $ echo $?
>   1
> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>

Yes, there is not too much point of checking if $(LLVM) is set or not
because we already check for the other tools.

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

> ---
> 
>  arch/Kconfig | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 2bb30673d8e6..2af10ebe5ed0 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -632,7 +632,6 @@ config HAS_LTO_CLANG
>  	def_bool y
>  	# Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510
>  	depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD
> -	depends on $(success,test $(LLVM) -eq 1)
>  	depends on $(success,test $(LLVM_IAS) -eq 1)
>  	depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
>  	depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
> -- 
> 2.27.0
>