linux-kernel - Re: [PATCH v3] Makefile: lld: tell clang to use lld

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Date:   Tue, 2 Apr 2019 00:27:26 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     yamada.masahiro@...ionext.com, keescook@...omium.org,
        clang-built-linux@...glegroups.com,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] Makefile: lld: tell clang to use lld

On Tue, Apr 02, 2019 at 12:08:58AM -0700, Nick Desaulniers wrote:
> This is needed because clang doesn't select which linker to use based on
> $LD but rather -fuse-ld={bfd,gold,lld,<absolute path to linker>}.  This
> is problematic especially for cc-ldoption, which checks for linker flag
> support via invoking the compiler, rather than the linker.
> 
> Select the linker via absolute path from $PATH via `which`. This allows
> you to build with:
> 
> $ make LD=ld.lld
> $ make LD=ld.lld-8
> $ make LD=/path/to/ld.lld
> 
> Add -Qunused-arguments to KBUILD_CPPFLAGS when using LLD, as otherwise
> Clang likes to complain about -fuse-lld= being unused when compiling but
> not linking (-c) such as when cc-option is used.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/342
> Link: https://github.com/ClangBuiltLinux/linux/issues/366
> Link: https://github.com/ClangBuiltLinux/linux/issues/357
> Suggested-by: Nathan Chancellor <natechancellor@...il.com>
> Suggested-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Changes V2->V3:
> * Use absolute path based on `which $LD` as per Masahiro.
> * Add -Qunused-arguments.
> * Drop tested-by/reviewed-by tags, since this patched has changed enough
>   to warrant re-testing/re-review, IMO.
> * Add more info to the commit message.
> Changes V1->V2:
> * add reviewed and tested by tags.
> * move this addition up 2 statments so that it's properly added to
>   KBUILD_*FLAGS as per Nathan.
> 
>  Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index 026fbc450906..895c076b6305 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -514,6 +514,10 @@ ifneq ($(GCC_TOOLCHAIN),)
>  CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
>  endif
>  CLANG_FLAGS	+= -no-integrated-as
> +ifneq ($(shell $(LD) --version 2>&1 | head -n 1 | grep LLD),)
> +CLANG_FLAGS	+= -fuse-ld=$(shell which $(LD))
> +KBUILD_CPPFLAGS += -Qunused-arguments

We should remove the -Qunused-arguments call in the second clang block
at the same time. With that change:

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>

> +endif
>  KBUILD_CFLAGS	+= $(CLANG_FLAGS)
>  KBUILD_AFLAGS	+= $(CLANG_FLAGS)
>  export CLANG_FLAGS
> -- 
> 2.21.0.392.gf8f6787159e-goog
>