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

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Date:   Tue, 2 Apr 2019 09:52:13 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <natechancellor@...il.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 2, 2019 at 9:09 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> 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>

Thanks for bringing this up again, Nick.

Suggested-by: Sedat Dilek <sedat.dilek@...il.com> (see [1]).

I suggest to do a separate patch on the move of  "KBUILD_CPPFLAGS +=
-Qunused-arguments".
( Patch was attached in [2],[3]. )

As pointed by Nathan this needs the removal in the original code-block.

Other than that:

Reviewed-by: Sedat Dilek <sedat.dilek@...il.com>

[1] https://github.com/ClangBuiltLinux/linux/issues/366#issuecomment-466836735
[2] https://github.com/ClangBuiltLinux/linux/issues/366#issuecomment-467118373
[3] https://github.com/ClangBuiltLinux/linux/files/2901865/kbuild-clang-lld-Move-Qunused-arguments-CPPFLAGS-patch.txt

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