linux-kernel - Re: [PATCH] Makefile: add -fuse-ld=lld to KBUILD

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Message-ID: <20200820222059.GA10485@Ryzen-9-3900X.localdomain>
Date:   Thu, 20 Aug 2020 15:20:59 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>, stable@...r.kernel.org,
        Matthias Maennich <maennich@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] Makefile: add -fuse-ld=lld to KBUILD_HOSTLDFLAGS when
 LLVM=1

On Thu, Aug 20, 2020 at 03:09:55PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> While moving Android kernels over to use LLVM=1, we observe the failure
> when building in a hermetic docker image:
>   HOSTCC  scripts/basic/fixdep
> clang: error: unable to execute command: Executable "ld" doesn't exist!
> 
> The is because the build of the host utility fixdep builds the fixdep
> executable in one step by invoking the compiler as the driver, rather
> than individual compile then link steps.
> 
> Clang when configured from source defaults to use the system's linker,
> and not LLVM's own LLD, unless the CMake config
> -DCLANG_DEFAULT_LINKER='lld' is set when configuring a build of clang
> itself.
> 
> Don't rely on the compiler's implicit default linker; be explicit.
> 
> Cc: stable@...r.kernel.org
> Fixes: commit a0d1c951ef08 ("kbuild: support LLVM=1 to switch the default tools to Clang/LLVM")

Minor nit, "commit" is unnecessary here and might be flagged by some tag
checking scripts.

> Reported-by: Matthias Maennich <maennich@...gle.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

Regardless of the above, this should work fine so:

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

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/Makefile b/Makefile
> index def590b743a9..b4e93b228a26 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -436,6 +436,7 @@ OBJDUMP		= llvm-objdump
>  READELF		= llvm-readelf
>  OBJSIZE		= llvm-size
>  STRIP		= llvm-strip
> +KBUILD_HOSTLDFLAGS	+= -fuse-ld=lld
>  else
>  CC		= $(CROSS_COMPILE)gcc
>  LD		= $(CROSS_COMPILE)ld
> -- 
> 2.28.0.297.g1956fa8f8d-goog
>