Re: [PATCH v4] Makefile: lld: tell clang to use lld

[Nathan Chancellor <natechancellor@gmail.com>]

On Tue, Apr 02, 2019 at 12:33:17AM -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 sooner, as otherwise
> Clang likes to complain about -fuse-lld= being unused when compiling but
> not linking (-c) such as when cc-option is used. There's no need to
> guard with cc-option.
> 
> 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@gmail.com>
> Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

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

> ---
> Changes V3->V4:
> * Unconditionally add -Qunused-arguments sooners, as per Nathan.
> * Slight modification to commit message for that point.
> Changes V2->V3:
> * Use absolute path based on `which $LD` as per Masahiro.
> * Add -Qunused-arguments.
> * Drop tested-by/reviewed-by tags, since this patched has changed enough
>   to warrant re-testing/re-review, IMO.
> * Add more info to the commit message.
> Changes V1->V2:
> * add reviewed and tested by tags.
> * move this addition up 2 statments so that it's properly added to
>   KBUILD_*FLAGS as per Nathan.
> 
>  Makefile | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/Makefile b/Makefile
> index 026fbc450906..b290e76e1ca5 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -514,6 +514,10 @@ ifneq ($(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))
> +endif
> +KBUILD_CPPFLAGS	+= -Qunused-arguments
>  KBUILD_CFLAGS	+= $(CLANG_FLAGS)
>  KBUILD_AFLAGS	+= $(CLANG_FLAGS)
>  export CLANG_FLAGS
> @@ -716,7 +720,6 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
>  KBUILD_CFLAGS += $(stackp-flags-y)
>  
>  ifdef CONFIG_CC_IS_CLANG
> -KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
>  KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
>  KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
>  KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
> -- 
> 2.21.0.392.gf8f6787159e-goog
>