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

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Date:   Tue, 2 Apr 2019 00:56:03 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        Clang-Built-Linux ML <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 09:52:13AM +0200, Sedat Dilek wrote:
> 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".

I think it makes sense to do it all in one patch as it doesn't make much
sense to do the move without this change. Nick already sent a v4 doing
this.

Cheers,
Nathan

> ( 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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