linux-kernel - Re: [PATCH 3/4] Makefile: lld: tell clang to use lld

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Date:   Fri, 8 Feb 2019 10:45:53 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, sedat.dilek@...il.com
Subject: Re: [PATCH 3/4] Makefile: lld: tell clang to use lld

On Fri, Feb 08, 2019 at 09:27:19AM -0800, Nick Desaulniers wrote:
> On Thu, Feb 7, 2019 at 9:51 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > On Thu, Feb 07, 2019 at 02:01:51PM -0800, ndesaulniers@...gle.com wrote:
> > > This is needed because clang doesn't select which linker to use based on
> > > $LD but rather -fuse-ld=$(LD). This is problematic especially for
> > > cc-ldoption, which checks for linker flag support via invoking the
> > > compiler, rather than the linker.
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/342
> > > Suggested-by: Nathan Chancellor <natechancellor@...il.com>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > > ---
> > >  Makefile | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/Makefile b/Makefile
> > > index 0eae4277206e..6307c17259ea 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -502,6 +502,9 @@ endif
> > >  CLANG_FLAGS  += -no-integrated-as
> > >  KBUILD_CFLAGS        += $(CLANG_FLAGS)
> > >  KBUILD_AFLAGS        += $(CLANG_FLAGS)
> > > +ifneq ($(shell $(LD) --version 2>&1 | head -n 1 | grep LLD),)
> > > +CLANG_FLAGS  += -fuse-ld=lld
> 
> Sorry, Sedat reports [0] that this is not quite correct; if $(LD)
> matches LLD, then we need to set -fuse-ld=$(LD), not -fuse-ld=lld,
> since you may have lld installed as lld-7 or lld-8, for example:
> https://github.com/ClangBuiltLinux/linux/issues/358
> 

We'll need to make sure that it is compatible with the way that Clang
expects arguments to '-fuse-ld=' because it looks like 'ld.' is
automatically appended.

https://github.com/llvm/llvm-project/blob/5be3dbdb082ffba4f846e3572ea802d156dd8d34/clang/lib/Driver/ToolChain.cpp#L438-L468

> > > +endif
> >
> > This section needs to be moved up above KBUILD_CFLAGS otherwise it never
> > actually gets passed along.
> >
> > With that change, please add:
> >
> > Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
> > Tested-by: Nathan Chancellor <natechancellor@...il.com>
> >
> > >  export CLANG_FLAGS
> > >  endif
> > >
> > > --
> > > 2.20.1.791.gb4d0f1c61a-goog
> > >
> 
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers