linux-kernel - Re: [PATCH] kbuild: clang: do not use CROSS

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Message-ID: <CAKwvOd=3LoK7_qSijzk6Ww265LMDOk2-gQ-k9r0RwvENLi9JBA@mail.gmail.com>
Date:   Fri, 7 Apr 2023 12:54:58 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        linux-kbuild@...r.kernel.org, Tom Rix <trix@...hat.com>,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Fangrui Song <maskray@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>
Subject: Re: [PATCH] kbuild: clang: do not use CROSS_COMPILE for target triple

On Mon, Apr 3, 2023 at 7:48 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Sun, Apr 02, 2023 at 02:01:17AM +0900, Masahiro Yamada wrote:
> > The target triple is overridden by the user-supplied CROSS_COMPILE,
> > but I do not see a good reason to support it. Users can use a new
> > architecture without adding CLANG_TARGET_FLAGS_*, but that would be
> > a rare case.
> >
> > Use the hard-coded and deterministic target triple all the time.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
>
> I know of one bug where the value of '--target' matters:
>
> https://github.com/ClangBuiltLinux/linux/issues/1244
>
> This was fixed in LLVM 12.0.0. We are not testing this in our CI though,
> so we would not get bit by this (we could bump the minimum supported
> version of LLVM to 12.0.0 for this, we have talked recently about doing
> it for other reasons).
>
> I guess I cannot really think of a good reason not to do this aside from
> that; the target triple should only affect code generation, rather than
> tool selection (i.e., this does not take away the ability to use a
> custom set of binutils with clang).
>
> However, Nick is currently OOO and I would like his opinion voiced
> before we commit to this. Consider this a tentative:

Yeah, nothing I could think of; at this point CROSS_COMPILE is only
necessary for LLVM_IAS=0 builds and s390 (since LLD lacks s390
support) IIUC.

A user is more likely to adjust the --target for the host, which they
can do via USERCFLAGS or USERLDFLAGS, but not for the target.  I don't
think the gnu vs musl for the target triple makes a difference; we
might even be able to omit that part of the triple but I haven't
grepped through LLVM sources to see if that would result in
differences for codegen.

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> Acked-by: Nathan Chancellor <nathan@...nel.org>
>
> > ---
> >
> >  scripts/Makefile.clang | 8 ++------
> >  1 file changed, 2 insertions(+), 6 deletions(-)
> >
> > diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> > index 70b354fa1cb4..9076cc939e87 100644
> > --- a/scripts/Makefile.clang
> > +++ b/scripts/Makefile.clang
> > @@ -13,15 +13,11 @@ CLANG_TARGET_FLAGS_x86            := x86_64-linux-gnu
> >  CLANG_TARGET_FLAGS_um                := $(CLANG_TARGET_FLAGS_$(SUBARCH))
> >  CLANG_TARGET_FLAGS           := $(CLANG_TARGET_FLAGS_$(SRCARCH))
> >
> > -ifeq ($(CROSS_COMPILE),)
> >  ifeq ($(CLANG_TARGET_FLAGS),)
> > -$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang)
> > +$(error add '--target=' option to scripts/Makefile.clang)
> >  else
> >  CLANG_FLAGS  += --target=$(CLANG_TARGET_FLAGS)
> > -endif # CLANG_TARGET_FLAGS
> > -else
> > -CLANG_FLAGS  += --target=$(notdir $(CROSS_COMPILE:%-=%))
> > -endif # CROSS_COMPILE
> > +endif
> >
> >  ifeq ($(LLVM_IAS),0)
> >  CLANG_FLAGS  += -fno-integrated-as
> > --
> > 2.37.2
> >



-- 
Thanks,
~Nick Desaulniers