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

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Message-ID: <CAK7LNAQsgB-SC2t+9ZpDwJbME+BVt+Gur1ydTnHfBfMAfn_y0w@mail.gmail.com>
Date:   Sun, 16 Apr 2023 22:02:12 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     linux-kbuild@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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 11:48 PM 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


I did not look into it closely, but if we say
"
Using either CROSS_COMPILE=powerpc64-linux-gnu- or
CROSS_COMPILE=powerpc-linux-gnu- fixes it.
Using KCFLAGS=-v reveals that powerpc64le-linux-gnu-as is not getting
the endianness information.
", why didn't we fix it like the following?


 diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
 index 70b354fa1cb4..8dda7dc69c93 100644
 --- a/scripts/Makefile.clang
 +++ b/scripts/Makefile.clang
 @@ -6,7 +6,7 @@ CLANG_TARGET_FLAGS_arm64        := aarch64-linux-gnu
  CLANG_TARGET_FLAGS_hexagon     := hexagon-linux-musl
  CLANG_TARGET_FLAGS_m68k                := m68k-linux-gnu
  CLANG_TARGET_FLAGS_mips                := mipsel-linux-gnu
 -CLANG_TARGET_FLAGS_powerpc     := powerpc64le-linux-gnu
 +CLANG_TARGET_FLAGS_powerpc     := powerpc64-linux-gnu
  CLANG_TARGET_FLAGS_riscv       := riscv64-linux-gnu
  CLANG_TARGET_FLAGS_s390                := s390x-linux-gnu
  CLANG_TARGET_FLAGS_x86         := x86_64-linux-gnu



We do not need to test all possible target triples.
We can just use the one that is known to work.


Anyway, I will apply this patch. Thanks.


>
> 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:
>
> 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
> >



-- 
Best Regards
Masahiro Yamada