linux-kernel - Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for LLVM=1 LLVM

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Date:   Wed, 21 Jul 2021 12:49:26 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for
 LLVM=1 LLVM_IAS=1

On Wed, Jul 21, 2021 at 2:30 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On 7/20/2021 1:04 AM, Masahiro Yamada wrote:
> > On Fri, Jul 9, 2021 at 8:25 AM 'Nick Desaulniers' via Clang Built
> > Linux <clang-built-linux@...glegroups.com> wrote:
> >>
> >> We get constant feedback that the command line invocation of make is too
> >> long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
> >> target triple, or is an absolute path outside of $PATH, but it's mostly
> >> redundant for a given SRCARCH. SRCARCH itself is derived from ARCH
> >> (normalized for a few different targets).
> >>
> >> If CROSS_COMPILE is not set, simply set --target= for CLANG_FLAGS,
> >> KBUILD_CFLAGS, and KBUILD_AFLAGS based on $SRCARCH.
> >>
> >> Previously, we'd cross compile via:
> >> $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make LLVM=1 LLVM_IAS=1
> >> Now:
> >> $ ARCH=arm64 make LLVM=1 LLVM_IAS=1
> >>
> >> For native builds (not involving cross compilation) we now explicitly
> >> specify a target triple rather than rely on the implicit host triple.
> >>
> >> Link: https://github.com/ClangBuiltLinux/linux/issues/1399
> >> Suggested-by: Arnd Bergmann <arnd@...nel.org>
> >> Suggested-by: Nathan Chancellor <nathan@...nel.org>
> >> Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> >> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> >> ---
> >> Changes v1 -> v2:
> >> * Fix typos in commit message as per Geert and Masahiro.
> >> * Use SRCARCH instead of ARCH, simplifying x86 handling, as per
> >>    Masahiro. Add his sugguested by tag.
> >> * change commit oneline from 'drop' to 'infer.'
> >> * Add detail about explicit host --target and relationship of ARCH to
> >>    SRCARCH, as per Masahiro.
> >>
> >> Changes RFC -> v1:
> >> * Rebase onto linux-kbuild/for-next
> >> * Keep full target triples since missing the gnueabi suffix messes up
> >>    32b ARM. Drop Fangrui's sugguested by tag. Update commit message to
> >>    drop references to arm64.
> >> * Flush out TODOS.
> >> * Add note about -EL/-EB, -m32/-m64.
> >> * Add note to Documentation/.
> >>
> >>   Documentation/kbuild/llvm.rst |  5 +++++
> >>   scripts/Makefile.clang        | 34 ++++++++++++++++++++++++++++++++--
> >>   2 files changed, 37 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> >> index b18401d2ba82..80c63dd9a6d1 100644
> >> --- a/Documentation/kbuild/llvm.rst
> >> +++ b/Documentation/kbuild/llvm.rst
> >> @@ -46,6 +46,11 @@ example: ::
> >>
> >>          clang --target=aarch64-linux-gnu foo.c
> >>
> >> +When both ``LLVM=1`` and ``LLVM_IAS=1`` are used, ``CROSS_COMPILE`` becomes
> >> +unnecessary and can be inferred from ``ARCH``. Example: ::
> >> +
> >> +       ARCH=arm64 make LLVM=1 LLVM_IAS=1
> >> +
> >>   LLVM Utilities
> >>   --------------
> >>
> >> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> >> index 297932e973d4..956603f56724 100644
> >> --- a/scripts/Makefile.clang
> >> +++ b/scripts/Makefile.clang
> >> @@ -1,6 +1,36 @@
> >> -ifneq ($(CROSS_COMPILE),)
> >> +# Individual arch/{arch}/Makfiles should use -EL/-EB to set intended endianness
> >> +# and -m32/-m64 to set word size based on Kconfigs instead of relying on the
> >> +# target triple.
> >> +ifeq ($(CROSS_COMPILE),)
> >> +ifneq ($(LLVM),)
> >
> >
> > Do you need to check $(LLVM) ?
> >
> >
> > LLVM=1 is a convenient switch to change all the
> > defaults, but yet you can flip each tool individually.
> >
> > Instead of LLVM=1, you still should be able to
> > get the equivalent setups by:
> >
> >
> >    make CC=clang LD=ld.lld AR=llvm-ar OBJCOPY=llvm-objcopy ...
> >
> >
> > The --target option is passed to only
> > KBUILD_CFLAGS and KBUILD_AFLAGS.
> >
> > So, when we talk about --target=,
> > we only care about whether $(CC) is Clang.
> > Not caring about $(AR), $(LD), or $(OBJCOPY).
> >
> >
> > scripts/Makefile.clang is already guarded by:
> >
> > ifneq ($(findstring clang,$(CC_VERSION_TEXT)),
>
> $ make ARCH=arm64 CC=clang LLVM_IAS=1
>
> will use the right compiler and assembler but none of the other binary
> tools because '--prefix=' will not be set so CROSS_COMPILE needs to be
> specified still, which defeats the purpose of this whole change. This
> patch is designed to work for the "normal" case of saying "I want to use
> all of the LLVM tools", not "I want to use clang by itself".


I disagree.

LLVM=1 is a shorthand.



make LLVM=1 LLVM_IAS=1

  should be equivalent to:

make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
  HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld \
  LLVM_IAS=1



We do not care about the origin of CC=clang,
whether it came from LLVM=1 or every tool was explicitly,
individually specified.



ifneq ($(LLVM),) is a garbage code
that checks a pointless thing.







> >> +ifeq ($(LLVM_IAS),1)
> >> +ifeq ($(SRCARCH),arm)
> >> +CLANG_FLAGS    += --target=arm-linux-gnueabi
> >> +else ifeq ($(SRCARCH),arm64)
> >> +CLANG_FLAGS    += --target=aarch64-linux-gnu
> >> +else ifeq ($(SRCARCH),hexagon)
> >> +CLANG_FLAGS    += --target=hexagon-linux-gnu
> >> +else ifeq ($(SRCARCH),m68k)
> >> +CLANG_FLAGS    += --target=m68k-linux-gnu
> >> +else ifeq ($(SRCARCH),mips)
> >> +CLANG_FLAGS    += --target=mipsel-linux-gnu
> >> +else ifeq ($(SRCARCH),powerpc)
> >> +CLANG_FLAGS    += --target=powerpc64le-linux-gnu
> >> +else ifeq ($(SRCARCH),riscv)
> >> +CLANG_FLAGS    += --target=riscv64-linux-gnu
> >> +else ifeq ($(SRCARCH),s390)
> >> +CLANG_FLAGS    += --target=s390x-linux-gnu
> >> +else ifeq ($(SRCARCH),x86)
> >> +CLANG_FLAGS    += --target=x86_64-linux-gnu
> >> +else
> >> +$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang)
> >> +endif # SRCARCH
> >> +endif # LLVM_IAS
> >> +endif # LLVM
> >> +else
> >>   CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
> >> -endif
> >> +endif # CROSS_COMPILE
> >> +
> >>   ifeq ($(LLVM_IAS),1)
> >>   CLANG_FLAGS    += -integrated-as
> >>   else
> >> --
> >> 2.32.0.93.g670b81a890-goog
> >>
> >> --
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> >> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210708232522.3118208-3-ndesaulniers%40google.com.
> >
> >
> >



-- 
Best Regards
Masahiro Yamada