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References: <20210708232522.3118208-1-ndesaulniers@google.com> <20210708232522.3118208-3-ndesaulniers@google.com>
In-Reply-To: <20210708232522.3118208-3-ndesaulniers@google.com>
From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Tue, 20 Jul 2021 17:04:58 +0900
Message-ID: <CAK7LNARye5Opc0AdXpn+DHB7hTaphoRSCUWxJgXu+sjuNjWUCg@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for
 LLVM=1 LLVM_IAS=1
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Miguel Ojeda <ojeda@kernel.org>, Fangrui Song <maskray@google.com>,
        Michal Marek <michal.lkml@markovi.net>,
        Arnd Bergmann <arnd@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Christoph Hellwig <hch@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Nathan Chancellor <nathan@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Jul 9, 2021 at 8:25 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.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@kernel.org>
> Suggested-by: Nathan Chancellor <nathan@kernel.org>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.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)),


> +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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-- 
Best Regards
Masahiro Yamada