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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Thu, 5 Aug 2021 22:57:44 +0900
Message-ID: <CAK7LNASkBNDzXWxweotPZGJH-z3J59rPQwGDV32rfH9hH+sVHQ@mail.gmail.com>
Subject: Re: [PATCH v6 3/3] Documentation/llvm: update CROSS_COMPILE inferencing
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>
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On Tue, Aug 3, 2021 at 3:39 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> As noted by Masahiro, document how we can generally infer CROSS_COMPILE
> (and the more specific details about --target and --prefix) based on
> ARCH.
>
> Change use of env vars to command line parameters.
>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Reviewed-by: Fangrui Song <maskray@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes v5 -> v6:
> * Pick up Fangrui's RB tag.
> * Change use of env vars to command line parameters for consistency.
>
>  Documentation/kbuild/llvm.rst | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index b18401d2ba82..f8a360958f4c 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -38,7 +38,7 @@ Cross Compiling
>  A single Clang compiler binary will typically contain all supported backends,
>  which can help simplify cross compiling. ::
>
> -       ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
> +       make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
>
>  ``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
>  ``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
> @@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
>  Currently, the integrated assembler is disabled by default. You can pass
>  ``LLVM_IAS=1`` to enable it.
>
> +Omitting CROSS_COMPILE
> +----------------------
> +
> +As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
> +
> +Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> +``--prefix=<path>`` to search for the GNU assembler and linker.


Is there any place where we rely on --prefix
to search for the linker?

In general, the compiler stops after generating an object
since it is passed with the -c option.
The linking stage is separated.

In the old days, VDSO was an exceptional case
where $(CC) was used as the linker driver, but
commit fe00e50b2db8c60e4ec90befad1f5bab8ca2c800 fixed it.


> +
> +If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
> +from ``ARCH``.
> +
> +That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
> +
> +For example, to cross-compile the arm64 kernel::
> +
> +       make ARCH=arm64 LLVM=1 LLVM_IAS=1
> +
>  Supported Architectures
>  -----------------------
>
> --
> 2.32.0.554.ge1b32706d8-goog
>
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-- 
Best Regards
Masahiro Yamada