Re: [PATCH v6 3/3] Documentation/llvm: update CROSS_COMPILE inferencing
From: Masahiro Yamada
Date: Thu Aug 05 2021 - 09:58:42 EST
On Tue, Aug 3, 2021 at 3:39 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> As noted by Masahiro, document how we can generally infer CROSS_COMPILE
> (and the more specific details about --target and --prefix) based on
> Change use of env vars to command line parameters.
> Suggested-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> 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
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