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 > ARCH. > > 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 > ----------------------- > > -- > 2.32.0.554.ge1b32706d8-goog > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210802183910.1802120-4-ndesaulniers%40google.com. -- Best Regards Masahiro Yamada
