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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 7 Mar 2022 11:08:29 -0800
Message-ID: <CAKwvOd=Q-7vPaRPj1wQagFsY3txcAKzrqU_D2UAX3h4ym91uUA@mail.gmail.com>
Subject: Re: [PATCH v2] kbuild: Make $(LLVM) more flexible
To:     Nathan Chancellor <nathan@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>
Cc:     Kees Cook <keescook@chromium.org>, Shuah Khan <shuah@kernel.org>,
        llvm@lists.linux.dev, linux-kbuild@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>
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On Fri, Mar 4, 2022 at 9:14 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index d32616891dcf..68b74416ec48 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -49,17 +49,36 @@ example: ::
>  LLVM Utilities
>  --------------
>
> -LLVM has substitutes for GNU binutils utilities. Kbuild supports ``LLVM=1``
> -to enable them. ::
> -
> -       make LLVM=1
> -
> -They can be enabled individually. The full list of the parameters: ::
> +LLVM has substitutes for GNU binutils utilities. They can be enabled individually.
> +The full list of supported make variables: ::
>
>         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
>
> +To simplify the above command, Kbuild supports the ``LLVM`` variable: ::
> +
> +       make LLVM=1
> +
> +If your LLVM tools are not available in your PATH, you can supply their
> +location using the LLVM variable with a trailing slash: ::
> +
> +       make LLVM=/path/to/llvm/
> +
> +which will use ``/path/to/llvm/clang``, ``/path/to/llvm/ld.lld``, etc.

I don't think we should do this; `PATH=/path/to/llvm/ make LLVM=1`
works and (my interpretation of what) Masahiro said "if anyone asks
for this, here's how we could do that."  I don't think I've seen an
explicit ask for that. I'd rather LLVM= have 2 behaviors than 3, but I
won't hold this patch up over that.  Either way:

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> +
> +If your LLVM tools have a version suffix and you want to test with that
> +explicit version rather than the unsuffixed executables like ``LLVM=1``, you
> +can pass the suffix using the ``LLVM`` variable: ::
> +
> +       make LLVM=-14
> +
> +which will use ``clang-14``, ``ld.lld-14``, etc.
> +
> +``LLVM=0`` is not the same as omitting ``LLVM`` altogether, it will behave like
> +``LLVM=1``.

Hmm... I can see someone's build wrappers setting LLVM=1, then them
being surprised that appending LLVM=0 doesn't disable LLVM=1 as they
might expect.  But Masahiro says let's fix this later which is fine.

--
Thanks,
~Nick Desaulniers