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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 20 Jul 2021 10:42:57 -0700
Message-ID: <CAHk-=wgGxu4_hgzdYpFuKd95SfnkJbPTWAQ9-fMgmMN1Oxs2xQ@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for
 LLVM=1 LLVM_IAS=1
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        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>,
        Nathan Chancellor <nathan@kernel.org>
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On Tue, Jul 20, 2021 at 1:05 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> LLVM=1 is a convenient switch to change all the
> defaults, but yet you can flip each tool individually.

Actually, I'd argue that "LLVM=1" is *not* a convenient switch.
Neither are the individual other command line settings.

When clang was the odd man out, and special, it all made sense.
Changing the path to CC was similar to changing the path to AWK. And
that's obviously why we have what we have.

But clang has become a primary compiler for some kernel communities,
and I think it might be time to just re-visit that entirely.

In particular, I think we should just make it a Kconfig option. I hate
the command flag stuff so much, that my clang tree literally has this
patch in it:

    -CC = $(CROSS_COMPILE)gcc
    +CC = $(CROSS_COMPILE)clang

so that I can just do the same "make -j128" in both my gcc tree and my
clang tree.

But each build tree already has its own .config file, so it would be a
lot more convenient if that was how the compiler was chosen, and then
"make oldconfig" would just DTRT.

We do most of the other heavy lifting in this area in Kconfig anyway,
why not add that compiler choice?

Obviously it would be gated by the tests to see which compilers are
_installed_ (and that they are valid versions), so that it doesn't ask
stupid things ("do you want gcc or clang" when only one of them is
installed and/or viable).

Hmm? So then any "LLVM=1" thing would be about the "make config"
stage, not the actual build stage.

(It has annoyed me for years that if you want to cross-compile, you
first have to do "make ARCH=xyz config" and then remember to do "make
ARCH=xyz" for the build too, but I cross-compile so seldom that I've
never really cared).

Let the flame wars^H^Hpolite discussions ensue..

                        Linus