linux-kernel - Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for LLVM=1 LLVM

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Message-ID: <CAKwvOdkvju7heeNpk87brsjkhXHbdKFsUgf63KWhXox9rDkQsA@mail.gmail.com>
Date:   Tue, 20 Jul 2021 14:29:35 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Christoph Hellwig <hch@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for
 LLVM=1 LLVM_IAS=1

On Tue, Jul 20, 2021 at 10:43 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> 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..

I will concede that "why do I need to respecify all of my command line
args/env vars to make when I already did so for a configure step?" is
a reasonable question to ask, and may be worth pursuing.

There's still some that we can eliminate for BOTH configure and actual
build, like CROSS_COMPILE. :^)
-- 
Thanks,
~Nick Desaulniers