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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 11 May 2020 12:52:31 -0700
Message-ID: <CAKwvOdn06jCe_FhKiF_wSQavytVVD++RXD-bHbMdqnEA-wO-9w@mail.gmail.com>
Subject: Re: [PATCH] x86: support i386 with Clang
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        David Woodhouse <dwmw2@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Dmitry Golovin <dima@golovin.in>,
        Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Mon, May 11, 2020 at 11:24 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Mon, May 11, 2020 at 11:12 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Would using "__builtin_choose_expr()" be able to avoid this whole issue?
>
> We actually have a fair amount of "pick expression based on size", so
> with a few helper macros we could make the code look better than the
> case statements too.
>
> Something (ENTIRELY UNTESTED!) like the attached patch, perhaps?
>
> NOTE! I only converted one single use to that "pick_size_xyz()" model.
> If this actually works for clang too, we could do the others.
>
> I guess I should just test it, since I have that clang tree.

Interesting approach.  Researching __builtin_choose_expr, it looks
like it was cited as prior art for C11's _Generic keyword.
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1404.htm

I'm kind of tempted now to play with _Generic and see if that makes a
difference, though I'm not hopeful.  Without checking, I don't know if
that will produce warnings with `-std=gnu89`.

...

I'm playing around with _Generic now to see if that's a possible
solution, but it looks like it can't be used for selecting inline asm
constraint strings.  But maybe there's another way to use _Generic
here.  TODO: more testing.

I don't quite understand the use of GNU C statement expressions in
pick_type_statement, though I'm guessing the return value is important
somewhere.  Maybe just looking at the preprocessed source would make
it clearer.

> "why do we cast to unsigned long and then to char,
> when we just checked that the size of the type is 1"?

Deja vu.  I don't remember who I discussed that with, maybe Arnd, but
that was something else I had asked at some point.  I must have
forgotten to look into it more before sending the patch.  Can likely
drop that at least.

--
Thanks,
~Nick Desaulniers