linux-kernel - Re: [PATCH] x86: support i386 with Clang

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Date:   Mon, 11 May 2020 13:01:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw2@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Dmitry Golovin <dima@...ovin.in>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86: support i386 with Clang

On Mon, May 11, 2020 at 12:52 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> Interesting approach.  Researching __builtin_choose_expr, it looks
> like it was cited as prior art for C11's _Generic keyword.

Well, the thing that made me think that __builtin_choose_expr() would
work is that unlike the switch statement, you absolutely _have_ to do
the choice in the front end. You can't leave it as some kind of
optimization for later phases, because the choice od expression ends
up also determining the type of the result, so it isn't just a local
choice - it affects everything around that expression.

But clang still doesn't like that "qi" constraint with a (non-chosen)
expression that has a "u64" type.

I guess we can take the stupid extra cast, but I think it would at
least need a comment (maybe through a helper function) about why "qi"
needs it, but "ri" does not, and why the cast to "unsigned long" is
needed, even though "clearly" the type is already just 8 bits.

Otherwise somebody will just remove that "obviously pointless" cast,
and gcc will eat the result happily, and clang will fail.

And nobody will notice for a while anyway, because this issue only
happens on 32-bit targets, and developers don't use those any more.

                 Linus