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

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Date:   Mon, 11 May 2020 21:03:28 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        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, 2020-05-11 at 13:01 -0700, Linus Torvalds wrote:
> 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.

I'm also mildly concerned that LLVM will start to whine about the 'ri'
case too. It's odd that it doesn't, even when GCC does.


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