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

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Message-ID: <CAHk-=wif=_ZomkWJAmQRCUAMHQ72V3NEQ-OteiPE56K7KoSjbQ@mail.gmail.com>
Date:   Mon, 11 May 2020 11:12:32 -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 10:24 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> Bumping for comment+review.
>
> On Mon, May 4, 2020 at 4:03 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> >
> > -                   : "qi" ((pto_T__)(val)));           \
> > +                   : "qi" ((unsigned char)(unsigned long)(val))); \

I find the extra casts really annoying and illogical.

I kind of see why they happen: just casting to 'unsigned char' can
cause warnings, when this case isn't even taken, and the sizeof(var)
is something else (and you cast a pointer to a char without going
through the 'unsigned long' first.

But that doesn't make this change any more sensible or readable.

Would using "__builtin_choose_expr()" be able to avoid this whole issue?

Because I'd rather re-write the whole thing with a different model
that doesn't have this issue, than make the code look insane.

And it does look insane, because that whole thing makes for a very
natural question: "why do we cast to unsigned long and then to char,
when we just checked that the size of the type is 1"?

                 Linus