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

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Message-ID: <CAMzpN2jcCM4m_HnJPc6kpVhnf588w6bPxYM_bCEd3OgbnvBvTg@mail.gmail.com>
Date:   Mon, 11 May 2020 18:54:00 -0400
From:   Brian Gerst <brgerst@...il.com>
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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 3:34 PM Brian Gerst <brgerst@...il.com> wrote:
>
> On Mon, May 11, 2020 at 2:46 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Mon, May 11, 2020 at 11:09 AM Brian Gerst <brgerst@...il.com> wrote:
> > > This looks like the same issue that we just discussed for bitops.h.
> > > Add the "b" operand size modifier to force it to use the 8-bit
> > > register names (and probably also needs the "w" modifier in the 16-bit
> > > case).
> >
> > While it does feel familiar, it is slightly different.
> > https://godbolt.org/z/Rme4Zg
> > That case was both compilers validating the inline asm, yet generating
> > assembly that the assembler would choke on.  This case is validation
> > in the front end failing.
>
> > long long ret;
> > switch (sizeof(ret)) {
> > case 1:
> >         asm ("movb $5, %0" : "=q" (ret));
> >         break;
> > case 8:;
> > }
>
> So if the issue here is that the output variable type is long long,
> what code is using a 64-bit percpu variable on a 32-bit kernel?  Can
> you give a specific file that fails to build with Clang?  If Clang is
> choking on it it may be silently miscompiling on GCC.

On further investigation, 64-bit percpu operations fall back to the
generic code on x86-32, so there is no problem with miscompiling here.

On a side note from looking at the preprocessed output of the percpu
macros: they generate a ton of extra dead code because the core macros
also have a switch on data size.  I will take a stab at cleaning that
up.

--
Brian Gerst