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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 11 May 2020 13:18:15 -0700
Message-ID: <CAKwvOd=dxX-KG3o6tyYmnoxMwVHvFvvHdaC5G+8ynGEFhZarVQ@mail.gmail.com>
Subject: Re: [PATCH] x86: support i386 with Clang
To:     Brian Gerst <brgerst@gmail.com>
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>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        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 12:34 PM Brian Gerst <brgerst@gmail.com> wrote:
>
> On Mon, May 11, 2020 at 2:46 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Mon, May 11, 2020 at 11:09 AM Brian Gerst <brgerst@gmail.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.

I'm not sure that's the case.  Applying this patch, undoing the hunk
in percpu_from_op() we get tons of errors.  Looking at one:

kernel/events/core.c:8679:8: error: invalid output size for constraint '=q'
./include/linux/percpu-defs.h:446:2: note: expanded from macro '__this_cpu_read'
        raw_cpu_read(pcp);                                              \
        ^
...

There's nothing wrong with this line, it's reading a percpu u64 into a
local u64.  The error comes from validating the inline asm in the dead
branch.
-- 
Thanks,
~Nick Desaulniers