Re: [PATCH] x86: support i386 with Clang

From: Brian Gerst <brgerst@gmail.com>
To: Nick Desaulniers <ndesaulniers@google.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>
Subject: Re: [PATCH] x86: support i386 with Clang
Date: Mon, 11 May 2020 15:34:47 -0400	[thread overview]
Message-ID: <CAMzpN2gTEwGh0U+L3_R6pC8Qmv1iY7bRTiTEXD86mF3u9Nnkqg@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOdnxV_KwC-q73e3basJvo4-9FCGeMUOrZLj5xyt6Yyeh2A@mail.gmail.com>

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.

--
Brian Gerst