Re: [PATCH] x86: support i386 with Clang

[Brian Gerst]

On Mon, May 11, 2020 at 2:46 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Mon, May 11, 2020 at 11:09 AM Brian Gerst <brgerst@xxxxxxxxx> 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