Re: [PATCH] x86: support i386 with Clang

[Brian Gerst <brgerst@gmail.com>]

On Mon, May 11, 2020 at 3:34 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.

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