Re: [PATCH v5] x86: bitops: fix build regression

[Brian Gerst <brgerst@gmail.com>]

On Sat, May 9, 2020 at 8:20 AM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Fri, May 8, 2020 at 9:35 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > This is easily reproducible via CC=clang+CONFIG_STAGING=y+CONFIG_VT6656=m.
> >
> > It turns out that if your config tickles __builtin_constant_p via
> > differences in choices to inline or not, these statements produce
> > invalid assembly:
> >
> > $ cat foo.c
> > long a(long b, long c) {
> >   asm("orb\t%1, %0" : "+q"(c): "r"(b));
> >   return c;
> > }
> > $ gcc foo.c
> > foo.c: Assembler messages:
> > foo.c:2: Error: `%rax' not allowed with `orb'
> >
> > Use the `%b` "x86 Operand Modifier" to instead force register allocation
> > to select a lower-8-bit GPR operand.
> >
> > The "q" constraint only has meaning on -m32 otherwise is treated as
> > "r". Not all GPRs have low-8-bit aliases for -m32.
> >
>
> Looks very good!
> One question though, does it work with minimum supported version of gcc?

Yes.  The operand width modifiers have been around a long time but not
well documented until more recently.

--
Brian Gerst