Re: [PATCH] x86/altcall: further refine clang workaround

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jul 25, 2024 at 03:18:29PM +0200, Jan Beulich wrote:
> On 25.07.2024 12:56, Roger Pau Monne wrote:
> > --- a/xen/arch/x86/include/asm/alternative.h
> > +++ b/xen/arch/x86/include/asm/alternative.h
> > @@ -184,11 +184,11 @@ extern void alternative_branches(void);
> >   * https://github.com/llvm/llvm-project/issues/82598
> >   */
> >  #define ALT_CALL_ARG(arg, n)                                            \
> > -    register union {                                                    \
> > -        typeof(arg) e[sizeof(long) / sizeof(arg)];                      \
> > -        unsigned long r;                                                \
> > +    register struct {                                                   \
> > +        typeof(arg) e;                                                  \
> > +        char pad[sizeof(void *) - sizeof(arg)];                         \
> 
> One thing that occurred to me only after our discussion, and I then forgot
> to mention this before you would send a patch: What if sizeof(void *) ==
> sizeof(arg)? Zero-sized arrays are explicitly something we're trying to
> get rid of.

I wondered about this, but I though it was only [] that we were trying
to get rid of, not [0].

> I was wondering whether we could get away resorting to bitfields, as those
> are well-defined when having a width of zero:
> 
>     register struct {                                                   \
>         typeof(arg) e;                                                  \
>         unsigned long pad:(sizeof(void *) - sizeof(arg)) * 8;           \
>     } ...
> 
> Yet when the width is zero, the field may not have name, whereas when the
> field uniformly doesn't have a name, Clang would, like also for
> 
>     register struct {                                                   \
>         typeof(arg) e;                                                  \
>         unsigned long :0;                                               \
>     } ...
> 
> regards that space as not needing any (re)init. Bottom line: For the
> moment I'm out of ideas.

Hm, I see.  I don't have any good ideas right now either.  Will put it
on the back burner and pick up later, already too much on my plate
right now to be playing clang games.  Thanks for your input.

Roger.