On Fri, Jul 26, 2024 at 09:36:15AM +0200, Jan Beulich wrote: > On 26.07.2024 09:31, Roger Pau Monné wrote: > > On Thu, Jul 25, 2024 at 05:00:22PM +0200, Jan Beulich wrote: > >> On 25.07.2024 16:54, Roger Pau Monné wrote: > >>> 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]. > >> > >> Sadly (here) it's actually the other way around, aiui. > > > > The only other option I have in mind is using an oversized array on > > the union, like: > > > > #define ALT_CALL_ARG(arg, n) \ > > union { \ > > typeof(arg) e[(sizeof(long) + sizeof(arg) - 1) / sizeof(arg)]; \ > > unsigned long r; \ > > } a ## n ## __ = { \ > > .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\ > > }; \ > > register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) = \ > > a ## n ## __.r > > Yet that's likely awful code-gen wise?
Seems OK: https://godbolt.org/z/nsdo5Gs8W > For the time being, can we perhaps > just tighten the BUILD_BUG_ON(), as iirc Alejandro had suggested? My main concern with tightening the BUILD_BUG_ON() is that then I would also like to do so for the GCC one, so that build fails uniformly. Thanks, Roger.
