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? For the time being, can we perhaps just tighten the BUILD_BUG_ON(), as iirc Alejandro had suggested? Jan
