On Fri Jul 26, 2024 at 3:17 PM BST, Alejandro Vallejo wrote: > On Fri Jul 26, 2024 at 9:05 AM BST, Jan Beulich wrote: > > On 26.07.2024 09:52, Roger Pau Monné wrote: > > > 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 > > > > In which case why not go this route. If the compiler is doing fine with > > that, maybe the array dimension expression could be further simplified, > > accepting yet more over-sizing? Like "sizeof(void *) / sizeof (arg) + 1" > > or even simply "sizeof(void *)"? Suitably commented of course ... > > > > >> 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. > > > > If we were to take that route, then yes, probably should constrain both > > (with a suitable comment on the gcc one). > > > > Jan > > Yet another way would be to have an intermediate `long` to cast onto. > Compilers > will optimise away the copy. It ignores the different-type aliasing rules in > the C spec, so there's an assumption that we have -fno-strict-aliasing. But I > belive we do? Otherwise it should pretty much work on anything. > > ``` > #define ALT_CALL_ARG(arg, n) \ > unsigned long __tmp = 0; \ > *(typeof(arg) *)&__tmp = \ > ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); }) \ > register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) = __tmp; \ > ``` > > fwiw, clang18 emits identical code compared with the previous godbolt link. > > Link: https://godbolt.org/z/facd1M9xa > > Cheers, > Alejandro
Bah. s/b/__tmp/ in line15. Same output though, so the point still stands. Cheers, Alejandro
