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 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.
Jan
