https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94703
--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to pskocik from comment #11)
> Thanks for the shot at a fix, Richard Biener.
>
> Since I have reported this, I think I should mentioned a related
> suboptimality that should probably be getting fixed alongside with this (if
> this one is getting fixed), namely that while
>
>
> int64_t zextend_int_to_int64_nospill(int *X)
> {
> union { int64_t _; } r = {0}; return memcpy(&r._,X,sizeof(*X)),r._;
> }
>
> (and hopefully later even
>
> int64_t zextend_int_to_int64_spill(int *X) { int64_t r = {0}; return
> memcpy(&r,X,sizeof(*X)),r; }
> )
>
> generates, on x86_64, the optimal
>
> zextend_int_to_int64_nospill:
> mov eax, DWORD PTR [rdi]
> ret
>
> for zeroextending promotions of sub-int types, an extra xor instruction gets
> generated, e.g.:
>
>
> int64_t zextend_short_to_int64_nospill_but_suboptimal(short *X)
> {
> union { int64_t _; } r ={0}; return memcpy(&r._,X,sizeof(*X)),r._;
> }
>
> =>
>
> zextend_short_to_int64_nospill_but_suboptimal:
> xor eax, eax
> mov ax, WORD PTR [rdi]
> ret
>
> which was surprising to me because it doesn't happen with zero-extending
> memcpy-based promotion from {,u}ints to larger types ({,u}{,l}longs).
>
> https://gcc.godbolt.org/z/ZjXaCw
I think this is PR93507 for which I have a patch queued as well.
