[Bug middle-end/94703] Small-sized memcpy leading to unnecessary register spillage unless done through a dummy union

["pskocik at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94703

--- Comment #11 from pskocik at gmail dot com ---
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