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
