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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/94703] Small-sized  memcpy leading to unnecessary
 register spillage unless done through a dummy union
Date: Thu, 14 May 2020 09:10:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94703

--- 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.
>=20
> 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
>=20
>=20
> int64_t zextend_int_to_int64_nospill(int *X)=20
> {=20
>     union { int64_t _; } r =3D {0}; return memcpy(&r._,X,sizeof(*X)),r._;
> }
>=20
> (and hopefully later even=20
>=20
> int64_t zextend_int_to_int64_spill(int *X) { int64_t r =3D {0}; return
> memcpy(&r,X,sizeof(*X)),r; }
> )
>=20
> generates, on x86_64, the optimal
>=20
> zextend_int_to_int64_nospill:
>         mov     eax, DWORD PTR [rdi]
>         ret
>=20
> for zeroextending promotions of sub-int types, an extra xor instruction g=
ets
> generated, e.g.:
>=20
>=20
> int64_t zextend_short_to_int64_nospill_but_suboptimal(short *X)=20
> {
> union { int64_t _; } r =3D{0}; return memcpy(&r._,X,sizeof(*X)),r._;
> }
>=20
> =3D>
>=20
> zextend_short_to_int64_nospill_but_suboptimal:
>         xor     eax, eax
>         mov     ax, WORD PTR [rdi]
>         ret
>=20
> 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).
>=20
> https://gcc.godbolt.org/z/ZjXaCw

I think this is PR93507 for which I have a patch queued as well.=