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From: "dartdart26 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101923] std::function's move ctor is slower
 than the copy one for empty source objects
Date: Tue, 17 Aug 2021 06:09:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101923

--- Comment #5 from Petar Ivanov <dartdart26 at gmail dot com> ---
(In reply to Andrew Pinski from comment #4)
> Hmm
>=20
>   __tmp =3D MEM[(union _Any_data & {ref-all})&f];
>   MEM[(union _Any_data * {ref-all})&f] =3D MEM[(union _Any_data &
> {ref-all})&moved];
>   MEM[(union _Any_data * {ref-all})&moved] =3D __tmp;
>   __tmp =3D{v} {CLOBBER};
>   _13 =3D MEM[(void (*type) (const union _Any_data & {ref-all}, const str=
uct
> Car &) &)&f + 24];
>   _14 =3D MEM[(void (*type) (const union _Any_data & {ref-all}, const str=
uct
> Car &) &)&moved + 24];
>   MEM[(void (*<Te9f8>) (const union _Any_data & {ref-all}, const struct C=
ar
> &) &)&f + 24] =3D _14;
>   MEM[(void (*<Te9f8>) (const union _Any_data & {ref-all}, const struct C=
ar
> &) &)&moved + 24] =3D _13;
>=20
> So a missed optimization at the gimple level.
> But note the arm64 compiler on godbolt is a few months old, 20210528.  Th=
ere
> might have been some fixes which improve this already.

I see, thank you.

Do you think the x86 results on quick bench are something worth improving? =
From
a user's perspective, I assume the expectation is that moves are at least as
fast as copies.

Could you please advise on how I can proceed with this report? Can a change=
 be
made in libstdc++ or should it be considered a compiler issue?

Thank you!=