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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101923] std::function's move ctor is slower
 than the copy one for empty source objects
Date: Mon, 16 Aug 2021 07:30:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101923

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
          Component|libstdc++                   |tree-optimization

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Hmm

  __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 struc=
t Car
&) &)&f + 24];
  _14 =3D MEM[(void (*type) (const union _Any_data & {ref-all}, const struc=
t Car
&) &)&moved + 24];
  MEM[(void (*<Te9f8>) (const union _Any_data & {ref-all}, const struct Car=
 &)
&)&f + 24] =3D _14;
  MEM[(void (*<Te9f8>) (const union _Any_data & {ref-all}, const struct Car=
 &)
&)&moved + 24] =3D _13;

So a missed optimization at the gimple level.
But note the arm64 compiler on godbolt is a few months old, 20210528.  There
might have been some fixes which improve this already.=