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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/109849] suboptimal code for vector walking loop
Date: Fri, 16 Jun 2023 14:20:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109849

--- Comment #14 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
One interesting situation is:
void std::vector<std::pair<unsigned int, unsigned int> >::push_back (struct
vector * const this, const struct value_type & __x)
{
  struct __normal_iterator D.27894;
  struct pair * _1;
  struct pair * _2;
  struct pair * _3;

  <bb 2> [local count: 1073741824]:
  _1 =3D this_6(D)->D.26707._M_impl.D.26014._M_finish;
  _2 =3D this_6(D)->D.26707._M_impl.D.26014._M_end_of_storage;
  if (_1 !=3D _2)
    goto <bb 3>; [82.57%]
  else
    goto <bb 4>; [17.43%]

  <bb 3> [local count: 886588625]:
  *_1 =3D MEM[(const struct pair &)__x_7(D)];
  _3 =3D _1 + 8;
  this_6(D)->D.26707._M_impl.D.26014._M_finish =3D _3;
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 187153200]:
  D.27894._M_current =3D _1;
  std::vector<std::pair<unsigned int, unsigned int> >::_M_realloc_insert<co=
nst
std::pair<unsigned int, unsigned int>&> (this_6(D), D.27894, __x_7(D));

  <bb 5> [local count: 1073741824]:
  return;

}
here we could do partial inlining and offline the call to _M_realloc_insert=
 but
we fail to cut since _1 is already load:

Split point at BB 4
  header time: 9.302800 header size: 9
  split time: 2.440200 split size: 5
  bbs: 4
  SSA names to pass: 1, 9, 11
  Refused: need to pass non-param values=20

It should be easy to insert code loading the parameter again in the split p=
art.
We still hit the SRA limitation since this would be still escaping, but it =
is
another missed optimization on this simple testcase.=