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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/110711] possible missed optimization for std::max
 with -march=znver2
Date: Wed, 19 Jul 2023 06:57:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110711

--- Comment #4 from Hongtao.liu <crazylht at gmail dot com> ---
  <bb 18> [local count: 105119324]:
  _6 =3D .LOOP_VECTORIZED (1, 2);
  if (_6 !=3D 0)
    goto <bb 19>; [100.00%]
  else
    goto <bb 20>; [100.00%]

  <bb 19> [local count: 105119324]:

  <bb 5> [local count: 955630227]:
  # i_21 =3D PHI <i_16(10), 1(19)>
  # prephitmp_19 =3D PHI <prephitmp_24(10), _1(19)>
  _2 =3D i_21 * 8;
  _3 =3D a_10(D) + _2;
  _17 =3D MEM[(const double &)_3];
  _29 =3D MAX_EXPR <_17, prephitmp_19>;
  prephitmp_24 =3D _29;
  i_16 =3D i_21 + 1;
  if (n_els_7(D) > i_16)
    goto <bb 10>; [89.00%]
  else
    goto <bb 13>; [11.00%]

  <bb 13> [local count: 105119324]:
  # prephitmp_15 =3D PHI <_29(5), prephitmp_27(16)>
  goto <bb 8>; [100.00%]

  <bb 10> [local count: 850510903]:
  goto <bb 5>; [100.00%]


test.C:8:26: note:   Analyze phi: prephitmp_19 =3D PHI <prephitmp_24(10), _=
1(19)>
test.C:8:26: note:   reduction path: prephitmp_24 _29 prephitmp_19=20
test.C:8:26: note:   reduction: unknown pattern
test.C:8:26: missed:   Unknown def-use cycle pattern.
test.C:8:26: note:   =3D=3D=3D vect_determine_precisions =3D=3D=3D

It looks like there's extra move generate by ifcvt which make vectorizer th=
ink
it's not a reduction. prephitmp_24 can be replaced with _29 since it's not =
used
elsewhere.=