[Bug tree-optimization/107247] New: SLP reduction results fail to reduce to a single accumulator

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107247

            Bug ID: 107247
           Summary: SLP reduction results fail to reduce to a single
                    accumulator
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

float fl[128];
int x[128];
float
foo (int n1)
{
  float sum0, sum1, sum2, sum3;
  sum0 = sum1 = sum2 = sum3 = 0.0f;

  int n = (n1 / 4) * 4;
  for (int i = 0; i < n; i += 4)
    {
      sum0 += fabs (fl[i]);
      sum1 += fabs (fl[i + 1]);
      sum2 += fabs (fl[i + 2]);
      sum3 += fabs (fl[i + 3]);
      x[i] = 1;
    }

  return sum0 + sum1 + sum2 + sum3;
}

shows how we fail to reduce the SLP reduction accumulators to a single one
before extracting the elements:

  <bb 3> [local count: 567644343]:
  # sum0_37 = PHI <sum0_29(7), 0.0(9)>
  # sum1_39 = PHI <sum1_30(7), 0.0(9)>
  # sum2_41 = PHI <sum2_31(7), 0.0(9)>
  # sum3_43 = PHI <sum3_32(7), 0.0(9)>
  # i_45 = PHI <i_34(7), 0(9)>
  # vectp_fl.8_89 = PHI <vectp_fl.8_90(7), &fl(9)>
  # vect_sum3_43.15_102 = PHI <vect_sum3_32.16_106(7), { 0.0, 0.0, 0.0, 0.0
}(9)>
  # vect_sum3_43.15_103 = PHI <vect_sum3_32.16_107(7), { 0.0, 0.0, 0.0, 0.0
}(9)>
  # vect_sum3_43.15_104 = PHI <vect_sum3_32.16_108(7), { 0.0, 0.0, 0.0, 0.0
}(9)>
  # vect_sum3_43.15_105 = PHI <vect_sum3_32.16_109(7), { 0.0, 0.0, 0.0, 0.0
}(9)>
...
  vect__12.14_98 = ABS_EXPR <vect__11.10_91>;
  vect__12.14_99 = ABS_EXPR <vect__11.11_93>;
  vect__12.14_100 = ABS_EXPR <vect__11.12_95>;
  vect__12.14_101 = ABS_EXPR <vect__11.13_97>;
  vect_sum3_32.16_106 = vect__12.14_98 + vect_sum3_43.15_102;
  vect_sum3_32.16_107 = vect__12.14_99 + vect_sum3_43.15_103;
  vect_sum3_32.16_108 = vect__12.14_100 + vect_sum3_43.15_104;
  vect_sum3_32.16_109 = vect__12.14_101 + vect_sum3_43.15_105;
...

  <bb 11> [local count: 94607391]:
  # sum0_48 = PHI <sum0_29(3)>
  # sum1_36 = PHI <sum1_30(3)>
  # sum2_35 = PHI <sum2_31(3)>
  # sum3_24 = PHI <sum3_32(3)>
  # vect_sum3_32.16_110 = PHI <vect_sum3_32.16_106(3)>
  # vect_sum3_32.16_111 = PHI <vect_sum3_32.16_107(3)>
  # vect_sum3_32.16_112 = PHI <vect_sum3_32.16_108(3)>
  # vect_sum3_32.16_113 = PHI <vect_sum3_32.16_109(3)>
  _114 = BIT_FIELD_REF <vect_sum3_32.16_110, 32, 0>;
  _115 = BIT_FIELD_REF <vect_sum3_32.16_110, 32, 32>;
  _116 = BIT_FIELD_REF <vect_sum3_32.16_110, 32, 64>;
  _117 = BIT_FIELD_REF <vect_sum3_32.16_110, 32, 96>;
  _118 = BIT_FIELD_REF <vect_sum3_32.16_111, 32, 0>;
  _119 = BIT_FIELD_REF <vect_sum3_32.16_111, 32, 32>;
  _120 = BIT_FIELD_REF <vect_sum3_32.16_111, 32, 64>;
  _121 = BIT_FIELD_REF <vect_sum3_32.16_111, 32, 96>;
  _122 = BIT_FIELD_REF <vect_sum3_32.16_112, 32, 0>;
  _123 = BIT_FIELD_REF <vect_sum3_32.16_112, 32, 32>;
  _124 = BIT_FIELD_REF <vect_sum3_32.16_112, 32, 64>;
  _125 = BIT_FIELD_REF <vect_sum3_32.16_112, 32, 96>;
  _126 = BIT_FIELD_REF <vect_sum3_32.16_113, 32, 0>;
  _127 = BIT_FIELD_REF <vect_sum3_32.16_113, 32, 32>;
  _128 = BIT_FIELD_REF <vect_sum3_32.16_113, 32, 64>;
  _129 = BIT_FIELD_REF <vect_sum3_32.16_113, 32, 96>;
  _130 = _114 + _118;
  _131 = _115 + _119;
  _132 = _116 + _120;
  _133 = _117 + _121;
  _134 = _130 + _122;
  _135 = _131 + _123;
  _136 = _132 + _124;
  _137 = _133 + _125;
  _138 = _134 + _126;
  _139 = _135 + _127;
  _140 = _136 + _128;
  _141 = _137 + _129;
...

instead of doing vector adds and a single series of extracts.