[Bug tree-optimization/101944] New: suboptimal SLP for reduced case from namd_r

["linkw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101944

            Bug ID: 101944
           Summary: suboptimal SLP for reduced case from namd_r
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: linkw at gcc dot gnu.org
  Target Milestone: ---

For SPEC2017 bmk 508.namd_r, it's observed that it degraded by -3.73% 
at -O2 -ftree-slp-vectorize vs baseline -O2 on Power9 with either default cost
model or very cheap cost model. By isolating functions, several functions are
responsible for it.  One typical case is the below reduced one:

------------- TEST CASE 

typedef double BigReal;

extern BigReal table_four_i[8];
extern void func1(BigReal *);
extern void func2(BigReal *);
extern void func3(BigReal, BigReal, BigReal, BigReal);

void foo(BigReal scaling, BigReal *A1, BigReal *B1, BigReal diffa) {

  BigReal vdwEnergy = 0;
  func1(&vdwEnergy);

  const BigReal A = scaling * *A1;
  const BigReal B = scaling * *B1;

  BigReal vdw_d = A * table_four_i[0] - B * table_four_i[2];
  BigReal vdw_c = A * table_four_i[1] - B * table_four_i[3];
  BigReal vdw_b = A * table_four_i[4] - B * table_four_i[6];
  BigReal vdw_a = A * table_four_i[5] - B * table_four_i[7];

  register BigReal vdw_val =
      ((diffa * vdw_d * (1 / 6.) + vdw_c * (1 / 4.)) * diffa +
       vdw_b * (1 / 2.)) * diffa + vdw_a;
  vdwEnergy -= vdw_val;

  func2 (&vdwEnergy);
  func3 (vdw_a, vdw_b, vdw_c, vdw_d);
}

-------------

Options: -O2 -ffast-math -ftree-slp-vectorize -mcpu=power9

Scalar version at optimized dumping:

  func1 (&vdwEnergy);
  _1 = *A1_32(D);
  A_34 = _1 * scaling_33(D);
  _2 = *B1_35(D);
  B_36 = _2 * scaling_33(D);
  _3 = table_four_i[0];
  _5 = table_four_i[2];
  _6 = _5 * B_36;
  vdw_d_37 = .FMS (_3, A_34, _6);
  _7 = table_four_i[1];
  _9 = table_four_i[3];
  _10 = _9 * B_36;
  vdw_c_38 = .FMS (_7, A_34, _10);
  _11 = table_four_i[4];
  _13 = table_four_i[6];
  _14 = _13 * B_36;
  vdw_b_39 = .FMS (_11, A_34, _14);
  _15 = table_four_i[5];
  _17 = table_four_i[7];
  _18 = _17 * B_36;
  vdw_a_40 = .FMS (_15, A_34, _18);
  _51 = diffa_41(D) *
1.666666666666666574148081281236954964697360992431640625e-1;
  _21 = vdw_c_38 * 2.5e-1;
  _22 = .FMA (vdw_d_37, _51, _21);
  _24 = vdw_b_39 * 5.0e-1;
  _25 = .FMA (_22, diffa_41(D), _24);
  _26 = _25 * diffa_41(D);
  vdwEnergy.0_27 = vdwEnergy;
  _49 = _18 + vdwEnergy.0_27;
  _52 = .FMA (_15, A_34, _26);
  _28 = _49 - _52;
  vdwEnergy = _28;
  func2 (&vdwEnergy);


Vector version at optimized dumping:

  func1 (&vdwEnergy);
  _1 = *A1_32(D);
  A_34 = _1 * scaling_33(D);
  _49 = {A_34, A_34};
  _2 = *B1_35(D);
  B_36 = _2 * scaling_33(D);
  _54 = {B_36, B_36};
  vect__3.6_48 = MEM <vector(2) double> [(double *)&table_four_i];
  vect__5.10_52 = MEM <vector(2) double> [(double *)&table_four_i + 16B];
  vect__6.11_55 = vect__5.10_52 * _54;
  vect_vdw_d_37.12_56 = .FMS (vect__3.6_48, _49, vect__6.11_55);
  _58 = BIT_FIELD_REF <vect_vdw_d_37.12_56, 64, 64>;
  _57 = BIT_FIELD_REF <vect_vdw_d_37.12_56, 64, 0>;
  _11 = table_four_i[4];
  _13 = table_four_i[6];
  _14 = _13 * B_36;
  vdw_b_39 = .FMS (_11, A_34, _14);
  _15 = table_four_i[5];
  _17 = table_four_i[7];
  _18 = _17 * B_36;
  vdw_a_40 = .FMS (_15, A_34, _18);
  _51 = diffa_41(D) *
1.666666666666666574148081281236954964697360992431640625e-1;
  _59 = {_51, 2.5e-1};
  vect__20.13_60 = vect_vdw_d_37.12_56 * _59;
  _61 = .REDUC_PLUS (vect__20.13_60);
  _24 = vdw_b_39 * 5.0e-1;
  _25 = .FMA (diffa_41(D), _61, _24);
  _26 = _25 * diffa_41(D);
  vdwEnergy.0_27 = vdwEnergy;
  _66 = _18 + vdwEnergy.0_27;
  _68 = .FMA (_15, A_34, _26);
  _28 = _66 - _68;
  vdwEnergy = _28;
  func2 (&vdwEnergy);

reduc.c:24:34: note: Cost model analysis for part in loop 0:
  Vector cost: 16
  Scalar cost: 17