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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/105216] New: [12 regression] 8% regression
 for m-queens compared to gcc11 O2
Date: Mon, 11 Apr 2022 07:57:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105216

            Bug ID: 105216
           Summary: [12 regression] 8% regression for m-queens compared to
                    gcc11 O2
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: crazylht at gmail dot com
  Target Milestone: ---

Created attachment 52778
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D52778&action=3Dedit
g++ -O2 main.c;./a.out

Regression happens in pass_pre

      while (posib !=3D UINT_FAST32_MAX) {
        // The standard trick for getting the rightmost bit in the mask
        uint_fast32_t bit =3D ~posib & (posib + 1);
        posib ^=3D bit; // Eliminate the tried possibility.
        uint_fast32_t new_diagl =3D (bit << 1) | diagl_shifted;
        uint_fast32_t new_diagr =3D (bit >> 1) | diagr_shifted;
        bit |=3D cols[d];
        uint_fast32_t new_posib =3D (bit | new_diagl | new_diagr);

        if (new_posib !=3D UINT_FAST32_MAX) {
            uint_fast32_t lookahead1 =3D (bit | (new_diagl << (LOOKAHEAD - =
2)) |
(new_diagr >> (LOOKAHEAD - 2)));
            uint_fast32_t lookahead2 =3D (bit | (new_diagl << (LOOKAHEAD - =
1)) |
(new_diagr >> (LOOKAHEAD - 1)));
            uint_fast32_t allowed2 =3D l_rest > (int8_t)0;

            if(allowed2 && ((lookahead2 =3D=3D UINT_FAST32_MAX) || (lookahe=
ad1 =3D=3D
UINT_FAST32_MAX))) {
                continue;
            }


          if(l_rest =3D=3D (STORE_LEVEL + 1)) {
            start_level4[level4_cnt][0] =3D bit;   // cols
            start_level4[level4_cnt][1] =3D new_diagl; // diagl
            start_level4[level4_cnt][2] =3D new_diagr; // diagr
            level4_cnt++;
            continue;
          }

          l_rest--;

          // The next two lines save stack depth + backtrack operations
          // when we passed the last possibility in a row.
          // Go lower in the stack, avoid branching by writing above the
current
          // position
          posibs[d] =3D posib;
          d +=3D posib !=3D UINT_FAST32_MAX; // avoid branching with this t=
rick
          posib =3D new_posib;


          // make values current
          cols[d] =3D bit;
          diagl[d] =3D new_diagl;
          diagr[d] =3D new_diagr;
          rest[d] =3D l_rest;
          diagl_shifted =3D new_diagl << 1;
          diagr_shifted =3D new_diagr >> 1;
        }
      }
      d--;
      posib =3D posibs[d]; // backtrack ...
    }

For bit |=3D cols[d]; cols[d] is loop invariant if loop latch from *continu=
e*,
GCC11 catches it, GCC12 not.=