[Bug tree-optimization/102436] New: [11/12 Regression] Lost Load/Store Motion

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102436

            Bug ID: 102436
           Summary: [11/12 Regression] Lost Load/Store Motion
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: law at gcc dot gnu.org
  Target Milestone: ---

Created attachment 51492
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51492&action=edit
Testcase

So consider this loop (-O2, lim2 dump, trunk, x86_64):



;;   basic block 3, loop depth 1
;;    pred:       2
;;                10
  # target_8 = PHI <target_13(D)(2), target_17(10)>
  _4 = board[target_8];
  if (_4 == 13)
    goto <bb 4>; [94.50%]
  else
    goto <bb 7>; [5.50%]
;;    succ:       4
;;                7

;;   basic block 4, loop depth 1
;;    pred:       3
  if (captures.32_5 == 0)
    goto <bb 5>; [33.00%]
  else
    goto <bb 6>; [67.00%]
;;    succ:       5
;;                6

;;   basic block 5, loop depth 1
;;    pred:       4
  numb_moves.1_21 = numb_moves;
  _22 = (long unsigned int) numb_moves.1_21;
  _23 = _22 * 24;
  _24 = (struct move_s *) _23;
  _24->from = gfrom.30_1;
  _24->target = target_8;
  _24->captured = 13;
  _24->castled = 0;
  _24->promoted = 0;
  _24->ep = 0;
  _26 = numb_moves.1_21 + 1;
  numb_moves = _26;
;;    succ:       6

;;   basic block 6, loop depth 1
;;    pred:       4
;;                5
  target_17 = target_8 + offset_14;
  _7 = board[target_17];
  if (_7 != 0)
    goto <bb 10>; [94.50%]
  else
    goto <bb 9>; [5.50%]
;;    succ:       10
;;                9

;;   basic block 10, loop depth 1
;;    pred:       6
  goto <bb 3>; [100.00%]
;;    succ:       3

In particular note the load from and store to numb_moves in block #5 within the
loop.  I don't immediately see an aliasing issue that would prevent LSM.  The
bigger problem is control flow, obviously the load/store may not be executed,
but I thought our LIM/LSM code handled that correctly.

If we look at gcc-10 we get something like this:


;;   basic block 3, loop depth 1
;;    pred:       2
;;                10
  # target_9 = PHI <target_14(D)(2), target_19(10)>
  # numb_moves_lsm.43_6 = PHI <numb_moves_lsm.43_34(2),
numb_moves_lsm.43_2(10)>
  # numb_moves_lsm_flag.44_20 = PHI <numb_moves_lsm_flag.44_35(2),
numb_moves_lsm_flag.44_18(10)>
  _4 = board[target_9];
  if (_4 == 13)
    goto <bb 4>; [94.50%]
  else
    goto <bb 7>; [5.50%]
;;    succ:       4
;;                7

;;   basic block 4, loop depth 1
;;    pred:       3
  if (captures.32_5 == 0)
    goto <bb 5>; [33.00%]
  else
    goto <bb 6>; [67.00%]
;;    succ:       5
;;                6

;;   basic block 5, loop depth 1
;;    pred:       4
  numb_moves.1_21 = numb_moves_lsm.43_6;
  _22 = (long unsigned int) numb_moves.1_21;
  _23 = _22 * 24;
  _24 = (struct move_s *) _23;
  _24->from = gfrom.30_1;
  _24->target = target_9;
  _24->captured = 13;
  _24->castled = 0;
  _24->promoted = 0;
  _24->ep = 0;
  _26 = numb_moves.1_21 + 1;
  numb_moves_lsm.43_37 = _26;
  numb_moves_lsm_flag.44_38 = 1;
;;    succ:       6

;;   basic block 6, loop depth 1
;;    pred:       4
;;                5
  # numb_moves_lsm.43_2 = PHI <numb_moves_lsm.43_6(4), numb_moves_lsm.43_37(5)>
  # numb_moves_lsm_flag.44_18 = PHI <numb_moves_lsm_flag.44_20(4),
numb_moves_lsm_flag.44_38(5)>
  target_19 = target_9 + offset_15;
  _8 = board[target_19];
  if (_8 != 0)
    goto <bb 10>; [94.50%]
  else
    goto <bb 11>; [5.50%]
;;    succ:       10
;;                11

[ ... ]
;;   basic block 10, loop depth 1
;;    pred:       6
  goto <bb 3>; [100.00%]
;;    succ:       3


Obviously with the load before the loop and the store after.