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.
