[Bug tree-optimization/102756] [12 Regression] Complete unrolling is too senative to PRE; c-c++-common/torture/vector-compare-2.c

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102756

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-10-15
           Keywords|                            |missed-optimization
            Summary|[12 Regression] Vectorizer  |[12 Regression] Complete
                   |change creates poor code    |unrolling is too senative
                   |for                         |to PRE;
                   |c-c++-common/torture/vector |c-c++-common/torture/vector
                   |-compare-2.c                |-compare-2.c
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
With -ftree-vectorize
size: 7-4, last_iteration: 7-4
  Loop size: 7
  Estimated size after unrolling: 8


  _1 = VIEW_CONVERT_EXPR<int[4]>(r)[i_10];


With -fno-tree-vectorize
size: 7-4, last_iteration: 6-4
  Loop size: 7
  Estimated size after unrolling: 7

  pretmp_2 = MEM[(vector(4) int *)&r][i_7];


Also -O2 -fno-tree-vectorize -fno-tree-pre produces the same as the -O2
-ftree-vectorize case.



------------------- CUT ----------------------------
Loop 1 iterates 3 times.
Loop 1 iterates at most 3 times.
Loop 1 likely iterates at most 3 times.
Estimating sizes for loop 1
 BB: 3, after_exit: 0
  size:   1 _1 = VIEW_CONVERT_EXPR<int[4]>(r)[i_10];
  size:   2 if (_1 != -3)
 BB: 7, after_exit: 1
 BB: 5, after_exit: 0
  size:   1 i_7 = i_10 + 1;
   Induction variable computation will be folded away.
  size:   1 ivtmp_9 = ivtmp_2 - 1;
   Induction variable computation will be folded away.
  size:   2 if (ivtmp_9 != 0)
   Exit condition will be eliminated in peeled copies.
   Exit condition will be eliminated in last copy.
   Constant conditional.
size: 7-4, last_iteration: 7-4
  Loop size: 7
  Estimated size after unrolling: 8
Not unrolling loop 1: size would grow.


vs:
Estimating sizes for loop 1
 BB: 3, after_exit: 0
  size:   2 if (prephitmp_9 != -3)
 BB: 6, after_exit: 1
  size:   1 pretmp_2 = MEM[(vector(4) int *)&r][i_7];
 BB: 5, after_exit: 0
  size:   1 i_7 = i_10 + 1;
   Induction variable computation will be folded away.
  size:   1 ivtmp_11 = ivtmp_1 - 1;
   Induction variable computation will be folded away.
  size:   2 if (ivtmp_11 != 0)
   Exit condition will be eliminated in peeled copies.
   Exit condition will be eliminated in last copy.
   Constant conditional.
size: 7-4, last_iteration: 6-4
  Loop size: 7
  Estimated size after unrolling: 7


PRE decides to do the load for MEM[(vector(4) int *)&r][0] which is why the
last iteration is 6-4 rather than 7-4.