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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/112612] Holding on the loop variable rather
 than a derived value which can replace it
Date: Mon, 20 Nov 2023 09:42:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112612

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-11-20

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

Candidate 1:
  Var befor: ivtmp.5
  Var after: ivtmp.5
  Incr POS: before exit test
  IV struct:
    Type:       unsigned long
    Base:       0
    Step:       1
    Biv:        N
    Overflowness wrto loop niter:       No-overflow
...
Candidate 8:
  Var befor: ivtmp.10
  Var after: ivtmp.10
  Incr POS: before exit test
  IV struct:
    Type:       unsigned int
    Base:       0
    Step:       2
    Biv:        N
    Overflowness wrto loop niter:       No-overflow

Candidate 9:
  Var befor: ivtmp.11
  Var after: ivtmp.11
  Incr POS: before exit test
  IV struct:
    Type:       unsigned long
    Base:       (unsigned long) a_8(D)
    Step:       4
    Object:     (void *) a_8(D)
    Biv:        N
    Overflowness wrto loop niter:       Overflow

so we do have this candidate.

Improved to:
  cost: 16 (complexity 0)
  reg_cost: 4
  cand_cost: 10
  cand_group_cost: 2 (complexity 0)
  candidates: 8, 9
   group:0 --> iv_cand:8, cost=3D(0,0)
   group:1 --> iv_cand:9, cost=3D(2,0)
   group:2 --> iv_cand:8, cost=3D(0,0)
  invariant variables:
  invariant expressions:

Initial set of candidates:
  cost: 15 (complexity 2)
  reg_cost: 3
  cand_cost: 5
  cand_group_cost: 7 (complexity 2)
  candidates: 1
   group:0 --> iv_cand:1, cost=3D(4,0)
   group:1 --> iv_cand:1, cost=3D(3,2)
   group:2 --> iv_cand:1, cost=3D(0,0)
  invariant variables: 1
  invariant expressions:

but somehow we fail to express(?) some of the uses with just candidate 8?

It "works" with -m32 added:

  <bb 3> [local count: 1063004408]:
  # ivtmp.10_12 =3D PHI <ivtmp.10_11(5), 0(2)>
  val_7 =3D (int) ivtmp.10_12;
  MEM[(int *)a_8(D) + ivtmp.10_12 * 2] =3D val_7;
  ivtmp.10_11 =3D ivtmp.10_12 + 2;
  if (ivtmp.10_11 !=3D 200)
    goto <bb 5>; [98.99%]

so it might be the 32->64bit promotion is what gets us off.  We might
possibly want to consider a 'unsigned long' candidate with step 2.

.L2:
        movl    %eax, (%edx,%eax,2)
        addl    $2, %eax
        cmpl    $200, %eax
        jne     .L2=