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From: "law at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94734] [10 Regression] Program crashes when
 compiled with -O2 since r10-1892-gb9ef6a2e04bfd013
Date: Fri, 24 Apr 2020 00:48:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94734

--- Comment #6 from Jeffrey A. Law <law at redhat dot com> ---
THe whole point of that change is to not require a dominating load if the
object comes from the stack.

We conditionally load from the same location, then have a PHI which selects
that loaded value or "1" and we store the result.  That's precisely what is
supposed to be happening.

The loop in question looks like:

;;   basic block 3, loop depth 1
;;    pred:       2
;;                6
  # ivtmp.13_9 =3D PHI <0(2), ivtmp.13_7(6)>
  _4 =3D MEM[base: in_21(D), index: ivtmp.13_9, step: 8, offset: 0B];
  if (_4 =3D=3D 0B)
    goto <bb 13>; [5.50%]
  else
    goto <bb 4>; [94.50%]
;;    succ:       13
;;                4

;;   basic block 4, loop depth 1
;;    pred:       3
  if (ivtmp.13_9 !=3D 2)
    goto <bb 6>; [28.10%]
  else
    goto <bb 5>; [71.90%]
;;    succ:       6
;;                5

;;   basic block 5, loop depth 1
;;    pred:       4
  cstore_32 =3D MEM[symbol: arr, index: ivtmp.13_9, step: 4, offset: 0B];
;;    succ:       6

;;   basic block 6, loop depth 1
;;    pred:       5
;;                4
  # cstore_31 =3D PHI <cstore_32(5), 1(4)>
  MEM[symbol: arr, index: ivtmp.13_9, step: 4, offset: 0B] =3D cstore_31;
  _40 =3D (unsigned int) ivtmp.13_9;
  _38 =3D _40 + 1;
  _37 =3D (int) _38;
  ivtmp.13_7 =3D ivtmp.13_9 + 1;
  sum_a_27 =3D (int) ivtmp.13_7;
  if (n_16(D) > sum_a_27)
    goto <bb 3>; [94.50%]
  else
    goto <bb 7>; [5.50%]
;;    succ:       3
;;                7

You can see the load from the same stock slot in bb5, the selecting PHI in =
bb6
and the store in bb6.

THe test in bb4 looks weird and is the source of the problem I believe.=