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From: "law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110931] [14 Regression] Dead Code Elimination
 Regression since r14-2890-gcc2003cd875
Date: Sat, 09 Mar 2024 06:50:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110931

--- Comment #5 from Jeffrey A. Law <law at gcc dot gnu.org> ---
One could also look at this as a failure to propagate.

  l.0_1 =3D l;
  t_4 =3D (short int) l.0_1;
  if (t_4 =3D=3D 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 6>; [50.00%]


We ought to be able to propagate l.0_1 into the conditional for t_4.  If we=
 do
that, then we'll be able to propagate 0 for l.0_1 in this block:

;;   basic block 3, loop depth 0, count 536870912 (estimated locally, freq
0.5000)
;;    prev block 2, next block 4, flags: (NEW, VISITED)
;;    pred:       2 [50.0% (guessed)]  count:536870912 (estimated locally, =
freq
0.5000) (TRUE_VALUE,EXECUTABLE)
  b_5 =3D (char) l.0_1;
  if (b_5 !=3D 1)
    goto <bb 4>; [0.00%]
  else
    goto <bb 5>; [100.00%]


And once we propagate the value 0 that will make the make bb5 (containing t=
he
call to foo) unreachable.

IIRC we have (had?) some code to do these kinds of propagations through
conversions.  IIRC it was fairly conservative, perhaps overly so.=