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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/109943] [13/14 Regression] Missed Dead Code
 Elimination when using __builtin_unreachable since r13-6834-g41ade3399bd
Date: Fri, 12 Jan 2024 13:50:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109943

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
With the __builtin_unreachable present (thus g >=3D 0) we simplify
(int)(char)d to d & 255 which causes the VN difference:

 Value numbering stmt =3D _9 =3D (int) _8;
+d.4_7 is available for d.4_7
+_2 is available for _2
 Setting value number of _9 to _9 (changed)
-Making available beyond BB2 _9 for value _9
-Value numbering stmt =3D _10 =3D (int) d.4_7;
-Setting value number of _10 to _9 (changed)
+Making available beyond BB4 _9 for value _9
+Value numbering stmt =3D _10 =3D b.3_6 & 255;
+_2 is available for _2
+Setting value number of _10 to _10 (changed)
+Making available beyond BB4 _10 for value _10
 Value numbering stmt =3D _11 =3D _9 % _10;

in particular we have

  _2 =3D b;
  b.3_6 =3D b;
  d.4_7 =3D (char) b.3_6;
  _8 =3D (char) _2;
  _9 =3D (int) _8;
  _10 =3D (int) d.4_7;
  _11 =3D _9 % _10;

vs.

  _2 =3D b;
  b.3_6 =3D b;
  d.4_7 =3D (char) b.3_6;
  _8 =3D (char) _2;
  if (d.4_7 <=3D 0)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [100.00%]

  <bb 3> [count: 0]:
  __builtin_unreachable ();

  <bb 4> [local count: 1073741824]:
  _9 =3D (int) _8;
  _10 =3D b.3_6 & 255;
  _11 =3D _9 % _10;

where the trivial equivalence is no longer visible.=