https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109943
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
With the __builtin_unreachable present (thus g >= 0) we simplify
(int)(char)d to d & 255 which causes the VN difference:
Value numbering stmt = _9 = (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 = _10 = (int) d.4_7;
-Setting value number of _10 to _9 (changed)
+Making available beyond BB4 _9 for value _9
+Value numbering stmt = _10 = 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 = _11 = _9 % _10;
in particular we have
_2 = b;
b.3_6 = b;
d.4_7 = (char) b.3_6;
_8 = (char) _2;
_9 = (int) _8;
_10 = (int) d.4_7;
_11 = _9 % _10;
vs.
_2 = b;
b.3_6 = b;
d.4_7 = (char) b.3_6;
_8 = (char) _2;
if (d.4_7 <= 0)
goto <bb 3>; [0.00%]
else
goto <bb 4>; [100.00%]
<bb 3> [count: 0]:
__builtin_unreachable ();
<bb 4> [local count: 1073741824]:
_9 = (int) _8;
_10 = b.3_6 & 255;
_11 = _9 % _10;
where the trivial equivalence is no longer visible.
