https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112444
--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
OK, so the reason is that we value-number tmp_46 in
<bb 3> [local count: 482002707]:
if (0 != 0)
goto <bb 5>; [33.00%]
else
goto <bb 4>; [67.00%]
<bb 4> [local count: 322941815]:
tmp_5 = .DEFERRED_INIT (1, 2, &"tmp"[0]);
...
goto <bb 6>;
<bb 5> [local count: 159060893]:
...
tmp_2 = .DEFERRED_INIT (1, 2, &"tmp"[0]);
...
tmp_46 = PHI <tmp_5(4), tmp_2(5)>
to tmp_2 because we use ssa_undefined_value_p () to check whether we are
dealing with an undefined value. And that returns true for tmp_5. This
makes us pick tmp_2 which we treat as VARYING since we didn't visit it
and we don't trust the not-executable state of its incoming edge (a missed
optimization, guess I can look at that as well). tmp_2 is also considered
undefined. We then have
/* If we saw only undefined values and VN_TOP use one of the
undefined values. */
else if (sameval == VN_TOP)
result = seen_undef ? seen_undef : sameval;
and "one of" puts us in an unlucky situation here.
I do have a sensible fix around this I think.
