https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61409
--- Comment #16 from Jeffrey A. Law <law at redhat dot com> ---
AFAICT tree-ssa-uninit won't look at the predicate associated with an undefined
PHI argument and test it against the predicate for the actual use.
ie, given this PHI from the testcase:
;; basic block 6, loop depth 0
;; pred: 9
;; 5
# ptr_1 = PHI <ptr_5(D)(9), ptr_9(5)>
We want to look at the control dependent path that leads to the edge (9,5).
For this test, that edge is control dependent on bb2:
;; basic block 2, loop depth 0
;; pred: ENTRY
if (i_4(D) != 0)
goto <bb 3>;
else
goto <bb 9>;
ie, we know that for ptr_1 to take the value ptr_5 that i_4 != 0 must be false.
So the guard for the edge (9,5) is NOT i_4 != 0.
And in this testcase, the actual use of ptr_1 is guarded by:
if (i_4(D) != 0)
goto <bb 7>;
else
goto <bb 11>;
When i_4 != 0 is true, then we'll get to the use. So the guard for the use is
i_4 != 0
Those two guards can never both be true. So there's no way at runtime for the
value ptr_5 to flow into ptr_1 and then into the use of ptr_1 in bb7.
And AFAICT, tree-ssa-uninit.c doesn't have the code to do that kind of
analysis.
