https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55217
--- Comment #5 from Michael Veksler <mickey.veksler at gmail dot com> ---
Running the delta.c example with -fdump-tree-all-all-lineno produces
delta.c.125t.vrp2.
For some reason, stop_9 (which is the first stop_.* in the file) is initialized
with stop_9 = barD.1593 (), but it should have been initialized with 0.
=============
# i_17 = PHI <[delta.c : 5:36] i_10(4), [delta.c : 5:14] 10(2)>
# .MEM_18 = PHI <.MEM_8(4), .MEM_4(D)(2)>
[delta.c : 6:13] # .MEM_8 = VDEF <.MEM_18>
# USE = nonlocal
# CLB = nonlocal
stop_9 = barD.1593 (); <====== Weird reorder
[delta.c : 5:36] i_10 = i_17 + -1;
[delta.c : 5:22] _5 = i_10 >= 0;
[delta.c : 5:29] _6 = stop_9 == 0;
[delta.c : 5:26] _7 = _6 & _5;
[delta.c : 5:5] if (_7 != 0)
goto <bb 4>;
else
goto <bb 5>;
==========
This seems wrong because the first time stop == 0 is checked at the source is:
int stop= 0;
for (int i=10 ; i>=0 && !stop; --i) {
^^^^^ <=== First time stop == 0 is checked.
stop= bar();
}
}
=====
This seems that VRP sees the call to bar() in the wrong place.
Another issue is that VRP sees "i>=0 && !stop", which it translates to:
Visiting statement:
=========== This gives a don't know: ===========
[delta.c : 5:26] _7 = _6 & _5;
Found new range for _7: [0, +INF]
[snip]
Predicate evaluates to: DON'T KNOW
==================================================================
>From there things go downhill. Instead of knowing that _7 implies _5 (i.e.,
i>=0), it loses this information. So VRP does not understand that in the loop
i>= 0.
=== This causes the following: ====
i_17: loop information indicates does not overflow
Induction variable (int) 9 + -1 * iteration does not wrap in statement i_10 =
i_17 + -1;
in loop 1.
Statement i_10 = i_17 + -1;
is executed at most 2147483657 (bounded by 2147483657) + 1 times in loop 1.
Found new range for i_17: [-INF, 10]
=================================
If it know that _7 implies _5 and hence i>=0 then it could have understood that
i_17: [0, 10].
===== This could lead to missed optimizations (in other cases), and bogus
warnings: ===
Visiting statement:
[delta.c : 5:36] i_10 = i_17 + -1;
Found new range for i_10: [-INF(OVF), 9]
