http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60363
--- Comment #7 from bin.cheng <amker.cheng at gmail dot com> ---
The problem has nothing to do with VRP, and might be a missed opportunity of
jump threading.
The dump after VRP but before jump threading at the end of VRP is like:
...
<bb 21>:
goto <bb 7>;
...
<bb 7>:
# kill_elt_3 = PHI <kill_elt_4(20), 0B(21), kill_elt_2(6)>
if (b_elt_11(D) != 0B)
goto <bb 8>;
else
goto <bb 14>;
<bb 8>:
if (kill_elt_3 != 0B)
goto <bb 9>;
else
goto <bb 22>;
<bb 22>:
goto <bb 14>;
...
<bb 14>:
# changed_19 = PHI <changed_1(7), changed_1(22), changed_26(12),
changed_1(11), changed_1(9)>
# kill_elt_18 = PHI <kill_elt_3(7), 0B(22), kill_elt_3(12), kill_elt_3(11),
kill_elt_3(9)>
<bb 17>:
# changed_1 = PHI <changed_19(14), 0(2)>
# kill_elt_4 = PHI <kill_elt_18(14), kill_elt_7(D)(2)>
if (a_elt_9(D) != 0B)
goto <bb 24>;
else
goto <bb 15>;
There is jump threading path like:
Registering jump thread: (21, 7) incoming edge; (7, 8) joiner; (8, 22)
normal;
After jump threading, the dump is like:
<bb 18>:
# kill_elt_2 = PHI <kill_elt_10(5), kill_elt_14(4)>
if (kill_elt_2 != 0B)
goto <bb 6>;
else
goto <bb 21>;
<bb 21>:
goto <bb 27>;
...
<bb 14>:
# changed_19 = PHI <changed_1(7), changed_1(22), changed_26(12),
changed_1(11), changed_1(9), changed_1(27), changed_1(29)>
# kill_elt_18 = PHI <kill_elt_3(7), 0B(26), kill_elt_3(12), kill_elt_3(11),
kill_elt_3(9), kill_elt_3(27), kill_elt_3(29)>
<bb 17>:
# changed_1 = PHI <changed_19(14), 0(2)>
# kill_elt_4 = PHI <kill_elt_18(14), kill_elt_7(D)(2)>
if (a_elt_9(D) != 0B)
goto <bb 24>;
else
goto <bb 15>;
...
<bb 26>:
goto <bb 14>;
<bb 27>:
# kill_elt_41 = PHI <0B(21)>
if (b_elt_11(D) != 0B)
goto <bb 26>;
else
goto <bb 14>;
During jump threading process, GCC updates phi node in destination basic block
like "kill_elt_18 = PHI <". Because bb27 is duplicated from bb7, the phi
argument "kill_elt_3(27)" is copied directly from "kill_elt_3(7)". Although
"kill_elt_3 == 0" holds when coming from edge <bb27, bb14>, gcc doesn't know
this fact.
At last, cfgcleanup tries to remove forwarder block bb26, but it can't make it
because the phi arguments from edge<bb27, bb14> and edge<bb26, bb14> doesn't
match.
I am thinking maybe jump threading can back trace the threading path and try to
add phi argument for edge<bb27, bb14> with the value(0) of kill_elt_3 (i.e.,
"0(27)"). The value is available along the jump threading path.
