https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69378
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ok, difference is in PRE:
--- a/pr61034.C.123t.pre 2016-01-20 10:27:14.052404275 +0100
+++ b/pr61034.C.123t.pre 2016-01-20 10:26:24.331842158 +0100
@@ -778,289 +778,344 @@
Value numbering store SR.21_304->count to 2
Setting value number of .MEM_132 to .MEM_132 (changed)
Value numbering _48 stmt = _48 = MEM[(struct O *)_248].count;
-Setting value number of _48 to 2 (changed)
+Setting value number of _48 to _48 (changed)
Value numbering _49 stmt = _49 = _48 + -1;
-Match-and-simplified _48 + -1 to 1
-RHS _48 + -1 simplified to 1
-Setting value number of _49 to 1 (changed)
+Setting value number of _49 to _49 (changed)
...
and the reason is that the dominance queries I do for the PR69117 fix do not
honor the edges we computed as unreachable. The first case we hit that ends
up pessimizing points-to info during VN is quite simple:
<bb 14>:
# RANGE [2, 2147483647] NONZERO 2147483647
_57 = _56 + 1;
MEM[(struct O *)_248].count = _57;
if (_75 > 1)
goto <bb 15>;
else
goto <bb 41>;
<bb 41>:
goto <bb 16>;
<bb 15>:
# PT = { D.5028 }
# ALIGN = 8, MISALIGN = 0
# USE = nonlocal
# CLB = nonlocal
_266 = __builtin_malloc (16);
MEM[(struct O *)_266].num = _202;
# RANGE [1, 2147483646] NONZERO 2147483647
_280 = _74;
MEM[(struct O *)_194].count = _74;
MEM[(struct O *)_266].count = 1;
D.4995 ={v} {CLOBBER};
<bb 16>:
# PT = { D.5024 D.5028 }
# ALIGN = 8, MISALIGN = 0
# SR.21_304 = PHI <_194(41), _266(15)>
with _75 > 1 known to be true and the edge 41 -> 16 not executable we
value-number SR.21_304 to _266 but clear points-to info of that on the
way unnecessarily.
So we can open-code a wrapper around dominated_by_p and handle some
simple CFG cases with not executable edges. I'm not sure it's worth
to covering everything like by looking up the common immediate dominator
and figuring out if there is an always executed path through both nodes
starting from it. [I believe it's enough to handle the case with
equal immediate dominator]
