https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81388
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|P3 |P2
Status|UNCONFIRMED |NEW
Keywords| |wrong-code
Last reconfirmed| |2017-07-17
CC| |amker at gcc dot gnu.org
Ever confirmed|0 |1
Summary|Incorrect code generation |[7/8 Regression] Incorrect
|with -O1 |code generation with -O1
|-fno-strict-overflow |
Target Milestone|--- |7.2
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Smells like a niter analysis issue to me. With -fno-tree-ivcanon I get
unsigned long ivtmp.3;
<bb 3> [100.00%]:
# ivtmp.3_7 = PHI <0(2), ivtmp.3_8(3)>
bar ();
ivtmp.3_8 = ivtmp.3_7 + 1;
if (ivtmp.3_8 != 9223372036854775808)
goto <bb 3>; [99.00%]
else
goto <bb 4>; [1.00%]
GCC 6 computes:
Loop 1 iterates at most -1 times.
while GCC 7 does:
Analyzing # of iterations of loop 1
exit condition [dst_3(D) + 2, + , 2](no_overflow) < dst_3(D)
bounds on difference of bases: -2 ... -2
Applying pattern match.pd:1308, generic-match.c:13648
result:
# of iterations 9223372036854775807, bounded by 9223372036854775807
so likely (T)P - (T)(P + A) -> -(T) A makes things go downhill here,
producing pointer (unsigned) typed negated 2 and us interpreting that
as unsigned. The same pattern is in GCC 6 so sth in niter analysis
broke. -fno-strict-overflow doesn't seem to be necessary btw, -fwrapv
does do the trick as well.
Bin?
