http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42108
--- Comment #54 from Richard Biener <rguenth at gcc dot gnu.org> 2013-01-16
12:36:52 UTC ---
Re-confirmed on trunk. The initial GFortran IL is still ... awkward. Apart
from the issue of using a canonicalized IV at all we have
D.1910 = i;
D.1911 = *nnd;
D.1912 = *n;
k = D.1910;
if (D.1912 > 0)
{
if (D.1911 < D.1910) goto L.6;
}
else
{
if (D.1911 > D.1910) goto L.6;
}
countm1.6 = (unsigned int) ((D.1911 - D.1910) *
(D.1912 < 0 ? -1 : 1)) / (unsigned int) ((D.1912 < 0 ? -1 : 1) * D.1912);
while (1)
{
...
do_exit.7 = countm1.6 == 0;
countm1.6 = countm1.6 + 4294967295;
if (do_exit.7) goto L.6;
}
}
L.6:;
in the computation of countm1.6 we have a redundant (D.1912 < 0 ? -1 : 1)
test which ends up complicating the CFG. It's also redundant again
with a test that was done just above. In fact it completely cancels out
in the countm1 compute. (the exit test via a do_exit temporary is
because of a local change in my tree ... eh)
Also note that
/* Calculate the loop count. to-from can overflow, so
we cast to unsigned. */
but we do
(unsigned)(to * step_sign - from * step_sign) / (unsigned) (step * step_sign)
that does not avoid overflow of step * step_sign (step == INT_MIN, step_sign ==
-1) nor overflow of to * step_sign - from * step_sign which we fold to
(to - from) * step_sign anyway (signed overflow is undefined, heh).
I believe we need to do
((unsigned)to - (unsigned)from) * (unsigned)step_sign / ((unsigned) step *
(unsigned) step_sign)
to avoid these issues.
