http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53168
--- Comment #5 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-05-03
14:17:32 UTC ---
I think it is because of how we do VN lookup / insert during phi_translation.
SCCVN does not guarantee availability of its lookup result, thus when
such availability is not guaranteed when we translate things we should rather
use the non-canonical expression (after all we translate expressions (sic!),
not values).
What happens is that we have (simplified)
if (b)
{
x_1 = a;
}
else
{
if (c)
for(;;)
{
x_2 = a;
a = 0;
}
}
and SCCVN value-numbers x_2 to x_1. PRE sees that 'a' is ANTIC_IN in the
for (;;) block - but it's not ANTIC_OUT in the if (c) block because it
gets cleaned out there as it is translated as x_1 which is not insertable.
I think the PRE algorithm does not expect that phi_translate (ANTIC_IN (succ))
will ever return something that is _not_ valid at the ANTIC_OUT position.
But our phi-translate implementation happily causes this situation.
My suggested change to check validity of the phi_translate result at
insert time is a workaround, but it only makes this a missed optimization.
This particular case is fixed by not turning the memory reference into
a conversion on translation.
