http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49234
--- Comment #15 from rguenther at suse dot de <rguenther at suse dot de> 2013-03-04 09:57:40 UTC --- On Fri, 1 Mar 2013, aldyh at redhat dot com wrote: > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49234 > > --- Comment #12 from Aldy Hernandez <aldyh at redhat dot com> 2013-03-01 > 19:17:32 UTC --- > > --- Comment #11 from Ian Lance Taylor <ian at airs dot com> 2013-03-01 > > 14:52:53 UTC --- > > I suspect we can handle this case by observing that all the incoming values > > to > > the PHI node are constants. > > Ian. > > They're not all constants. In this particular case we have phis like this: > > state_2 = PHI <0(6), state_3(12), 2(5)> > > I suppose we could chase the SSA def chain and if all the phi arguments > are either constants, or known to point to an SSA that has been > previously analyzed as constant, then we can avoid generating an > overflow. But we'd have to keep track of states across calls to > vrp_visit_phi_node. Is this what you had in mind, or am I > misunderstanding something? > > Obviously, Richi's idea is much simpler :). With his suggestion we > could probably do with: > > @@ -8111,11 +8109,9 @@ vrp_visit_phi_node (gimple phi) > if (cmp_max < 0 || cmp_max > 0) > { > if (!needs_overflow_infinity (TREE_TYPE (vr_result.max)) > - || !vrp_var_may_overflow (lhs, phi)) > + || !vrp_var_may_overflow (lhs, phi) > + || supports_overflow_infinity (TREE_TYPE (vr_result.max))) > vr_result.max = TYPE_MAX_VALUE (TREE_TYPE (vr_result.max)); > - else if (supports_overflow_infinity (TREE_TYPE (vr_result.max))) > - vr_result.max = > - positive_overflow_infinity (TREE_TYPE (vr_result.max)); > } > > And similarly for MIN. In the above, supports_overflow_infinity is just > there to make sure we have a CONSTANT_CLASS_P. If that's not needed, we > could even do: > > if (cmp_max < 0 || cmp_max > 0) > vr_result.max = TYPE_MAX_VALUE (TREE_TYPE (vr_result.max)); > > and be done with it. That's what I was suggesting. As for strict-overflow warnings in VRP my suggestion was that we want to warn only when a folding result _changes_. Thus, propagate -fwrapv and -fno-wrapv in "parallel" using two lattices and compare the final result. That's way less prone to false positives. Richard.
