------- Comment #10 from law at redhat dot com 2006-02-22 16:22 ------- Subject: Re: Fowardprop does harm for VRP to figure out if a point is non zero
On Wed, 2006-02-22 at 12:47 +0000, rguenth at gcc dot gnu dot org wrote: A little history... DOM was pretty clever in that it had the ability to backwards propagate some non-null ranges. That code was written to make DOM's non-null tracking relatively immune to things like comparison simplification. It was quick, simple and relatively effective. We *really* don't want to do that in VRP. First it violates a fundamental principle designed to ensure VRP terminates. Namely that we don't move backward in the lattice. ie, we don't allow VR_VARYING -> VR_RANGE/VR_ANTI_RANGE state transitions. I briefly toyed with the idea of doing the backward range propagation after all the forward propagation was done, but before substitution/simplifications. There's a handful of implementation issues with this approach and it will likely result in a measurable compile-time hit due to the extra ASSERT_EXPRs. It's something I'm still pondering, but it's not my favored solution ATM. What I'm seriously looking at and still evaluating is delaying the forwprop pass. For the initial stuff I looked at it seems like a *much* better solution -- not only does it allow VRP to catch more of the non-null stuff, but it seems to help forwprop and the following DOM pass as well. I'll be returning to this once we've reached closure on the Ada regressions. jeff -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26406