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From: "law at redhat dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/26406] Fowardprop does harm for VRP to figure out if a point is non zero Date: Wed, 22 Feb 2006 16:23:00 -0000 [thread overview] Message-ID: <20060222162259.19448.qmail@sourceware.org> (raw) In-Reply-To: <bug-26406-6528@http.gcc.gnu.org/bugzilla/> ------- 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
next prev parent reply other threads:[~2006-02-22 16:23 UTC|newest] Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top 2006-02-21 21:35 [Bug tree-optimization/26406] New: " pinskia at gcc dot gnu dot org 2006-02-21 21:45 ` [Bug tree-optimization/26406] " pinskia at gcc dot gnu dot org 2006-02-22 7:36 ` steven at gcc dot gnu dot org 2006-02-22 10:05 ` rguenth at gcc dot gnu dot org 2006-02-22 10:33 ` rguenth at gcc dot gnu dot org 2006-02-22 12:47 ` rguenth at gcc dot gnu dot org 2006-02-22 12:49 ` rguenth at gcc dot gnu dot org 2006-02-22 12:58 ` pinskia at gcc dot gnu dot org 2006-02-22 13:10 ` patchapp at dberlin dot org 2006-02-22 15:24 ` law at redhat dot com 2006-02-22 16:23 ` law at redhat dot com [this message] 2006-02-22 20:55 ` rguenth at gcc dot gnu dot org 2006-02-22 21:45 ` law at redhat dot com 2006-02-25 15:55 ` [Bug tree-optimization/26406] [4.2 Regression] Forwardprop " pinskia at gcc dot gnu dot org 2006-02-25 18:01 ` pinskia at gcc dot gnu dot org 2006-02-25 18:37 ` law at redhat dot com 2006-03-08 16:43 ` law at redhat dot com
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