Nick Lewycky
2011-Jan-18 08:32 UTC
[LLVMdev] induction variable computation not preserving scev
Hi, I tracked down a bug in indvars where we weren't updating SCEV properly. The attached patch shows the fix to this bug with a testcase, but it also causes five new test failures. Would someone be willing to take a look at the failures and figure out why we're getting worse output once this patch is applied? If I had to hazard a guess, I'd say that indvars is probably emitting code that can't be analyzed by SCEV as easily as the original. Nick -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20110118/d41d3189/attachment.html> -------------- next part -------------- A non-text attachment was scrubbed... Name: indvars.patch Type: text/x-patch Size: 2351 bytes Desc: not available URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20110118/d41d3189/attachment.bin>
Dan Gohman
2011-Jan-19 21:01 UTC
[LLVMdev] induction variable computation not preserving scev
On Jan 18, 2011, at 12:32 AM, Nick Lewycky wrote:> Hi, > > I tracked down a bug in indvars where we weren't updating SCEV properly. The attached patch shows the fix to this bug with a testcase, but it also causes five new test failures.Indvars isn't restructuring the loop there, so it ideally shouldn't need to call forgetLoop(). Offhand, is it possible that the problem here is the same as the one in PR8037?> > Would someone be willing to take a look at the failures and figure out why we're getting worse output once this patch is applied? If I had to hazard a guess, I'd say that indvars is probably emitting code that can't be analyzed by SCEV as easily as the original.It's certainly plausible. It's easy to loose track of things like overflow behavior when folding casts into arithmetic, for example. Dan
Nick Lewycky
2011-Jan-19 22:03 UTC
[LLVMdev] induction variable computation not preserving scev
On 19 January 2011 13:01, Dan Gohman <gohman at apple.com> wrote:> > On Jan 18, 2011, at 12:32 AM, Nick Lewycky wrote: > > > Hi, > > > > I tracked down a bug in indvars where we weren't updating SCEV properly. > The attached patch shows the fix to this bug with a testcase, but it also > causes five new test failures. > > Indvars isn't restructuring the loop there, so it ideally shouldn't > need to call forgetLoop().Hmm? It certainly is, it's changing the condition on the branch instruction that makes up the loop latch! Offhand, is it possible that the problem> here is the same as the one in PR8037? >No, that only shows up when you try to run "opt -analyze -indvars -scalar-evolution" by causing a crash. With that patch in place, you can see that "opt -analyze -indvars -scalar-evolution" produces different values for the SCEVs than you get with "opt -indvars | opt -analyze -scalar-evolution" which led me to add the forgetLoop here.> > Would someone be willing to take a look at the failures and figure out > why we're getting worse output once this patch is applied? If I had to > hazard a guess, I'd say that indvars is probably emitting code that can't be > analyzed by SCEV as easily as the original. > > It's certainly plausible. It's easy to loose track of things like > overflow behavior when folding casts into arithmetic, for example. > > Dan > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20110119/32cd4172/attachment.html>
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