Hi, I think this should be optimized as expected on latest trunk: https://godbolt.org/z/vW7K7f Cheers > On Apr 6, 2020, at 16:53, Stefanos Baziotis <stefanos.baziotis at gmail.com> wrote: > > Hi, > > > But the value can't be [0,7] due to the earlier branch. When I said it was guaranteed to wrap, I only meant for the range of values that were possible after the first branch. > Indeed, for some reason I remembered that the left check is >= 0 when I wrote this. > > Thanks a lot for the breakdown in CorrelatedValuePropagation. I had no idea about this pass. > > > It was able to see that the input to add was in the range [8,14) in the call to LVI->getConstantRange in processBinOp. > > I could not reproduce that. I get: > BinOp: %4 = add nsw i32 %2, -8 > LRange: [0,-2147483648) > RRange: [-8,-7) > > > processCmp skips calling LVI for the select's icmp because the input isn't in the same basic block and isn't a phi. > Do you maybe mean: _is_ in the same BB (and isn't a PHI). > > By the way, I don't get the reasoning in the comment above: > // As a policy choice, we choose not to waste compile time on anything where > // the comparison is testing local values. > > > I think this is because the code executed for getConstant doesn't handle icmp even when it can prove the input is in a constant range. > > Maybe we ended up on the same thing. I'm not sure I followed that correctly but getValueFromICmpCondition() should have been able to handle that. > > Best, > Stefanos > > > ΣÏÎ¹Ï ÎÎµÏ , 6 ÎÏÏ 2020 ÏÏÎ¹Ï 5:22 Ï.μ., ο/η Craig Topper <craig.topper at gmail.com <mailto:craig.topper at gmail.com>> ÎγÏαÏε: > On Sun, Apr 5, 2020 at 6:34 PM Stefanos Baziotis <stefanos.baziotis at gmail.com <mailto:stefanos.baziotis at gmail.com>> wrote: > Hi Craig, > > > Adding a nuw to the add -8 is incorrect. > Yeah, I didn't mean to say it was correct. It was just an observation that with nuw the optimization was happened and I asked if someone thought it was somehow connected. > > > From the perspective of the unsigned math, -8 is treated a very large positive number. The input to the add is [8,13) and adding a large positive number to it wraps around past 0. So that is guaranteed unsigned wrap > I understand yes, but I don't think it is guaranteed. Unless I miss something, for values in [0, 7] it won't wrap. But past that and up to (and including in the original source code) 13, it will wrap yes. > > But the value can't be [0,7] due to the earlier branch. When I said it was guaranteed to wrap, I only meant for the range of values that were possible after the first branch. > > In theory, the CorrelatedValuePropagation pass should have been able to optimize the select. It was able to see that the input to add was in the range [8,14) in the call to LVI->getConstantRange in processBinOp. processCmp skips calling LVI for the select's icmp because the input isn't in the same basic block and isn't a phi. And the call to LVI->getConstant for the select in processSelect didn't return a constant. I think this is because the code executed for getConstant doesn't handle icmp even when it can prove the input is in a constant range. I tried removing the local value check in processCmp so that getPredicateAt would called, but that didn't help either. > > > Best, > - Stefanos > > > ΣÏÎ¹Ï ÎÎµÏ , 6 ÎÏÏ 2020 ÏÏÎ¹Ï 3:10 Ï.μ., ο/η Craig Topper <craig.topper at gmail.com <mailto:craig.topper at gmail.com>> ÎγÏαÏε: > Adding a nuw to the add -8 is incorrect. From the perspective of the unsigned math, -8 is treated a very large positive number. The input to the add is [8,13) and adding a large positive number to it wraps around past 0. So that is guaranteed unsigned wrap. On the other hand, a sub nuw 8 would be correct. > > ~Craig > > > On Sun, Apr 5, 2020 at 3:27 PM Stefanos Baziotis via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote: > Thanks, I didn't know that! Indeed, it's instruction combine that does the job. > > - Stefanos > > ΣÏÎ¹Ï ÎÎµÏ , 6 ÎÏÏ 2020 ÏÏÎ¹Ï 12:38 Ï.μ., ο/η Florian Hahn <florian_hahn at apple.com <mailto:florian_hahn at apple.com>> ÎγÏαÏε: > > > > On Apr 5, 2020, at 22:20, Stefanos Baziotis <stefanos.baziotis at gmail.com <mailto:stefanos.baziotis at gmail.com>> wrote: > > > > > Any idea about how the compiler could remove the lshr and use a add -16? > > Actually, I just figured that doing this test is like solving this: > > > > 8 <= x/2 <= 13 > > 16 <= x <= 26 > > 0 <= x - 16 <= 10 => 0 <= x < 11 > > The left part is know since it's unsigned > > The right part could be done x <= 11 => x < 12 because it's actually an integer division. > > Wow... I would be really happy to know what pass does that. > > Iâd guess a combination of instcombine rules together with some other transforms. You could probably use `-print-after-all` (`clang -mllvm -print-after-all` if you are using clang) to track down the relevant passes/steps. > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200711/d8362118/attachment.html>
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