[llvm-dev] Branch is not optimized because of right shift

[Florian Hahn via llvm-dev]

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
> 
> 
> Στις Δευ, 6 Απρ 2020 στις 5:22 π.μ., ο/η Craig Topper <craig.topper at gmail.com <mailto:craig.topper at gmail.com>> έγραψε:
> 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
> 
> 
> Στις Δευ, 6 Απρ 2020 στις 3:10 π.μ., ο/η Craig Topper <craig.topper at gmail.com <mailto:craig.topper at gmail.com>> έγραψε:
> 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
> 
> Στις Δευ, 6 Απρ 2020 στις 12:38 π.μ., ο/η Florian Hahn <florian_hahn at apple.com <mailto:florian_hahn at apple.com>> έγραψε:
> 
> 
> > 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.
> 
> I’d 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.
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