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Hi everyone, In a twitch chat someone redirected me to an example that is not optimized: https://godbolt.org/z/BL-4jL I included the original source code and this is after -O2. We both thought that the -8 branch could be optimized out. I added a nuw in the subtraction and it actually does it. Any thoughts on why that doesn't happen already? Best, Stefanos Baziotis -------------- next part -------------- An...

...: > > http://volta.cs.utah.edu:8080/z/b7Yj7j > > John > > > > On 4/4/20 6:29 PM, Stefanos Baziotis via llvm-dev wrote: > > Hi everyone, > > > > In a twitch chat someone redirected me to an example that is not > > optimized: https://godbolt.org/z/BL-4jL > > > > I included the original source code and this is after -O2. We both > > thought that the -8 branch could be optimized out. I added a nuw in the > > subtraction and it actually does it. > > > > Any thoughts on why that doesn't happen already? > >...

...do about it. > > > > A clearer version of the .ll: https://godbolt.org/z/2t4RU5 > > I think the IR in both of your examples makes things harder for the > compiler than expected from the original C source. > > With the IR in your original example (https://godbolt.org/z/BL-4jL), I > think the problem is that the branch condition is '%0 - 16 < 12’, which > allows us to limit the range of `%0 - 16` easily, but in the branch only %0 > is used. Sinking the lshr too early made the analysis harder. > > In the second example (https://godbolt.org/z/2t4RU5),...