[PATCH] D139598: [InstCombine] Fold (X << Z) / (X * Y) -> (1 << Z) / Y

[Chenbing.Zheng via Phabricator via llvm-commits]

Chenbing.Zheng added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp:1062
+      return BinaryOperator::CreateUDiv(
+          Builder.CreateShl(ConstantInt::get(Ty, 1), Z), Y);
+  }
----------------
nikic wrote:
> Chenbing.Zheng wrote:
> > RKSimon wrote:
> > > arsenm wrote:
> > > > Can you preserve NUW and exact?
> > > Move the !IsSigned check outside to avoid the cost of match calls?
> > I preserve exact, but it seems that NUW is unnecessary when Op0 of SHL is 1 ?
> You're right that we can infer nuw for `1 << x`, but based on your tests it doesn't look like we actually do this...
Yer, I think 'shl nuw 1 << x' is equivalent to 'shl 1 << x', so I tend to don't  keep 'NUW'.  Or what is the benefit of keeping 'NUW' ?


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  https://reviews.llvm.org/D139598/new/

https://reviews.llvm.org/D139598