[PATCH] D153148: [InstCombine] Fold `(-1 + A) & B` into `A ? 0 : B` where A is effectively a bool

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


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Comment at: llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp:2609
+      return SelectInst::Create(A, Constant::getNullValue(Ty), B);
+    if (computeKnownBits(A, /* Depth */ 0, &I).countMaxActiveBits() <= 1) {
+      return SelectInst::Create(
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nikic wrote:
> goldstein.w.n wrote:
> > `computeKnownBits(A, /* Depth */ 0, &I).countMaxActiveBits() <= 1` -> `isKnownToBeAPowerOfTwo(A,..., /*OrZero*/true, ...)`
> That's not the same thing though? The current check only allows 0 and 1, which result in -1 and 0 after the add. If it's just any power of two we have 0 and 2^x instead, which become -1 and 2^x-1.
Yup ignore my incorrect comment. I misread as a population count check :(


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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D153148/new/

https://reviews.llvm.org/D153148