[PATCH] D145890: [InstCombine] Generate better code for std::bit_floor from libstdc++

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp:3296
+//
+//   X == 0 ? 0 : (1 << (32 - ctlz(X >> 1)))
+//
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'32' -> 'BitWidth'.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp:3304
+// replacing one of the select operands.
+static Instruction *foldBitFloor(SelectInst &SI,
+                                 InstCombiner::BuilderTy &Builder) {
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I would prefer this as a more generic function for replacing `1 << (BitWidth - ctlz(X >> 1))` -> `(SignBit >> ctlz(X)` which we can do anytime we know `X` is non-zero: https://alive2.llvm.org/ce/z/PDxghx

Then use the helper for the select case, but then it can also be used with an `IsKnownNonZero` query.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D145890/new/

https://reviews.llvm.org/D145890