[PATCH] D25913: [InstCombine] Fold nuw left-shifts in `ugt`/`ule` comparisons.

bryant via llvm-commits llvm-commits at lists.llvm.org
Wed Oct 26 15:24:49 PDT 2016


bryant added inline comments.


================
Comment at: lib/Transforms/InstCombine/InstCombineCompares.cpp:1963
+    // (X << S) <=u C = X <=u (C >> S)
+    // (X << S) <u (C + 1) = X <u (C >> S) + 1 if C < - 1
+    // (X << S) <u C = X <u ((C - 1) >> S) + 1 if C > 0
----------------
bryant wrote:
> efriedma wrote:
> > "// (X << S) <u (C + 1) = X <u (C >> S) + 1 if C < - 1"
> > 
> > This comment is confusing... when exactly is "C < - 1" true?
> -1 == all ones. Assuming that addition wraps, `X <=u C` is equivalent to `X <=u (C + 1)` if C + 1 doesn't wrap, or if C is less than all ones.
Replace the second `<=u` with `<u`.


Repository:
  rL LLVM

https://reviews.llvm.org/D25913





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