[PATCH] D25913: [InstCombine] Fold nuw left-shifts in `ugt`/`ule` comparisons.
Eli Friedman via llvm-commits
llvm-commits at lists.llvm.org
Wed Oct 26 15:25:53 PDT 2016
efriedma 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`.
Ah, okay. You could probably write that in a way that's a little more obvious.
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