[llvm] [InstCombine] Fold the `log2_ceil` idiom (PR #76661)

[Nikita Popov via llvm-commits]

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@@ -1730,6 +1730,28 @@ Instruction *InstCombinerImpl::visitAdd(BinaryOperator &I) {
         I, Builder.CreateIntrinsic(Intrinsic::ctpop, {I.getType()},
                                    {Builder.CreateOr(A, B)}));
 
+  // Fold the log2_ceil idiom:
+  // zext(ctpop(A, true) >u/!= 1) + (ctlz(A) ^ (BW - 1)) --> BW - ctlz(A - 1)
+  // TODO: Add one-use checks?
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nikic wrote:

I'd add the one-use checks along the ctlz path, so that we at worst replace one ctlz with another.

https://github.com/llvm/llvm-project/pull/76661