[llvm] [InstCombine] Fold `(ct{t, l}z Pow2)` -> `Log2(Pow2)` (PR #122620)

[Nikita Popov via llvm-commits]

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@@ -588,6 +588,22 @@ static Instruction *foldCttzCtlz(IntrinsicInst &II, InstCombinerImpl &IC) {
     }
   }
 
+  // cttz(Pow2) -> Log2(Pow2)
+  // ctlz(Pow2) -> BitWidth - 1 - Log2(Pow2)
+  if (IsTZ || II.hasOneUse()) {
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nikic wrote:

Your new condition effectively drops ctlz entirely :)

https://github.com/llvm/llvm-project/pull/122620