[llvm] [SimplifyCFG] Add optimization for switches of powers of two (PR #70977)

[Nikita Popov via llvm-commits]

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@@ -6839,6 +6836,81 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
   return true;
 }
 
+/// Tries to transform switch of powers of two to reduce switch range.
+/// For example, switch like:
+/// switch (C) { case 1: case 2: case 64: case 128: }
+/// will be transformed to:
+/// switch ( count_trailing_zeros(C) ) { case 0: case 1: case 6: case 7: }
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nikic wrote:

```suggestion
/// switch (count_trailing_zeros(C)) { case 0: case 1: case 6: case 7: }
```

https://github.com/llvm/llvm-project/pull/70977