[llvm] [SimplifyCFG]: Switch on umin replaces default (PR #164097)

[Nikita Popov via llvm-commits]

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@@ -7540,6 +7540,60 @@ static bool reduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
   return true;
 }
 
+/// Tries to transform the switch when the condition is umin and a constant.
+/// In that case, the default branch can be replaced by the constant's branch.
+/// For example:
+/// switch(umin(a, 3)) {
+/// case 0:
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nikic wrote:

I think this needs to be explicitly checked in the transform, we shouldn't rely on eliminateDeadSwitchCases having removed such cases (looking at the implementation, it uses known bits rather than ranges, so I think it may not eliminate all dead cases if the umin is not at a power of two boundary).

https://github.com/llvm/llvm-project/pull/164097