[llvm] [SimplifyCFG] Simplify switch with implicit default (PR #95665)

[via llvm-commits]

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@@ -7104,6 +7104,47 @@ static bool simplifySwitchOfPowersOfTwo(SwitchInst *SI, IRBuilder<> &Builder,
   return true;
 }
 
+// Try to fold switch with manually selected default branch.
+// For example, for the switch
+//   switch (v) { case 0: case 1: case 2: ... case MaxC: default: }
+//     (continuous cases value from 0 to MaxC)
+//   and, v = select (x u<= MaxC), x, AnotherC.
+// so "AnotherC" is the de-facto default branch, and it will be folded to
+//   switch (x) { case 0: case 1: case 2: ... case MaxC:  default -> AnotherC: }
+static bool simplifySwitchWithImplicitDefault(SwitchInst *SI) {
+  Value *Cond;
+  ConstantInt *Bound, *DefaultCase;
+  ICmpInst::Predicate Pred;
+  if (!match(SI->getCondition(),
+             m_Select(m_ICmp(Pred, m_Value(Cond), m_ConstantInt(Bound)),
+                      m_Deferred(Cond), m_ConstantInt(DefaultCase))) ||
+      Pred != CmpInst::ICMP_ULT)
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goldsteinn wrote:

Hmm, truthfully would just say make a helper that covers all unsigned preds. Shouldn't be that hard and then we don't need to spend what is probably more effort finding all the cases that each pred might be emitted in.

https://github.com/llvm/llvm-project/pull/95665