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

[Nikita Popov via llvm-commits]

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@@ -7104,6 +7104,48 @@ static bool simplifySwitchOfPowersOfTwo(SwitchInst *SI, IRBuilder<> &Builder,
   return true;
 }
 
+// Try to fold switch with manually selecting 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))
+    return false;
+
+  // In an ideal situation, the range of switch cases is continuous,
+  // and should match the range of ICmp. That is,
+  //   MaxCase (declared below) + 1 = SI->getNumCases() = OuterRange.Upper.
+  // Checking it partially here can be a fast-fail path
+  if (Bound->getValue() != SI->getNumCases())
+    return false;
+
+  SmallVector<uint64_t, 4> Values;
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nikic wrote:

Use APInt. Please add a test with i128, which will currently assert.

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