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

[Yingwei Zheng via llvm-commits]

================
@@ -7104,6 +7104,61 @@ 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))))
+    return false;
+
+  ConstantRange OuterRange =
+      ConstantRange::makeExactICmpRegion(Pred, Bound->getValue());
+  if (!OuterRange.getLower().isZero())
+    return false;
+
+  bool HasDefault =
+      !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
+
+  // In an ideal situation, if default is reachable, the range of switch cases
+  // should be 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 (HasDefault && OuterRange.getUpper() != SI->getNumCases())
+    return false;
+
+  APInt Min = SI->case_begin()->getCaseValue()->getValue(), Max = Min;
+  for (const auto &Case : SI->cases()) {
----------------
dtcxzyw wrote:

Can we use `drop_front` here?


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