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

[via llvm-commits]

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@@ -7540,6 +7540,81 @@ static bool reduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
   return true;
 }
 
+/// Tries to transform the switch when the condition is umin with a constant.
+/// In that case, the default branch can be replaced by the constant's branch.
+/// This method also removes dead cases when the simplification cannot replace
+/// the default branch.
+///
+/// For example:
+/// switch(umin(a, 3)) {
+/// case 0:
+/// case 1:
+/// case 2:
+/// case 3:
+/// case 4:
+///   // ...
+/// default:
+///   unreachable
+/// }
+///
+/// Transforms into:
+///
+/// switch(a) {
+/// case 0:
+/// case 1:
+/// case 2:
+/// default:
+///   // This is case 3
+/// }
+static bool simplifySwitchWhenUMin(SwitchInst *SI, DomTreeUpdater *DTU) {
+  Value *A;
+  ConstantInt *Constant;
+
+  if (!match(SI->getCondition(), m_UMin(m_Value(A), m_ConstantInt(Constant))))
+    return false;
+
+  SmallVector<DominatorTree::UpdateType> Updates;
+  SwitchInstProfUpdateWrapper SIW(*SI);
+  BasicBlock *BB = SIW->getParent();
+
+  // Dead cases are removed even when the simplification fails.
+  // A case is dead when its value is higher than the Constant.
+  for (auto I = SI->case_begin(), E = SI->case_end(); I != E;) {
+    if (!I->getCaseValue()->getValue().ugt(Constant->getValue())) {
+      ++I;
+      continue;
+    }
+    BasicBlock *DeadCaseBB = I->getCaseSuccessor();
+    DeadCaseBB->removePredecessor(BB);
+    Updates.push_back({DominatorTree::Delete, BB, DeadCaseBB});
+    I = SI->removeCase(I);
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kper wrote:

done

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