1 files changed, 28 insertions, 7 deletions
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 55e3756..61d891d 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5414,11 +5414,13 @@ static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
 }
 
 static void createUnreachableSwitchDefault(SwitchInst *Switch,
-                                           DomTreeUpdater *DTU) {
+                                           DomTreeUpdater *DTU,
+                                           bool RemoveOrigDefaultBlock = true) {
   LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
   auto *BB = Switch->getParent();
   auto *OrigDefaultBlock = Switch->getDefaultDest();
-  OrigDefaultBlock->removePredecessor(BB);
+  if (RemoveOrigDefaultBlock)
+    OrigDefaultBlock->removePredecessor(BB);
   BasicBlock *NewDefaultBlock = BasicBlock::Create(
       BB->getContext(), BB->getName() + ".unreachabledefault", BB->getParent(),
       OrigDefaultBlock);
@@ -5427,7 +5429,8 @@ static void createUnreachableSwitchDefault(SwitchInst *Switch,
   if (DTU) {
     SmallVector<DominatorTree::UpdateType, 2> Updates;
     Updates.push_back({DominatorTree::Insert, BB, &*NewDefaultBlock});
-    if (!is_contained(successors(BB), OrigDefaultBlock))
+    if (RemoveOrigDefaultBlock &&
+        !is_contained(successors(BB), OrigDefaultBlock))
       Updates.push_back({DominatorTree::Delete, BB, &*OrigDefaultBlock});
     DTU->applyUpdates(Updates);
   }
@@ -5609,10 +5612,28 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
       Known.getBitWidth() - (Known.Zero | Known.One).popcount();
   assert(NumUnknownBits <= Known.getBitWidth());
   if (HasDefault && DeadCases.empty() &&
-      NumUnknownBits < 64 /* avoid overflow */ &&
-      SI->getNumCases() == (1ULL << NumUnknownBits)) {
-    createUnreachableSwitchDefault(SI, DTU);
-    return true;
+      NumUnknownBits < 64 /* avoid overflow */) {
+    uint64_t AllNumCases = 1ULL << NumUnknownBits;
+    if (SI->getNumCases() == AllNumCases) {
+      createUnreachableSwitchDefault(SI, DTU);
+      return true;
+    }
+    // When only one case value is missing, replace default with that case.
+    // Eliminating the default branch will provide more opportunities for
+    // optimization, such as lookup tables.
+    if (SI->getNumCases() == AllNumCases - 1) {
+      assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
+      uint64_t MissingCaseVal = 0;
+      for (const auto &Case : SI->cases())
+        MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
+      auto *MissingCase =
+          cast<ConstantInt>(ConstantInt::get(Cond->getType(), MissingCaseVal));
+      SwitchInstProfUpdateWrapper SIW(*SI);
+      SIW.addCase(MissingCase, SI->getDefaultDest(), SIW.getSuccessorWeight(0));
+      createUnreachableSwitchDefault(SI, DTU, /*RemoveOrigDefaultBlock*/ false);
+      SIW.setSuccessorWeight(0, 0);
+      return true;
+    }
   }
 
   if (DeadCases.empty())