[SimplifyCFG] switchToSelect(): don't forget to insert DomTree edge iff needed

DestBB might or might not already be a successor of SelectBB,
and it wasn't we need to ensure that we record the fact in DomTree.

The testcase used to crash in lazy domtree updater mode + non-per-function
domtree validity checks disabled.

1 files changed, 11 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 4c0427e..559830e 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5394,20 +5394,25 @@ static void RemoveSwitchAfterSelectConversion(SwitchInst *SI, PHINode *PHI,
                                               Value *SelectValue,
                                               IRBuilder<> &Builder,
                                               DomTreeUpdater *DTU) {
+  std::vector<DominatorTree::UpdateType> Updates;
+
   BasicBlock *SelectBB = SI->getParent();
+  BasicBlock *DestBB = PHI->getParent();
+
+  if (!is_contained(predecessors(DestBB), SelectBB))
+    Updates.push_back({DominatorTree::Insert, SelectBB, DestBB});
+  Builder.CreateBr(DestBB);
+
+  // Remove the switch.
+
   while (PHI->getBasicBlockIndex(SelectBB) >= 0)
     PHI->removeIncomingValue(SelectBB);
   PHI->addIncoming(SelectValue, SelectBB);
 
-  Builder.CreateBr(PHI->getParent());
-
-  std::vector<DominatorTree::UpdateType> Updates;
-
-  // Remove the switch.
   for (unsigned i = 0, e = SI->getNumSuccessors(); i < e; ++i) {
     BasicBlock *Succ = SI->getSuccessor(i);
 
-    if (Succ == PHI->getParent())
+    if (Succ == DestBB)
       continue;
     Succ->removePredecessor(SelectBB);
     Updates.push_back({DominatorTree::Delete, SelectBB, Succ});