[SimplifyCFG] avoid crashing after simplifying a switch (PR42737)

Later code in TryToSimplifyUncondBranchFromEmptyBlock() assumes that
we have cleaned up unreachable blocks, but that was not happening
with this switch transform.

llvm-svn: 367037

1 files changed, 17 insertions, 8 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 11651d040dc..f5bcdc74a08 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -4276,6 +4276,17 @@ static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
   return true;
 }
 
+static void createUnreachableSwitchDefault(SwitchInst *Switch) {
+  LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
+  BasicBlock *NewDefaultBlock =
+     SplitBlockPredecessors(Switch->getDefaultDest(), Switch->getParent(), "");
+  Switch->setDefaultDest(&*NewDefaultBlock);
+  SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front());
+  auto *NewTerminator = NewDefaultBlock->getTerminator();
+  new UnreachableInst(Switch->getContext(), NewTerminator);
+  EraseTerminatorAndDCECond(NewTerminator);
+}
+
 /// Turn a switch with two reachable destinations into an integer range
 /// comparison and branch.
 static bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder) {
@@ -4384,6 +4395,11 @@ static bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder) {
       cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
   }
 
+  // Clean up the default block - it may have phis or other instructions before
+  // the unreachable terminator.
+  if (!HasDefault)
+    createUnreachableSwitchDefault(SI);
+
   // Drop the switch.
   SI->eraseFromParent();
 
@@ -4428,14 +4444,7 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, AssumptionCache *AC,
   if (HasDefault && DeadCases.empty() &&
       NumUnknownBits < 64 /* avoid overflow */ &&
       SI->getNumCases() == (1ULL << NumUnknownBits)) {
-    LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
-    BasicBlock *NewDefault =
-        SplitBlockPredecessors(SI->getDefaultDest(), SI->getParent(), "");
-    SI->setDefaultDest(&*NewDefault);
-    SplitBlock(&*NewDefault, &NewDefault->front());
-    auto *OldTI = NewDefault->getTerminator();
-    new UnreachableInst(SI->getContext(), OldTI);
-    EraseTerminatorAndDCECond(OldTI);
+    createUnreachableSwitchDefault(SI);
     return true;
   }