[MemorySSA] Check for unreachable blocks when getting last definition.

If a single predecessor is found, still check if the block is
unreachable. The test that found this had a self loop unreachable block.
Resolves PR43493.

llvm-svn: 373383

1 files changed, 3 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/MemorySSAUpdater.cpp b/llvm/lib/Analysis/MemorySSAUpdater.cpp
index 315037bcc04..e0b27f1d501 100644
--- a/llvm/lib/Analysis/MemorySSAUpdater.cpp
+++ b/llvm/lib/Analysis/MemorySSAUpdater.cpp
@@ -839,6 +839,9 @@ void MemorySSAUpdater::applyInsertUpdates(ArrayRef<CFGUpdate> Updates,
       } else {
         // Single predecessor, BB cannot be dead. GetLastDef of Pred.
         assert(Count == 1 && Pred && "Single predecessor expected.");
+        // BB can be unreachable though, return LoE if that is the case.
+        if (!DT.getNode(BB))
+          return MSSA->getLiveOnEntryDef();
         BB = Pred;
       }
     };