diff --git a/lib/Transforms/Scalar/ADCE.cpp b/lib/Transforms/Scalar/ADCE.cpp
index 22b4c347dcb..c7d349932c9 100644
--- a/lib/Transforms/Scalar/ADCE.cpp
+++ b/lib/Transforms/Scalar/ADCE.cpp
@@ -91,9 +91,8 @@ private:
   TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
 
   inline void markInstructionLive(Instruction *I) {
-    if (LiveSet.count(I)) return;
+    if (!LiveSet.insert(I).second) return;
     DEBUG(std::cerr << "Insn Live: " << *I);
-    LiveSet.insert(I);
     WorkList.push_back(I);
   }