diff --git a/lib/Transforms/Scalar/ADCE.cpp b/lib/Transforms/Scalar/ADCE.cpp
index 7fafe26d4a9..e1f228d6fab 100644
--- a/lib/Transforms/Scalar/ADCE.cpp
+++ b/lib/Transforms/Scalar/ADCE.cpp
@@ -234,6 +234,7 @@ bool ADCE::doADCE() {
         }
       } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
                  isa<UnwindInst>(I)) {
+        // Unreachable instructions are not marked intrinsically live here.
 	markInstructionLive(I);
       } else if (isInstructionTriviallyDead(I)) {
         // Remove the instruction from it's basic block...