diff --git a/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/lib/Transforms/Instrumentation/AddressSanitizer.cpp
index a67a9a217a4..a98439b6b39 100644
--- a/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ b/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -1738,6 +1738,9 @@ void FunctionStackPoisoner::poisonStack() {
   IRBuilder<> IRB(InsBefore);
   IRB.SetCurrentDebugLocation(EntryDebugLocation);
 
+  // Make sure non-instrumented allocas stay in the first basic block.
+  // Otherwise, debug info is broken, because only first-basic-block allocas are
+  // treated as regular stack slots.
   for (auto *AI : NonInstrumentedStaticAllocaVec) AI->moveBefore(InsBefore);
 
   SmallVector<ASanStackVariableDescription, 16> SVD;