diff --git a/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp b/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp
index 2ee776711587..25c438c9615a 100644
--- a/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp
+++ b/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp
@@ -403,8 +403,18 @@ void AggressiveAntiDepBreaker::PrescanInstruction(MachineInstr *MI,
       continue;
 
     // Update def for Reg and aliases.
-    for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
+    for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
+      // We need to be careful here not to define already-live super registers.
+      // If the super register is already live, then this definition is not
+      // a definition of the whole super register (just a partial insertion
+      // into it). Earlier subregister definitions (which we've not yet visited
+      // because we're iterating bottom-up) need to be linked to the same group
+      // as this definition.
+      if (TRI->isSuperRegister(Reg, *AI) && State->IsLive(*AI))
+        continue;
+
       DefIndices[*AI] = Count;
+    }
   }
 }