diff --git a/llvm/lib/CodeGen/ScoreboardHazardRecognizer.cpp b/llvm/lib/CodeGen/ScoreboardHazardRecognizer.cpp
--- a/llvm/lib/CodeGen/ScoreboardHazardRecognizer.cpp
+++ b/llvm/lib/CodeGen/ScoreboardHazardRecognizer.cpp
@@ -143,6 +143,13 @@
       }
 
       InstrStage::FuncUnits freeUnits = IS->getUnits();
+      if (!freeUnits) {
+        // No functional units are required by this stage, so cannot conflict.
+        // In addition, prevent endless loop when running the post-RA-sched
+        // pass.
+        continue;
+      }
+
       switch (IS->getReservationKind()) {
       case InstrStage::Required:
         // Required FUs conflict with both reserved and required ones