Index: llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -122,6 +122,11 @@
     cl::desc(
         "Set the max unroll count for full unrolling, for testing purposes"));
 
+static cl::opt<unsigned> UnrollFullSizeThreshold(
+    "unroll-full-size-threshold", cl::init(1000), cl::Hidden,
+    cl::desc(
+        "Set the size threshold for full unrolling, for testing purposes"));
+
 static cl::opt<bool>
     UnrollAllowPartial("unroll-allow-partial", cl::Hidden,
                        cl::desc("Allows loops to be partially unrolled until "
@@ -1120,6 +1125,11 @@
     return LoopUnrollResult::Unmodified;
   }
 
+  if (LoopSize > UnrollFullSizeThreshold) {
+    LLVM_DEBUG(dbgs() << "  Not unrolling loop with as large loop size.\n");
+    return LoopUnrollResult::Unmodified;
+  }
+
   // Find the smallest exact trip count for any exit. This is an upper bound
   // on the loop trip count, but an exit at an earlier iteration is still
   // possible. An unroll by the smallest exact trip count guarantees that all