diff --git a/lib/Transforms/Utils/LoopUnroll.cpp b/lib/Transforms/Utils/LoopUnroll.cpp
index b1cc76859e9..7b066453cee 100644
--- a/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/lib/Transforms/Utils/LoopUnroll.cpp
@@ -301,13 +301,41 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
 
   for (unsigned It = 1; It != Count; ++It) {
     std::vector<BasicBlock*> NewBlocks;
+    SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
+    NewLoops[L] = L;
 
     for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
       ValueToValueMapTy VMap;
       BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
       Header->getParent()->getBasicBlockList().push_back(New);
 
-      L->addBasicBlockToLoop(New, LI->getBase());
+      // Tell LI about New.
+      if (*BB == Header) {
+        assert(LI->getLoopFor(*BB) == L && "Header should not be in a sub-loop");
+        L->addBasicBlockToLoop(New, LI->getBase());
+      } else {
+        // Figure out which loop New is in.
+        const Loop *OldLoop = LI->getLoopFor(*BB);
+        assert(OldLoop && "Should (at least) be in the loop being unrolled!");
+
+        Loop *&NewLoop = NewLoops[OldLoop];
+        if (!NewLoop) {
+          // Found a new sub-loop.
+          assert(*BB == OldLoop->getHeader() &&
+                 "Header should be first in RPO");
+
+          Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
+          assert(NewLoopParent &&
+                 "Expected parent loop before sub-loop in RPO");
+          NewLoop = new Loop;
+          NewLoopParent->addChildLoop(NewLoop);
+
+          // Forget the old loop, since its inputs may have changed.
+          if (SE)
+            SE->forgetLoop(OldLoop);
+        }
+        NewLoop->addBasicBlockToLoop(New, LI->getBase());
+      }
 
       if (*BB == Header)
         // Loop over all of the PHI nodes in the block, changing them to use
diff --git a/test/Transforms/LoopUnroll/update-loop-info-in-subloops.ll b/test/Transforms/LoopUnroll/update-loop-info-in-subloops.ll
new file mode 100644
index 00000000000..adbf47defe8
--- /dev/null
+++ b/test/Transforms/LoopUnroll/update-loop-info-in-subloops.ll
@@ -0,0 +1,35 @@
+; RUN: opt -S < %s -loop-unroll -block-freq | FileCheck %s
+; Crasher from PR20987.
+
+; CHECK: define void @update_loop_info_in_subloops
+; CHECK: entry:
+; CHECK: L:
+; CHECK: L.inner:
+; CHECK: L.inner.latch:
+; CHECK: L.latch:
+; CHECK: L.inner.1:
+; CHECK: L.inner.latch.1:
+; CHECK: L.latch.1:
+
+define void @update_loop_info_in_subloops() {
+entry:
+  br label %L
+
+L:
+  %0 = phi i64 [ 1, %entry ], [ %1, %L.latch ]
+  br label %L.inner
+
+L.inner:
+  br label %L.inner.latch
+
+L.inner.latch:
+  br i1 false, label %L.latch, label %L.inner
+
+L.latch:
+  %1 = add i64 %0, 1
+  %2 = icmp eq i64 %1, 3
+  br i1 %2, label %exit, label %L
+
+exit:
+  ret void
+}