diff --git a/llvm/include/llvm/Transforms/Utils/LoopUtils.h b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
index 8c2527b6ae68a..e561a9c584073 100644
--- a/llvm/include/llvm/Transforms/Utils/LoopUtils.h
+++ b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
@@ -79,7 +79,8 @@ bool formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
 ///
 /// Looks at all instructions in the loop which have uses outside of the
 /// current loop. For each, an LCSSA PHI node is inserted and the uses outside
-/// the loop are rewritten to use this node.
+/// the loop are rewritten to use this node. Sub-loops must be in LCSSA form
+/// already.
 ///
 /// LoopInfo and DominatorTree are required and preserved.
 ///
diff --git a/llvm/lib/Transforms/Utils/LCSSA.cpp b/llvm/lib/Transforms/Utils/LCSSA.cpp
index 53d444b309d5d..eeeaadc253f29 100644
--- a/llvm/lib/Transforms/Utils/LCSSA.cpp
+++ b/llvm/lib/Transforms/Utils/LCSSA.cpp
@@ -325,6 +325,10 @@ bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
   // Look at all the instructions in the loop, checking to see if they have uses
   // outside the loop.  If so, put them into the worklist to rewrite those uses.
   for (BasicBlock *BB : BlocksDominatingExits) {
+    // Skip blocks that are part of any sub-loops, they must be in LCSSA
+    // already.
+    if (LI->getLoopFor(BB) != &L)
+      continue;
     for (Instruction &I : *BB) {
       // Reject two common cases fast: instructions with no uses (like stores)
       // and instructions with one use that is in the same block as this.