Index: lib/Analysis/LoopInfo.cpp
===================================================================
--- lib/Analysis/LoopInfo.cpp
+++ lib/Analysis/LoopInfo.cpp
@@ -25,6 +25,7 @@
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Metadata.h"
+#include "llvm/IR/PredIteratorCache.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include <algorithm>
@@ -438,6 +439,8 @@
   UnloopUpdater(Loop *UL, LoopInfo *LInfo) :
     Unloop(UL), LI(LInfo), DFS(UL), FoundIB(false) {}
 
+  void fixLCSSAForDetachedBlock(BasicBlock *BB);
+
   void updateBlockParents();
 
   void removeBlocksFromAncestors();
@@ -449,6 +452,34 @@
 };
 } // end anonymous namespace
 
+/// fixLCSSAForDetachedBlocks - Fix LCSSA if basic block was removed from the
+/// loop getting unrolled and now not inside any loop.
+void UnloopUpdater::fixLCSSAForDetachedBlock(BasicBlock *BB) {
+  assert(!LI->getLoopFor(BB) && "block part of other loop");
+  for (BasicBlock::iterator I = BB->begin(), ie = BB->end(); I != ie; ++I) {
+    Instruction *Inst = dyn_cast<Instruction>(&*I);
+    for (User::op_iterator Op = Inst->op_begin(), Oe = Inst->op_end(); Op != Oe;
+         ++Op)
+      if (Instruction *Oi = dyn_cast<Instruction>(*Op))
+        if (!Unloop->contains(LI->getLoopFor(Oi->getParent()))) {
+          PredIteratorCache PIC;
+          if (PIC.GetNumPreds(BB)) {
+            PHINode *OiPN =
+                PHINode::Create(Oi->getType(), PIC.GetNumPreds(BB),
+                                Oi->getName() + ".lcssa", BB->getFirstNonPHI());
+            for (BasicBlock **PI = PIC.GetPreds(BB); *PI; ++PI)
+              OiPN->addIncoming(Oi, *PI);
+
+            for (Value::user_iterator U = Oi->user_begin(), Ue = Oi->user_end();
+                 U != Ue; ++U)
+              if (Instruction *Ui = dyn_cast<Instruction>(*U))
+                if (OiPN != Ui && Ui->getParent() == BB)
+                  Ui->replaceUsesOfWith(Oi, OiPN);
+          }
+        }
+  }
+}
+
 /// updateBlockParents - Update the parent loop for all blocks that are directly
 /// contained within the original "unloop".
 void UnloopUpdater::updateBlockParents() {
@@ -467,6 +498,9 @@
         assert((NL != Unloop && (!NL || NL->contains(Unloop))) &&
                "uninitialized successor");
         LI->changeLoopFor(*POI, NL);
+
+        if (!NL)
+          fixLCSSAForDetachedBlock(*POI);
       }
       else {
         // Or the current block is part of a subloop, in which case its parent
@@ -493,6 +527,10 @@
         assert(NL != Unloop && (!NL || NL->contains(Unloop)) &&
                "uninitialized successor");
         LI->changeLoopFor(*POI, NL);
+
+        if (!NL)
+          fixLCSSAForDetachedBlock(*POI);
+
         Changed = true;
       }
     }
Index: test/Transforms/LoopUnroll/pr18861.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopUnroll/pr18861.ll
@@ -0,0 +1,43 @@
+; RUN: opt < %s -loop-unroll -indvars -disable-output
+
+@b = external global i32, align 4
+
+; Function Attrs: nounwind uwtable
+define void @fn1() #0 {
+entry:
+  br label %for.cond1thread-pre-split
+
+for.cond1thread-pre-split:                        ; preds = %for.inc8, %entry
+  %storemerge1 = phi i32 [ 0, %entry ], [ %inc9, %for.inc8 ]
+  br i1 undef, label %for.inc8, label %for.cond2.preheader.lr.ph
+
+for.cond2.preheader.lr.ph:                        ; preds = %for.cond1thread-pre-split
+  br label %for.cond2.preheader
+
+for.cond2.preheader:                              ; preds = %for.inc5, %for.cond2.preheader.lr.ph
+  br label %for.cond2
+
+for.cond2:                                        ; preds = %for.body3, %for.cond2.preheader
+  %storemerge = phi i32 [ %add, %for.body3 ], [ 0, %for.cond2.preheader ]
+  %cmp = icmp slt i32 %storemerge, 1
+  br i1 %cmp, label %for.body3, label %for.inc5
+
+for.body3:                                        ; preds = %for.cond2
+  %tobool4 = icmp eq i32 %storemerge, 0
+  %add = add nsw i32 %storemerge, 1
+  br i1 %tobool4, label %for.cond2, label %if.then
+
+if.then:                                          ; preds = %for.body3
+  store i32 %storemerge1, i32* @b, align 4
+  ret void
+
+for.inc5:                                         ; preds = %for.cond2
+  br i1 undef, label %for.cond1.for.inc8_crit_edge, label %for.cond2.preheader
+
+for.cond1.for.inc8_crit_edge:                     ; preds = %for.inc5
+  br label %for.inc8
+
+for.inc8:                                         ; preds = %for.cond1.for.inc8_crit_edge, %for.cond1thread-pre-split
+  %inc9 = add nsw i32 %storemerge1, 1
+  br label %for.cond1thread-pre-split
+}