diff --git a/include/llvm/Analysis/LoopInfo.h b/include/llvm/Analysis/LoopInfo.h
index 7ebcd6b8498..7c89744123f 100644
--- a/include/llvm/Analysis/LoopInfo.h
+++ b/include/llvm/Analysis/LoopInfo.h
@@ -708,9 +708,13 @@ public:
   /// specified loop.  This should be used by transformations that restructure
   /// the loop hierarchy tree.
   void changeLoopFor(BlockT *BB, LoopT *L) {
-    LoopT *&OldLoop = BBMap[BB];
-    assert(OldLoop && "Block not in a loop yet!");
-    OldLoop = L;
+    if (!L) {
+      typename DenseMap<BlockT *, LoopT *>::iterator I = BBMap.find(BB);
+      if (I != BBMap.end())
+        BBMap.erase(I);
+      return;
+    }
+    BBMap[BB] = L;
   }
 
   /// changeTopLevelLoop - Replace the specified loop in the top-level loops
@@ -1029,6 +1033,12 @@ public:
     LI.removeBlock(BB);
   }
 
+  /// updateUnloop - Update LoopInfo after removing the last backedge from a
+  /// loop--now the "unloop". This updates the loop forest and parent loops for
+  /// each block so that Unloop is no longer referenced, but the caller must
+  /// actually delete the Unloop object.
+  void updateUnloop(Loop *Unloop);
+
   /// replacementPreservesLCSSAForm - Returns true if replacing From with To
   /// everywhere is guaranteed to preserve LCSSA form.
   bool replacementPreservesLCSSAForm(Instruction *From, Value *To) {
diff --git a/lib/Analysis/LoopInfo.cpp b/lib/Analysis/LoopInfo.cpp
index 528b3cf87b6..6bebae57407 100644
--- a/lib/Analysis/LoopInfo.cpp
+++ b/lib/Analysis/LoopInfo.cpp
@@ -383,6 +383,174 @@ void Loop::dump() const {
   print(dbgs());
 }
 
+//===----------------------------------------------------------------------===//
+// UnloopUpdater implementation
+//
+
+/// Find the new parent loop for all blocks within the "unloop" whose last
+/// backedges has just been removed.
+class UnloopUpdater {
+  Loop *Unloop;
+  LoopInfo *LI;
+
+  LoopBlocksDFS DFS;
+
+  // Map unloop's immediate subloops to their nearest reachable parents. Nested
+  // loops within these subloops will not change parents. However, an immediate
+  // subloop's new parent will be the nearest loop reachable from either its own
+  // exits *or* any of its nested loop's exits.
+  DenseMap<Loop*, Loop*> SubloopParents;
+
+  // Flag the presence of an irreducible backedge whose destination is a block
+  // directly contained by the original unloop.
+  bool FoundIB;
+
+public:
+  UnloopUpdater(Loop *UL, LoopInfo *LInfo) :
+    Unloop(UL), LI(LInfo), DFS(UL), FoundIB(false) {}
+
+  void updateBlockParents();
+
+  void updateSubloopParents();
+
+protected:
+  Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
+};
+
+/// updateBlockParents - Update the parent loop for all blocks that are directly
+/// contained within the original "unloop".
+void UnloopUpdater::updateBlockParents() {
+  if (Unloop->getNumBlocks()) {
+    // Perform a post order CFG traversal of all blocks within this loop,
+    // propagating the nearest loop from sucessors to predecessors.
+    LoopBlocksTraversal Traversal(DFS, LI);
+    for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
+           POE = Traversal.end(); POI != POE; ++POI) {
+
+      Loop *L = LI->getLoopFor(*POI);
+      Loop *NL = getNearestLoop(*POI, L);
+
+      if (NL != L) {
+        // For reducible loops, NL is now an ancestor of Unloop.
+        assert((NL != Unloop && (!NL || NL->contains(Unloop))) &&
+               "uninitialized successor");
+        LI->changeLoopFor(*POI, NL);
+      }
+      else {
+        // Or the current block is part of a subloop, in which case its parent
+        // is unchanged.
+        assert((FoundIB || Unloop->contains(L)) && "uninitialized successor");
+      }
+    }
+  }
+  // Each irreducible loop within the unloop induces a round of iteration using
+  // the DFS result cached by Traversal.
+  bool Changed = FoundIB;
+  for (unsigned NIters = 0; Changed; ++NIters) {
+    assert(NIters < Unloop->getNumBlocks() && "runaway iterative algorithm");
+
+    // Iterate over the postorder list of blocks, propagating the nearest loop
+    // from successors to predecessors as before.
+    Changed = false;
+    for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
+           POE = DFS.endPostorder(); POI != POE; ++POI) {
+
+      Loop *L = LI->getLoopFor(*POI);
+      Loop *NL = getNearestLoop(*POI, L);
+      if (NL != L) {
+        assert(NL != Unloop && (!NL || NL->contains(Unloop)) &&
+               "uninitialized successor");
+        LI->changeLoopFor(*POI, NL);
+        Changed = true;
+      }
+    }
+  }
+}
+
+/// updateSubloopParents - Update the parent loop for all subloops directly
+/// nested within unloop.
+void UnloopUpdater::updateSubloopParents() {
+  while (!Unloop->empty()) {
+    Loop *Subloop = *(Unloop->end()-1);
+    Unloop->removeChildLoop(Unloop->end()-1);
+
+    assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
+    if (SubloopParents[Subloop])
+      SubloopParents[Subloop]->addChildLoop(Subloop);
+  }
+}
+
+/// getNearestLoop - Return the nearest parent loop among this block's
+/// successors. If a successor is a subloop header, consider its parent to be
+/// the nearest parent of the subloop's exits.
+///
+/// For subloop blocks, simply update SubloopParents and return NULL.
+Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
+
+  // Initialy for blocks directly contained by Unloop, NearLoop == Unloop and is
+  // considered uninitialized.
+  Loop *NearLoop = BBLoop;
+
+  Loop *Subloop = 0;
+  if (NearLoop != Unloop && Unloop->contains(NearLoop)) {
+    Subloop = NearLoop;
+    // Find the subloop ancestor that is directly contained within Unloop.
+    while (Subloop->getParentLoop() != Unloop) {
+      Subloop = Subloop->getParentLoop();
+      assert(Subloop && "subloop is not an ancestor of the original loop");
+    }
+    // Get the current nearest parent of the Subloop exits, initially Unloop.
+    if (!SubloopParents.count(Subloop))
+      SubloopParents[Subloop] = Unloop;
+    NearLoop = SubloopParents[Subloop];
+  }
+
+  succ_iterator I = succ_begin(BB), E = succ_end(BB);
+  if (I == E) {
+    assert(!Subloop && "subloop blocks must have a successor");
+    NearLoop = 0; // unloop blocks may now exit the function.
+  }
+  for (; I != E; ++I) {
+    if (*I == BB)
+      continue; // self loops are uninteresting
+
+    Loop *L = LI->getLoopFor(*I);
+    if (L == Unloop) {
+      // This successor has not been processed. This path must lead to an
+      // irreducible backedge.
+      assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
+      FoundIB = true;
+    }
+    if (L != Unloop && Unloop->contains(L)) {
+      // Successor is in a subloop.
+      if (Subloop)
+        continue; // Branching within subloops. Ignore it.
+
+      // BB branches from the original into a subloop header.
+      assert(L->getParentLoop() == Unloop && "cannot skip into nested loops");
+
+      // Get the current nearest parent of the Subloop's exits.
+      L = SubloopParents[L];
+      // L could be Unloop if the only exit was an irreducible backedge.
+    }
+    if (L == Unloop) {
+      continue;
+    }
+    // Handle critical edges from Unloop into a sibling loop.
+    if (L && !L->contains(Unloop)) {
+      L = L->getParentLoop();
+    }
+    // Remember the nearest parent loop among successors or subloop exits.
+    if (NearLoop == Unloop || !NearLoop || NearLoop->contains(L))
+      NearLoop = L;
+  }
+  if (Subloop) {
+    SubloopParents[Subloop] = NearLoop;
+    return BBLoop;
+  }
+  return NearLoop;
+}
+
 //===----------------------------------------------------------------------===//
 // LoopInfo implementation
 //
@@ -392,6 +560,82 @@ bool LoopInfo::runOnFunction(Function &) {
   return false;
 }
 
+/// updateUnloop - The last backedge has been removed from a loop--now the
+/// "unloop". Find a new parent for the blocks contained within unloop and
+/// update the loop tree. We don't necessarilly have valid dominators at this
+/// point, but LoopInfo is still valid except for the removal of this loop.
+///
+/// Note that Unloop may now be an empty loop. Calling Loop::getHeader without
+/// checking first is illegal.
+void LoopInfo::updateUnloop(Loop *Unloop) {
+
+  // First handle the special case of no parent loop to simplify the algorithm.
+  if (!Unloop->getParentLoop()) {
+    // Since BBLoop had no parent, Unloop blocks are no longer in a loop.
+    for (Loop::block_iterator I = Unloop->block_begin(),
+         E = Unloop->block_end(); I != E; ++I) {
+
+      // Don't reparent blocks in subloops.
+      if (getLoopFor(*I) != Unloop)
+        continue;
+
+      // Blocks no longer have a parent but are still referenced by Unloop until
+      // the Unloop object is deleted.
+      LI.changeLoopFor(*I, 0);
+    }
+
+    // Remove the loop from the top-level LoopInfo object.
+    for (LoopInfo::iterator I = LI.begin(), E = LI.end();; ++I) {
+      assert(I != E && "Couldn't find loop");
+      if (*I == Unloop) {
+        LI.removeLoop(I);
+        break;
+      }
+    }
+
+    // Move all of the subloops to the top-level.
+    while (!Unloop->empty())
+      LI.addTopLevelLoop(Unloop->removeChildLoop(Unloop->end()-1));
+
+    return;
+  }
+
+  // Update the parent loop for all blocks within the loop. Blocks within
+  // subloops will not change parents.
+  UnloopUpdater Updater(Unloop, this);
+  Updater.updateBlockParents();
+
+  // Remove unloop's blocks from all ancestors below their new parents.
+  for (Loop::block_iterator BI = Unloop->block_begin(),
+         BE = Unloop->block_end(); BI != BE; ++BI) {
+    Loop *NewParent = getLoopFor(*BI);
+    // If this block is in a subloop, skip it.
+    if (Unloop->contains(NewParent))
+      continue;
+
+    // Remove blocks from former Ancestors except Unloop itself which will be
+    // deleted.
+    for (Loop *OldParent = Unloop->getParentLoop(); OldParent != NewParent;
+         OldParent = OldParent->getParentLoop()) {
+      assert(OldParent && "new loop is not an ancestor of the original");
+      OldParent->removeBlockFromLoop(*BI);
+    }
+  }
+
+  // Add direct subloops as children in their new parent loop.
+  Updater.updateSubloopParents();
+
+  // Remove unloop from its parent loop.
+  Loop *ParentLoop = Unloop->getParentLoop();
+  for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();; ++I) {
+    assert(I != E && "Couldn't find loop");
+    if (*I == Unloop) {
+      ParentLoop->removeChildLoop(I);
+      break;
+    }
+  }
+}
+
 void LoopInfo::verifyAnalysis() const {
   // LoopInfo is a FunctionPass, but verifying every loop in the function
   // each time verifyAnalysis is called is very expensive. The
diff --git a/lib/Analysis/LoopPass.cpp b/lib/Analysis/LoopPass.cpp
index f8abf3c074c..7ba3268b9ef 100644
--- a/lib/Analysis/LoopPass.cpp
+++ b/lib/Analysis/LoopPass.cpp
@@ -84,62 +84,18 @@ LPPassManager::LPPassManager(int Depth)
 /// Delete loop from the loop queue and loop hierarchy (LoopInfo).
 void LPPassManager::deleteLoopFromQueue(Loop *L) {
 
-  if (Loop *ParentLoop = L->getParentLoop()) { // Not a top-level loop.
-    // Reparent all of the blocks in this loop.  Since BBLoop had a parent,
-    // they are now all in it.
-    for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
-         I != E; ++I)
-      if (LI->getLoopFor(*I) == L)    // Don't change blocks in subloops.
-        LI->changeLoopFor(*I, ParentLoop);
-
-    // Remove the loop from its parent loop.
-    for (Loop::iterator I = ParentLoop->begin(), E = ParentLoop->end();;
-         ++I) {
-      assert(I != E && "Couldn't find loop");
-      if (*I == L) {
-        ParentLoop->removeChildLoop(I);
-        break;
-      }
-    }
-
-    // Move all subloops into the parent loop.
-    while (!L->empty())
-      ParentLoop->addChildLoop(L->removeChildLoop(L->end()-1));
-  } else {
-    // Reparent all of the blocks in this loop.  Since BBLoop had no parent,
-    // they no longer in a loop at all.
-
-    for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
-      // Don't change blocks in subloops.
-      if (LI->getLoopFor(L->getBlocks()[i]) == L) {
-        LI->removeBlock(L->getBlocks()[i]);
-        --i;
-      }
-    }
-
-    // Remove the loop from the top-level LoopInfo object.
-    for (LoopInfo::iterator I = LI->begin(), E = LI->end();; ++I) {
-      assert(I != E && "Couldn't find loop");
-      if (*I == L) {
-        LI->removeLoop(I);
-        break;
-      }
-    }
-
-    // Move all of the subloops to the top-level.
-    while (!L->empty())
-      LI->addTopLevelLoop(L->removeChildLoop(L->end()-1));
-  }
-
-  delete L;
+  LI->updateUnloop(L);
 
   // If L is current loop then skip rest of the passes and let
   // runOnFunction remove L from LQ. Otherwise, remove L from LQ now
   // and continue applying other passes on CurrentLoop.
-  if (CurrentLoop == L) {
+  if (CurrentLoop == L)
     skipThisLoop = true;
+
+  delete L;
+
+  if (skipThisLoop)
     return;
-  }
 
   for (std::deque<Loop *>::iterator I = LQ.begin(),
          E = LQ.end(); I != E; ++I) {