[LPM] Make LoopSimplify no longer a LoopPass and instead both a utility

function and a FunctionPass.

This has many benefits. The motivating use case was to be able to
compute function analysis passes *after* running LoopSimplify (to avoid
invalidating them) and then to run other passes which require
LoopSimplify. Specifically passes like unrolling and vectorization are
critical to wire up to BranchProbabilityInfo and BlockFrequencyInfo so
that they can be profile aware. For the LoopVectorize pass the only
things in the way are LoopSimplify and LCSSA. This fixes LoopSimplify
and LCSSA is next on my list.

There are also a bunch of other benefits of doing this:
- It is now very feasible to make more passes *preserve* LoopSimplify
  because they can simply run it after changing a loop. Because
  subsequence passes can assume LoopSimplify is preserved we can reduce
  the runs of this pass to the times when we actually mutate a loop
  structure.
- The new pass manager should be able to more easily support loop passes
  factored in this way.
- We can at long, long last observe that LoopSimplify is preserved
  across SCEV. This *halves* the number of times we run LoopSimplify!!!

Now, getting here wasn't trivial. First off, the interfaces used by
LoopSimplify are all over the map regarding how analysis are updated. We
end up with weird "pass" parameters as a consequence. I'll try to clean
at least some of this up later -- I'll have to have it all clean for the
new pass manager.

Next up I discovered a really frustrating bug. LoopUnroll *claims* to
preserve LoopSimplify. That's actually a lie. But the way the
LoopPassManager ends up running the passes, it always ran LoopSimplify
on the unrolled-into loop, rectifying this oversight before any
verification could kick in and point out that in fact nothing was
preserved. So I've added code to the unroller to *actually* simplify the
surrounding loop when it succeeds at unrolling.

The only functional change in the test suite is that we now catch a case
that was previously missed because SCEV and other loop transforms see
their containing loops as simplified and thus don't miss some
opportunities. One test case has been converted to check that we catch
this case rather than checking that we miss it but at least don't get
the wrong answer.

Note that I have #if-ed out all of the verification logic in
LoopSimplify! This is a temporary workaround while extracting these bits
from the LoopPassManager. Currently, there is no way to have a pass in
the LoopPassManager which preserves LoopSimplify along with one which
does not. The LPM will try to verify on each loop in the nest that
LoopSimplify holds but the now-Function-pass cannot distinguish what
loop is being verified and so must try to verify all of them. The inner
most loop is clearly no longer simplified as there is a pass which
didn't even *attempt* to preserve it. =/ Once I get LCSSA out (and maybe
LoopVectorize and some other fixes) I'll be able to re-enable this check
and catch any places where we are still failing to preserve
LoopSimplify. If this causes problems I can back this out and try to
commit *all* of this at once, but so far this seems to work and allow
much more incremental progress.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199884 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 453 insertions, 386 deletions

diff --git a/lib/Transforms/Scalar/LoopUnrollPass.cpp b/lib/Transforms/Scalar/LoopUnrollPass.cpp
index a6963a8d2b..658413497a 100644
--- a/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -254,7 +254,7 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
   }
 
   // Unroll the loop.
-  if (!UnrollLoop(L, Count, TripCount, Runtime, TripMultiple, LI, &LPM))
+  if (!UnrollLoop(L, Count, TripCount, Runtime, TripMultiple, LI, this, &LPM))
     return false;
 
   return true;
diff --git a/lib/Transforms/Utils/LoopSimplify.cpp b/lib/Transforms/Utils/LoopSimplify.cpp
index 1ad4d768d1..15500e6eb4 100644
--- a/lib/Transforms/Utils/LoopSimplify.cpp
+++ b/lib/Transforms/Utils/LoopSimplify.cpp
@@ -42,11 +42,12 @@
 #include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/SetOperations.h"
 #include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/DependenceAnalysis.h"
 #include "llvm/Analysis/InstructionSimplify.h"
-#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Dominators.h"
@@ -65,310 +66,41 @@ using namespace llvm;
 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
 STATISTIC(NumNested  , "Number of nested loops split out");
 
-namespace {
-  struct LoopSimplify : public LoopPass {
-    static char ID; // Pass identification, replacement for typeid
-    LoopSimplify() : LoopPass(ID) {
-      initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
-    }
-
-    // AA - If we have an alias analysis object to update, this is it, otherwise
-    // this is null.
-    AliasAnalysis *AA;
-    LoopInfo *LI;
-    DominatorTree *DT;
-    ScalarEvolution *SE;
-    Loop *L;
-    virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
-
-    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
-      // We need loop information to identify the loops...
-      AU.addRequired<DominatorTreeWrapperPass>();
-      AU.addPreserved<DominatorTreeWrapperPass>();
-
-      AU.addRequired<LoopInfo>();
-      AU.addPreserved<LoopInfo>();
-
-      AU.addPreserved<AliasAnalysis>();
-      AU.addPreserved<ScalarEvolution>();
-      AU.addPreserved<DependenceAnalysis>();
-      AU.addPreservedID(BreakCriticalEdgesID);  // No critical edges added.
-    }
-
-    /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
-    void verifyAnalysis() const;
-
-  private:
-    bool ProcessLoop(Loop *L, LPPassManager &LPM);
-    BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
-    Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM,
-                             BasicBlock *Preheader);
-    BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
-  };
-}
-
-static void PlaceSplitBlockCarefully(BasicBlock *NewBB,
-                                     SmallVectorImpl<BasicBlock*> &SplitPreds,
-                                     Loop *L);
-
-char LoopSimplify::ID = 0;
-INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
-                "Canonicalize natural loops", true, false)
-INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
-INITIALIZE_PASS_DEPENDENCY(LoopInfo)
-INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
-                "Canonicalize natural loops", true, false)
-
-// Publicly exposed interface to pass...
-char &llvm::LoopSimplifyID = LoopSimplify::ID;
-Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
-
-/// runOnLoop - Run down all loops in the CFG (recursively, but we could do
-/// it in any convenient order) inserting preheaders...
-///
-bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) {
-  L = l;
-  bool Changed = false;
-  LI = &getAnalysis<LoopInfo>();
-  AA = getAnalysisIfAvailable<AliasAnalysis>();
-  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
-  SE = getAnalysisIfAvailable<ScalarEvolution>();
-
-  Changed |= ProcessLoop(L, LPM);
-
-  return Changed;
-}
-
-/// ProcessLoop - Walk the loop structure in depth first order, ensuring that
-/// all loops have preheaders.
-///
-bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) {
-  bool Changed = false;
-ReprocessLoop:
-
-  // Check to see that no blocks (other than the header) in this loop have
-  // predecessors that are not in the loop.  This is not valid for natural
-  // loops, but can occur if the blocks are unreachable.  Since they are
-  // unreachable we can just shamelessly delete those CFG edges!
-  for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
-       BB != E; ++BB) {
-    if (*BB == L->getHeader()) continue;
-
-    SmallPtrSet<BasicBlock*, 4> BadPreds;
-    for (pred_iterator PI = pred_begin(*BB),
-         PE = pred_end(*BB); PI != PE; ++PI) {
-      BasicBlock *P = *PI;
-      if (!L->contains(P))
-        BadPreds.insert(P);
-    }
-
-    // Delete each unique out-of-loop (and thus dead) predecessor.
-    for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
-         E = BadPreds.end(); I != E; ++I) {
-
-      DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
-                   << (*I)->getName() << "\n");
-
-      // Inform each successor of each dead pred.
-      for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
-        (*SI)->removePredecessor(*I);
-      // Zap the dead pred's terminator and replace it with unreachable.
-      TerminatorInst *TI = (*I)->getTerminator();
-       TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
-      (*I)->getTerminator()->eraseFromParent();
-      new UnreachableInst((*I)->getContext(), *I);
-      Changed = true;
-    }
-  }
-
-  // If there are exiting blocks with branches on undef, resolve the undef in
-  // the direction which will exit the loop. This will help simplify loop
-  // trip count computations.
-  SmallVector<BasicBlock*, 8> ExitingBlocks;
-  L->getExitingBlocks(ExitingBlocks);
-  for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
-       E = ExitingBlocks.end(); I != E; ++I)
-    if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
-      if (BI->isConditional()) {
-        if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
-
-          DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
-                       << (*I)->getName() << "\n");
-
-          BI->setCondition(ConstantInt::get(Cond->getType(),
-                                            !L->contains(BI->getSuccessor(0))));
-
-          // This may make the loop analyzable, force SCEV recomputation.
-          if (SE)
-            SE->forgetLoop(L);
-
-          Changed = true;
-        }
-      }
-
-  // Does the loop already have a preheader?  If so, don't insert one.
-  BasicBlock *Preheader = L->getLoopPreheader();
-  if (!Preheader) {
-    Preheader = InsertPreheaderForLoop(L, this);
-    if (Preheader) {
-      ++NumInserted;
-      Changed = true;
-    }
-  }
-
-  // Next, check to make sure that all exit nodes of the loop only have
-  // predecessors that are inside of the loop.  This check guarantees that the
-  // loop preheader/header will dominate the exit blocks.  If the exit block has
-  // predecessors from outside of the loop, split the edge now.
-  SmallVector<BasicBlock*, 8> ExitBlocks;
-  L->getExitBlocks(ExitBlocks);
-
-  SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
-                                               ExitBlocks.end());
-  for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
-         E = ExitBlockSet.end(); I != E; ++I) {
-    BasicBlock *ExitBlock = *I;
-    for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
-         PI != PE; ++PI)
-      // Must be exactly this loop: no subloops, parent loops, or non-loop preds
-      // allowed.
-      if (!L->contains(*PI)) {
-        if (RewriteLoopExitBlock(L, ExitBlock)) {
-          ++NumInserted;
-          Changed = true;
-        }
-        break;
-      }
-  }
-
-  // If the header has more than two predecessors at this point (from the
-  // preheader and from multiple backedges), we must adjust the loop.
-  BasicBlock *LoopLatch = L->getLoopLatch();
-  if (!LoopLatch) {
-    // If this is really a nested loop, rip it out into a child loop.  Don't do
-    // this for loops with a giant number of backedges, just factor them into a
-    // common backedge instead.
-    if (L->getNumBackEdges() < 8) {
-      if (SeparateNestedLoop(L, LPM, Preheader)) {
-        ++NumNested;
-        // This is a big restructuring change, reprocess the whole loop.
-        Changed = true;
-        // GCC doesn't tail recursion eliminate this.
-        goto ReprocessLoop;
-      }
-    }
-
-    // If we either couldn't, or didn't want to, identify nesting of the loops,
-    // insert a new block that all backedges target, then make it jump to the
-    // loop header.
-    LoopLatch = InsertUniqueBackedgeBlock(L, Preheader);
-    if (LoopLatch) {
-      ++NumInserted;
-      Changed = true;
-    }
+// If the block isn't already, move the new block to right after some 'outside
+// block' block.  This prevents the preheader from being placed inside the loop
+// body, e.g. when the loop hasn't been rotated.
+static void placeSplitBlockCarefully(BasicBlock *NewBB,
+                                     SmallVectorImpl<BasicBlock *> &SplitPreds,
+                                     Loop *L) {
+  // Check to see if NewBB is already well placed.
+  Function::iterator BBI = NewBB; --BBI;
+  for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
+    if (&*BBI == SplitPreds[i])
+      return;
   }
 
-  // Scan over the PHI nodes in the loop header.  Since they now have only two
-  // incoming values (the loop is canonicalized), we may have simplified the PHI
-  // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
-  PHINode *PN;
-  for (BasicBlock::iterator I = L->getHeader()->begin();
-       (PN = dyn_cast<PHINode>(I++)); )
-    if (Value *V = SimplifyInstruction(PN, 0, 0, DT)) {
-      if (AA) AA->deleteValue(PN);
-      if (SE) SE->forgetValue(PN);
-      PN->replaceAllUsesWith(V);
-      PN->eraseFromParent();
-    }
-
-  // If this loop has multiple exits and the exits all go to the same
-  // block, attempt to merge the exits. This helps several passes, such
-  // as LoopRotation, which do not support loops with multiple exits.
-  // SimplifyCFG also does this (and this code uses the same utility
-  // function), however this code is loop-aware, where SimplifyCFG is
-  // not. That gives it the advantage of being able to hoist
-  // loop-invariant instructions out of the way to open up more
-  // opportunities, and the disadvantage of having the responsibility
-  // to preserve dominator information.
-  bool UniqueExit = true;
-  if (!ExitBlocks.empty())
-    for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
-      if (ExitBlocks[i] != ExitBlocks[0]) {
-        UniqueExit = false;
-        break;
-      }
-  if (UniqueExit) {
-    for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
-      BasicBlock *ExitingBlock = ExitingBlocks[i];
-      if (!ExitingBlock->getSinglePredecessor()) continue;
-      BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
-      if (!BI || !BI->isConditional()) continue;
-      CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
-      if (!CI || CI->getParent() != ExitingBlock) continue;
-
-      // Attempt to hoist out all instructions except for the
-      // comparison and the branch.
-      bool AllInvariant = true;
-      bool AnyInvariant = false;
-      for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
-        Instruction *Inst = I++;
-        // Skip debug info intrinsics.
-        if (isa<DbgInfoIntrinsic>(Inst))
-          continue;
-        if (Inst == CI)
-          continue;
-        if (!L->makeLoopInvariant(Inst, AnyInvariant,
-                                 Preheader ? Preheader->getTerminator() : 0)) {
-          AllInvariant = false;
-          break;
-        }
-      }
-      if (AnyInvariant) {
-        Changed = true;
-        // The loop disposition of all SCEV expressions that depend on any
-        // hoisted values have also changed.
-        if (SE)
-          SE->forgetLoopDispositions(L);
-      }
-      if (!AllInvariant) continue;
-
-      // The block has now been cleared of all instructions except for
-      // a comparison and a conditional branch. SimplifyCFG may be able
-      // to fold it now.
-      if (!FoldBranchToCommonDest(BI)) continue;
-
-      // Success. The block is now dead, so remove it from the loop,
-      // update the dominator tree and delete it.
-      DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
-                   << ExitingBlock->getName() << "\n");
-
-      // Notify ScalarEvolution before deleting this block. Currently assume the
-      // parent loop doesn't change (spliting edges doesn't count). If blocks,
-      // CFG edges, or other values in the parent loop change, then we need call
-      // to forgetLoop() for the parent instead.
-      if (SE)
-        SE->forgetLoop(L);
-
-      assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
-      Changed = true;
-      LI->removeBlock(ExitingBlock);
-
-      DomTreeNode *Node = DT->getNode(ExitingBlock);
-      const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
-        Node->getChildren();
-      while (!Children.empty()) {
-        DomTreeNode *Child = Children.front();
-        DT->changeImmediateDominator(Child, Node->getIDom());
-      }
-      DT->eraseNode(ExitingBlock);
+  // If it isn't already after an outside block, move it after one.  This is
+  // always good as it makes the uncond branch from the outside block into a
+  // fall-through.
 
-      BI->getSuccessor(0)->removePredecessor(ExitingBlock);
-      BI->getSuccessor(1)->removePredecessor(ExitingBlock);
-      ExitingBlock->eraseFromParent();
+  // Figure out *which* outside block to put this after.  Prefer an outside
+  // block that neighbors a BB actually in the loop.
+  BasicBlock *FoundBB = 0;
+  for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
+    Function::iterator BBI = SplitPreds[i];
+    if (++BBI != NewBB->getParent()->end() &&
+        L->contains(BBI)) {
+      FoundBB = SplitPreds[i];
+      break;
     }
   }
 
-  return Changed;
+  // If our heuristic for a *good* bb to place this after doesn't find
+  // anything, just pick something.  It's likely better than leaving it within
+  // the loop.
+  if (!FoundBB)
+    FoundBB = SplitPreds[0];
+  NewBB->moveAfter(FoundBB);
 }
 
 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
@@ -413,15 +145,16 @@ BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, Pass *PP) {
 
   // Make sure that NewBB is put someplace intelligent, which doesn't mess up
   // code layout too horribly.
-  PlaceSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
+  placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
 
   return PreheaderBB;
 }
 
-/// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
-/// blocks.  This method is used to split exit blocks that have predecessors
-/// outside of the loop.
-BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
+/// \brief Ensure that the loop preheader dominates all exit blocks.
+///
+/// This method is used to split exit blocks that have predecessors outside of
+/// the loop.
+static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit, Pass *PP) {
   SmallVector<BasicBlock*, 8> LoopBlocks;
   for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
     BasicBlock *P = *I;
@@ -441,10 +174,10 @@ BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
     SplitLandingPadPredecessors(Exit, ArrayRef<BasicBlock*>(&LoopBlocks[0],
                                                             LoopBlocks.size()),
                                 ".loopexit", ".nonloopexit",
-                                this, NewBBs);
+                                PP, NewBBs);
     NewExitBB = NewBBs[0];
   } else {
-    NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", this);
+    NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", PP);
   }
 
   DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
@@ -452,29 +185,29 @@ BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
   return NewExitBB;
 }
 
-/// AddBlockAndPredsToSet - Add the specified block, and all of its
-/// predecessors, to the specified set, if it's not already in there.  Stop
-/// predecessor traversal when we reach StopBlock.
-static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
+/// Add the specified block, and all of its predecessors, to the specified set,
+/// if it's not already in there.  Stop predecessor traversal when we reach
+/// StopBlock.
+static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
                                   std::set<BasicBlock*> &Blocks) {
-  std::vector<BasicBlock *> WorkList;
-  WorkList.push_back(InputBB);
+  SmallVector<BasicBlock *, 8> Worklist;
+  Worklist.push_back(InputBB);
   do {
-    BasicBlock *BB = WorkList.back(); WorkList.pop_back();
+    BasicBlock *BB = Worklist.pop_back_val();
     if (Blocks.insert(BB).second && BB != StopBlock)
       // If BB is not already processed and it is not a stop block then
       // insert its predecessor in the work list
       for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
         BasicBlock *WBB = *I;
-        WorkList.push_back(WBB);
+        Worklist.push_back(WBB);
       }
-  } while(!WorkList.empty());
+  } while (!Worklist.empty());
 }
 
-/// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
-/// PHI node that tells us how to partition the loops.
-static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
-                                        AliasAnalysis *AA, LoopInfo *LI) {
+/// \brief The first part of loop-nestification is to find a PHI node that tells
+/// us how to partition the loops.
+static PHINode *findPHIToPartitionLoops(Loop *L, AliasAnalysis *AA,
+                                        DominatorTree *DT) {
   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
     PHINode *PN = cast<PHINode>(I);
     ++I;
@@ -496,46 +229,10 @@ static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
   return 0;
 }
 
-// PlaceSplitBlockCarefully - If the block isn't already, move the new block to
-// right after some 'outside block' block.  This prevents the preheader from
-// being placed inside the loop body, e.g. when the loop hasn't been rotated.
-void PlaceSplitBlockCarefully(BasicBlock *NewBB,
-                              SmallVectorImpl<BasicBlock*> &SplitPreds,
-                              Loop *L) {
-  // Check to see if NewBB is already well placed.
-  Function::iterator BBI = NewBB; --BBI;
-  for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
-    if (&*BBI == SplitPreds[i])
-      return;
-  }
-
-  // If it isn't already after an outside block, move it after one.  This is
-  // always good as it makes the uncond branch from the outside block into a
-  // fall-through.
-
-  // Figure out *which* outside block to put this after.  Prefer an outside
-  // block that neighbors a BB actually in the loop.
-  BasicBlock *FoundBB = 0;
-  for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
-    Function::iterator BBI = SplitPreds[i];
-    if (++BBI != NewBB->getParent()->end() &&
-        L->contains(BBI)) {
-      FoundBB = SplitPreds[i];
-      break;
-    }
-  }
-
-  // If our heuristic for a *good* bb to place this after doesn't find
-  // anything, just pick something.  It's likely better than leaving it within
-  // the loop.
-  if (!FoundBB)
-    FoundBB = SplitPreds[0];
-  NewBB->moveAfter(FoundBB);
-}
-
-
-/// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
-/// them out into a nested loop.  This is important for code that looks like
+/// \brief If this loop has multiple backedges, try to pull one of them out into
+/// a nested loop.
+///
+/// This is important for code that looks like
 /// this:
 ///
 ///  Loop:
@@ -551,8 +248,9 @@ void PlaceSplitBlockCarefully(BasicBlock *NewBB,
 /// If we are able to separate out a loop, return the new outer loop that was
 /// created.
 ///
-Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM,
-                                       BasicBlock *Preheader) {
+static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
+                                AliasAnalysis *AA, DominatorTree *DT,
+                                LoopInfo *LI, ScalarEvolution *SE, Pass *PP) {
   // Don't try to separate loops without a preheader.
   if (!Preheader)
     return 0;
@@ -561,7 +259,7 @@ Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM,
   assert(!L->getHeader()->isLandingPad() &&
          "Can't insert backedge to landing pad");
 
-  PHINode *PN = FindPHIToPartitionLoops(L, DT, AA, LI);
+  PHINode *PN = findPHIToPartitionLoops(L, AA, DT);
   if (PN == 0) return 0;  // No known way to partition.
 
   // Pull out all predecessors that have varying values in the loop.  This
@@ -587,11 +285,11 @@ Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM,
 
   BasicBlock *Header = L->getHeader();
   BasicBlock *NewBB =
-    SplitBlockPredecessors(Header, OuterLoopPreds,  ".outer", this);
+    SplitBlockPredecessors(Header, OuterLoopPreds,  ".outer", PP);
 
   // Make sure that NewBB is put someplace intelligent, which doesn't mess up
   // code layout too horribly.
-  PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
+  placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
 
   // Create the new outer loop.
   Loop *NewOuter = new Loop();
@@ -605,9 +303,6 @@ Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM,
   // L is now a subloop of our outer loop.
   NewOuter->addChildLoop(L);
 
-  // Add the new loop to the pass manager queue.
-  LPM.insertLoopIntoQueue(NewOuter);
-
   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
        I != E; ++I)
     NewOuter->addBlockEntry(*I);
@@ -622,7 +317,7 @@ Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM,
   for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
     BasicBlock *P = *PI;
     if (DT->dominates(Header, P))
-      AddBlockAndPredsToSet(P, Header, BlocksInL);
+      addBlockAndPredsToSet(P, Header, BlocksInL);
   }
 
   // Scan all of the loop children of L, moving them to OuterLoop if they are
@@ -650,15 +345,15 @@ Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM,
   return NewOuter;
 }
 
-
-
-/// InsertUniqueBackedgeBlock - This method is called when the specified loop
-/// has more than one backedge in it.  If this occurs, revector all of these
-/// backedges to target a new basic block and have that block branch to the loop
-/// header.  This ensures that loops have exactly one backedge.
+/// \brief This method is called when the specified loop has more than one
+/// backedge in it.
 ///
-BasicBlock *
-LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
+/// If this occurs, revector all of these backedges to target a new basic block
+/// and have that block branch to the loop header.  This ensures that loops
+/// have exactly one backedge.
+static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
+                                             AliasAnalysis *AA,
+                                             DominatorTree *DT, LoopInfo *LI) {
   assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
 
   // Get information about the loop
@@ -769,7 +464,349 @@ LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) {
   return BEBlock;
 }
 
-void LoopSimplify::verifyAnalysis() const {
+/// \brief Simplify one loop and queue further loops for simplification.
+///
+/// FIXME: Currently this accepts both lots of analyses that it uses and a raw
+/// Pass pointer. The Pass pointer is used by numerous utilities to update
+/// specific analyses. Rather than a pass it would be much cleaner and more
+/// explicit if they accepted the analysis directly and then updated it.
+static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
+                            AliasAnalysis *AA, DominatorTree *DT, LoopInfo *LI,
+                            ScalarEvolution *SE, Pass *PP) {
+  bool Changed = false;
+ReprocessLoop:
+
+  // Check to see that no blocks (other than the header) in this loop have
+  // predecessors that are not in the loop.  This is not valid for natural
+  // loops, but can occur if the blocks are unreachable.  Since they are
+  // unreachable we can just shamelessly delete those CFG edges!
+  for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
+       BB != E; ++BB) {
+    if (*BB == L->getHeader()) continue;
+
+    SmallPtrSet<BasicBlock*, 4> BadPreds;
+    for (pred_iterator PI = pred_begin(*BB),
+         PE = pred_end(*BB); PI != PE; ++PI) {
+      BasicBlock *P = *PI;
+      if (!L->contains(P))
+        BadPreds.insert(P);
+    }
+
+    // Delete each unique out-of-loop (and thus dead) predecessor.
+    for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
+         E = BadPreds.end(); I != E; ++I) {
+
+      DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
+                   << (*I)->getName() << "\n");
+
+      // Inform each successor of each dead pred.
+      for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
+        (*SI)->removePredecessor(*I);
+      // Zap the dead pred's terminator and replace it with unreachable.
+      TerminatorInst *TI = (*I)->getTerminator();
+       TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
+      (*I)->getTerminator()->eraseFromParent();
+      new UnreachableInst((*I)->getContext(), *I);
+      Changed = true;
+    }
+  }
+
+  // If there are exiting blocks with branches on undef, resolve the undef in
+  // the direction which will exit the loop. This will help simplify loop
+  // trip count computations.
+  SmallVector<BasicBlock*, 8> ExitingBlocks;
+  L->getExitingBlocks(ExitingBlocks);
+  for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
+       E = ExitingBlocks.end(); I != E; ++I)
+    if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
+      if (BI->isConditional()) {
+        if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
+
+          DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
+                       << (*I)->getName() << "\n");
+
+          BI->setCondition(ConstantInt::get(Cond->getType(),
+                                            !L->contains(BI->getSuccessor(0))));
+
+          // This may make the loop analyzable, force SCEV recomputation.
+          if (SE)
+            SE->forgetLoop(L);
+
+          Changed = true;
+        }
+      }
+
+  // Does the loop already have a preheader?  If so, don't insert one.
+  BasicBlock *Preheader = L->getLoopPreheader();
+  if (!Preheader) {
+    Preheader = InsertPreheaderForLoop(L, PP);
+    if (Preheader) {
+      ++NumInserted;
+      Changed = true;
+    }
+  }
+
+  // Next, check to make sure that all exit nodes of the loop only have
+  // predecessors that are inside of the loop.  This check guarantees that the
+  // loop preheader/header will dominate the exit blocks.  If the exit block has
+  // predecessors from outside of the loop, split the edge now.
+  SmallVector<BasicBlock*, 8> ExitBlocks;
+  L->getExitBlocks(ExitBlocks);
+
+  SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
+                                               ExitBlocks.end());
+  for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
+         E = ExitBlockSet.end(); I != E; ++I) {
+    BasicBlock *ExitBlock = *I;
+    for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
+         PI != PE; ++PI)
+      // Must be exactly this loop: no subloops, parent loops, or non-loop preds
+      // allowed.
+      if (!L->contains(*PI)) {
+        if (rewriteLoopExitBlock(L, ExitBlock, PP)) {
+          ++NumInserted;
+          Changed = true;
+        }
+        break;
+      }
+  }
+
+  // If the header has more than two predecessors at this point (from the
+  // preheader and from multiple backedges), we must adjust the loop.
+  BasicBlock *LoopLatch = L->getLoopLatch();
+  if (!LoopLatch) {
+    // If this is really a nested loop, rip it out into a child loop.  Don't do
+    // this for loops with a giant number of backedges, just factor them into a
+    // common backedge instead.
+    if (L->getNumBackEdges() < 8) {
+      if (Loop *OuterL = separateNestedLoop(L, Preheader, AA, DT, LI, SE, PP)) {
+        ++NumNested;
+        // Enqueue the outer loop as it should be processed next in our
+        // depth-first nest walk.
+        Worklist.push_back(OuterL);
+
+        // This is a big restructuring change, reprocess the whole loop.
+        Changed = true;
+        // GCC doesn't tail recursion eliminate this.
+        // FIXME: It isn't clear we can't rely on LLVM to TRE this.
+        goto ReprocessLoop;
+      }
+    }
+
+    // If we either couldn't, or didn't want to, identify nesting of the loops,
+    // insert a new block that all backedges target, then make it jump to the
+    // loop header.
+    LoopLatch = insertUniqueBackedgeBlock(L, Preheader, AA, DT, LI);
+    if (LoopLatch) {
+      ++NumInserted;
+      Changed = true;
+    }
+  }
+
+  // Scan over the PHI nodes in the loop header.  Since they now have only two
+  // incoming values (the loop is canonicalized), we may have simplified the PHI
+  // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
+  PHINode *PN;
+  for (BasicBlock::iterator I = L->getHeader()->begin();
+       (PN = dyn_cast<PHINode>(I++)); )
+    if (Value *V = SimplifyInstruction(PN, 0, 0, DT)) {
+      if (AA) AA->deleteValue(PN);
+      if (SE) SE->forgetValue(PN);
+      PN->replaceAllUsesWith(V);
+      PN->eraseFromParent();
+    }
+
+  // If this loop has multiple exits and the exits all go to the same
+  // block, attempt to merge the exits. This helps several passes, such
+  // as LoopRotation, which do not support loops with multiple exits.
+  // SimplifyCFG also does this (and this code uses the same utility
+  // function), however this code is loop-aware, where SimplifyCFG is
+  // not. That gives it the advantage of being able to hoist
+  // loop-invariant instructions out of the way to open up more
+  // opportunities, and the disadvantage of having the responsibility
+  // to preserve dominator information.
+  bool UniqueExit = true;
+  if (!ExitBlocks.empty())
+    for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
+      if (ExitBlocks[i] != ExitBlocks[0]) {
+        UniqueExit = false;
+        break;
+      }
+  if (UniqueExit) {
+    for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
+      BasicBlock *ExitingBlock = ExitingBlocks[i];
+      if (!ExitingBlock->getSinglePredecessor()) continue;
+      BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
+      if (!BI || !BI->isConditional()) continue;
+      CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
+      if (!CI || CI->getParent() != ExitingBlock) continue;
+
+      // Attempt to hoist out all instructions except for the
+      // comparison and the branch.
+      bool AllInvariant = true;
+      bool AnyInvariant = false;
+      for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
+        Instruction *Inst = I++;
+        // Skip debug info intrinsics.
+        if (isa<DbgInfoIntrinsic>(Inst))
+          continue;
+        if (Inst == CI)
+          continue;
+        if (!L->makeLoopInvariant(Inst, AnyInvariant,
+                                 Preheader ? Preheader->getTerminator() : 0)) {
+          AllInvariant = false;
+          break;
+        }
+      }
+      if (AnyInvariant) {
+        Changed = true;
+        // The loop disposition of all SCEV expressions that depend on any
+        // hoisted values have also changed.
+        if (SE)
+          SE->forgetLoopDispositions(L);
+      }
+      if (!AllInvariant) continue;
+
+      // The block has now been cleared of all instructions except for
+      // a comparison and a conditional branch. SimplifyCFG may be able
+      // to fold it now.
+      if (!FoldBranchToCommonDest(BI)) continue;
+
+      // Success. The block is now dead, so remove it from the loop,
+      // update the dominator tree and delete it.
+      DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
+                   << ExitingBlock->getName() << "\n");
+
+      // Notify ScalarEvolution before deleting this block. Currently assume the
+      // parent loop doesn't change (spliting edges doesn't count). If blocks,
+      // CFG edges, or other values in the parent loop change, then we need call
+      // to forgetLoop() for the parent instead.
+      if (SE)
+        SE->forgetLoop(L);
+
+      assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
+      Changed = true;
+      LI->removeBlock(ExitingBlock);
+
+      DomTreeNode *Node = DT->getNode(ExitingBlock);
+      const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
+        Node->getChildren();
+      while (!Children.empty()) {
+        DomTreeNode *Child = Children.front();
+        DT->changeImmediateDominator(Child, Node->getIDom());
+      }
+      DT->eraseNode(ExitingBlock);
+
+      BI->getSuccessor(0)->removePredecessor(ExitingBlock);
+      BI->getSuccessor(1)->removePredecessor(ExitingBlock);
+      ExitingBlock->eraseFromParent();
+    }
+  }
+
+  return Changed;
+}
+
+bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP,
+                        AliasAnalysis *AA, ScalarEvolution *SE) {
+  bool Changed = false;
+
+  // Worklist maintains our depth-first queue of loops in this nest to process.
+  SmallVector<Loop *, 4> Worklist;
+  Worklist.push_back(L);
+
+  // Walk the worklist from front to back, pushing newly found sub loops onto
+  // the back. This will let us process loops from back to front in depth-first
+  // order. We can use this simple process because loops form a tree.
+  for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
+    Loop *L2 = Worklist[Idx];
+    for (Loop::iterator I = L2->begin(), E = L2->end(); I != E; ++I)
+      Worklist.push_back(*I);
+  }
+
+  while (!Worklist.empty())
+    Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, AA, DT, LI, SE, PP);
+
+  return Changed;
+}
+
+namespace {
+  struct LoopSimplify : public FunctionPass {
+    static char ID; // Pass identification, replacement for typeid
+    LoopSimplify() : FunctionPass(ID) {
+      initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
+    }
+
+    // AA - If we have an alias analysis object to update, this is it, otherwise
+    // this is null.
+    AliasAnalysis *AA;
+    DominatorTree *DT;
+    LoopInfo *LI;
+    ScalarEvolution *SE;
+
+    virtual bool runOnFunction(Function &F);
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      // We need loop information to identify the loops...
+      AU.addRequired<DominatorTreeWrapperPass>();
+      AU.addPreserved<DominatorTreeWrapperPass>();
+
+      AU.addRequired<LoopInfo>();
+      AU.addPreserved<LoopInfo>();
+
+      AU.addPreserved<AliasAnalysis>();
+      AU.addPreserved<ScalarEvolution>();
+      AU.addPreserved<DependenceAnalysis>();
+      AU.addPreservedID(BreakCriticalEdgesID);  // No critical edges added.
+    }
+
+    /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
+    void verifyAnalysis() const;
+
+  private:
+    bool ProcessLoop(Loop *L);
+    BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
+    Loop *SeparateNestedLoop(Loop *L, BasicBlock *Preheader);
+    BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader);
+  };
+}
+
+char LoopSimplify::ID = 0;
+INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
+                "Canonicalize natural loops", true, false)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopInfo)
+INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
+                "Canonicalize natural loops", true, false)
+
+// Publicly exposed interface to pass...
+char &llvm::LoopSimplifyID = LoopSimplify::ID;
+Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
+
+/// runOnLoop - Run down all loops in the CFG (recursively, but we could do
+/// it in any convenient order) inserting preheaders...
+///
+bool LoopSimplify::runOnFunction(Function &F) {
+  bool Changed = false;
+  AA = getAnalysisIfAvailable<AliasAnalysis>();
+  LI = &getAnalysis<LoopInfo>();
+  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+  SE = getAnalysisIfAvailable<ScalarEvolution>();
+
+  // Simplify each loop nest in the function.
+  for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
+    Changed |= simplifyLoop(*I, DT, LI, this, AA, SE);
+
+  return Changed;
+}
+
+// FIXME: Restore this code when we re-enable verification in verifyAnalysis
+// below.
+#if 0
+static void verifyLoop(Loop *L) {
+  // Verify subloops.
+  for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
+    verifyLoop(*I);
+
   // It used to be possible to just assert L->isLoopSimplifyForm(), however
   // with the introduction of indirectbr, there are now cases where it's
   // not possible to transform a loop as necessary. We can at least check
@@ -806,3 +843,15 @@ void LoopSimplify::verifyAnalysis() const {
     (void)HasIndBrExiting;
   }
 }
+#endif
+
+void LoopSimplify::verifyAnalysis() const {
+  // FIXME: This routine is being called mid-way through the loop pass manager
+  // as loop passes destroy this analysis. That's actually fine, but we have no
+  // way of expressing that here. Once all of the passes that destroy this are
+  // hoisted out of the loop pass manager we can add back verification here.
+#if 0
+  for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
+    verifyLoop(*I);
+#endif
+}
diff --git a/lib/Transforms/Utils/LoopUnroll.cpp b/lib/Transforms/Utils/LoopUnroll.cpp
index 3c792dd136..3c43fbbe82 100644
--- a/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/lib/Transforms/Utils/LoopUnroll.cpp
@@ -30,6 +30,7 @@
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Cloning.h"
 #include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/SimplifyIndVar.h"
 using namespace llvm;
 
@@ -138,10 +139,10 @@ static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI,
 /// removed from the LoopPassManager as well. LPM can also be NULL.
 ///
 /// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are
-/// available it must also preserve those analyses.
+/// available from the Pass it must also preserve those analyses.
 bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
                       bool AllowRuntime, unsigned TripMultiple,
-                      LoopInfo *LI, LPPassManager *LPM) {
+                      LoopInfo *LI, Pass *PP, LPPassManager *LPM) {
   BasicBlock *Preheader = L->getLoopPreheader();
   if (!Preheader) {
     DEBUG(dbgs() << "  Can't unroll; loop preheader-insertion failed.\n");
@@ -209,8 +210,8 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
 
   // Notify ScalarEvolution that the loop will be substantially changed,
   // if not outright eliminated.
-  if (LPM) {
-    ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
+  if (PP) {
+    ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>();
     if (SE)
       SE->forgetLoop(L);
   }
@@ -410,15 +411,18 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
     }
   }
 
-  if (LPM) {
+  DominatorTree *DT = 0;
+  if (PP) {
     // FIXME: Reconstruct dom info, because it is not preserved properly.
     // Incrementally updating domtree after loop unrolling would be easy.
     if (DominatorTreeWrapperPass *DTWP =
-            LPM->getAnalysisIfAvailable<DominatorTreeWrapperPass>())
-      DTWP->getDomTree().recalculate(*L->getHeader()->getParent());
+            PP->getAnalysisIfAvailable<DominatorTreeWrapperPass>()) {
+      DT = &DTWP->getDomTree();
+      DT->recalculate(*L->getHeader()->getParent());
+    }
 
     // Simplify any new induction variables in the partially unrolled loop.
-    ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
+    ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>();
     if (SE && !CompletelyUnroll) {
       SmallVector<WeakVH, 16> DeadInsts;
       simplifyLoopIVs(L, SE, LPM, DeadInsts);
@@ -451,9 +455,23 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
 
   NumCompletelyUnrolled += CompletelyUnroll;
   ++NumUnrolled;
+
+  Loop *OuterL = L->getParentLoop();
   // Remove the loop from the LoopPassManager if it's completely removed.
   if (CompletelyUnroll && LPM != NULL)
     LPM->deleteLoopFromQueue(L);
 
+  // If we have a pass and a DominatorTree we should re-simplify impacted loops
+  // to ensure subsequent analyses can rely on this form. We want to simplify
+  // at least one layer outside of the loop that was unrolled so that any
+  // changes to the parent loop exposed by the unrolling are considered.
+  if (PP && DT) {
+    if (!OuterL && !CompletelyUnroll)
+      OuterL = L;
+    if (OuterL)
+      simplifyLoop(OuterL, DT, LI, PP, /*AliasAnalysis*/ 0,
+                   PP->getAnalysisIfAvailable<ScalarEvolution>());
+  }
+
   return true;
 }