Add -unroll-runtime for unrolling loops with run-time trip counts.

Patch by Brendon Cahoon!

This extends the existing LoopUnroll and LoopUnrollPass. Brendon
measured no regressions in the llvm test suite with -unroll-runtime
enabled. This implementation works by using the existing loop
unrolling code to unroll the loop by a power-of-two (default 8). It
generates an if-then-else sequence of code prior to the loop to
execute the extra iterations before entering the unrolled loop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146245 91177308-0d34-0410-b5e6-96231b3b80d8

9 files changed, 644 insertions, 15 deletions

diff --git a/include/llvm/Transforms/Utils/UnrollLoop.h b/include/llvm/Transforms/Utils/UnrollLoop.h
index 7212a8c760..f175e8371e 100644
--- a/include/llvm/Transforms/Utils/UnrollLoop.h
+++ b/include/llvm/Transforms/Utils/UnrollLoop.h
@@ -22,9 +22,12 @@ class Loop;
 class LoopInfo;
 class LPPassManager;
 
-bool UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
+bool UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, bool AllowRuntime,
                 unsigned TripMultiple, LoopInfo* LI, LPPassManager* LPM);
 
+bool UnrollRuntimeLoopProlog(Loop *L, unsigned Count, LoopInfo *LI,
+                             LPPassManager* LPM);
+
 }
 
 #endif
diff --git a/lib/Transforms/Scalar/LoopUnrollPass.cpp b/lib/Transforms/Scalar/LoopUnrollPass.cpp
index f3fdd4f3c5..22dbfe326c 100644
--- a/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -40,6 +40,10 @@ UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
   cl::desc("Allows loops to be partially unrolled until "
            "-unroll-threshold loop size is reached."));
 
+static cl::opt<bool>
+UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
+  cl::desc("Unroll loops with run-time trip counts"));
+
 namespace {
   class LoopUnroll : public LoopPass {
   public:
@@ -63,6 +67,10 @@ namespace {
     // explicit -unroll-threshold).
     static const unsigned OptSizeUnrollThreshold = 50;
 
+    // Default unroll count for loops with run-time trip count if
+    // -unroll-count is not set
+    static const unsigned UnrollRuntimeCount = 8;
+
     unsigned CurrentCount;
     unsigned CurrentThreshold;
     bool     CurrentAllowPartial;
@@ -151,8 +159,13 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
     TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
     TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
   }
-  // Automatically select an unroll count.
+  // Use a default unroll-count if the user doesn't specify a value
+  // and the trip count is a run-time value.  The default is different
+  // for run-time or compile-time trip count loops.
   unsigned Count = CurrentCount;
+  if (UnrollRuntime && CurrentCount == 0 && TripCount == 0)
+    Count = UnrollRuntimeCount;
+
   if (Count == 0) {
     // Conservative heuristic: if we know the trip count, see if we can
     // completely unroll (subject to the threshold, checked below); otherwise
@@ -177,15 +190,23 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
     if (TripCount != 1 && Size > Threshold) {
       DEBUG(dbgs() << "  Too large to fully unroll with count: " << Count
             << " because size: " << Size << ">" << Threshold << "\n");
-      if (!CurrentAllowPartial) {
+      if (!CurrentAllowPartial && !(UnrollRuntime && TripCount == 0)) {
         DEBUG(dbgs() << "  will not try to unroll partially because "
               << "-unroll-allow-partial not given\n");
         return false;
       }
-      // Reduce unroll count to be modulo of TripCount for partial unrolling
-      Count = Threshold / LoopSize;
-      while (Count != 0 && TripCount%Count != 0) {
-        Count--;
+      if (TripCount) {
+        // Reduce unroll count to be modulo of TripCount for partial unrolling
+        Count = CurrentThreshold / LoopSize;
+        while (Count != 0 && TripCount%Count != 0)
+          Count--;
+      }
+      else if (UnrollRuntime) {
+        // Reduce unroll count to be a lower power-of-two value
+        while (Count != 0 && Size > CurrentThreshold) {
+          Count >>= 1;
+          Size = LoopSize*Count;
+        }
       }
       if (Count < 2) {
         DEBUG(dbgs() << "  could not unroll partially\n");
@@ -196,7 +217,7 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
   }
 
   // Unroll the loop.
-  if (!UnrollLoop(L, Count, TripCount, TripMultiple, LI, &LPM))
+  if (!UnrollLoop(L, Count, TripCount, UnrollRuntime, TripMultiple, LI, &LPM))
     return false;
 
   return true;
diff --git a/lib/Transforms/Utils/CMakeLists.txt b/lib/Transforms/Utils/CMakeLists.txt
index d43a275aa8..d96f59c982 100644
--- a/lib/Transforms/Utils/CMakeLists.txt
+++ b/lib/Transforms/Utils/CMakeLists.txt
@@ -14,6 +14,7 @@ add_llvm_library(LLVMTransformUtils
   Local.cpp
   LoopSimplify.cpp
   LoopUnroll.cpp
+  LoopUnrollRuntime.cpp
   LowerExpectIntrinsic.cpp
   LowerInvoke.cpp
   LowerSwitch.cpp
diff --git a/lib/Transforms/Utils/LoopUnroll.cpp b/lib/Transforms/Utils/LoopUnroll.cpp
index 62e4fa2953..b96f14b144 100644
--- a/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/lib/Transforms/Utils/LoopUnroll.cpp
@@ -135,7 +135,8 @@ static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI,
 /// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are
 /// available it must also preserve those analyses.
 bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
-                      unsigned TripMultiple, LoopInfo *LI, LPPassManager *LPM) {
+                      bool AllowRuntime, unsigned TripMultiple,
+                      LoopInfo *LI, LPPassManager *LPM) {
   BasicBlock *Preheader = L->getLoopPreheader();
   if (!Preheader) {
     DEBUG(dbgs() << "  Can't unroll; loop preheader-insertion failed.\n");
@@ -165,12 +166,6 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
     return false;
   }
 
-  // Notify ScalarEvolution that the loop will be substantially changed,
-  // if not outright eliminated.
-  ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
-  if (SE)
-    SE->forgetLoop(L);
-
   if (TripCount != 0)
     DEBUG(dbgs() << "  Trip Count = " << TripCount << "\n");
   if (TripMultiple != 1)
@@ -188,6 +183,20 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
   // Are we eliminating the loop control altogether?
   bool CompletelyUnroll = Count == TripCount;
 
+  // We assume a run-time trip count if the compiler cannot
+  // figure out the loop trip count and the unroll-runtime
+  // flag is specified.
+  bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);
+
+  if (RuntimeTripCount && !UnrollRuntimeLoopProlog(L, Count, LI, LPM))
+    return false;
+
+  // Notify ScalarEvolution that the loop will be substantially changed,
+  // if not outright eliminated.
+  ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
+  if (SE)
+    SE->forgetLoop(L);
+
   // If we know the trip count, we know the multiple...
   unsigned BreakoutTrip = 0;
   if (TripCount != 0) {
@@ -209,6 +218,8 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
       DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
     } else if (TripMultiple != 1) {
       DEBUG(dbgs() << " with " << TripMultiple << " trips per branch");
+    } else if (RuntimeTripCount) {
+      DEBUG(dbgs() << " with run-time trip count");
     }
     DEBUG(dbgs() << "!\n");
   }
@@ -332,6 +343,10 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
     BasicBlock *Dest = Headers[j];
     bool NeedConditional = true;
 
+    if (RuntimeTripCount && j != 0) {
+      NeedConditional = false;
+    }
+
     // For a complete unroll, make the last iteration end with a branch
     // to the exit block.
     if (CompletelyUnroll && j == 0) {
diff --git a/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/lib/Transforms/Utils/LoopUnrollRuntime.cpp
new file mode 100644
index 0000000000..2b92e59c3d
--- /dev/null
+++ b/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -0,0 +1,375 @@
+//===-- UnrollLoopRuntime.cpp - Runtime Loop unrolling utilities ----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements some loop unrolling utilities for loops with run-time
+// trip counts.  See LoopUnroll.cpp for unrolling loops with compile-time
+// trip counts.
+//
+// The functions in this file are used to generate extra code when the 
+// run-time trip count modulo the unroll factor is not 0.  When this is the
+// case, we need to generate code to execute these 'left over' iterations.
+//
+// The current strategy generates an if-then-else sequence prior to the 
+// unrolled loop to execute the 'left over' iterations.  Other strategies
+// include generate a loop before or after the unrolled loop.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "loop-unroll"
+#include "llvm/Transforms/Utils/UnrollLoop.h"
+#include "llvm/BasicBlock.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/LoopIterator.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionExpander.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include <algorithm>
+
+using namespace llvm;
+
+STATISTIC(NumRuntimeUnrolled, 
+          "Number of loops unrolled with run-time trip counts");
+
+/// Connect the unrolling prolog code to the original loop.
+/// The unrolling prolog code contains code to execute the
+/// 'extra' iterations if the run-time trip count modulo the
+/// unroll count is non-zero.
+///
+/// This function performs the following:
+/// - Create PHI nodes at prolog end block to combine values
+///   that exit the prolog code and jump around the prolog.
+/// - Add a PHI operand to a PHI node at the loop exit block
+///   for values that exit the prolog and go around the loop.
+/// - Branch around the original loop if the trip count is less
+///   than the unroll factor.
+///
+static void ConnectProlog(Loop *L, Value *TripCount, unsigned Count,
+                          BasicBlock *LastPrologBB, BasicBlock *PrologEnd,
+                          BasicBlock *OrigPH, BasicBlock *NewPH,
+                          ValueToValueMapTy &LVMap, Pass *P) {
+  BasicBlock *Latch = L->getLoopLatch();
+  assert(Latch != 0 && "Loop must have a latch");
+
+  // Create a PHI node for each outgoing value from the original loop
+  // (which means it is an outgoing value from the prolog code too).
+  // The new PHI node is inserted in the prolog end basic block.
+  // The new PHI name is added as an operand of a PHI node in either
+  // the loop header or the loop exit block.
+  for (succ_iterator SBI = succ_begin(Latch), SBE = succ_end(Latch);
+       SBI != SBE; ++SBI) {
+    for (BasicBlock::iterator BBI = (*SBI)->begin();
+         PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
+
+      // Add a new PHI node to the prolog end block and add the
+      // appropriate incoming values.
+      PHINode *NewPN = PHINode::Create(PN->getType(), 2, PN->getName()+".unr",
+                                       PrologEnd->getTerminator());
+      // Adding a value to the new PHI node from the original loop preheader.
+      // This is the value that skips all the prolog code.
+      if (L->contains(PN)) {
+        NewPN->addIncoming(PN->getIncomingValueForBlock(NewPH), OrigPH);
+      } else {
+        NewPN->addIncoming(Constant::getNullValue(PN->getType()), OrigPH);
+      }
+      Value *OrigVal = PN->getIncomingValueForBlock(Latch);
+      Value *V = OrigVal;
+      if (Instruction *I = dyn_cast<Instruction>(V)) {
+        if (L->contains(I)) {
+          V = LVMap[I];
+        }
+      }
+      // Adding a value to the new PHI node from the last prolog block
+      // that was created.
+      NewPN->addIncoming(V, LastPrologBB);
+
+      // Update the existing PHI node operand with the value from the
+      // new PHI node.  How this is done depends on if the existing
+      // PHI node is in the original loop block, or the exit block.
+      if (L->contains(PN)) {
+        PN->setIncomingValue(PN->getBasicBlockIndex(NewPH), NewPN);
+      } else {
+        PN->addIncoming(NewPN, PrologEnd);
+      }
+    }
+  }
+
+  // Create a branch around the orignal loop, which is taken if the
+  // trip count is less than the unroll factor.
+  Instruction *InsertPt = PrologEnd->getTerminator();
+  Instruction *BrLoopExit =
+    new ICmpInst(InsertPt, ICmpInst::ICMP_ULT, TripCount,
+                 ConstantInt::get(TripCount->getType(), Count));
+  BasicBlock *Exit = L->getUniqueExitBlock();
+  assert(Exit != 0 && "Loop must have a single exit block only");
+  // Split the exit to maintain loop canonicalization guarantees
+  SmallVector<BasicBlock*, 4> Preds(pred_begin(Exit), pred_end(Exit));
+  if (!Exit->isLandingPad()) {
+    SplitBlockPredecessors(Exit, Preds.data(), Preds.size(),
+                           ".unr-lcssa", P);
+  } else {
+    SmallVector<BasicBlock*, 2> NewBBs;
+    SplitLandingPadPredecessors(Exit, Preds, ".unr1-lcssa", ".unr2-lcssa",
+                                P, NewBBs);
+  }
+  // Add the branch to the exit block (around the unrolled loop)
+  BranchInst::Create(Exit, NewPH, BrLoopExit, InsertPt);
+  InsertPt->eraseFromParent();
+}
+
+/// Create a clone of the blocks in a loop and connect them together.
+/// This function doesn't create a clone of the loop structure.
+///
+/// There are two value maps that are defined and used.  VMap is
+/// for the values in the current loop instance.  LVMap contains
+/// the values from the last loop instance.  We need the LVMap values
+/// to update the inital values for the current loop instance.
+///
+static void CloneLoopBlocks(Loop *L,
+                            bool FirstCopy,
+                            BasicBlock *InsertTop,
+                            BasicBlock *InsertBot,
+                            std::vector<BasicBlock *> &NewBlocks,
+                            LoopBlocksDFS &LoopBlocks,
+                            ValueToValueMapTy &VMap,
+                            ValueToValueMapTy &LVMap,
+                            LoopInfo *LI) {
+
+  BasicBlock *Preheader = L->getLoopPreheader();
+  BasicBlock *Header = L->getHeader();
+  BasicBlock *Latch = L->getLoopLatch();
+  Function *F = Header->getParent();
+  LoopBlocksDFS::RPOIterator BlockBegin = LoopBlocks.beginRPO();
+  LoopBlocksDFS::RPOIterator BlockEnd = LoopBlocks.endRPO();
+  // For each block in the original loop, create a new copy,
+  // and update the value map with the newly created values.
+  for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
+    BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, ".unr", F);
+    NewBlocks.push_back(NewBB);
+
+    if (Loop *ParentLoop = L->getParentLoop())
+      ParentLoop->addBasicBlockToLoop(NewBB, LI->getBase());
+
+    VMap[*BB] = NewBB;
+    if (Header == *BB) {
+      // For the first block, add a CFG connection to this newly
+      // created block
+      InsertTop->getTerminator()->setSuccessor(0, NewBB);
+
+      // Change the incoming values to the ones defined in the
+      // previously cloned loop.
+      for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
+        PHINode *NewPHI = cast<PHINode>(VMap[I]);
+        if (FirstCopy) {
+          // We replace the first phi node with the value from the preheader
+          VMap[I] = NewPHI->getIncomingValueForBlock(Preheader);
+          NewBB->getInstList().erase(NewPHI);
+        } else {
+          // Update VMap with values from the previous block
+          unsigned idx = NewPHI->getBasicBlockIndex(Latch);
+          Value *InVal = NewPHI->getIncomingValue(idx);
+          if (Instruction *I = dyn_cast<Instruction>(InVal))
+            if (L->contains(I))
+              InVal = LVMap[InVal];
+          NewPHI->setIncomingValue(idx, InVal);
+          NewPHI->setIncomingBlock(idx, InsertTop);
+        }
+      }
+    }
+
+    if (Latch == *BB) {
+      VMap.erase((*BB)->getTerminator());
+      NewBB->getTerminator()->eraseFromParent();
+      BranchInst::Create(InsertBot, NewBB);
+    }
+  }
+  // LastValueMap is updated with the values for the current loop
+  // which are used the next time this function is called.
+  for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
+       VI != VE; ++VI) {
+    LVMap[VI->first] = VI->second;
+  }
+}
+
+/// Insert code in the prolog code when unrolling a loop with a
+/// run-time trip-count.
+///
+/// This method assumes that the loop unroll factor is total number
+/// of loop bodes in the loop after unrolling. (Some folks refer
+/// to the unroll factor as the number of *extra* copies added).
+/// We assume also that the loop unroll factor is a power-of-two. So, after
+/// unrolling the loop, the number of loop bodies executed is 2,
+/// 4, 8, etc.  Note - LLVM converts the if-then-sequence to a switch 
+/// instruction in SimplifyCFG.cpp.  Then, the backend decides how code for
+/// the switch instruction is generated.
+///
+///    extraiters = tripcount % loopfactor
+///    if (extraiters == 0) jump Loop:
+///    if (extraiters == loopfactor) jump L1
+///    if (extraiters == loopfactor-1) jump L2
+///    ...
+///    L1:  LoopBody;
+///    L2:  LoopBody;
+///    ...
+///    if tripcount < loopfactor jump End
+///    Loop:
+///    ...
+///    End:
+///
+bool llvm::UnrollRuntimeLoopProlog(Loop *L, unsigned Count, LoopInfo *LI,
+                                   LPPassManager *LPM) {
+  // for now, only unroll loops that contain a single exit
+  SmallVector<BasicBlock*, 4> ExitingBlocks;
+  L->getExitingBlocks(ExitingBlocks);
+  if (ExitingBlocks.size() > 1)
+    return false;
+
+  // Make sure the loop is in canonical form, and there is a single
+  // exit block only.
+  if (!L->isLoopSimplifyForm() || L->getUniqueExitBlock() == 0)
+    return false;
+
+  // Use Scalar Evolution to compute the trip count.  This allows more
+  // loops to be unrolled than relying on induction var simplification
+  ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
+  if (SE == 0)
+    return false;
+
+  // Only unroll loops with a computable trip count and the trip count needs
+  // to be an int value (allowing a pointer type is a TODO item)
+  const SCEV *BECount = SE->getBackedgeTakenCount(L);
+  if (isa<SCEVCouldNotCompute>(BECount) || !BECount->getType()->isIntegerTy())
+    return false;
+
+  // Add 1 since the backedge count doesn't include the first loop iteration
+  const SCEV *TripCountSC = 
+    SE->getAddExpr(BECount, SE->getConstant(BECount->getType(), 1));
+  if (isa<SCEVCouldNotCompute>(TripCountSC))
+    return false;
+
+  // We only handle cases when the unroll factor is a power of 2.
+  // Count is the loop unroll factor, the number of extra copies added + 1.
+  if ((Count & (Count-1)) != 0)
+    return false;
+
+  // If this loop is nested, then the loop unroller changes the code in
+  // parent loop, so the Scalar Evolution pass needs to be run again
+  if (Loop *ParentLoop = L->getParentLoop())
+    SE->forgetLoop(ParentLoop);
+
+  BasicBlock *PH = L->getLoopPreheader();
+  BasicBlock *Header = L->getHeader();
+  BasicBlock *Latch = L->getLoopLatch();
+  // It helps to splits the original preheader twice, one for the end of the
+  // prolog code and one for a new loop preheader
+  BasicBlock *PEnd = SplitEdge(PH, Header, LPM->getAsPass());
+  BasicBlock *NewPH = SplitBlock(PEnd, PEnd->getTerminator(), LPM->getAsPass());
+  BranchInst *PreHeaderBR = cast<BranchInst>(PH->getTerminator());
+
+  // Compute the number of extra iterations required, which is:
+  //  extra iterations = run-time trip count % (loop unroll factor + 1)
+  SCEVExpander Expander(*SE, "loop-unroll");
+  Value *TripCount = Expander.expandCodeFor(TripCountSC, TripCountSC->getType(),
+                                            PreHeaderBR);
+  Type *CountTy = TripCount->getType();
+  BinaryOperator *ModVal =
+    BinaryOperator::CreateURem(TripCount,
+                               ConstantInt::get(CountTy, Count),
+                               "xtraiter");
+  ModVal->insertBefore(PreHeaderBR);
+
+  // Check if for no extra iterations, then jump to unrolled loop
+  Value *BranchVal = new ICmpInst(PreHeaderBR,
+                                  ICmpInst::ICMP_NE, ModVal,
+                                  ConstantInt::get(CountTy, 0), "lcmp");
+  // Branch to either the extra iterations or the unrolled loop
+  // We will fix up the true branch label when adding loop body copies
+  BranchInst::Create(PEnd, PEnd, BranchVal, PreHeaderBR);
+  assert(PreHeaderBR->isUnconditional() && 
+         PreHeaderBR->getSuccessor(0) == PEnd && 
+         "CFG edges in Preheader are not correct");
+  PreHeaderBR->eraseFromParent();
+
+  ValueToValueMapTy LVMap;
+  Function *F = Header->getParent();
+  // These variables are used to update the CFG links in each iteration
+  BasicBlock *CompareBB = 0;
+  BasicBlock *LastLoopBB = PH;
+  // Get an ordered list of blocks in the loop to help with the ordering of the
+  // cloned blocks in the prolog code
+  LoopBlocksDFS LoopBlocks(L);
+  LoopBlocks.perform(LI);
+
+  //
+  // For each extra loop iteration, create a copy of the loop's basic blocks
+  // and generate a condition that branches to the copy depending on the
+  // number of 'left over' iterations.
+  //
+  for (unsigned leftOverIters = Count-1; leftOverIters > 0; --leftOverIters) {
+    std::vector<BasicBlock*> NewBlocks;
+    ValueToValueMapTy VMap;
+
+    // Clone all the basic blocks in the loop, but we don't clone the loop
+    // This function adds the appropriate CFG connections.
+    CloneLoopBlocks(L, (leftOverIters == Count-1), LastLoopBB, PEnd, NewBlocks,
+                    LoopBlocks, VMap, LVMap, LI);
+    LastLoopBB = cast<BasicBlock>(VMap[Latch]);
+
+    // Insert the cloned blocks into function just before the original loop
+    F->getBasicBlockList().splice(PEnd, F->getBasicBlockList(),
+                                  NewBlocks[0], F->end());
+
+    // Generate the code for the comparison which determines if the loop
+    // prolog code needs to be executed.
+    if (leftOverIters == Count-1) {
+      // There is no compare block for the fall-thru case when for the last
+      // left over iteration
+      CompareBB = NewBlocks[0];
+    } else {
+      // Create a new block for the comparison
+      BasicBlock *NewBB = BasicBlock::Create(CompareBB->getContext(), "unr.cmp",
+                                             F, CompareBB);
+      if (Loop *ParentLoop = L->getParentLoop()) {
+        // Add the new block to the parent loop, if needed
+        ParentLoop->addBasicBlockToLoop(NewBB, LI->getBase());
+      }
+
+      // The comparison w/ the extra iteration value and branch
+      Value *BranchVal = new ICmpInst(*NewBB, ICmpInst::ICMP_EQ, ModVal,
+                                      ConstantInt::get(CountTy, leftOverIters),
+                                      "un.tmp");
+      // Branch to either the extra iterations or the unrolled loop
+      BranchInst::Create(NewBlocks[0], CompareBB,
+                         BranchVal, NewBB);
+      CompareBB = NewBB;
+      PH->getTerminator()->setSuccessor(0, NewBB);
+      VMap[NewPH] = CompareBB;
+    }
+
+    // Rewrite the cloned instruction operands to use the values
+    // created when the clone is created.
+    for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) {
+      for (BasicBlock::iterator I = NewBlocks[i]->begin(),
+             E = NewBlocks[i]->end(); I != E; ++I) {
+        RemapInstruction(I, VMap, 
+                         RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
+      }
+    }
+  }
+
+  // Connect the prolog code to the original loop and update the
+  // PHI functions.
+  ConnectProlog(L, TripCount, Count, LastLoopBB, PEnd, PH, NewPH, LVMap,
+                LPM->getAsPass());
+  NumRuntimeUnrolled++;
+  return true;
+}
diff --git a/test/Transforms/LoopUnroll/runtime-loop.ll b/test/Transforms/LoopUnroll/runtime-loop.ll
new file mode 100644
index 0000000000..d8bbea9f10
--- /dev/null
+++ b/test/Transforms/LoopUnroll/runtime-loop.ll
@@ -0,0 +1,109 @@
+; RUN: opt < %s -S -loop-unroll -unroll-runtime=true | FileCheck %s
+
+; Tests for unrolling loops with run-time trip counts
+
+; CHECK: unr.cmp{{.*}}:
+; CHECK: for.body.unr{{.*}}:
+; CHECK: for.body:
+; CHECK: br i1 %exitcond.7, label %for.end.loopexit{{.*}}, label %for.body
+
+define i32 @test(i32* nocapture %a, i32 %n) nounwind uwtable readonly {
+entry:
+  %cmp1 = icmp eq i32 %n, 0
+  br i1 %cmp1, label %for.end, label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %sum.02 = phi i32 [ %add, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %add = add nsw i32 %0, %sum.02
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  %sum.0.lcssa = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  ret i32 %sum.0.lcssa
+}
+
+
+; Still try to completely unroll loops with compile-time trip counts
+; even if the -unroll-runtime is specified
+
+; CHECK: for.body:
+; CHECK-NOT: for.body.unr:
+
+define i32 @test1(i32* nocapture %a) nounwind uwtable readonly {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %sum.01 = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %add = add nsw i32 %0, %sum.01
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, 5
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret i32 %add
+}
+
+; This is test 2007-05-09-UnknownTripCount.ll which can be unrolled now
+; if the -unroll-runtime option is turned on
+
+; CHECK: bb72.2:
+
+define void @foo(i32 %trips) {
+entry:
+        br label %cond_true.outer
+
+cond_true.outer:
+        %indvar1.ph = phi i32 [ 0, %entry ], [ %indvar.next2, %bb72 ]
+        br label %bb72
+
+bb72:
+        %indvar.next2 = add i32 %indvar1.ph, 1
+        %exitcond3 = icmp eq i32 %indvar.next2, %trips
+        br i1 %exitcond3, label %cond_true138, label %cond_true.outer
+
+cond_true138:
+        ret void
+}
+
+
+; Test run-time unrolling for a loop that counts down by -2.
+
+; CHECK: for.body.unr:
+; CHECK: br i1 %cmp.7, label %for.cond.for.end_crit_edge{{.*}}, label %for.body
+
+define zeroext i16 @down(i16* nocapture %p, i32 %len) nounwind uwtable readonly {
+entry:
+  %cmp2 = icmp eq i32 %len, 0
+  br i1 %cmp2, label %for.end, label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %p.addr.05 = phi i16* [ %incdec.ptr, %for.body ], [ %p, %entry ]
+  %len.addr.04 = phi i32 [ %sub, %for.body ], [ %len, %entry ]
+  %res.03 = phi i32 [ %add, %for.body ], [ 0, %entry ]
+  %incdec.ptr = getelementptr inbounds i16* %p.addr.05, i64 1
+  %0 = load i16* %p.addr.05, align 2
+  %conv = zext i16 %0 to i32
+  %add = add i32 %conv, %res.03
+  %sub = add nsw i32 %len.addr.04, -2
+  %cmp = icmp eq i32 %sub, 0
+  br i1 %cmp, label %for.cond.for.end_crit_edge, label %for.body
+
+for.cond.for.end_crit_edge:                       ; preds = %for.body
+  %phitmp = trunc i32 %add to i16
+  br label %for.end
+
+for.end:                                          ; preds = %for.cond.for.end_crit_edge, %entry
+  %res.0.lcssa = phi i16 [ %phitmp, %for.cond.for.end_crit_edge ], [ 0, %entry ]
+  ret i16 %res.0.lcssa
+}
diff --git a/test/Transforms/LoopUnroll/runtime-loop1.ll b/test/Transforms/LoopUnroll/runtime-loop1.ll
new file mode 100644
index 0000000000..ad99b8cd9c
--- /dev/null
+++ b/test/Transforms/LoopUnroll/runtime-loop1.ll
@@ -0,0 +1,30 @@
+; RUN: opt < %s -S -loop-unroll -unroll-runtime -unroll-count=4 | FileCheck %s
+
+; This tests that setting the unroll count works
+
+; CHECK: unr.cmp:
+; CHECK: for.body.unr:
+; CHECK: for.body:
+; CHECK: br i1 %exitcond.3, label %for.end.loopexit{{.*}}, label %for.body
+; CHECK-NOT: br i1 %exitcond.4, label %for.end.loopexit{{.*}}, label %for.body
+
+define i32 @test(i32* nocapture %a, i32 %n) nounwind uwtable readonly {
+entry:
+  %cmp1 = icmp eq i32 %n, 0
+  br i1 %cmp1, label %for.end, label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %sum.02 = phi i32 [ %add, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %add = add nsw i32 %0, %sum.02
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  %sum.0.lcssa = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  ret i32 %sum.0.lcssa
+}
diff --git a/test/Transforms/LoopUnroll/runtime-loop2.ll b/test/Transforms/LoopUnroll/runtime-loop2.ll
new file mode 100644
index 0000000000..cbc7af58ff
--- /dev/null
+++ b/test/Transforms/LoopUnroll/runtime-loop2.ll
@@ -0,0 +1,31 @@
+; RUN: opt < %s -S -loop-unroll -unroll-threshold=50 -unroll-runtime -unroll-count=8 | FileCheck %s
+
+; Choose a smaller, power-of-two, unroll count if the loop is too large.
+; This test makes sure we're not unrolling 'odd' counts
+
+; CHECK: unr.cmp:
+; CHECK: for.body.unr:
+; CHECK: for.body:
+; CHECK: br i1 %exitcond.3, label %for.end.loopexit{{.*}}, label %for.body
+; CHECK-NOT: br i1 %exitcond.4, label %for.end.loopexit{{.*}}, label %for.body
+
+define i32 @test(i32* nocapture %a, i32 %n) nounwind uwtable readonly {
+entry:
+  %cmp1 = icmp eq i32 %n, 0
+  br i1 %cmp1, label %for.end, label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
+  %sum.02 = phi i32 [ %add, %for.body ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
+  %0 = load i32* %arrayidx, align 4
+  %add = add nsw i32 %0, %sum.02
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body, %entry
+  %sum.0.lcssa = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  ret i32 %sum.0.lcssa
+}
diff --git a/test/Transforms/LoopUnroll/runtime-loop3.ll b/test/Transforms/LoopUnroll/runtime-loop3.ll
new file mode 100644
index 0000000000..55cf22373e
--- /dev/null
+++ b/test/Transforms/LoopUnroll/runtime-loop3.ll
@@ -0,0 +1,44 @@
+; RUN: opt < %s -disable-output -stats -loop-unroll -unroll-runtime -unroll-threshold=400 -info-output-file - | FileCheck %s --check-prefix=STATS
+
+; Test that nested loops can be unrolled.  We need to increase threshold to do it
+
+; STATS: 2 loop-unroll - Number of loops unrolled (completely or otherwise)
+
+define i32 @nested(i32* nocapture %a, i32 %n, i32 %m) nounwind uwtable readonly {
+entry:
+  %cmp11 = icmp sgt i32 %n, 0
+  br i1 %cmp11, label %for.cond1.preheader.lr.ph, label %for.end7
+
+for.cond1.preheader.lr.ph:                        ; preds = %entry
+  %cmp28 = icmp sgt i32 %m, 0
+  br label %for.cond1.preheader
+
+for.cond1.preheader:                              ; preds = %for.inc5, %for.cond1.preheader.lr.ph
+  %indvars.iv16 = phi i64 [ 0, %for.cond1.preheader.lr.ph ], [ %indvars.iv.next17, %for.inc5 ]
+  %sum.012 = phi i32 [ 0, %for.cond1.preheader.lr.ph ], [ %sum.1.lcssa, %for.inc5 ]
+  br i1 %cmp28, label %for.body3, label %for.inc5
+
+for.body3:                                        ; preds = %for.cond1.preheader, %for.body3
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body3 ], [ 0, %for.cond1.preheader ]
+  %sum.19 = phi i32 [ %add4, %for.body3 ], [ %sum.012, %for.cond1.preheader ]
+  %0 = add nsw i64 %indvars.iv, %indvars.iv16
+  %arrayidx = getelementptr inbounds i32* %a, i64 %0
+  %1 = load i32* %arrayidx, align 4
+  %add4 = add nsw i32 %1, %sum.19
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %m
+  br i1 %exitcond, label %for.inc5, label %for.body3
+
+for.inc5:                                         ; preds = %for.body3, %for.cond1.preheader
+  %sum.1.lcssa = phi i32 [ %sum.012, %for.cond1.preheader ], [ %add4, %for.body3 ]
+  %indvars.iv.next17 = add i64 %indvars.iv16, 1
+  %lftr.wideiv18 = trunc i64 %indvars.iv.next17 to i32
+  %exitcond19 = icmp eq i32 %lftr.wideiv18, %n
+  br i1 %exitcond19, label %for.end7, label %for.cond1.preheader
+
+for.end7:                                         ; preds = %for.inc5, %entry
+  %sum.0.lcssa = phi i32 [ 0, %entry ], [ %sum.1.lcssa, %for.inc5 ]
+  ret i32 %sum.0.lcssa
+}
+