Add a new optimization pass: Stack Coloring, that merges disjoint static allocations (allocas). Allocas are known to be

disjoint if they are marked by disjoint lifetime markers (@llvm.lifetime.XXX intrinsics). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163299 91177308-0d34-0410-b5e6-96231b3b80d8
2025-06-16 12:24:03 +00:00 · 2012-09-06 09:17:37 +00:00
parent 6c822eea47
commit c05d30601c
22 changed files with 1058 additions and 17 deletions
--- a/lib/CodeGen/StackColoring.cpp
+++ b/lib/CodeGen/StackColoring.cpp
@ -0,0 +1,657 @@
+//===-- StackColoring.cpp -------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass implements the stack-coloring optimization that looks for
+// lifetime markers machine instructions (LIFESTART_BEGIN and LIFESTART_END),
+// which represent the possible lifetime of stack slots. It attempts to
+// merge disjoint stack slots and reduce the used stack space.
+// NOTE: This pass is not StackSlotColoring, which optimizes spill slots.
+//
+// TODO: In the future we plan to improve stack coloring in the following ways:
+// 1. Allow merging multiple small slots into a single larger slot at different
+//    offsets.
+// 2. Merge this pass with StackSlotColoring and allow merging of allocas with
+//    spill slots.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "stackcoloring"
+#include "MachineTraceMetrics.h"
+#include "llvm/Function.h"
+#include "llvm/Module.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SparseSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/LiveInterval.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
+#include "llvm/CodeGen/MachineMemOperand.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/SlotIndexes.h"
+#include "llvm/DebugInfo.h"
+#include "llvm/MC/MCInstrItineraries.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+static cl::opt<bool>
+DisableColoring("no-stack-coloring",
+               cl::init(false), cl::Hidden,
+               cl::desc("Suppress stack coloring"));
+
+STATISTIC(NumMarkerSeen,  "Number of life markers found.");
+STATISTIC(StackSpaceSaved, "Number of bytes saved due to merging slots.");
+STATISTIC(StackSlotMerged, "Number of stack slot merged.");
+
+//===----------------------------------------------------------------------===//
+//                           StackColoring Pass
+//===----------------------------------------------------------------------===//
+
+namespace {
+/// StackColoring - A machine pass for merging disjoint stack allocations,
+/// marked by the LIFETIME_START and LIFETIME_END pseudo instructions.
+class StackColoring : public MachineFunctionPass {
+  MachineFrameInfo *MFI;
+  MachineFunction *MF;
+
+  /// A class representing liveness information for a single basic block.
+  /// Each bit in the BitVector represents the liveness property
+  /// for a different stack slot.
+  struct BlockLifetimeInfo {
+    /// Which slots BEGINs in each basic block.
+    BitVector Begin;
+    /// Which slots ENDs in each basic block.
+    BitVector End;
+    /// Which slots are marked as LIVE_IN, coming into each basic block.
+    BitVector LiveIn;
+    /// Which slots are marked as LIVE_OUT, coming out of each basic block.
+    BitVector LiveOut;
+  };
+
+  /// Maps active slots (per bit) for each basic block.
+  DenseMap<MachineBasicBlock*, BlockLifetimeInfo> BlockLiveness;
+
+  /// Maps serial numbers to basic blocks.
+  DenseMap<MachineBasicBlock*, int> BasicBlocks;
+  /// Maps basic blocks to a serial number.
+  SmallVector<MachineBasicBlock*, 8> BasicBlockNumbering;
+
+  /// Maps liveness intervals for each slot.
+  SmallVector<LiveInterval*, 16> Intervals;
+  /// VNInfo is used for the construction of LiveIntervals.
+  VNInfo::Allocator VNInfoAllocator;
+  /// SlotIndex analysis object.
+  SlotIndexes* Indexes;
+
+  /// The list of lifetime markers found. These markers are to be removed
+  /// once the coloring is done.
+  SmallVector<MachineInstr*, 8> Markers;
+
+  /// SlotSizeSorter - A Sort utility for arranging stack slots according
+  /// to their size.
+  struct SlotSizeSorter {
+    MachineFrameInfo *MFI;
+    SlotSizeSorter(MachineFrameInfo *mfi) : MFI(mfi) { }
+    bool operator()(int LHS, int RHS) {
+      // We use -1 to denote a uninteresting slot. Place these slots at the end.
+      if (LHS == -1) return false;
+      if (RHS == -1) return true;
+      // Sort according to size.
+      return MFI->getObjectSize(LHS) > MFI->getObjectSize(RHS);
+  }
+};
+
+public:
+  static char ID;
+  StackColoring() : MachineFunctionPass(ID) {
+    initializeStackColoringPass(*PassRegistry::getPassRegistry());
+  }
+  void getAnalysisUsage(AnalysisUsage &AU) const;
+  bool runOnMachineFunction(MachineFunction &MF);
+
+private:
+  /// Debug.
+  void dump();
+
+  /// Removes all of the lifetime marker instructions from the function.
+  /// \returns true if any markers were removed.
+  bool removeAllMarkers();
+
+  /// Scan the machine function and find all of the lifetime markers.
+  /// Record the findings in the BEGIN and END vectors.
+  /// \returns the number of markers found.
+  unsigned collectMarkers(unsigned NumSlot);
+
+  /// Perform the dataflow calculation and calculate the lifetime for each of
+  /// the slots, based on the BEGIN/END vectors. Set the LifetimeLIVE_IN and
+  /// LifetimeLIVE_OUT maps that represent which stack slots are live coming
+  /// in and out blocks.
+  void calculateLocalLiveness();
+
+  /// Construct the LiveIntervals for the slots.
+  void calculateLiveIntervals(unsigned NumSlots);
+
+  /// Go over the machine function and change instructions which use stack
+  /// slots to use the joint slots.
+  void remapInstructions(DenseMap<int, int> &SlotRemap);
+
+  /// Map entries which point to other entries to their destination.
+  ///   A->B->C becomes A->C.
+   void expungeSlotMap(DenseMap<int, int> &SlotRemap, unsigned NumSlots);
+};
+} // end anonymous namespace
+
+char StackColoring::ID = 0;
+char &llvm::StackColoringID = StackColoring::ID;
+
+INITIALIZE_PASS_BEGIN(StackColoring,
+                   "stack-coloring", "Merge disjoint stack slots", false, false)
+INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
+INITIALIZE_PASS_END(StackColoring,
+                   "stack-coloring", "Merge disjoint stack slots", false, false)
+
+void StackColoring::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequired<MachineDominatorTree>();
+  AU.addPreserved<MachineDominatorTree>();
+  AU.addRequired<SlotIndexes>();
+  MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+void StackColoring::dump() {
+  for (df_iterator<MachineFunction*> FI = df_begin(MF), FE = df_end(MF);
+       FI != FE; ++FI) {
+    unsigned Num = BasicBlocks[*FI];
+    DEBUG(dbgs()<<"Inspecting block #"<<Num<<" ["<<FI->getName()<<"]\n");
+    Num = 0;
+    DEBUG(dbgs()<<"BEGIN  : {");
+    for (unsigned i=0; i < BlockLiveness[*FI].Begin.size(); ++i)
+      DEBUG(dbgs()<<BlockLiveness[*FI].Begin.test(i)<<" ");
+    DEBUG(dbgs()<<"}\n");
+
+    DEBUG(dbgs()<<"END    : {");
+    for (unsigned i=0; i < BlockLiveness[*FI].End.size(); ++i)
+      DEBUG(dbgs()<<BlockLiveness[*FI].End.test(i)<<" ");
+
+    DEBUG(dbgs()<<"}\n");
+
+    DEBUG(dbgs()<<"LIVE_IN: {");
+    for (unsigned i=0; i < BlockLiveness[*FI].LiveIn.size(); ++i)
+      DEBUG(dbgs()<<BlockLiveness[*FI].LiveIn.test(i)<<" ");
+
+    DEBUG(dbgs()<<"}\n");
+    DEBUG(dbgs()<<"LIVEOUT: {");
+    for (unsigned i=0; i < BlockLiveness[*FI].LiveOut.size(); ++i)
+      DEBUG(dbgs()<<BlockLiveness[*FI].LiveOut.test(i)<<" ");
+    DEBUG(dbgs()<<"}\n");
+  }
+}
+
+unsigned StackColoring::collectMarkers(unsigned NumSlot) {
+  unsigned MarkersFound = 0;
+  // Scan the function to find all lifetime markers.
+  // NOTE: We use the a reverse-post-order iteration to ensure that we obtain a
+  // deterministic numbering, and because we'll need a post-order iteration
+  // later for solving the liveness dataflow problem.
+  for (df_iterator<MachineFunction*> FI = df_begin(MF), FE = df_end(MF);
+       FI != FE; ++FI) {
+
+    // Assign a serial number to this basic block.
+    BasicBlocks[*FI] = BasicBlockNumbering.size();;
+    BasicBlockNumbering.push_back(*FI);
+
+    BlockLiveness[*FI].Begin.resize(NumSlot);
+    BlockLiveness[*FI].End.resize(NumSlot);
+
+    for (MachineBasicBlock::iterator BI = (*FI)->begin(), BE = (*FI)->end();
+         BI != BE; ++BI) {
+
+      if (BI->getOpcode() != TargetOpcode::LIFETIME_START &&
+          BI->getOpcode() != TargetOpcode::LIFETIME_END)
+        continue;
+
+      Markers.push_back(BI);
+
+      bool IsStart = BI->getOpcode() == TargetOpcode::LIFETIME_START;
+      MachineOperand &MI = BI->getOperand(0);
+      unsigned Slot = MI.getIndex();
+
+      MarkersFound++;
+
+      const Value* Allocation = MFI->getObjectAllocation(Slot);
+      if (Allocation) {
+        DEBUG(dbgs()<<"Found lifetime marker for allocation: "<<
+              Allocation->getName()<<"\n");
+      }
+
+      if (IsStart) {
+        BlockLiveness[*FI].Begin.set(Slot);
+      } else {
+        if (BlockLiveness[*FI].Begin.test(Slot)) {
+          // Allocas that start and end within a single block are handled
+          // specially when computing the LiveIntervals to avoid pessimizing
+          // the liveness propagation.
+          BlockLiveness[*FI].Begin.reset(Slot);
+        } else {
+          BlockLiveness[*FI].End.set(Slot);
+        }
+      }
+    }
+  }
+
+  // Update statistics.
+  NumMarkerSeen += MarkersFound;
+  return MarkersFound;
+}
+
+void StackColoring::calculateLocalLiveness() {
+  // Perform a standard reverse dataflow computation to solve for
+  // global liveness.  The BEGIN set here is equivalent to KILL in the standard
+  // formulation, and END is equivalent to GEN.  The result of this computation
+  // is a map from blocks to bitvectors where the bitvectors represent which
+  // allocas are live in/out of that block.
+  SmallPtrSet<MachineBasicBlock*, 8> BBSet(BasicBlockNumbering.begin(),
+                                           BasicBlockNumbering.end());
+  unsigned NumSSMIters = 0;
+  bool changed = true;
+  while (changed) {
+    changed = false;
+    ++NumSSMIters;
+
+    SmallPtrSet<MachineBasicBlock*, 8> NextBBSet;
+
+    for (SmallVector<MachineBasicBlock*, 8>::iterator
+         PI = BasicBlockNumbering.begin(), PE = BasicBlockNumbering.end();
+         PI != PE; ++PI) {
+
+      MachineBasicBlock *BB = *PI;
+      if (!BBSet.count(BB)) continue;
+
+      BitVector LocalLiveIn;
+      BitVector LocalLiveOut;
+
+      // Forward propagation from begins to ends.
+      for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
+           PE = BB->pred_end(); PI != PE; ++PI)
+        LocalLiveIn |= BlockLiveness[*PI].LiveOut;
+      LocalLiveIn |= BlockLiveness[BB].End;
+      LocalLiveIn.reset(BlockLiveness[BB].Begin);
+
+      // Reverse propagation from ends to begins.
+      for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
+           SE = BB->succ_end(); SI != SE; ++SI)
+        LocalLiveOut |= BlockLiveness[*SI].LiveIn;
+      LocalLiveOut |= BlockLiveness[BB].Begin;
+      LocalLiveOut.reset(BlockLiveness[BB].End);
+
+      LocalLiveIn |= LocalLiveOut;
+      LocalLiveOut |= LocalLiveIn;
+
+      // After adopting the live bits, we need to turn-off the bits which
+      // are de-activated in this block.
+      LocalLiveOut.reset(BlockLiveness[BB].End);
+      LocalLiveIn.reset(BlockLiveness[BB].Begin);
+
+      if (LocalLiveIn.test(BlockLiveness[BB].LiveIn)) {
+        changed = true;
+        BlockLiveness[BB].LiveIn |= LocalLiveIn;
+
+        for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
+             PE = BB->pred_end(); PI != PE; ++PI)
+          NextBBSet.insert(*PI);
+      }
+
+      if (LocalLiveOut.test(BlockLiveness[BB].LiveOut)) {
+        changed = true;
+        BlockLiveness[BB].LiveOut |= LocalLiveOut;
+
+        for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
+             SE = BB->succ_end(); SI != SE; ++SI)
+          NextBBSet.insert(*SI);
+      }
+    }
+
+    BBSet = NextBBSet;
+  }// while changed.
+}
+
+void StackColoring::calculateLiveIntervals(unsigned NumSlots) {
+  SmallVector<SlotIndex, 16> Starts;
+  SmallVector<SlotIndex, 16> Finishes;
+
+  // For each block, find which slots are active within this block
+  // and update the live intervals.
+  for (MachineFunction::iterator MBB = MF->begin(), MBBe = MF->end();
+       MBB != MBBe; ++MBB) {
+    Starts.clear();
+    Starts.resize(NumSlots);
+    Finishes.clear();
+    Finishes.resize(NumSlots);
+
+    BitVector Alive = BlockLiveness[MBB].LiveIn;
+    Alive |= BlockLiveness[MBB].LiveOut;
+
+    if (Alive.any()) {
+      for (int pos = Alive.find_first(); pos != -1;
+           pos = Alive.find_next(pos)) {
+        Starts[pos] = Indexes->getMBBStartIdx(MBB);
+        Finishes[pos] = Indexes->getMBBEndIdx(MBB);
+      }
+    }
+
+    for (SmallVector<MachineInstr*, 8>::iterator it = Markers.begin(),
+         e = Markers.end(); it != e; ++it) {
+      MachineInstr *MI = *it;
+      assert((MI->getOpcode() == TargetOpcode::LIFETIME_START ||
+              MI->getOpcode() == TargetOpcode::LIFETIME_END) &&
+             "Invalid Lifetime marker");
+
+      if (MI->getParent() == MBB) {
+        bool IsStart = MI->getOpcode() == TargetOpcode::LIFETIME_START;
+        MachineOperand &Mo = MI->getOperand(0);
+        int Slot = Mo.getIndex();
+        assert(Slot >= 0 && "Invalid slot");
+        if (IsStart) {
+          Starts[Slot] = Indexes->getInstructionIndex(MI);
+        } else {
+          Finishes[Slot] = Indexes->getInstructionIndex(MI);
+        }
+      }
+    }
+
+    for (unsigned i = 0; i < NumSlots; ++i) {
+      assert(!!Starts[i] == !!Finishes[i] && "Unmatched range");
+      if (Starts[i] == Finishes[i])
+        continue;
+
+      assert(Starts[i] && Finishes[i] && "Invalid interval");
+      VNInfo *ValNum = Intervals[i]->getValNumInfo(0);
+      SlotIndex S = Starts[i];
+      SlotIndex F = Finishes[i];
+      if (S < F) {
+        // We have a single consecutive region.
+        Intervals[i]->addRange(LiveRange(S, F, ValNum));
+      } else {
+        // We have two non consecutive regions. This happens when
+        // LIFETIME_START appears after the LIFETIME_END marker.
+        SlotIndex NewStart = Indexes->getMBBStartIdx(MBB);
+        SlotIndex NewFin = Indexes->getMBBEndIdx(MBB);
+        Intervals[i]->addRange(LiveRange(NewStart, F, ValNum));
+        Intervals[i]->addRange(LiveRange(S, NewFin, ValNum));
+      }
+    }
+  }
+}
+
+bool StackColoring::removeAllMarkers() {
+  unsigned Count = 0;
+  for (unsigned i = 0; i < Markers.size(); ++i) {
+    Markers[i]->eraseFromParent();
+    Count++;
+  }
+  Markers.clear();
+
+  DEBUG(dbgs()<<"Removed "<<Count<<" markers.\n");
+  return Count;
+}
+
+void StackColoring::remapInstructions(DenseMap<int, int> &SlotRemap) {
+  unsigned FixedInstr = 0;
+  unsigned FixedMemOp = 0;
+  unsigned FixedDbg = 0;
+  MachineModuleInfo *MMI = &MF->getMMI();
+
+  // Remap debug information that refers to stack slots.
+  MachineModuleInfo::VariableDbgInfoMapTy &VMap = MMI->getVariableDbgInfo();
+  for (MachineModuleInfo::VariableDbgInfoMapTy::iterator VI = VMap.begin(),
+       VE = VMap.end(); VI != VE; ++VI) {
+    const MDNode *Var = VI->first;
+    if (!Var) continue;
+    std::pair<unsigned, DebugLoc> &VP = VI->second;
+    if (SlotRemap.count(VP.first)) {
+      DEBUG(dbgs()<<"Remapping debug info for ["<<Var->getName()<<"].\n");
+      VP.first = SlotRemap[VP.first];
+      FixedDbg++;
+    }
+  }
+
+  // Keep a list of *allocas* which need to be remapped.
+  DenseMap<const Value*, const Value*> Allocas;
+  for (DenseMap<int, int>::iterator it = SlotRemap.begin(),
+       e = SlotRemap.end(); it != e; ++it) {
+    const Value* From = MFI->getObjectAllocation(it->first);
+    const Value* To = MFI->getObjectAllocation(it->second);
+    assert(To && From && "Invalid allocation object");
+    Allocas[From] = To;
+  }
+
+  // Remap all instructions to the new stack slots.
+  MachineFunction::iterator BB, BBE;
+  MachineBasicBlock::iterator I, IE;
+  for (BB = MF->begin(), BBE = MF->end(); BB != BBE; ++BB)
+    for (I = BB->begin(), IE = BB->end(); I != IE; ++I) {
+
+      // Update the MachineMemOperand to use the new alloca.
+      for (MachineInstr::mmo_iterator MM = I->memoperands_begin(),
+           E = I->memoperands_end(); MM != E; ++MM) {
+        MachineMemOperand *MMO = *MM;
+
+        const Value *V = MMO->getValue();
+
+        if (!V)
+          continue;
+
+        // Climb up and find the original alloca.
+        V = GetUnderlyingObject(V);
+        // If we did not find one, or if the one that we found is not in our
+        // map, then move on.
+        if (!V || !Allocas.count(V))
+          continue;
+
+        MMO->setValue(Allocas[V]);
+        FixedMemOp++;
+      }
+
+      // Update all of the machine instruction operands.
+      for (unsigned i = 0 ; i <  I->getNumOperands(); ++i) {
+        MachineOperand &MO = I->getOperand(i);
+
+        if (!MO.isFI())
+          continue;
+        int FromSlot = MO.getIndex();
+
+        // Don't touch arguments.
+        if (FromSlot<0)
+          continue;
+
+        // Only look at mapped slots.
+        if (!SlotRemap.count(FromSlot))
+          continue;
+
+        // Fix the machine instructions.
+        int ToSlot = SlotRemap[FromSlot];
+        MO.setIndex(ToSlot);
+        FixedInstr++;
+      }
+    }
+
+  DEBUG(dbgs()<<"Fixed "<<FixedMemOp<<" machine memory operands.\n");
+  DEBUG(dbgs()<<"Fixed "<<FixedDbg<<" debug locations.\n");
+  DEBUG(dbgs()<<"Fixed "<<FixedInstr<<" machine instructions.\n");
+}
+
+void StackColoring::expungeSlotMap(DenseMap<int, int> &SlotRemap,
+                                   unsigned NumSlots) {
+  // Expunge slot remap map.
+  for (unsigned i=0; i < NumSlots; ++i) {
+    // If we are remapping i
+    if (SlotRemap.count(i)) {
+      int Target = SlotRemap[i];
+      // As long as our target is mapped to something else, follow it.
+      while (SlotRemap.count(Target)) {
+        Target = SlotRemap[Target];
+        SlotRemap[i] = Target;
+      }
+    }
+  }
+}
+
+bool StackColoring::runOnMachineFunction(MachineFunction &Func) {
+  DEBUG(dbgs() << "********** Stack Coloring **********\n"
+               << "********** Function: "
+               << ((Value*)Func.getFunction())->getName() << '\n');
+  MF = &Func;
+  MFI = MF->getFrameInfo();
+  Indexes = &getAnalysis<SlotIndexes>();
+  BlockLiveness.clear();
+  BasicBlocks.clear();
+  BasicBlockNumbering.clear();
+  Markers.clear();
+  Intervals.clear();
+  VNInfoAllocator.Reset();
+
+  unsigned NumSlots = MFI->getObjectIndexEnd();
+
+  // If there are no stack slots then there are no markers to remove.
+  if (!NumSlots)
+    return false;
+
+  SmallVector<int, 8> SortedSlots;
+
+  SortedSlots.reserve(NumSlots);
+  Intervals.reserve(NumSlots);
+
+  unsigned NumMarkers = collectMarkers(NumSlots);
+
+  unsigned TotalSize = 0;
+  DEBUG(dbgs()<<"Found "<<NumMarkers<<" markers and "<<NumSlots<<" slots\n");
+  DEBUG(dbgs()<<"Slot structure:\n");
+
+  for (int i=0; i < MFI->getObjectIndexEnd(); ++i) {
+    DEBUG(dbgs()<<"Slot #"<<i<<" - "<<MFI->getObjectSize(i)<<" bytes.\n");
+    TotalSize += MFI->getObjectSize(i);
+  }
+
+  DEBUG(dbgs()<<"Total Stack size: "<<TotalSize<<" bytes\n\n");
+
+  // Don't continue because there are not enough lifetime markers, or the
+  // stack or too small, or we are told not to optimize the slots.
+  if (NumMarkers < 2 || TotalSize < 16 || DisableColoring) {
+    DEBUG(dbgs()<<"Will not try to merge slots.\n");
+    return removeAllMarkers();
+  }
+
+  for (unsigned i=0; i < NumSlots; ++i) {
+    LiveInterval *LI = new LiveInterval(i, 0);
+    Intervals.push_back(LI);
+    LI->getNextValue(Indexes->getZeroIndex(), VNInfoAllocator);
+    SortedSlots.push_back(i);
+  }
+
+  // Calculate the liveness of each block.
+  calculateLocalLiveness();
+
+  // Propagate the liveness information.
+  calculateLiveIntervals(NumSlots);
+
+  // Maps old slots to new slots.
+  DenseMap<int, int> SlotRemap;
+  unsigned RemovedSlots = 0;
+  unsigned ReducedSize = 0;
+
+  // Do not bother looking at empty intervals.
+  for (unsigned I = 0; I < NumSlots; ++I) {
+    if (Intervals[SortedSlots[I]]->empty())
+      SortedSlots[I] = -1;
+  }
+
+  // This is a simple greedy algorithm for merging allocas. First, sort the
+  // slots, placing the largest slots first. Next, perform an n^2 scan and look
+  // for disjoint slots. When you find disjoint slots, merge the samller one
+  // into the bigger one and update the live interval. Remove the small alloca
+  // and continue.
+
+  // Sort the slots according to their size. Place unused slots at the end.
+  std::sort(SortedSlots.begin(), SortedSlots.end(), SlotSizeSorter(MFI));
+
+  bool Chanded = true;
+  while (Chanded) {
+    Chanded = false;
+    for (unsigned I = 0; I < NumSlots; ++I) {
+      if (SortedSlots[I] == -1)
+        continue;
+
+      for (unsigned J=0; J < NumSlots; ++J) {
+        if (SortedSlots[J] == -1)
+          continue;
+
+        int FirstSlot = SortedSlots[I];
+        int SecondSlot = SortedSlots[J];
+        LiveInterval *First = Intervals[FirstSlot];
+        LiveInterval *Second = Intervals[SecondSlot];
+        assert (!First->empty() && !Second->empty() && "Found an empty range");
+
+        // Merge disjoint slots.
+        if (!First->overlaps(*Second)) {
+          Chanded = true;
+          First->MergeRangesInAsValue(*Second, First->getValNumInfo(0));
+          SlotRemap[SecondSlot] = FirstSlot;
+          SortedSlots[J] = -1;
+          DEBUG(dbgs()<<"Merging #"<<I<<" and slots #"<<J<<" together.\n");
+          unsigned MaxAlignment = std::max(MFI->getObjectAlignment(FirstSlot),
+                                           MFI->getObjectAlignment(SecondSlot));
+
+          assert(MFI->getObjectSize(FirstSlot) >=
+                 MFI->getObjectSize(SecondSlot) &&
+                 "Merging a small object into a larger one");
+
+          RemovedSlots+=1;
+          ReducedSize += MFI->getObjectSize(SecondSlot);
+          MFI->setObjectAlignment(FirstSlot, MaxAlignment);
+          MFI->RemoveStackObject(SecondSlot);
+        }
+      }
+    }
+  }// While changed.
+
+  // Record statistics.
+  StackSpaceSaved += ReducedSize;
+  StackSlotMerged += RemovedSlots;
+  DEBUG(dbgs()<<"Merge "<<RemovedSlots<<" slots. Saved "<<
+        ReducedSize<<" bytes\n");
+
+  // Scan the entire function and update all machine operands that use frame
+  // indices to use the remapped frame index.
+  expungeSlotMap(SlotRemap, NumSlots);
+  remapInstructions(SlotRemap);
+
+  // Release the intervals.
+  for (unsigned I = 0; I < NumSlots; ++I) {
+    delete Intervals[I];
+  }
+
+  return removeAllMarkers();
+}