StackColoring.cpp revision c05d30601ced172b55be81bb529df6be91d6ae15 
1//===-- StackColoring.cpp -------------------------------------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass implements the stack-coloring optimization that looks for
11// lifetime markers machine instructions (LIFESTART_BEGIN and LIFESTART_END),
12// which represent the possible lifetime of stack slots. It attempts to
13// merge disjoint stack slots and reduce the used stack space.
14// NOTE: This pass is not StackSlotColoring, which optimizes spill slots.
15//
16// TODO: In the future we plan to improve stack coloring in the following ways:
17// 1. Allow merging multiple small slots into a single larger slot at different
18//    offsets.
19// 2. Merge this pass with StackSlotColoring and allow merging of allocas with
20//    spill slots.
21//
22//===----------------------------------------------------------------------===//
23
24#define DEBUG_TYPE "stackcoloring"
25#include "MachineTraceMetrics.h"
26#include "llvm/Function.h"
27#include "llvm/Module.h"
28#include "llvm/ADT/BitVector.h"
29#include "llvm/Analysis/Dominators.h"
30#include "llvm/Analysis/ValueTracking.h"
31#include "llvm/ADT/DepthFirstIterator.h"
32#include "llvm/ADT/PostOrderIterator.h"
33#include "llvm/ADT/SetVector.h"
34#include "llvm/ADT/SmallPtrSet.h"
35#include "llvm/ADT/SparseSet.h"
36#include "llvm/ADT/Statistic.h"
37#include "llvm/CodeGen/LiveInterval.h"
38#include "llvm/CodeGen/MachineLoopInfo.h"
39#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
40#include "llvm/CodeGen/MachineDominators.h"
41#include "llvm/CodeGen/MachineBasicBlock.h"
42#include "llvm/CodeGen/MachineFunctionPass.h"
43#include "llvm/CodeGen/MachineLoopInfo.h"
44#include "llvm/CodeGen/MachineModuleInfo.h"
45#include "llvm/CodeGen/MachineRegisterInfo.h"
46#include "llvm/CodeGen/MachineFrameInfo.h"
47#include "llvm/CodeGen/MachineMemOperand.h"
48#include "llvm/CodeGen/Passes.h"
49#include "llvm/CodeGen/SlotIndexes.h"
50#include "llvm/DebugInfo.h"
51#include "llvm/MC/MCInstrItineraries.h"
52#include "llvm/Target/TargetInstrInfo.h"
53#include "llvm/Target/TargetRegisterInfo.h"
54#include "llvm/Support/CommandLine.h"
55#include "llvm/Support/Debug.h"
56#include "llvm/Support/raw_ostream.h"
57
58using namespace llvm;
59
60static cl::opt<bool>
61DisableColoring("no-stack-coloring",
62               cl::init(false), cl::Hidden,
63               cl::desc("Suppress stack coloring"));
64
65STATISTIC(NumMarkerSeen,  "Number of life markers found.");
66STATISTIC(StackSpaceSaved, "Number of bytes saved due to merging slots.");
67STATISTIC(StackSlotMerged, "Number of stack slot merged.");
68
69//===----------------------------------------------------------------------===//
70//                           StackColoring Pass
71//===----------------------------------------------------------------------===//
72
73namespace {
74/// StackColoring - A machine pass for merging disjoint stack allocations,
75/// marked by the LIFETIME_START and LIFETIME_END pseudo instructions.
76class StackColoring : public MachineFunctionPass {
77  MachineFrameInfo *MFI;
78  MachineFunction *MF;
79
80  /// A class representing liveness information for a single basic block.
81  /// Each bit in the BitVector represents the liveness property
82  /// for a different stack slot.
83  struct BlockLifetimeInfo {
84    /// Which slots BEGINs in each basic block.
85    BitVector Begin;
86    /// Which slots ENDs in each basic block.
87    BitVector End;
88    /// Which slots are marked as LIVE_IN, coming into each basic block.
89    BitVector LiveIn;
90    /// Which slots are marked as LIVE_OUT, coming out of each basic block.
91    BitVector LiveOut;
92  };
93
94  /// Maps active slots (per bit) for each basic block.
95  DenseMap<MachineBasicBlock*, BlockLifetimeInfo> BlockLiveness;
96
97  /// Maps serial numbers to basic blocks.
98  DenseMap<MachineBasicBlock*, int> BasicBlocks;
99  /// Maps basic blocks to a serial number.
100  SmallVector<MachineBasicBlock*, 8> BasicBlockNumbering;
101
102  /// Maps liveness intervals for each slot.
103  SmallVector<LiveInterval*, 16> Intervals;
104  /// VNInfo is used for the construction of LiveIntervals.
105  VNInfo::Allocator VNInfoAllocator;
106  /// SlotIndex analysis object.
107  SlotIndexes* Indexes;
108
109  /// The list of lifetime markers found. These markers are to be removed
110  /// once the coloring is done.
111  SmallVector<MachineInstr*, 8> Markers;
112
113  /// SlotSizeSorter - A Sort utility for arranging stack slots according
114  /// to their size.
115  struct SlotSizeSorter {
116    MachineFrameInfo *MFI;
117    SlotSizeSorter(MachineFrameInfo *mfi) : MFI(mfi) { }
118    bool operator()(int LHS, int RHS) {
119      // We use -1 to denote a uninteresting slot. Place these slots at the end.
120      if (LHS == -1) return false;
121      if (RHS == -1) return true;
122      // Sort according to size.
123      return MFI->getObjectSize(LHS) > MFI->getObjectSize(RHS);
124  }
125};
126
127public:
128  static char ID;
129  StackColoring() : MachineFunctionPass(ID) {
130    initializeStackColoringPass(*PassRegistry::getPassRegistry());
131  }
132  void getAnalysisUsage(AnalysisUsage &AU) const;
133  bool runOnMachineFunction(MachineFunction &MF);
134
135private:
136  /// Debug.
137  void dump();
138
139  /// Removes all of the lifetime marker instructions from the function.
140  /// \returns true if any markers were removed.
141  bool removeAllMarkers();
142
143  /// Scan the machine function and find all of the lifetime markers.
144  /// Record the findings in the BEGIN and END vectors.
145  /// \returns the number of markers found.
146  unsigned collectMarkers(unsigned NumSlot);
147
148  /// Perform the dataflow calculation and calculate the lifetime for each of
149  /// the slots, based on the BEGIN/END vectors. Set the LifetimeLIVE_IN and
150  /// LifetimeLIVE_OUT maps that represent which stack slots are live coming
151  /// in and out blocks.
152  void calculateLocalLiveness();
153
154  /// Construct the LiveIntervals for the slots.
155  void calculateLiveIntervals(unsigned NumSlots);
156
157  /// Go over the machine function and change instructions which use stack
158  /// slots to use the joint slots.
159  void remapInstructions(DenseMap<int, int> &SlotRemap);
160
161  /// Map entries which point to other entries to their destination.
162  ///   A->B->C becomes A->C.
163   void expungeSlotMap(DenseMap<int, int> &SlotRemap, unsigned NumSlots);
164};
165} // end anonymous namespace
166
167char StackColoring::ID = 0;
168char &llvm::StackColoringID = StackColoring::ID;
169
170INITIALIZE_PASS_BEGIN(StackColoring,
171                   "stack-coloring", "Merge disjoint stack slots", false, false)
172INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
173INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
174INITIALIZE_PASS_END(StackColoring,
175                   "stack-coloring", "Merge disjoint stack slots", false, false)
176
177void StackColoring::getAnalysisUsage(AnalysisUsage &AU) const {
178  AU.addRequired<MachineDominatorTree>();
179  AU.addPreserved<MachineDominatorTree>();
180  AU.addRequired<SlotIndexes>();
181  MachineFunctionPass::getAnalysisUsage(AU);
182}
183
184void StackColoring::dump() {
185  for (df_iterator<MachineFunction*> FI = df_begin(MF), FE = df_end(MF);
186       FI != FE; ++FI) {
187    unsigned Num = BasicBlocks[*FI];
188    DEBUG(dbgs()<<"Inspecting block #"<<Num<<" ["<<FI->getName()<<"]\n");
189    Num = 0;
190    DEBUG(dbgs()<<"BEGIN  : {");
191    for (unsigned i=0; i < BlockLiveness[*FI].Begin.size(); ++i)
192      DEBUG(dbgs()<<BlockLiveness[*FI].Begin.test(i)<<" ");
193    DEBUG(dbgs()<<"}\n");
194
195    DEBUG(dbgs()<<"END    : {");
196    for (unsigned i=0; i < BlockLiveness[*FI].End.size(); ++i)
197      DEBUG(dbgs()<<BlockLiveness[*FI].End.test(i)<<" ");
198
199    DEBUG(dbgs()<<"}\n");
200
201    DEBUG(dbgs()<<"LIVE_IN: {");
202    for (unsigned i=0; i < BlockLiveness[*FI].LiveIn.size(); ++i)
203      DEBUG(dbgs()<<BlockLiveness[*FI].LiveIn.test(i)<<" ");
204
205    DEBUG(dbgs()<<"}\n");
206    DEBUG(dbgs()<<"LIVEOUT: {");
207    for (unsigned i=0; i < BlockLiveness[*FI].LiveOut.size(); ++i)
208      DEBUG(dbgs()<<BlockLiveness[*FI].LiveOut.test(i)<<" ");
209    DEBUG(dbgs()<<"}\n");
210  }
211}
212
213unsigned StackColoring::collectMarkers(unsigned NumSlot) {
214  unsigned MarkersFound = 0;
215  // Scan the function to find all lifetime markers.
216  // NOTE: We use the a reverse-post-order iteration to ensure that we obtain a
217  // deterministic numbering, and because we'll need a post-order iteration
218  // later for solving the liveness dataflow problem.
219  for (df_iterator<MachineFunction*> FI = df_begin(MF), FE = df_end(MF);
220       FI != FE; ++FI) {
221
222    // Assign a serial number to this basic block.
223    BasicBlocks[*FI] = BasicBlockNumbering.size();;
224    BasicBlockNumbering.push_back(*FI);
225
226    BlockLiveness[*FI].Begin.resize(NumSlot);
227    BlockLiveness[*FI].End.resize(NumSlot);
228
229    for (MachineBasicBlock::iterator BI = (*FI)->begin(), BE = (*FI)->end();
230         BI != BE; ++BI) {
231
232      if (BI->getOpcode() != TargetOpcode::LIFETIME_START &&
233          BI->getOpcode() != TargetOpcode::LIFETIME_END)
234        continue;
235
236      Markers.push_back(BI);
237
238      bool IsStart = BI->getOpcode() == TargetOpcode::LIFETIME_START;
239      MachineOperand &MI = BI->getOperand(0);
240      unsigned Slot = MI.getIndex();
241
242      MarkersFound++;
243
244      const Value* Allocation = MFI->getObjectAllocation(Slot);
245      if (Allocation) {
246        DEBUG(dbgs()<<"Found lifetime marker for allocation: "<<
247              Allocation->getName()<<"\n");
248      }
249
250      if (IsStart) {
251        BlockLiveness[*FI].Begin.set(Slot);
252      } else {
253        if (BlockLiveness[*FI].Begin.test(Slot)) {
254          // Allocas that start and end within a single block are handled
255          // specially when computing the LiveIntervals to avoid pessimizing
256          // the liveness propagation.
257          BlockLiveness[*FI].Begin.reset(Slot);
258        } else {
259          BlockLiveness[*FI].End.set(Slot);
260        }
261      }
262    }
263  }
264
265  // Update statistics.
266  NumMarkerSeen += MarkersFound;
267  return MarkersFound;
268}
269
270void StackColoring::calculateLocalLiveness() {
271  // Perform a standard reverse dataflow computation to solve for
272  // global liveness.  The BEGIN set here is equivalent to KILL in the standard
273  // formulation, and END is equivalent to GEN.  The result of this computation
274  // is a map from blocks to bitvectors where the bitvectors represent which
275  // allocas are live in/out of that block.
276  SmallPtrSet<MachineBasicBlock*, 8> BBSet(BasicBlockNumbering.begin(),
277                                           BasicBlockNumbering.end());
278  unsigned NumSSMIters = 0;
279  bool changed = true;
280  while (changed) {
281    changed = false;
282    ++NumSSMIters;
283
284    SmallPtrSet<MachineBasicBlock*, 8> NextBBSet;
285
286    for (SmallVector<MachineBasicBlock*, 8>::iterator
287         PI = BasicBlockNumbering.begin(), PE = BasicBlockNumbering.end();
288         PI != PE; ++PI) {
289
290      MachineBasicBlock *BB = *PI;
291      if (!BBSet.count(BB)) continue;
292
293      BitVector LocalLiveIn;
294      BitVector LocalLiveOut;
295
296      // Forward propagation from begins to ends.
297      for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
298           PE = BB->pred_end(); PI != PE; ++PI)
299        LocalLiveIn |= BlockLiveness[*PI].LiveOut;
300      LocalLiveIn |= BlockLiveness[BB].End;
301      LocalLiveIn.reset(BlockLiveness[BB].Begin);
302
303      // Reverse propagation from ends to begins.
304      for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
305           SE = BB->succ_end(); SI != SE; ++SI)
306        LocalLiveOut |= BlockLiveness[*SI].LiveIn;
307      LocalLiveOut |= BlockLiveness[BB].Begin;
308      LocalLiveOut.reset(BlockLiveness[BB].End);
309
310      LocalLiveIn |= LocalLiveOut;
311      LocalLiveOut |= LocalLiveIn;
312
313      // After adopting the live bits, we need to turn-off the bits which
314      // are de-activated in this block.
315      LocalLiveOut.reset(BlockLiveness[BB].End);
316      LocalLiveIn.reset(BlockLiveness[BB].Begin);
317
318      if (LocalLiveIn.test(BlockLiveness[BB].LiveIn)) {
319        changed = true;
320        BlockLiveness[BB].LiveIn |= LocalLiveIn;
321
322        for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
323             PE = BB->pred_end(); PI != PE; ++PI)
324          NextBBSet.insert(*PI);
325      }
326
327      if (LocalLiveOut.test(BlockLiveness[BB].LiveOut)) {
328        changed = true;
329        BlockLiveness[BB].LiveOut |= LocalLiveOut;
330
331        for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
332             SE = BB->succ_end(); SI != SE; ++SI)
333          NextBBSet.insert(*SI);
334      }
335    }
336
337    BBSet = NextBBSet;
338  }// while changed.
339}
340
341void StackColoring::calculateLiveIntervals(unsigned NumSlots) {
342  SmallVector<SlotIndex, 16> Starts;
343  SmallVector<SlotIndex, 16> Finishes;
344
345  // For each block, find which slots are active within this block
346  // and update the live intervals.
347  for (MachineFunction::iterator MBB = MF->begin(), MBBe = MF->end();
348       MBB != MBBe; ++MBB) {
349    Starts.clear();
350    Starts.resize(NumSlots);
351    Finishes.clear();
352    Finishes.resize(NumSlots);
353
354    BitVector Alive = BlockLiveness[MBB].LiveIn;
355    Alive |= BlockLiveness[MBB].LiveOut;
356
357    if (Alive.any()) {
358      for (int pos = Alive.find_first(); pos != -1;
359           pos = Alive.find_next(pos)) {
360        Starts[pos] = Indexes->getMBBStartIdx(MBB);
361        Finishes[pos] = Indexes->getMBBEndIdx(MBB);
362      }
363    }
364
365    for (SmallVector<MachineInstr*, 8>::iterator it = Markers.begin(),
366         e = Markers.end(); it != e; ++it) {
367      MachineInstr *MI = *it;
368      assert((MI->getOpcode() == TargetOpcode::LIFETIME_START ||
369              MI->getOpcode() == TargetOpcode::LIFETIME_END) &&
370             "Invalid Lifetime marker");
371
372      if (MI->getParent() == MBB) {
373        bool IsStart = MI->getOpcode() == TargetOpcode::LIFETIME_START;
374        MachineOperand &Mo = MI->getOperand(0);
375        int Slot = Mo.getIndex();
376        assert(Slot >= 0 && "Invalid slot");
377        if (IsStart) {
378          Starts[Slot] = Indexes->getInstructionIndex(MI);
379        } else {
380          Finishes[Slot] = Indexes->getInstructionIndex(MI);
381        }
382      }
383    }
384
385    for (unsigned i = 0; i < NumSlots; ++i) {
386      assert(!!Starts[i] == !!Finishes[i] && "Unmatched range");
387      if (Starts[i] == Finishes[i])
388        continue;
389
390      assert(Starts[i] && Finishes[i] && "Invalid interval");
391      VNInfo *ValNum = Intervals[i]->getValNumInfo(0);
392      SlotIndex S = Starts[i];
393      SlotIndex F = Finishes[i];
394      if (S < F) {
395        // We have a single consecutive region.
396        Intervals[i]->addRange(LiveRange(S, F, ValNum));
397      } else {
398        // We have two non consecutive regions. This happens when
399        // LIFETIME_START appears after the LIFETIME_END marker.
400        SlotIndex NewStart = Indexes->getMBBStartIdx(MBB);
401        SlotIndex NewFin = Indexes->getMBBEndIdx(MBB);
402        Intervals[i]->addRange(LiveRange(NewStart, F, ValNum));
403        Intervals[i]->addRange(LiveRange(S, NewFin, ValNum));
404      }
405    }
406  }
407}
408
409bool StackColoring::removeAllMarkers() {
410  unsigned Count = 0;
411  for (unsigned i = 0; i < Markers.size(); ++i) {
412    Markers[i]->eraseFromParent();
413    Count++;
414  }
415  Markers.clear();
416
417  DEBUG(dbgs()<<"Removed "<<Count<<" markers.\n");
418  return Count;
419}
420
421void StackColoring::remapInstructions(DenseMap<int, int> &SlotRemap) {
422  unsigned FixedInstr = 0;
423  unsigned FixedMemOp = 0;
424  unsigned FixedDbg = 0;
425  MachineModuleInfo *MMI = &MF->getMMI();
426
427  // Remap debug information that refers to stack slots.
428  MachineModuleInfo::VariableDbgInfoMapTy &VMap = MMI->getVariableDbgInfo();
429  for (MachineModuleInfo::VariableDbgInfoMapTy::iterator VI = VMap.begin(),
430       VE = VMap.end(); VI != VE; ++VI) {
431    const MDNode *Var = VI->first;
432    if (!Var) continue;
433    std::pair<unsigned, DebugLoc> &VP = VI->second;
434    if (SlotRemap.count(VP.first)) {
435      DEBUG(dbgs()<<"Remapping debug info for ["<<Var->getName()<<"].\n");
436      VP.first = SlotRemap[VP.first];
437      FixedDbg++;
438    }
439  }
440
441  // Keep a list of *allocas* which need to be remapped.
442  DenseMap<const Value*, const Value*> Allocas;
443  for (DenseMap<int, int>::iterator it = SlotRemap.begin(),
444       e = SlotRemap.end(); it != e; ++it) {
445    const Value* From = MFI->getObjectAllocation(it->first);
446    const Value* To = MFI->getObjectAllocation(it->second);
447    assert(To && From && "Invalid allocation object");
448    Allocas[From] = To;
449  }
450
451  // Remap all instructions to the new stack slots.
452  MachineFunction::iterator BB, BBE;
453  MachineBasicBlock::iterator I, IE;
454  for (BB = MF->begin(), BBE = MF->end(); BB != BBE; ++BB)
455    for (I = BB->begin(), IE = BB->end(); I != IE; ++I) {
456
457      // Update the MachineMemOperand to use the new alloca.
458      for (MachineInstr::mmo_iterator MM = I->memoperands_begin(),
459           E = I->memoperands_end(); MM != E; ++MM) {
460        MachineMemOperand *MMO = *MM;
461
462        const Value *V = MMO->getValue();
463
464        if (!V)
465          continue;
466
467        // Climb up and find the original alloca.
468        V = GetUnderlyingObject(V);
469        // If we did not find one, or if the one that we found is not in our
470        // map, then move on.
471        if (!V || !Allocas.count(V))
472          continue;
473
474        MMO->setValue(Allocas[V]);
475        FixedMemOp++;
476      }
477
478      // Update all of the machine instruction operands.
479      for (unsigned i = 0 ; i <  I->getNumOperands(); ++i) {
480        MachineOperand &MO = I->getOperand(i);
481
482        if (!MO.isFI())
483          continue;
484        int FromSlot = MO.getIndex();
485
486        // Don't touch arguments.
487        if (FromSlot<0)
488          continue;
489
490        // Only look at mapped slots.
491        if (!SlotRemap.count(FromSlot))
492          continue;
493
494        // Fix the machine instructions.
495        int ToSlot = SlotRemap[FromSlot];
496        MO.setIndex(ToSlot);
497        FixedInstr++;
498      }
499    }
500
501  DEBUG(dbgs()<<"Fixed "<<FixedMemOp<<" machine memory operands.\n");
502  DEBUG(dbgs()<<"Fixed "<<FixedDbg<<" debug locations.\n");
503  DEBUG(dbgs()<<"Fixed "<<FixedInstr<<" machine instructions.\n");
504}
505
506void StackColoring::expungeSlotMap(DenseMap<int, int> &SlotRemap,
507                                   unsigned NumSlots) {
508  // Expunge slot remap map.
509  for (unsigned i=0; i < NumSlots; ++i) {
510    // If we are remapping i
511    if (SlotRemap.count(i)) {
512      int Target = SlotRemap[i];
513      // As long as our target is mapped to something else, follow it.
514      while (SlotRemap.count(Target)) {
515        Target = SlotRemap[Target];
516        SlotRemap[i] = Target;
517      }
518    }
519  }
520}
521
522bool StackColoring::runOnMachineFunction(MachineFunction &Func) {
523  DEBUG(dbgs() << "********** Stack Coloring **********\n"
524               << "********** Function: "
525               << ((Value*)Func.getFunction())->getName() << '\n');
526  MF = &Func;
527  MFI = MF->getFrameInfo();
528  Indexes = &getAnalysis<SlotIndexes>();
529  BlockLiveness.clear();
530  BasicBlocks.clear();
531  BasicBlockNumbering.clear();
532  Markers.clear();
533  Intervals.clear();
534  VNInfoAllocator.Reset();
535
536  unsigned NumSlots = MFI->getObjectIndexEnd();
537
538  // If there are no stack slots then there are no markers to remove.
539  if (!NumSlots)
540    return false;
541
542  SmallVector<int, 8> SortedSlots;
543
544  SortedSlots.reserve(NumSlots);
545  Intervals.reserve(NumSlots);
546
547  unsigned NumMarkers = collectMarkers(NumSlots);
548
549  unsigned TotalSize = 0;
550  DEBUG(dbgs()<<"Found "<<NumMarkers<<" markers and "<<NumSlots<<" slots\n");
551  DEBUG(dbgs()<<"Slot structure:\n");
552
553  for (int i=0; i < MFI->getObjectIndexEnd(); ++i) {
554    DEBUG(dbgs()<<"Slot #"<<i<<" - "<<MFI->getObjectSize(i)<<" bytes.\n");
555    TotalSize += MFI->getObjectSize(i);
556  }
557
558  DEBUG(dbgs()<<"Total Stack size: "<<TotalSize<<" bytes\n\n");
559
560  // Don't continue because there are not enough lifetime markers, or the
561  // stack or too small, or we are told not to optimize the slots.
562  if (NumMarkers < 2 || TotalSize < 16 || DisableColoring) {
563    DEBUG(dbgs()<<"Will not try to merge slots.\n");
564    return removeAllMarkers();
565  }
566
567  for (unsigned i=0; i < NumSlots; ++i) {
568    LiveInterval *LI = new LiveInterval(i, 0);
569    Intervals.push_back(LI);
570    LI->getNextValue(Indexes->getZeroIndex(), VNInfoAllocator);
571    SortedSlots.push_back(i);
572  }
573
574  // Calculate the liveness of each block.
575  calculateLocalLiveness();
576
577  // Propagate the liveness information.
578  calculateLiveIntervals(NumSlots);
579
580  // Maps old slots to new slots.
581  DenseMap<int, int> SlotRemap;
582  unsigned RemovedSlots = 0;
583  unsigned ReducedSize = 0;
584
585  // Do not bother looking at empty intervals.
586  for (unsigned I = 0; I < NumSlots; ++I) {
587    if (Intervals[SortedSlots[I]]->empty())
588      SortedSlots[I] = -1;
589  }
590
591  // This is a simple greedy algorithm for merging allocas. First, sort the
592  // slots, placing the largest slots first. Next, perform an n^2 scan and look
593  // for disjoint slots. When you find disjoint slots, merge the samller one
594  // into the bigger one and update the live interval. Remove the small alloca
595  // and continue.
596
597  // Sort the slots according to their size. Place unused slots at the end.
598  std::sort(SortedSlots.begin(), SortedSlots.end(), SlotSizeSorter(MFI));
599
600  bool Chanded = true;
601  while (Chanded) {
602    Chanded = false;
603    for (unsigned I = 0; I < NumSlots; ++I) {
604      if (SortedSlots[I] == -1)
605        continue;
606
607      for (unsigned J=0; J < NumSlots; ++J) {
608        if (SortedSlots[J] == -1)
609          continue;
610
611        int FirstSlot = SortedSlots[I];
612        int SecondSlot = SortedSlots[J];
613        LiveInterval *First = Intervals[FirstSlot];
614        LiveInterval *Second = Intervals[SecondSlot];
615        assert (!First->empty() && !Second->empty() && "Found an empty range");
616
617        // Merge disjoint slots.
618        if (!First->overlaps(*Second)) {
619          Chanded = true;
620          First->MergeRangesInAsValue(*Second, First->getValNumInfo(0));
621          SlotRemap[SecondSlot] = FirstSlot;
622          SortedSlots[J] = -1;
623          DEBUG(dbgs()<<"Merging #"<<I<<" and slots #"<<J<<" together.\n");
624          unsigned MaxAlignment = std::max(MFI->getObjectAlignment(FirstSlot),
625                                           MFI->getObjectAlignment(SecondSlot));
626
627          assert(MFI->getObjectSize(FirstSlot) >=
628                 MFI->getObjectSize(SecondSlot) &&
629                 "Merging a small object into a larger one");
630
631          RemovedSlots+=1;
632          ReducedSize += MFI->getObjectSize(SecondSlot);
633          MFI->setObjectAlignment(FirstSlot, MaxAlignment);
634          MFI->RemoveStackObject(SecondSlot);
635        }
636      }
637    }
638  }// While changed.
639
640  // Record statistics.
641  StackSpaceSaved += ReducedSize;
642  StackSlotMerged += RemovedSlots;
643  DEBUG(dbgs()<<"Merge "<<RemovedSlots<<" slots. Saved "<<
644        ReducedSize<<" bytes\n");
645
646  // Scan the entire function and update all machine operands that use frame
647  // indices to use the remapped frame index.
648  expungeSlotMap(SlotRemap, NumSlots);
649  remapInstructions(SlotRemap);
650
651  // Release the intervals.
652  for (unsigned I = 0; I < NumSlots; ++I) {
653    delete Intervals[I];
654  }
655
656  return removeAllMarkers();
657}
658