StackColoring.cpp revision 6165dba25f3374ce340b420ab9a360623c26fdc3 
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(true), 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 we have both BEGIN and END markers in the same basic block then
319      // we know that the BEGIN marker comes after the END, because we already
320      // handle the case where the BEGIN comes before the END when collecting
321      // the markers (and building the BEGIN/END vectore).
322      // Want to enable the LIVE_IN and LIVE_OUT of slots that have both
323      // BEGIN and END because it means that the value lives before and after
324      // this basic block.
325      BitVector LocalEndBegin = BlockLiveness[BB].End;
326      LocalEndBegin &= BlockLiveness[BB].Begin;
327      LocalLiveIn |= LocalEndBegin;
328      LocalLiveOut |= LocalEndBegin;
329
330      if (LocalLiveIn.test(BlockLiveness[BB].LiveIn)) {
331        changed = true;
332        BlockLiveness[BB].LiveIn |= LocalLiveIn;
333
334        for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
335             PE = BB->pred_end(); PI != PE; ++PI)
336          NextBBSet.insert(*PI);
337      }
338
339      if (LocalLiveOut.test(BlockLiveness[BB].LiveOut)) {
340        changed = true;
341        BlockLiveness[BB].LiveOut |= LocalLiveOut;
342
343        for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
344             SE = BB->succ_end(); SI != SE; ++SI)
345          NextBBSet.insert(*SI);
346      }
347    }
348
349    BBSet = NextBBSet;
350  }// while changed.
351}
352
353void StackColoring::calculateLiveIntervals(unsigned NumSlots) {
354  SmallVector<SlotIndex, 16> Starts;
355  SmallVector<SlotIndex, 16> Finishes;
356
357  // For each block, find which slots are active within this block
358  // and update the live intervals.
359  for (MachineFunction::iterator MBB = MF->begin(), MBBe = MF->end();
360       MBB != MBBe; ++MBB) {
361    Starts.clear();
362    Starts.resize(NumSlots);
363    Finishes.clear();
364    Finishes.resize(NumSlots);
365
366    // Create the interval for the basic blocks with lifetime markers in them.
367    for (SmallVector<MachineInstr*, 8>::iterator it = Markers.begin(),
368         e = Markers.end(); it != e; ++it) {
369      MachineInstr *MI = *it;
370      if (MI->getParent() != MBB)
371        continue;
372
373      assert((MI->getOpcode() == TargetOpcode::LIFETIME_START ||
374              MI->getOpcode() == TargetOpcode::LIFETIME_END) &&
375             "Invalid Lifetime marker");
376
377      bool IsStart = MI->getOpcode() == TargetOpcode::LIFETIME_START;
378      MachineOperand &Mo = MI->getOperand(0);
379      int Slot = Mo.getIndex();
380      assert(Slot >= 0 && "Invalid slot");
381
382      SlotIndex ThisIndex = Indexes->getInstructionIndex(MI);
383
384      if (IsStart) {
385        if (!Starts[Slot].isValid() || Starts[Slot] > ThisIndex)
386          Starts[Slot] = ThisIndex;
387      } else {
388        if (!Finishes[Slot].isValid() || Finishes[Slot] < ThisIndex)
389          Finishes[Slot] = ThisIndex;
390      }
391    }
392
393    // Create the interval of the blocks that we previously found to be 'alive'.
394    BitVector Alive = BlockLiveness[MBB].LiveIn;
395    Alive |= BlockLiveness[MBB].LiveOut;
396
397    if (Alive.any()) {
398      for (int pos = Alive.find_first(); pos != -1;
399           pos = Alive.find_next(pos)) {
400        if (!Starts[pos].isValid())
401          Starts[pos] = Indexes->getMBBStartIdx(MBB);
402        if (!Finishes[pos].isValid())
403          Finishes[pos] = Indexes->getMBBEndIdx(MBB);
404      }
405    }
406
407    for (unsigned i = 0; i < NumSlots; ++i) {
408      assert(Starts[i].isValid() == Finishes[i].isValid() && "Unmatched range");
409      if (!Starts[i].isValid())
410        continue;
411
412      assert(Starts[i] && Finishes[i] && "Invalid interval");
413      VNInfo *ValNum = Intervals[i]->getValNumInfo(0);
414      SlotIndex S = Starts[i];
415      SlotIndex F = Finishes[i];
416      if (S < F) {
417        // We have a single consecutive region.
418        Intervals[i]->addRange(LiveRange(S, F, ValNum));
419      } else {
420        // We have two non consecutive regions. This happens when
421        // LIFETIME_START appears after the LIFETIME_END marker.
422        SlotIndex NewStart = Indexes->getMBBStartIdx(MBB);
423        SlotIndex NewFin = Indexes->getMBBEndIdx(MBB);
424        Intervals[i]->addRange(LiveRange(NewStart, F, ValNum));
425        Intervals[i]->addRange(LiveRange(S, NewFin, ValNum));
426      }
427    }
428  }
429}
430
431bool StackColoring::removeAllMarkers() {
432  unsigned Count = 0;
433  for (unsigned i = 0; i < Markers.size(); ++i) {
434    Markers[i]->eraseFromParent();
435    Count++;
436  }
437  Markers.clear();
438
439  DEBUG(dbgs()<<"Removed "<<Count<<" markers.\n");
440  return Count;
441}
442
443void StackColoring::remapInstructions(DenseMap<int, int> &SlotRemap) {
444  unsigned FixedInstr = 0;
445  unsigned FixedMemOp = 0;
446  unsigned FixedDbg = 0;
447  MachineModuleInfo *MMI = &MF->getMMI();
448
449  // Remap debug information that refers to stack slots.
450  MachineModuleInfo::VariableDbgInfoMapTy &VMap = MMI->getVariableDbgInfo();
451  for (MachineModuleInfo::VariableDbgInfoMapTy::iterator VI = VMap.begin(),
452       VE = VMap.end(); VI != VE; ++VI) {
453    const MDNode *Var = VI->first;
454    if (!Var) continue;
455    std::pair<unsigned, DebugLoc> &VP = VI->second;
456    if (SlotRemap.count(VP.first)) {
457      DEBUG(dbgs()<<"Remapping debug info for ["<<Var->getName()<<"].\n");
458      VP.first = SlotRemap[VP.first];
459      FixedDbg++;
460    }
461  }
462
463  // Keep a list of *allocas* which need to be remapped.
464  DenseMap<const Value*, const Value*> Allocas;
465  for (DenseMap<int, int>::iterator it = SlotRemap.begin(),
466       e = SlotRemap.end(); it != e; ++it) {
467    const Value *From = MFI->getObjectAllocation(it->first);
468    const Value *To = MFI->getObjectAllocation(it->second);
469    assert(To && From && "Invalid allocation object");
470    Allocas[From] = To;
471  }
472
473  // Remap all instructions to the new stack slots.
474  MachineFunction::iterator BB, BBE;
475  MachineBasicBlock::iterator I, IE;
476  for (BB = MF->begin(), BBE = MF->end(); BB != BBE; ++BB)
477    for (I = BB->begin(), IE = BB->end(); I != IE; ++I) {
478
479      // Skip lifetime markers. We'll remove them soon.
480      if (I->getOpcode() == TargetOpcode::LIFETIME_START ||
481          I->getOpcode() == TargetOpcode::LIFETIME_END)
482        continue;
483
484      // Update the MachineMemOperand to use the new alloca.
485      for (MachineInstr::mmo_iterator MM = I->memoperands_begin(),
486           E = I->memoperands_end(); MM != E; ++MM) {
487        MachineMemOperand *MMO = *MM;
488
489        const Value *V = MMO->getValue();
490
491        if (!V)
492          continue;
493
494        // Climb up and find the original alloca.
495        V = GetUnderlyingObject(V);
496        // If we did not find one, or if the one that we found is not in our
497        // map, then move on.
498        if (!V || !Allocas.count(V))
499          continue;
500
501        MMO->setValue(Allocas[V]);
502        FixedMemOp++;
503      }
504
505      // Update all of the machine instruction operands.
506      for (unsigned i = 0 ; i <  I->getNumOperands(); ++i) {
507        MachineOperand &MO = I->getOperand(i);
508
509        if (!MO.isFI())
510          continue;
511        int FromSlot = MO.getIndex();
512
513        // Don't touch arguments.
514        if (FromSlot<0)
515          continue;
516
517        // Only look at mapped slots.
518        if (!SlotRemap.count(FromSlot))
519          continue;
520
521        // In a debug build, check that the instruction that we are modifying is
522        // inside the expected live range. If the instruction is not inside
523        // the calculated range then it means that the alloca usage moved
524        // outside of the lifetime markers.
525#ifndef NDEBUG
526        SlotIndex Index = Indexes->getInstructionIndex(I);
527        LiveInterval* Interval = Intervals[FromSlot];
528        assert(Interval->find(Index) != Interval->end() &&
529               "Found instruction usage outside of live range.");
530#endif
531
532        // Fix the machine instructions.
533        int ToSlot = SlotRemap[FromSlot];
534        MO.setIndex(ToSlot);
535        FixedInstr++;
536      }
537    }
538
539  DEBUG(dbgs()<<"Fixed "<<FixedMemOp<<" machine memory operands.\n");
540  DEBUG(dbgs()<<"Fixed "<<FixedDbg<<" debug locations.\n");
541  DEBUG(dbgs()<<"Fixed "<<FixedInstr<<" machine instructions.\n");
542}
543
544void StackColoring::expungeSlotMap(DenseMap<int, int> &SlotRemap,
545                                   unsigned NumSlots) {
546  // Expunge slot remap map.
547  for (unsigned i=0; i < NumSlots; ++i) {
548    // If we are remapping i
549    if (SlotRemap.count(i)) {
550      int Target = SlotRemap[i];
551      // As long as our target is mapped to something else, follow it.
552      while (SlotRemap.count(Target)) {
553        Target = SlotRemap[Target];
554        SlotRemap[i] = Target;
555      }
556    }
557  }
558}
559
560bool StackColoring::runOnMachineFunction(MachineFunction &Func) {
561  DEBUG(dbgs() << "********** Stack Coloring **********\n"
562               << "********** Function: "
563               << ((const Value*)Func.getFunction())->getName() << '\n');
564  MF = &Func;
565  MFI = MF->getFrameInfo();
566  Indexes = &getAnalysis<SlotIndexes>();
567  BlockLiveness.clear();
568  BasicBlocks.clear();
569  BasicBlockNumbering.clear();
570  Markers.clear();
571  Intervals.clear();
572  VNInfoAllocator.Reset();
573
574  unsigned NumSlots = MFI->getObjectIndexEnd();
575
576  // If there are no stack slots then there are no markers to remove.
577  if (!NumSlots)
578    return false;
579
580  SmallVector<int, 8> SortedSlots;
581
582  SortedSlots.reserve(NumSlots);
583  Intervals.reserve(NumSlots);
584
585  unsigned NumMarkers = collectMarkers(NumSlots);
586
587  unsigned TotalSize = 0;
588  DEBUG(dbgs()<<"Found "<<NumMarkers<<" markers and "<<NumSlots<<" slots\n");
589  DEBUG(dbgs()<<"Slot structure:\n");
590
591  for (int i=0; i < MFI->getObjectIndexEnd(); ++i) {
592    DEBUG(dbgs()<<"Slot #"<<i<<" - "<<MFI->getObjectSize(i)<<" bytes.\n");
593    TotalSize += MFI->getObjectSize(i);
594  }
595
596  DEBUG(dbgs()<<"Total Stack size: "<<TotalSize<<" bytes\n\n");
597
598  // Don't continue because there are not enough lifetime markers, or the
599  // stack or too small, or we are told not to optimize the slots.
600  if (NumMarkers < 2 || TotalSize < 16 || DisableColoring) {
601    DEBUG(dbgs()<<"Will not try to merge slots.\n");
602    return removeAllMarkers();
603  }
604
605  for (unsigned i=0; i < NumSlots; ++i) {
606    LiveInterval *LI = new LiveInterval(i, 0);
607    Intervals.push_back(LI);
608    LI->getNextValue(Indexes->getZeroIndex(), VNInfoAllocator);
609    SortedSlots.push_back(i);
610  }
611
612  // Calculate the liveness of each block.
613  calculateLocalLiveness();
614
615  // Propagate the liveness information.
616  calculateLiveIntervals(NumSlots);
617
618  // Maps old slots to new slots.
619  DenseMap<int, int> SlotRemap;
620  unsigned RemovedSlots = 0;
621  unsigned ReducedSize = 0;
622
623  // Do not bother looking at empty intervals.
624  for (unsigned I = 0; I < NumSlots; ++I) {
625    if (Intervals[SortedSlots[I]]->empty())
626      SortedSlots[I] = -1;
627  }
628
629  // This is a simple greedy algorithm for merging allocas. First, sort the
630  // slots, placing the largest slots first. Next, perform an n^2 scan and look
631  // for disjoint slots. When you find disjoint slots, merge the samller one
632  // into the bigger one and update the live interval. Remove the small alloca
633  // and continue.
634
635  // Sort the slots according to their size. Place unused slots at the end.
636  std::sort(SortedSlots.begin(), SortedSlots.end(), SlotSizeSorter(MFI));
637
638  bool Chanded = true;
639  while (Chanded) {
640    Chanded = false;
641    for (unsigned I = 0; I < NumSlots; ++I) {
642      if (SortedSlots[I] == -1)
643        continue;
644
645      for (unsigned J=I+1; J < NumSlots; ++J) {
646        if (SortedSlots[J] == -1)
647          continue;
648
649        int FirstSlot = SortedSlots[I];
650        int SecondSlot = SortedSlots[J];
651        LiveInterval *First = Intervals[FirstSlot];
652        LiveInterval *Second = Intervals[SecondSlot];
653        assert (!First->empty() && !Second->empty() && "Found an empty range");
654
655        // Merge disjoint slots.
656        if (!First->overlaps(*Second)) {
657          Chanded = true;
658          First->MergeRangesInAsValue(*Second, First->getValNumInfo(0));
659          SlotRemap[SecondSlot] = FirstSlot;
660          SortedSlots[J] = -1;
661          DEBUG(dbgs()<<"Merging #"<<FirstSlot<<" and slots #"<<
662                SecondSlot<<" together.\n");
663          unsigned MaxAlignment = std::max(MFI->getObjectAlignment(FirstSlot),
664                                           MFI->getObjectAlignment(SecondSlot));
665
666          assert(MFI->getObjectSize(FirstSlot) >=
667                 MFI->getObjectSize(SecondSlot) &&
668                 "Merging a small object into a larger one");
669
670          RemovedSlots+=1;
671          ReducedSize += MFI->getObjectSize(SecondSlot);
672          MFI->setObjectAlignment(FirstSlot, MaxAlignment);
673          MFI->RemoveStackObject(SecondSlot);
674        }
675      }
676    }
677  }// While changed.
678
679  // Record statistics.
680  StackSpaceSaved += ReducedSize;
681  StackSlotMerged += RemovedSlots;
682  DEBUG(dbgs()<<"Merge "<<RemovedSlots<<" slots. Saved "<<
683        ReducedSize<<" bytes\n");
684
685  // Scan the entire function and update all machine operands that use frame
686  // indices to use the remapped frame index.
687  expungeSlotMap(SlotRemap, NumSlots);
688  remapInstructions(SlotRemap);
689
690  // Release the intervals.
691  for (unsigned I = 0; I < NumSlots; ++I) {
692    delete Intervals[I];
693  }
694
695  return removeAllMarkers();
696}
697