StackSlotColoring.cpp revision 04fa35ab13afbbc5b2f12437a256db84a27485d2 
1//===-- StackSlotColoring.cpp - Stack slot coloring pass. -----------------===//
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 file implements the stack slot coloring pass.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "stackcoloring"
15#include "VirtRegMap.h"
16#include "llvm/CodeGen/Passes.h"
17#include "llvm/CodeGen/LiveIntervalAnalysis.h"
18#include "llvm/CodeGen/LiveStackAnalysis.h"
19#include "llvm/CodeGen/MachineFrameInfo.h"
20#include "llvm/CodeGen/MachineLoopInfo.h"
21#include "llvm/CodeGen/MachineRegisterInfo.h"
22#include "llvm/CodeGen/PseudoSourceValue.h"
23#include "llvm/Support/CommandLine.h"
24#include "llvm/Support/Compiler.h"
25#include "llvm/Support/Debug.h"
26#include "llvm/Target/TargetInstrInfo.h"
27#include "llvm/Target/TargetMachine.h"
28#include "llvm/ADT/BitVector.h"
29#include "llvm/ADT/SmallVector.h"
30#include "llvm/ADT/Statistic.h"
31#include <vector>
32using namespace llvm;
33
34static cl::opt<bool>
35DisableSharing("no-stack-slot-sharing",
36             cl::init(false), cl::Hidden,
37             cl::desc("Suppress slot sharing during stack coloring"));
38
39static cl::opt<bool>
40ColorWithRegs("color-ss-with-regs",
41             cl::init(false), cl::Hidden,
42             cl::desc("Color stack slots with free registers"));
43
44
45static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
46
47STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
48STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
49STATISTIC(NumRegRepl,    "Number of stack slot refs replaced with reg refs");
50
51namespace {
52  class VISIBILITY_HIDDEN StackSlotColoring : public MachineFunctionPass {
53    LiveStacks* LS;
54    VirtRegMap* VRM;
55    MachineFrameInfo *MFI;
56    MachineRegisterInfo *MRI;
57    const TargetInstrInfo  *TII;
58    const TargetRegisterInfo *TRI;
59    const MachineLoopInfo *loopInfo;
60
61    // SSIntervals - Spill slot intervals.
62    std::vector<LiveInterval*> SSIntervals;
63
64    // SSRefs - Keep a list of frame index references for each spill slot.
65    SmallVector<SmallVector<MachineInstr*, 8>, 16> SSRefs;
66
67    // OrigAlignments - Alignments of stack objects before coloring.
68    SmallVector<unsigned, 16> OrigAlignments;
69
70    // OrigSizes - Sizess of stack objects before coloring.
71    SmallVector<unsigned, 16> OrigSizes;
72
73    // AllColors - If index is set, it's a spill slot, i.e. color.
74    // FIXME: This assumes PEI locate spill slot with smaller indices
75    // closest to stack pointer / frame pointer. Therefore, smaller
76    // index == better color.
77    BitVector AllColors;
78
79    // NextColor - Next "color" that's not yet used.
80    int NextColor;
81
82    // UsedColors - "Colors" that have been assigned.
83    BitVector UsedColors;
84
85    // Assignments - Color to intervals mapping.
86    SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
87
88  public:
89    static char ID; // Pass identification
90    StackSlotColoring() : MachineFunctionPass(&ID), NextColor(-1) {}
91
92    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
93      AU.addRequired<LiveStacks>();
94      AU.addRequired<VirtRegMap>();
95      AU.addPreserved<VirtRegMap>();
96      AU.addRequired<MachineLoopInfo>();
97      AU.addPreserved<MachineLoopInfo>();
98      AU.addPreservedID(MachineDominatorsID);
99      MachineFunctionPass::getAnalysisUsage(AU);
100    }
101
102    virtual bool runOnMachineFunction(MachineFunction &MF);
103    virtual const char* getPassName() const {
104      return "Stack Slot Coloring";
105    }
106
107  private:
108    void InitializeSlots();
109    void ScanForSpillSlotRefs(MachineFunction &MF);
110    bool OverlapWithAssignments(LiveInterval *li, int Color) const;
111    int ColorSlot(LiveInterval *li);
112    bool ColorSlots(MachineFunction &MF);
113    bool ColorSlotsWithFreeRegs(SmallVector<int, 16> &SlotMapping,
114                                SmallVector<SmallVector<int, 4>, 16> &RevMap,
115                                BitVector &SlotIsReg);
116    void RewriteInstruction(MachineInstr *MI, int OldFI, int NewFI,
117                            MachineFunction &MF);
118    void UnfoldAndRewriteInstruction(MachineInstr *MI, int OldFI,
119                                     unsigned Reg, MachineFunction &MF);
120    bool AllMemRefsCanBeUnfolded(int SS);
121    bool RemoveDeadStores(MachineBasicBlock* MBB);
122  };
123} // end anonymous namespace
124
125char StackSlotColoring::ID = 0;
126
127static RegisterPass<StackSlotColoring>
128X("stack-slot-coloring", "Stack Slot Coloring");
129
130FunctionPass *llvm::createStackSlotColoringPass() {
131  return new StackSlotColoring();
132}
133
134namespace {
135  // IntervalSorter - Comparison predicate that sort live intervals by
136  // their weight.
137  struct IntervalSorter {
138    bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
139      return LHS->weight > RHS->weight;
140    }
141  };
142}
143
144/// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
145/// references and update spill slot weights.
146void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
147  SSRefs.resize(MFI->getObjectIndexEnd());
148
149  // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
150  for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
151       MBBI != E; ++MBBI) {
152    MachineBasicBlock *MBB = &*MBBI;
153    unsigned loopDepth = loopInfo->getLoopDepth(MBB);
154    for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
155         MII != EE; ++MII) {
156      MachineInstr *MI = &*MII;
157      for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
158        MachineOperand &MO = MI->getOperand(i);
159        if (!MO.isFI())
160          continue;
161        int FI = MO.getIndex();
162        if (FI < 0)
163          continue;
164        if (!LS->hasInterval(FI))
165          continue;
166        LiveInterval &li = LS->getInterval(FI);
167        li.weight += LiveIntervals::getSpillWeight(false, true, loopDepth);
168        SSRefs[FI].push_back(MI);
169      }
170    }
171  }
172}
173
174/// InitializeSlots - Process all spill stack slot liveintervals and add them
175/// to a sorted (by weight) list.
176void StackSlotColoring::InitializeSlots() {
177  int LastFI = MFI->getObjectIndexEnd();
178  OrigAlignments.resize(LastFI);
179  OrigSizes.resize(LastFI);
180  AllColors.resize(LastFI);
181  UsedColors.resize(LastFI);
182  Assignments.resize(LastFI);
183
184  // Gather all spill slots into a list.
185  DOUT << "Spill slot intervals:\n";
186  for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
187    LiveInterval &li = i->second;
188    DEBUG(li.dump());
189    int FI = li.getStackSlotIndex();
190    if (MFI->isDeadObjectIndex(FI))
191      continue;
192    SSIntervals.push_back(&li);
193    OrigAlignments[FI] = MFI->getObjectAlignment(FI);
194    OrigSizes[FI]      = MFI->getObjectSize(FI);
195    AllColors.set(FI);
196  }
197  DOUT << '\n';
198
199  // Sort them by weight.
200  std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
201
202  // Get first "color".
203  NextColor = AllColors.find_first();
204}
205
206/// OverlapWithAssignments - Return true if LiveInterval overlaps with any
207/// LiveIntervals that have already been assigned to the specified color.
208bool
209StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
210  const SmallVector<LiveInterval*,4> &OtherLIs = Assignments[Color];
211  for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
212    LiveInterval *OtherLI = OtherLIs[i];
213    if (OtherLI->overlaps(*li))
214      return true;
215  }
216  return false;
217}
218
219/// ColorSlotsWithFreeRegs - If there are any free registers available, try
220/// replacing spill slots references with registers instead.
221bool
222StackSlotColoring::ColorSlotsWithFreeRegs(SmallVector<int, 16> &SlotMapping,
223                                   SmallVector<SmallVector<int, 4>, 16> &RevMap,
224                                   BitVector &SlotIsReg) {
225  if (!ColorWithRegs || !VRM->HasUnusedRegisters())
226    return false;
227
228  bool Changed = false;
229  DOUT << "Assigning unused registers to spill slots:\n";
230  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
231    LiveInterval *li = SSIntervals[i];
232    int SS = li->getStackSlotIndex();
233    if (!UsedColors[SS])
234      continue;
235    // Get the largest common sub- register class of all the stack slots that
236    // are colored to this stack slot.
237    const TargetRegisterClass *RC = 0;
238    for (unsigned j = 0, ee = RevMap[SS].size(); j != ee; ++j) {
239      int RSS = RevMap[SS][j];
240      const TargetRegisterClass *RRC = LS->getIntervalRegClass(RSS);
241      if (!RC)
242        RC = RRC;
243      else
244        RC = getCommonSubClass(RC, RRC);
245    }
246
247    // If it's not colored to another stack slot, try coloring it
248    // to a "free" register.
249    if (!RC)
250      continue;
251    unsigned Reg = VRM->getFirstUnusedRegister(RC);
252    if (!Reg)
253      continue;
254    bool IsSafe = true;
255    for (unsigned j = 0, ee = RevMap[SS].size(); j != ee; ++j) {
256      int RSS = RevMap[SS][j];
257      if (!AllMemRefsCanBeUnfolded(RSS)) {
258        IsSafe = false;
259        break;
260      }
261    }
262    if (!IsSafe)
263      // Try color the next spill slot.
264      continue;
265
266    DOUT << "Assigning fi#" << SS << " to " << TRI->getName(Reg)
267         << ", which in turn means...\n";
268    // Register and its sub-registers are no longer free.
269    VRM->setRegisterUsed(Reg);
270    // If reg is a callee-saved register, it will have to be spilled in
271    // the prologue.
272    MRI->setPhysRegUsed(Reg);
273    for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS) {
274      VRM->setRegisterUsed(*AS);
275      MRI->setPhysRegUsed(*AS);
276    }
277    // This spill slot is dead after the rewrites
278    MFI->RemoveStackObject(SS);
279
280    // Remember all these FI references will have to be unfolded.
281    for (unsigned j = 0, ee = RevMap[SS].size(); j != ee; ++j) {
282      int RSS = RevMap[SS][j];
283      DOUT << "  Assigning fi#" << RSS << " to " << TRI->getName(Reg) << '\n';
284      SlotMapping[RSS] = Reg;
285      SlotIsReg.set(RSS);
286    }
287
288    ++NumEliminated;
289    Changed = true;
290  }
291  DOUT << '\n';
292
293  return Changed;
294}
295
296/// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
297///
298int StackSlotColoring::ColorSlot(LiveInterval *li) {
299  int Color = -1;
300  bool Share = false;
301  if (!DisableSharing) {
302    // Check if it's possible to reuse any of the used colors.
303    Color = UsedColors.find_first();
304    while (Color != -1) {
305      if (!OverlapWithAssignments(li, Color)) {
306        Share = true;
307        ++NumEliminated;
308        break;
309      }
310      Color = UsedColors.find_next(Color);
311    }
312  }
313
314  // Assign it to the first available color (assumed to be the best) if it's
315  // not possible to share a used color with other objects.
316  if (!Share) {
317    assert(NextColor != -1 && "No more spill slots?");
318    Color = NextColor;
319    UsedColors.set(Color);
320    NextColor = AllColors.find_next(NextColor);
321  }
322
323  // Record the assignment.
324  Assignments[Color].push_back(li);
325  int FI = li->getStackSlotIndex();
326  DOUT << "Assigning fi#" << FI << " to fi#" << Color << "\n";
327
328  // Change size and alignment of the allocated slot. If there are multiple
329  // objects sharing the same slot, then make sure the size and alignment
330  // are large enough for all.
331  unsigned Align = OrigAlignments[FI];
332  if (!Share || Align > MFI->getObjectAlignment(Color))
333    MFI->setObjectAlignment(Color, Align);
334  int64_t Size = OrigSizes[FI];
335  if (!Share || Size > MFI->getObjectSize(Color))
336    MFI->setObjectSize(Color, Size);
337  return Color;
338}
339
340/// Colorslots - Color all spill stack slots and rewrite all frameindex machine
341/// operands in the function.
342bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
343  unsigned NumObjs = MFI->getObjectIndexEnd();
344  SmallVector<int, 16> SlotMapping(NumObjs, -1);
345  SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
346  SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
347  BitVector SlotIsReg(NumObjs);
348  BitVector UsedColors(NumObjs);
349
350  DOUT << "Color spill slot intervals:\n";
351  bool Changed = false;
352  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
353    LiveInterval *li = SSIntervals[i];
354    int SS = li->getStackSlotIndex();
355    int NewSS = ColorSlot(li);
356    assert(NewSS >= 0 && "Stack coloring failed?");
357    SlotMapping[SS] = NewSS;
358    RevMap[NewSS].push_back(SS);
359    SlotWeights[NewSS] += li->weight;
360    UsedColors.set(NewSS);
361    Changed |= (SS != NewSS);
362  }
363
364  DOUT << "\nSpill slots after coloring:\n";
365  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
366    LiveInterval *li = SSIntervals[i];
367    int SS = li->getStackSlotIndex();
368    li->weight = SlotWeights[SS];
369  }
370  // Sort them by new weight.
371  std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
372
373#ifndef NDEBUG
374  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
375    DEBUG(SSIntervals[i]->dump());
376  DOUT << '\n';
377#endif
378
379  // Can we "color" a stack slot with a unused register?
380  Changed |= ColorSlotsWithFreeRegs(SlotMapping, RevMap, SlotIsReg);
381
382  if (!Changed)
383    return false;
384
385  // Rewrite all MO_FrameIndex operands.
386  for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
387    bool isReg = SlotIsReg[SS];
388    int NewFI = SlotMapping[SS];
389    if (NewFI == -1 || (NewFI == (int)SS && !isReg))
390      continue;
391
392    SmallVector<MachineInstr*, 8> &RefMIs = SSRefs[SS];
393    for (unsigned i = 0, e = RefMIs.size(); i != e; ++i)
394      if (isReg)
395        // Rewrite to use a register instead.
396        UnfoldAndRewriteInstruction(RefMIs[i], SS, NewFI, MF);
397      else
398        RewriteInstruction(RefMIs[i], SS, NewFI, MF);
399  }
400
401  // Delete unused stack slots.
402  while (NextColor != -1) {
403    DOUT << "Removing unused stack object fi#" << NextColor << "\n";
404    MFI->RemoveStackObject(NextColor);
405    NextColor = AllColors.find_next(NextColor);
406  }
407
408  return true;
409}
410
411/// AllMemRefsCanBeUnfolded - Return true if all references of the specified
412/// spill slot index can be unfolded.
413bool StackSlotColoring::AllMemRefsCanBeUnfolded(int SS) {
414  SmallVector<MachineInstr*, 8> &RefMIs = SSRefs[SS];
415  for (unsigned i = 0, e = RefMIs.size(); i != e; ++i) {
416    MachineInstr *MI = RefMIs[i];
417    if (!TII->getOpcodeAfterMemoryUnfold(MI->getOpcode(), false, false))
418      return false;
419    for (unsigned j = 0, ee = MI->getNumOperands(); j != ee; ++j) {
420      MachineOperand &MO = MI->getOperand(j);
421      if (MO.isFI() && MO.getIndex() != SS)
422        // If it uses another frameindex, we can, currently* unfold it.
423        return false;
424    }
425  }
426  return true;
427}
428
429/// RewriteInstruction - Rewrite specified instruction by replacing references
430/// to old frame index with new one.
431void StackSlotColoring::RewriteInstruction(MachineInstr *MI, int OldFI,
432                                           int NewFI, MachineFunction &MF) {
433  for (unsigned i = 0, ee = MI->getNumOperands(); i != ee; ++i) {
434    MachineOperand &MO = MI->getOperand(i);
435    if (!MO.isFI())
436      continue;
437    int FI = MO.getIndex();
438    if (FI != OldFI)
439      continue;
440    MO.setIndex(NewFI);
441  }
442
443  // Update the MachineMemOperand for the new memory location.
444  // FIXME: We need a better method of managing these too.
445  SmallVector<MachineMemOperand, 2> MMOs(MI->memoperands_begin(),
446                                         MI->memoperands_end());
447  MI->clearMemOperands(MF);
448  const Value *OldSV = PseudoSourceValue::getFixedStack(OldFI);
449  for (unsigned i = 0, ee = MMOs.size(); i != ee; ++i) {
450    if (MMOs[i].getValue() != OldSV)
451      MI->addMemOperand(MF, MMOs[i]);
452    else {
453      MachineMemOperand MMO(PseudoSourceValue::getFixedStack(NewFI),
454                            MMOs[i].getFlags(), MMOs[i].getOffset(),
455                            MMOs[i].getSize(),  MMOs[i].getAlignment());
456      MI->addMemOperand(MF, MMO);
457    }
458  }
459}
460
461/// UnfoldAndRewriteInstruction - Rewrite specified instruction by unfolding
462/// folded memory references and replacing those references with register
463/// references instead.
464void StackSlotColoring::UnfoldAndRewriteInstruction(MachineInstr *MI, int OldFI,
465                                                    unsigned Reg,
466                                                    MachineFunction &MF) {
467  MachineBasicBlock *MBB = MI->getParent();
468  SmallVector<MachineInstr*, 4> NewMIs;
469  bool Success = TII->unfoldMemoryOperand(MF, MI, Reg, false, false, NewMIs);
470  assert(Success && "Failed to unfold!");
471  MBB->insert(MI, NewMIs[0]);
472  MBB->erase(MI);
473  ++NumRegRepl;
474}
475
476/// RemoveDeadStores - Scan through a basic block and look for loads followed
477/// by stores.  If they're both using the same stack slot, then the store is
478/// definitely dead.  This could obviously be much more aggressive (consider
479/// pairs with instructions between them), but such extensions might have a
480/// considerable compile time impact.
481bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
482  // FIXME: This could be much more aggressive, but we need to investigate
483  // the compile time impact of doing so.
484  bool changed = false;
485
486  SmallVector<MachineInstr*, 4> toErase;
487
488  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
489       I != E; ++I) {
490    if (DCELimit != -1 && (int)NumDead >= DCELimit)
491      break;
492
493    MachineBasicBlock::iterator NextMI = next(I);
494    if (NextMI == MBB->end()) continue;
495
496    int FirstSS, SecondSS;
497    unsigned LoadReg = 0;
498    unsigned StoreReg = 0;
499    if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
500    if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
501    if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
502
503    ++NumDead;
504    changed = true;
505
506    if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
507      ++NumDead;
508      toErase.push_back(I);
509    }
510
511    toErase.push_back(NextMI);
512    ++I;
513  }
514
515  for (SmallVector<MachineInstr*, 4>::iterator I = toErase.begin(),
516       E = toErase.end(); I != E; ++I)
517    (*I)->eraseFromParent();
518
519  return changed;
520}
521
522
523bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
524  DOUT << "********** Stack Slot Coloring **********\n";
525
526  MFI = MF.getFrameInfo();
527  MRI = &MF.getRegInfo();
528  TII = MF.getTarget().getInstrInfo();
529  TRI = MF.getTarget().getRegisterInfo();
530  LS = &getAnalysis<LiveStacks>();
531  VRM = &getAnalysis<VirtRegMap>();
532  loopInfo = &getAnalysis<MachineLoopInfo>();
533
534  bool Changed = false;
535
536  unsigned NumSlots = LS->getNumIntervals();
537  if (NumSlots < 2) {
538    if (NumSlots == 0 || !VRM->HasUnusedRegisters())
539      // Nothing to do!
540      return false;
541  }
542
543  // Gather spill slot references
544  ScanForSpillSlotRefs(MF);
545  InitializeSlots();
546  Changed = ColorSlots(MF);
547
548  NextColor = -1;
549  SSIntervals.clear();
550  for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
551    SSRefs[i].clear();
552  SSRefs.clear();
553  OrigAlignments.clear();
554  OrigSizes.clear();
555  AllColors.clear();
556  UsedColors.clear();
557  for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
558    Assignments[i].clear();
559  Assignments.clear();
560
561  if (Changed) {
562    for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
563      Changed |= RemoveDeadStores(I);
564  }
565
566  return Changed;
567}
568