StackSlotColoring.cpp revision 71804149a3a6f6c081b874869b27fafe7d3288ce 
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 "stackslotcoloring"
15#include "llvm/CodeGen/Passes.h"
16#include "llvm/ADT/BitVector.h"
17#include "llvm/ADT/SmallVector.h"
18#include "llvm/ADT/Statistic.h"
19#include "llvm/CodeGen/LiveIntervalAnalysis.h"
20#include "llvm/CodeGen/LiveStackAnalysis.h"
21#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
22#include "llvm/CodeGen/MachineFrameInfo.h"
23#include "llvm/CodeGen/MachineInstrBuilder.h"
24#include "llvm/CodeGen/MachineMemOperand.h"
25#include "llvm/CodeGen/MachineRegisterInfo.h"
26#include "llvm/CodeGen/PseudoSourceValue.h"
27#include "llvm/IR/Module.h"
28#include "llvm/Support/CommandLine.h"
29#include "llvm/Support/Debug.h"
30#include "llvm/Support/raw_ostream.h"
31#include "llvm/Target/TargetInstrInfo.h"
32#include "llvm/Target/TargetMachine.h"
33#include <vector>
34using namespace llvm;
35
36static cl::opt<bool>
37DisableSharing("no-stack-slot-sharing",
38             cl::init(false), cl::Hidden,
39             cl::desc("Suppress slot sharing during stack coloring"));
40
41static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
42
43STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
44STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
45
46namespace {
47  class StackSlotColoring : public MachineFunctionPass {
48    LiveStacks* LS;
49    MachineFrameInfo *MFI;
50    const TargetInstrInfo  *TII;
51    const MachineBlockFrequencyInfo *MBFI;
52
53    // SSIntervals - Spill slot intervals.
54    std::vector<LiveInterval*> SSIntervals;
55
56    // SSRefs - Keep a list of MachineMemOperands for each spill slot.
57    // MachineMemOperands can be shared between instructions, so we need
58    // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
59    // become FI0 -> FI1 -> FI2.
60    SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
61
62    // OrigAlignments - Alignments of stack objects before coloring.
63    SmallVector<unsigned, 16> OrigAlignments;
64
65    // OrigSizes - Sizess of stack objects before coloring.
66    SmallVector<unsigned, 16> OrigSizes;
67
68    // AllColors - If index is set, it's a spill slot, i.e. color.
69    // FIXME: This assumes PEI locate spill slot with smaller indices
70    // closest to stack pointer / frame pointer. Therefore, smaller
71    // index == better color.
72    BitVector AllColors;
73
74    // NextColor - Next "color" that's not yet used.
75    int NextColor;
76
77    // UsedColors - "Colors" that have been assigned.
78    BitVector UsedColors;
79
80    // Assignments - Color to intervals mapping.
81    SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
82
83  public:
84    static char ID; // Pass identification
85    StackSlotColoring() :
86      MachineFunctionPass(ID), NextColor(-1) {
87        initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
88      }
89
90    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
91      AU.setPreservesCFG();
92      AU.addRequired<SlotIndexes>();
93      AU.addPreserved<SlotIndexes>();
94      AU.addRequired<LiveStacks>();
95      AU.addRequired<MachineBlockFrequencyInfo>();
96      AU.addPreserved<MachineBlockFrequencyInfo>();
97      AU.addPreservedID(MachineDominatorsID);
98      MachineFunctionPass::getAnalysisUsage(AU);
99    }
100
101    virtual bool runOnMachineFunction(MachineFunction &MF);
102
103  private:
104    void InitializeSlots();
105    void ScanForSpillSlotRefs(MachineFunction &MF);
106    bool OverlapWithAssignments(LiveInterval *li, int Color) const;
107    int ColorSlot(LiveInterval *li);
108    bool ColorSlots(MachineFunction &MF);
109    void RewriteInstruction(MachineInstr *MI, SmallVector<int, 16> &SlotMapping,
110                            MachineFunction &MF);
111    bool RemoveDeadStores(MachineBasicBlock* MBB);
112  };
113} // end anonymous namespace
114
115char StackSlotColoring::ID = 0;
116char &llvm::StackSlotColoringID = StackSlotColoring::ID;
117
118INITIALIZE_PASS_BEGIN(StackSlotColoring, "stack-slot-coloring",
119                "Stack Slot Coloring", false, false)
120INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
121INITIALIZE_PASS_DEPENDENCY(LiveStacks)
122INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
123INITIALIZE_PASS_END(StackSlotColoring, "stack-slot-coloring",
124                "Stack Slot Coloring", false, false)
125
126namespace {
127  // IntervalSorter - Comparison predicate that sort live intervals by
128  // their weight.
129  struct IntervalSorter {
130    bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
131      return LHS->weight > RHS->weight;
132    }
133  };
134}
135
136/// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
137/// references and update spill slot weights.
138void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
139  SSRefs.resize(MFI->getObjectIndexEnd());
140
141  // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
142  for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
143       MBBI != E; ++MBBI) {
144    MachineBasicBlock *MBB = &*MBBI;
145    BlockFrequency Freq = MBFI->getBlockFreq(MBB);
146    for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
147         MII != EE; ++MII) {
148      MachineInstr *MI = &*MII;
149      for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
150        MachineOperand &MO = MI->getOperand(i);
151        if (!MO.isFI())
152          continue;
153        int FI = MO.getIndex();
154        if (FI < 0)
155          continue;
156        if (!LS->hasInterval(FI))
157          continue;
158        LiveInterval &li = LS->getInterval(FI);
159        if (!MI->isDebugValue())
160          li.weight += LiveIntervals::getSpillWeight(false, true, Freq);
161      }
162      for (MachineInstr::mmo_iterator MMOI = MI->memoperands_begin(),
163           EE = MI->memoperands_end(); MMOI != EE; ++MMOI) {
164        MachineMemOperand *MMO = *MMOI;
165        if (const Value *V = MMO->getValue()) {
166          if (const FixedStackPseudoSourceValue *FSV =
167              dyn_cast<FixedStackPseudoSourceValue>(V)) {
168            int FI = FSV->getFrameIndex();
169            if (FI >= 0)
170              SSRefs[FI].push_back(MMO);
171          }
172        }
173      }
174    }
175  }
176}
177
178/// InitializeSlots - Process all spill stack slot liveintervals and add them
179/// to a sorted (by weight) list.
180void StackSlotColoring::InitializeSlots() {
181  int LastFI = MFI->getObjectIndexEnd();
182  OrigAlignments.resize(LastFI);
183  OrigSizes.resize(LastFI);
184  AllColors.resize(LastFI);
185  UsedColors.resize(LastFI);
186  Assignments.resize(LastFI);
187
188  // Gather all spill slots into a list.
189  DEBUG(dbgs() << "Spill slot intervals:\n");
190  for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
191    LiveInterval &li = i->second;
192    DEBUG(li.dump());
193    int FI = TargetRegisterInfo::stackSlot2Index(li.reg);
194    if (MFI->isDeadObjectIndex(FI))
195      continue;
196    SSIntervals.push_back(&li);
197    OrigAlignments[FI] = MFI->getObjectAlignment(FI);
198    OrigSizes[FI]      = MFI->getObjectSize(FI);
199    AllColors.set(FI);
200  }
201  DEBUG(dbgs() << '\n');
202
203  // Sort them by weight.
204  std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
205
206  // Get first "color".
207  NextColor = AllColors.find_first();
208}
209
210/// OverlapWithAssignments - Return true if LiveInterval overlaps with any
211/// LiveIntervals that have already been assigned to the specified color.
212bool
213StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
214  const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
215  for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
216    LiveInterval *OtherLI = OtherLIs[i];
217    if (OtherLI->overlaps(*li))
218      return true;
219  }
220  return false;
221}
222
223/// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
224///
225int StackSlotColoring::ColorSlot(LiveInterval *li) {
226  int Color = -1;
227  bool Share = false;
228  if (!DisableSharing) {
229    // Check if it's possible to reuse any of the used colors.
230    Color = UsedColors.find_first();
231    while (Color != -1) {
232      if (!OverlapWithAssignments(li, Color)) {
233        Share = true;
234        ++NumEliminated;
235        break;
236      }
237      Color = UsedColors.find_next(Color);
238    }
239  }
240
241  // Assign it to the first available color (assumed to be the best) if it's
242  // not possible to share a used color with other objects.
243  if (!Share) {
244    assert(NextColor != -1 && "No more spill slots?");
245    Color = NextColor;
246    UsedColors.set(Color);
247    NextColor = AllColors.find_next(NextColor);
248  }
249
250  // Record the assignment.
251  Assignments[Color].push_back(li);
252  int FI = TargetRegisterInfo::stackSlot2Index(li->reg);
253  DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
254
255  // Change size and alignment of the allocated slot. If there are multiple
256  // objects sharing the same slot, then make sure the size and alignment
257  // are large enough for all.
258  unsigned Align = OrigAlignments[FI];
259  if (!Share || Align > MFI->getObjectAlignment(Color))
260    MFI->setObjectAlignment(Color, Align);
261  int64_t Size = OrigSizes[FI];
262  if (!Share || Size > MFI->getObjectSize(Color))
263    MFI->setObjectSize(Color, Size);
264  return Color;
265}
266
267/// Colorslots - Color all spill stack slots and rewrite all frameindex machine
268/// operands in the function.
269bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
270  unsigned NumObjs = MFI->getObjectIndexEnd();
271  SmallVector<int, 16> SlotMapping(NumObjs, -1);
272  SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
273  SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
274  BitVector UsedColors(NumObjs);
275
276  DEBUG(dbgs() << "Color spill slot intervals:\n");
277  bool Changed = false;
278  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
279    LiveInterval *li = SSIntervals[i];
280    int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
281    int NewSS = ColorSlot(li);
282    assert(NewSS >= 0 && "Stack coloring failed?");
283    SlotMapping[SS] = NewSS;
284    RevMap[NewSS].push_back(SS);
285    SlotWeights[NewSS] += li->weight;
286    UsedColors.set(NewSS);
287    Changed |= (SS != NewSS);
288  }
289
290  DEBUG(dbgs() << "\nSpill slots after coloring:\n");
291  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
292    LiveInterval *li = SSIntervals[i];
293    int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
294    li->weight = SlotWeights[SS];
295  }
296  // Sort them by new weight.
297  std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
298
299#ifndef NDEBUG
300  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
301    DEBUG(SSIntervals[i]->dump());
302  DEBUG(dbgs() << '\n');
303#endif
304
305  if (!Changed)
306    return false;
307
308  // Rewrite all MachineMemOperands.
309  for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
310    int NewFI = SlotMapping[SS];
311    if (NewFI == -1 || (NewFI == (int)SS))
312      continue;
313
314    const Value *NewSV = PseudoSourceValue::getFixedStack(NewFI);
315    SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
316    for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
317      RefMMOs[i]->setValue(NewSV);
318  }
319
320  // Rewrite all MO_FrameIndex operands.  Look for dead stores.
321  for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
322       MBBI != E; ++MBBI) {
323    MachineBasicBlock *MBB = &*MBBI;
324    for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
325         MII != EE; ++MII)
326      RewriteInstruction(MII, SlotMapping, MF);
327    RemoveDeadStores(MBB);
328  }
329
330  // Delete unused stack slots.
331  while (NextColor != -1) {
332    DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
333    MFI->RemoveStackObject(NextColor);
334    NextColor = AllColors.find_next(NextColor);
335  }
336
337  return true;
338}
339
340/// RewriteInstruction - Rewrite specified instruction by replacing references
341/// to old frame index with new one.
342void StackSlotColoring::RewriteInstruction(MachineInstr *MI,
343                                           SmallVector<int, 16> &SlotMapping,
344                                           MachineFunction &MF) {
345  // Update the operands.
346  for (unsigned i = 0, ee = MI->getNumOperands(); i != ee; ++i) {
347    MachineOperand &MO = MI->getOperand(i);
348    if (!MO.isFI())
349      continue;
350    int OldFI = MO.getIndex();
351    if (OldFI < 0)
352      continue;
353    int NewFI = SlotMapping[OldFI];
354    if (NewFI == -1 || NewFI == OldFI)
355      continue;
356    MO.setIndex(NewFI);
357  }
358
359  // The MachineMemOperands have already been updated.
360}
361
362
363/// RemoveDeadStores - Scan through a basic block and look for loads followed
364/// by stores.  If they're both using the same stack slot, then the store is
365/// definitely dead.  This could obviously be much more aggressive (consider
366/// pairs with instructions between them), but such extensions might have a
367/// considerable compile time impact.
368bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
369  // FIXME: This could be much more aggressive, but we need to investigate
370  // the compile time impact of doing so.
371  bool changed = false;
372
373  SmallVector<MachineInstr*, 4> toErase;
374
375  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
376       I != E; ++I) {
377    if (DCELimit != -1 && (int)NumDead >= DCELimit)
378      break;
379
380    int FirstSS, SecondSS;
381    if (TII->isStackSlotCopy(I, FirstSS, SecondSS) &&
382        FirstSS == SecondSS &&
383        FirstSS != -1) {
384      ++NumDead;
385      changed = true;
386      toErase.push_back(I);
387      continue;
388    }
389
390    MachineBasicBlock::iterator NextMI = llvm::next(I);
391    if (NextMI == MBB->end()) continue;
392
393    unsigned LoadReg = 0;
394    unsigned StoreReg = 0;
395    if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
396    if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
397    if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
398
399    ++NumDead;
400    changed = true;
401
402    if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
403      ++NumDead;
404      toErase.push_back(I);
405    }
406
407    toErase.push_back(NextMI);
408    ++I;
409  }
410
411  for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
412       E = toErase.end(); I != E; ++I)
413    (*I)->eraseFromParent();
414
415  return changed;
416}
417
418
419bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
420  DEBUG({
421      dbgs() << "********** Stack Slot Coloring **********\n"
422             << "********** Function: " << MF.getName() << '\n';
423    });
424
425  MFI = MF.getFrameInfo();
426  TII = MF.getTarget().getInstrInfo();
427  LS = &getAnalysis<LiveStacks>();
428  MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
429
430  bool Changed = false;
431
432  unsigned NumSlots = LS->getNumIntervals();
433  if (NumSlots == 0)
434    // Nothing to do!
435    return false;
436
437  // If there are calls to setjmp or sigsetjmp, don't perform stack slot
438  // coloring. The stack could be modified before the longjmp is executed,
439  // resulting in the wrong value being used afterwards. (See
440  // <rdar://problem/8007500>.)
441  if (MF.exposesReturnsTwice())
442    return false;
443
444  // Gather spill slot references
445  ScanForSpillSlotRefs(MF);
446  InitializeSlots();
447  Changed = ColorSlots(MF);
448
449  NextColor = -1;
450  SSIntervals.clear();
451  for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
452    SSRefs[i].clear();
453  SSRefs.clear();
454  OrigAlignments.clear();
455  OrigSizes.clear();
456  AllColors.clear();
457  UsedColors.clear();
458  for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
459    Assignments[i].clear();
460  Assignments.clear();
461
462  return Changed;
463}
464