StackSlotColoring.cpp revision 1edf7ee80259c8a99c5544405cb2746443fab832 
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 "llvm/CodeGen/Passes.h"
16#include "llvm/CodeGen/LiveStackAnalysis.h"
17#include "llvm/CodeGen/MachineFrameInfo.h"
18#include "llvm/CodeGen/PseudoSourceValue.h"
19#include "llvm/Support/CommandLine.h"
20#include "llvm/Support/Compiler.h"
21#include "llvm/Support/Debug.h"
22#include "llvm/Target/TargetInstrInfo.h"
23#include "llvm/Target/TargetMachine.h"
24#include "llvm/ADT/BitVector.h"
25#include "llvm/ADT/SmallVector.h"
26#include "llvm/ADT/Statistic.h"
27#include <vector>
28using namespace llvm;
29
30static cl::opt<bool>
31DisableSharing("no-stack-slot-sharing",
32             cl::init(false), cl::Hidden,
33             cl::desc("Surpress slot sharing during stack coloring"));
34
35static cl::opt<bool>
36EnableDCE("enable-ssc-dce",
37               cl::init(false), cl::Hidden,
38               cl::desc("Enable slot coloring DCE"));
39
40STATISTIC(NumEliminated,   "Number of stack slots eliminated due to coloring");
41STATISTIC(NumDeadAccesses,
42                          "Number of trivially dead stack accesses eliminated");
43
44namespace {
45  class VISIBILITY_HIDDEN StackSlotColoring : public MachineFunctionPass {
46    LiveStacks* LS;
47    MachineFrameInfo *MFI;
48    const TargetInstrInfo  *TII;
49
50    // SSIntervals - Spill slot intervals.
51    std::vector<LiveInterval*> SSIntervals;
52
53    // OrigAlignments - Alignments of stack objects before coloring.
54    SmallVector<unsigned, 16> OrigAlignments;
55
56    // OrigSizes - Sizess of stack objects before coloring.
57    SmallVector<unsigned, 16> OrigSizes;
58
59    // AllColors - If index is set, it's a spill slot, i.e. color.
60    // FIXME: This assumes PEI locate spill slot with smaller indices
61    // closest to stack pointer / frame pointer. Therefore, smaller
62    // index == better color.
63    BitVector AllColors;
64
65    // NextColor - Next "color" that's not yet used.
66    int NextColor;
67
68    // UsedColors - "Colors" that have been assigned.
69    BitVector UsedColors;
70
71    // Assignments - Color to intervals mapping.
72    SmallVector<SmallVector<LiveInterval*,4>,16> Assignments;
73
74  public:
75    static char ID; // Pass identification
76    StackSlotColoring() : MachineFunctionPass(&ID), NextColor(-1) {}
77
78    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
79      AU.addRequired<LiveStacks>();
80
81      AU.addPreservedID(MachineLoopInfoID);
82      AU.addPreservedID(MachineDominatorsID);
83      MachineFunctionPass::getAnalysisUsage(AU);
84    }
85
86    virtual bool runOnMachineFunction(MachineFunction &MF);
87    virtual const char* getPassName() const {
88      return "Stack Slot Coloring";
89    }
90
91  private:
92    bool InitializeSlots();
93    bool OverlapWithAssignments(LiveInterval *li, int Color) const;
94    int ColorSlot(LiveInterval *li);
95    bool ColorSlots(MachineFunction &MF);
96    bool removeDeadStores(MachineBasicBlock* MBB);
97  };
98} // end anonymous namespace
99
100char StackSlotColoring::ID = 0;
101
102static RegisterPass<StackSlotColoring>
103X("stack-slot-coloring", "Stack Slot Coloring");
104
105FunctionPass *llvm::createStackSlotColoringPass() {
106  return new StackSlotColoring();
107}
108
109namespace {
110  // IntervalSorter - Comparison predicate that sort live intervals by
111  // their weight.
112  struct IntervalSorter {
113    bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
114      return LHS->weight > RHS->weight;
115    }
116  };
117}
118
119/// InitializeSlots - Process all spill stack slot liveintervals and add them
120/// to a sorted (by weight) list.
121bool StackSlotColoring::InitializeSlots() {
122  if (LS->getNumIntervals() < 2)
123    return false;
124
125  int LastFI = MFI->getObjectIndexEnd();
126  OrigAlignments.resize(LastFI);
127  OrigSizes.resize(LastFI);
128  AllColors.resize(LastFI);
129  UsedColors.resize(LastFI);
130  Assignments.resize(LastFI);
131
132  // Gather all spill slots into a list.
133  for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
134    LiveInterval &li = i->second;
135    int FI = li.getStackSlotIndex();
136    if (MFI->isDeadObjectIndex(FI))
137      continue;
138    SSIntervals.push_back(&li);
139    OrigAlignments[FI] = MFI->getObjectAlignment(FI);
140    OrigSizes[FI]      = MFI->getObjectSize(FI);
141    AllColors.set(FI);
142  }
143
144  // Sort them by weight.
145  std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
146
147  // Get first "color".
148  NextColor = AllColors.find_first();
149  return true;
150}
151
152/// OverlapWithAssignments - Return true if LiveInterval overlaps with any
153/// LiveIntervals that have already been assigned to the specified color.
154bool
155StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
156  const SmallVector<LiveInterval*,4> &OtherLIs = Assignments[Color];
157  for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
158    LiveInterval *OtherLI = OtherLIs[i];
159    if (OtherLI->overlaps(*li))
160      return true;
161  }
162  return false;
163}
164
165/// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
166///
167int StackSlotColoring::ColorSlot(LiveInterval *li) {
168  int Color = -1;
169  bool Share = false;
170  if (!DisableSharing) {
171    // Check if it's possible to reuse any of the used colors.
172    Color = UsedColors.find_first();
173    while (Color != -1) {
174      if (!OverlapWithAssignments(li, Color)) {
175        Share = true;
176        ++NumEliminated;
177        break;
178      }
179      Color = UsedColors.find_next(Color);
180    }
181  }
182
183  // Assign it to the first available color (assumed to be the best) if it's
184  // not possible to share a used color with other objects.
185  if (!Share) {
186    assert(NextColor != -1 && "No more spill slots?");
187    Color = NextColor;
188    UsedColors.set(Color);
189    NextColor = AllColors.find_next(NextColor);
190  }
191
192  // Record the assignment.
193  Assignments[Color].push_back(li);
194  int FI = li->getStackSlotIndex();
195  DOUT << "Assigning fi#" << FI << " to fi#" << Color << "\n";
196
197  // Change size and alignment of the allocated slot. If there are multiple
198  // objects sharing the same slot, then make sure the size and alignment
199  // are large enough for all.
200  unsigned Align = OrigAlignments[FI];
201  if (!Share || Align > MFI->getObjectAlignment(Color))
202    MFI->setObjectAlignment(Color, Align);
203  int64_t Size = OrigSizes[FI];
204  if (!Share || Size > MFI->getObjectSize(Color))
205    MFI->setObjectSize(Color, Size);
206  return Color;
207}
208
209/// Colorslots - Color all spill stack slots and rewrite all frameindex machine
210/// operands in the function.
211bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
212  unsigned NumObjs = MFI->getObjectIndexEnd();
213  std::vector<int> SlotMapping(NumObjs, -1);
214
215  bool Changed = false;
216  for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
217    LiveInterval *li = SSIntervals[i];
218    int SS = li->getStackSlotIndex();
219    int NewSS = ColorSlot(li);
220    SlotMapping[SS] = NewSS;
221    Changed |= (SS != NewSS);
222  }
223
224  if (!Changed)
225    return false;
226
227  // Rewrite all MO_FrameIndex operands.
228  // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
229  for (MachineFunction::iterator MBB = MF.begin(), E = MF.end();
230       MBB != E; ++MBB) {
231    for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
232         MII != EE; ++MII) {
233      MachineInstr &MI = *MII;
234      for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
235        MachineOperand &MO = MI.getOperand(i);
236        if (!MO.isFI())
237          continue;
238        int FI = MO.getIndex();
239        if (FI < 0)
240          continue;
241        int NewFI = SlotMapping[FI];
242        if (NewFI == -1)
243          continue;
244        MO.setIndex(NewFI);
245
246        // Update the MachineMemOperand for the new memory location.
247        // FIXME: We need a better method of managing these too.
248        SmallVector<MachineMemOperand, 2> MMOs(MI.memoperands_begin(),
249                                               MI.memoperands_end());
250        MI.clearMemOperands(MF);
251        const Value *OldSV = PseudoSourceValue::getFixedStack(FI);
252        for (unsigned i = 0, e = MMOs.size(); i != e; ++i) {
253          if (MMOs[i].getValue() == OldSV) {
254            MachineMemOperand MMO(PseudoSourceValue::getFixedStack(NewFI),
255                                  MMOs[i].getFlags(), MMOs[i].getOffset(),
256                                  MMOs[i].getSize(), MMOs[i].getAlignment());
257            MI.addMemOperand(MF, MMO);
258          } else
259            MI.addMemOperand(MF, MMOs[i]);
260        }
261      }
262    }
263  }
264
265  // Delete unused stack slots.
266  while (NextColor != -1) {
267    DOUT << "Removing unused stack object fi#" << NextColor << "\n";
268    MFI->RemoveStackObject(NextColor);
269    NextColor = AllColors.find_next(NextColor);
270  }
271
272  return true;
273}
274
275/// removeDeadStores - Scan through a basic block and look for loads followed
276/// by stores.  If they're both using the same stack slot, then the store is
277/// definitely dead.  This could obviously be much more aggressive (consider
278/// pairs with instructions between them), but such extensions might have a
279/// considerable compile time impact.
280bool StackSlotColoring::removeDeadStores(MachineBasicBlock* MBB) {
281  // FIXME: This could be much more aggressive, but we need to investigate
282  // the compile time impact of doing so.
283  bool changed = false;
284
285  SmallVector<MachineInstr*, 4> toErase;
286
287  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
288       I != E; ++I) {
289    MachineBasicBlock::iterator NextMI = next(I);
290    if (NextMI == MBB->end()) continue;
291
292    int FirstSS, SecondSS;
293    unsigned LoadReg = 0;
294    unsigned StoreReg = 0;
295    if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
296    if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
297    if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
298
299    ++NumDeadAccesses;
300    changed = true;
301
302    if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
303      ++NumDeadAccesses;
304      toErase.push_back(I);
305    }
306
307    toErase.push_back(NextMI);
308    ++I;
309  }
310
311  for (SmallVector<MachineInstr*, 4>::iterator I = toErase.begin(),
312       E = toErase.end(); I != E; ++I)
313    (*I)->eraseFromParent();
314
315  return changed;
316}
317
318
319bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
320  DOUT << "********** Stack Slot Coloring **********\n";
321
322  MFI = MF.getFrameInfo();
323  TII = MF.getTarget().getInstrInfo();
324  LS = &getAnalysis<LiveStacks>();
325
326  bool Changed = false;
327  if (InitializeSlots())
328    Changed = ColorSlots(MF);
329
330  NextColor = -1;
331  SSIntervals.clear();
332  OrigAlignments.clear();
333  OrigSizes.clear();
334  AllColors.clear();
335  UsedColors.clear();
336  for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
337    Assignments[i].clear();
338  Assignments.clear();
339
340  if (EnableDCE) {
341    for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
342      Changed |= removeDeadStores(I);
343  }
344
345  return Changed;
346}
347