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>
34 using namespace llvm;
35 
36 static cl::opt<bool>
37 DisableSharing("no-stack-slot-sharing",
38              cl::init(false), cl::Hidden,
39              cl::desc("Suppress slot sharing during stack coloring"));
40 
41 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
42 
43 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
44 STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
45 
46 namespace {
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     void getAnalysisUsage(AnalysisUsage &AU) const override {
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     bool runOnMachineFunction(MachineFunction &MF) override;
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, SmallVectorImpl<int> &SlotMapping,
110                             MachineFunction &MF);
111     bool RemoveDeadStores(MachineBasicBlock* MBB);
112   };
113 } // end anonymous namespace
114 
115 char StackSlotColoring::ID = 0;
116 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
117 
118 INITIALIZE_PASS_BEGIN(StackSlotColoring, "stack-slot-coloring",
119                 "Stack Slot Coloring", false, false)
120 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
121 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
122 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
123 INITIALIZE_PASS_END(StackSlotColoring, "stack-slot-coloring",
124                 "Stack Slot Coloring", false, false)
125 
126 namespace {
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.
138 void 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     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
146          MII != EE; ++MII) {
147       MachineInstr *MI = &*MII;
148       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
149         MachineOperand &MO = MI->getOperand(i);
150         if (!MO.isFI())
151           continue;
152         int FI = MO.getIndex();
153         if (FI < 0)
154           continue;
155         if (!LS->hasInterval(FI))
156           continue;
157         LiveInterval &li = LS->getInterval(FI);
158         if (!MI->isDebugValue())
159           li.weight += LiveIntervals::getSpillWeight(false, true, MBFI, MI);
160       }
161       for (MachineInstr::mmo_iterator MMOI = MI->memoperands_begin(),
162            EE = MI->memoperands_end(); MMOI != EE; ++MMOI) {
163         MachineMemOperand *MMO = *MMOI;
164         if (const FixedStackPseudoSourceValue *FSV =
165             dyn_cast_or_null<FixedStackPseudoSourceValue>(
166                 MMO->getPseudoValue())) {
167           int FI = FSV->getFrameIndex();
168           if (FI >= 0)
169             SSRefs[FI].push_back(MMO);
170         }
171       }
172     }
173   }
174 }
175 
176 /// InitializeSlots - Process all spill stack slot liveintervals and add them
177 /// to a sorted (by weight) list.
178 void StackSlotColoring::InitializeSlots() {
179   int LastFI = MFI->getObjectIndexEnd();
180   OrigAlignments.resize(LastFI);
181   OrigSizes.resize(LastFI);
182   AllColors.resize(LastFI);
183   UsedColors.resize(LastFI);
184   Assignments.resize(LastFI);
185 
186   // Gather all spill slots into a list.
187   DEBUG(dbgs() << "Spill slot intervals:\n");
188   for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
189     LiveInterval &li = i->second;
190     DEBUG(li.dump());
191     int FI = TargetRegisterInfo::stackSlot2Index(li.reg);
192     if (MFI->isDeadObjectIndex(FI))
193       continue;
194     SSIntervals.push_back(&li);
195     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
196     OrigSizes[FI]      = MFI->getObjectSize(FI);
197     AllColors.set(FI);
198   }
199   DEBUG(dbgs() << '\n');
200 
201   // Sort them by weight.
202   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
203 
204   // Get first "color".
205   NextColor = AllColors.find_first();
206 }
207 
208 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
209 /// LiveIntervals that have already been assigned to the specified color.
210 bool
211 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
212   const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
213   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
214     LiveInterval *OtherLI = OtherLIs[i];
215     if (OtherLI->overlaps(*li))
216       return true;
217   }
218   return false;
219 }
220 
221 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
222 ///
223 int StackSlotColoring::ColorSlot(LiveInterval *li) {
224   int Color = -1;
225   bool Share = false;
226   if (!DisableSharing) {
227     // Check if it's possible to reuse any of the used colors.
228     Color = UsedColors.find_first();
229     while (Color != -1) {
230       if (!OverlapWithAssignments(li, Color)) {
231         Share = true;
232         ++NumEliminated;
233         break;
234       }
235       Color = UsedColors.find_next(Color);
236     }
237   }
238 
239   // Assign it to the first available color (assumed to be the best) if it's
240   // not possible to share a used color with other objects.
241   if (!Share) {
242     assert(NextColor != -1 && "No more spill slots?");
243     Color = NextColor;
244     UsedColors.set(Color);
245     NextColor = AllColors.find_next(NextColor);
246   }
247 
248   // Record the assignment.
249   Assignments[Color].push_back(li);
250   int FI = TargetRegisterInfo::stackSlot2Index(li->reg);
251   DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
252 
253   // Change size and alignment of the allocated slot. If there are multiple
254   // objects sharing the same slot, then make sure the size and alignment
255   // are large enough for all.
256   unsigned Align = OrigAlignments[FI];
257   if (!Share || Align > MFI->getObjectAlignment(Color))
258     MFI->setObjectAlignment(Color, Align);
259   int64_t Size = OrigSizes[FI];
260   if (!Share || Size > MFI->getObjectSize(Color))
261     MFI->setObjectSize(Color, Size);
262   return Color;
263 }
264 
265 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
266 /// operands in the function.
267 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
268   unsigned NumObjs = MFI->getObjectIndexEnd();
269   SmallVector<int, 16> SlotMapping(NumObjs, -1);
270   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
271   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
272   BitVector UsedColors(NumObjs);
273 
274   DEBUG(dbgs() << "Color spill slot intervals:\n");
275   bool Changed = false;
276   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
277     LiveInterval *li = SSIntervals[i];
278     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
279     int NewSS = ColorSlot(li);
280     assert(NewSS >= 0 && "Stack coloring failed?");
281     SlotMapping[SS] = NewSS;
282     RevMap[NewSS].push_back(SS);
283     SlotWeights[NewSS] += li->weight;
284     UsedColors.set(NewSS);
285     Changed |= (SS != NewSS);
286   }
287 
288   DEBUG(dbgs() << "\nSpill slots after coloring:\n");
289   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
290     LiveInterval *li = SSIntervals[i];
291     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
292     li->weight = SlotWeights[SS];
293   }
294   // Sort them by new weight.
295   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
296 
297 #ifndef NDEBUG
298   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
299     DEBUG(SSIntervals[i]->dump());
300   DEBUG(dbgs() << '\n');
301 #endif
302 
303   if (!Changed)
304     return false;
305 
306   // Rewrite all MachineMemOperands.
307   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
308     int NewFI = SlotMapping[SS];
309     if (NewFI == -1 || (NewFI == (int)SS))
310       continue;
311 
312     const PseudoSourceValue *NewSV = PseudoSourceValue::getFixedStack(NewFI);
313     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
314     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
315       RefMMOs[i]->setValue(NewSV);
316   }
317 
318   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
319   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
320        MBBI != E; ++MBBI) {
321     MachineBasicBlock *MBB = &*MBBI;
322     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
323          MII != EE; ++MII)
324       RewriteInstruction(MII, SlotMapping, MF);
325     RemoveDeadStores(MBB);
326   }
327 
328   // Delete unused stack slots.
329   while (NextColor != -1) {
330     DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
331     MFI->RemoveStackObject(NextColor);
332     NextColor = AllColors.find_next(NextColor);
333   }
334 
335   return true;
336 }
337 
338 /// RewriteInstruction - Rewrite specified instruction by replacing references
339 /// to old frame index with new one.
340 void StackSlotColoring::RewriteInstruction(MachineInstr *MI,
341                                            SmallVectorImpl<int> &SlotMapping,
342                                            MachineFunction &MF) {
343   // Update the operands.
344   for (unsigned i = 0, ee = MI->getNumOperands(); i != ee; ++i) {
345     MachineOperand &MO = MI->getOperand(i);
346     if (!MO.isFI())
347       continue;
348     int OldFI = MO.getIndex();
349     if (OldFI < 0)
350       continue;
351     int NewFI = SlotMapping[OldFI];
352     if (NewFI == -1 || NewFI == OldFI)
353       continue;
354     MO.setIndex(NewFI);
355   }
356 
357   // The MachineMemOperands have already been updated.
358 }
359 
360 
361 /// RemoveDeadStores - Scan through a basic block and look for loads followed
362 /// by stores.  If they're both using the same stack slot, then the store is
363 /// definitely dead.  This could obviously be much more aggressive (consider
364 /// pairs with instructions between them), but such extensions might have a
365 /// considerable compile time impact.
366 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
367   // FIXME: This could be much more aggressive, but we need to investigate
368   // the compile time impact of doing so.
369   bool changed = false;
370 
371   SmallVector<MachineInstr*, 4> toErase;
372 
373   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
374        I != E; ++I) {
375     if (DCELimit != -1 && (int)NumDead >= DCELimit)
376       break;
377 
378     int FirstSS, SecondSS;
379     if (TII->isStackSlotCopy(I, FirstSS, SecondSS) &&
380         FirstSS == SecondSS &&
381         FirstSS != -1) {
382       ++NumDead;
383       changed = true;
384       toErase.push_back(I);
385       continue;
386     }
387 
388     MachineBasicBlock::iterator NextMI = std::next(I);
389     if (NextMI == MBB->end()) continue;
390 
391     unsigned LoadReg = 0;
392     unsigned StoreReg = 0;
393     if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
394     if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
395     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
396 
397     ++NumDead;
398     changed = true;
399 
400     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
401       ++NumDead;
402       toErase.push_back(I);
403     }
404 
405     toErase.push_back(NextMI);
406     ++I;
407   }
408 
409   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
410        E = toErase.end(); I != E; ++I)
411     (*I)->eraseFromParent();
412 
413   return changed;
414 }
415 
416 
417 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
418   DEBUG({
419       dbgs() << "********** Stack Slot Coloring **********\n"
420              << "********** Function: " << MF.getName() << '\n';
421     });
422 
423   MFI = MF.getFrameInfo();
424   TII = MF.getTarget().getInstrInfo();
425   LS = &getAnalysis<LiveStacks>();
426   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
427 
428   bool Changed = false;
429 
430   unsigned NumSlots = LS->getNumIntervals();
431   if (NumSlots == 0)
432     // Nothing to do!
433     return false;
434 
435   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
436   // coloring. The stack could be modified before the longjmp is executed,
437   // resulting in the wrong value being used afterwards. (See
438   // <rdar://problem/8007500>.)
439   if (MF.exposesReturnsTwice())
440     return false;
441 
442   // Gather spill slot references
443   ScanForSpillSlotRefs(MF);
444   InitializeSlots();
445   Changed = ColorSlots(MF);
446 
447   NextColor = -1;
448   SSIntervals.clear();
449   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
450     SSRefs[i].clear();
451   SSRefs.clear();
452   OrigAlignments.clear();
453   OrigSizes.clear();
454   AllColors.clear();
455   UsedColors.clear();
456   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
457     Assignments[i].clear();
458   Assignments.clear();
459 
460   return Changed;
461 }
462