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 Value *V = MMO->getValue()) {
165           if (const FixedStackPseudoSourceValue *FSV =
166               dyn_cast<FixedStackPseudoSourceValue>(V)) {
167             int FI = FSV->getFrameIndex();
168             if (FI >= 0)
169               SSRefs[FI].push_back(MMO);
170           }
171         }
172       }
173     }
174   }
175 }
176 
177 /// InitializeSlots - Process all spill stack slot liveintervals and add them
178 /// to a sorted (by weight) list.
179 void StackSlotColoring::InitializeSlots() {
180   int LastFI = MFI->getObjectIndexEnd();
181   OrigAlignments.resize(LastFI);
182   OrigSizes.resize(LastFI);
183   AllColors.resize(LastFI);
184   UsedColors.resize(LastFI);
185   Assignments.resize(LastFI);
186 
187   // Gather all spill slots into a list.
188   DEBUG(dbgs() << "Spill slot intervals:\n");
189   for (LiveStacks::iterator i = LS->begin(), e = LS->end(); i != e; ++i) {
190     LiveInterval &li = i->second;
191     DEBUG(li.dump());
192     int FI = TargetRegisterInfo::stackSlot2Index(li.reg);
193     if (MFI->isDeadObjectIndex(FI))
194       continue;
195     SSIntervals.push_back(&li);
196     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
197     OrigSizes[FI]      = MFI->getObjectSize(FI);
198     AllColors.set(FI);
199   }
200   DEBUG(dbgs() << '\n');
201 
202   // Sort them by weight.
203   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
204 
205   // Get first "color".
206   NextColor = AllColors.find_first();
207 }
208 
209 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
210 /// LiveIntervals that have already been assigned to the specified color.
211 bool
212 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
213   const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
214   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
215     LiveInterval *OtherLI = OtherLIs[i];
216     if (OtherLI->overlaps(*li))
217       return true;
218   }
219   return false;
220 }
221 
222 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
223 ///
224 int StackSlotColoring::ColorSlot(LiveInterval *li) {
225   int Color = -1;
226   bool Share = false;
227   if (!DisableSharing) {
228     // Check if it's possible to reuse any of the used colors.
229     Color = UsedColors.find_first();
230     while (Color != -1) {
231       if (!OverlapWithAssignments(li, Color)) {
232         Share = true;
233         ++NumEliminated;
234         break;
235       }
236       Color = UsedColors.find_next(Color);
237     }
238   }
239 
240   // Assign it to the first available color (assumed to be the best) if it's
241   // not possible to share a used color with other objects.
242   if (!Share) {
243     assert(NextColor != -1 && "No more spill slots?");
244     Color = NextColor;
245     UsedColors.set(Color);
246     NextColor = AllColors.find_next(NextColor);
247   }
248 
249   // Record the assignment.
250   Assignments[Color].push_back(li);
251   int FI = TargetRegisterInfo::stackSlot2Index(li->reg);
252   DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
253 
254   // Change size and alignment of the allocated slot. If there are multiple
255   // objects sharing the same slot, then make sure the size and alignment
256   // are large enough for all.
257   unsigned Align = OrigAlignments[FI];
258   if (!Share || Align > MFI->getObjectAlignment(Color))
259     MFI->setObjectAlignment(Color, Align);
260   int64_t Size = OrigSizes[FI];
261   if (!Share || Size > MFI->getObjectSize(Color))
262     MFI->setObjectSize(Color, Size);
263   return Color;
264 }
265 
266 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
267 /// operands in the function.
268 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
269   unsigned NumObjs = MFI->getObjectIndexEnd();
270   SmallVector<int, 16> SlotMapping(NumObjs, -1);
271   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
272   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
273   BitVector UsedColors(NumObjs);
274 
275   DEBUG(dbgs() << "Color spill slot intervals:\n");
276   bool Changed = false;
277   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
278     LiveInterval *li = SSIntervals[i];
279     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
280     int NewSS = ColorSlot(li);
281     assert(NewSS >= 0 && "Stack coloring failed?");
282     SlotMapping[SS] = NewSS;
283     RevMap[NewSS].push_back(SS);
284     SlotWeights[NewSS] += li->weight;
285     UsedColors.set(NewSS);
286     Changed |= (SS != NewSS);
287   }
288 
289   DEBUG(dbgs() << "\nSpill slots after coloring:\n");
290   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
291     LiveInterval *li = SSIntervals[i];
292     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
293     li->weight = SlotWeights[SS];
294   }
295   // Sort them by new weight.
296   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
297 
298 #ifndef NDEBUG
299   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
300     DEBUG(SSIntervals[i]->dump());
301   DEBUG(dbgs() << '\n');
302 #endif
303 
304   if (!Changed)
305     return false;
306 
307   // Rewrite all MachineMemOperands.
308   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
309     int NewFI = SlotMapping[SS];
310     if (NewFI == -1 || (NewFI == (int)SS))
311       continue;
312 
313     const Value *NewSV = PseudoSourceValue::getFixedStack(NewFI);
314     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
315     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
316       RefMMOs[i]->setValue(NewSV);
317   }
318 
319   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
320   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
321        MBBI != E; ++MBBI) {
322     MachineBasicBlock *MBB = &*MBBI;
323     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
324          MII != EE; ++MII)
325       RewriteInstruction(MII, SlotMapping, MF);
326     RemoveDeadStores(MBB);
327   }
328 
329   // Delete unused stack slots.
330   while (NextColor != -1) {
331     DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
332     MFI->RemoveStackObject(NextColor);
333     NextColor = AllColors.find_next(NextColor);
334   }
335 
336   return true;
337 }
338 
339 /// RewriteInstruction - Rewrite specified instruction by replacing references
340 /// to old frame index with new one.
341 void StackSlotColoring::RewriteInstruction(MachineInstr *MI,
342                                            SmallVectorImpl<int> &SlotMapping,
343                                            MachineFunction &MF) {
344   // Update the operands.
345   for (unsigned i = 0, ee = MI->getNumOperands(); i != ee; ++i) {
346     MachineOperand &MO = MI->getOperand(i);
347     if (!MO.isFI())
348       continue;
349     int OldFI = MO.getIndex();
350     if (OldFI < 0)
351       continue;
352     int NewFI = SlotMapping[OldFI];
353     if (NewFI == -1 || NewFI == OldFI)
354       continue;
355     MO.setIndex(NewFI);
356   }
357 
358   // The MachineMemOperands have already been updated.
359 }
360 
361 
362 /// RemoveDeadStores - Scan through a basic block and look for loads followed
363 /// by stores.  If they're both using the same stack slot, then the store is
364 /// definitely dead.  This could obviously be much more aggressive (consider
365 /// pairs with instructions between them), but such extensions might have a
366 /// considerable compile time impact.
367 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
368   // FIXME: This could be much more aggressive, but we need to investigate
369   // the compile time impact of doing so.
370   bool changed = false;
371 
372   SmallVector<MachineInstr*, 4> toErase;
373 
374   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
375        I != E; ++I) {
376     if (DCELimit != -1 && (int)NumDead >= DCELimit)
377       break;
378 
379     int FirstSS, SecondSS;
380     if (TII->isStackSlotCopy(I, FirstSS, SecondSS) &&
381         FirstSS == SecondSS &&
382         FirstSS != -1) {
383       ++NumDead;
384       changed = true;
385       toErase.push_back(I);
386       continue;
387     }
388 
389     MachineBasicBlock::iterator NextMI = std::next(I);
390     if (NextMI == MBB->end()) continue;
391 
392     unsigned LoadReg = 0;
393     unsigned StoreReg = 0;
394     if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
395     if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
396     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
397 
398     ++NumDead;
399     changed = true;
400 
401     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
402       ++NumDead;
403       toErase.push_back(I);
404     }
405 
406     toErase.push_back(NextMI);
407     ++I;
408   }
409 
410   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
411        E = toErase.end(); I != E; ++I)
412     (*I)->eraseFromParent();
413 
414   return changed;
415 }
416 
417 
418 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
419   DEBUG({
420       dbgs() << "********** Stack Slot Coloring **********\n"
421              << "********** Function: " << MF.getName() << '\n';
422     });
423 
424   MFI = MF.getFrameInfo();
425   TII = MF.getTarget().getInstrInfo();
426   LS = &getAnalysis<LiveStacks>();
427   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
428 
429   bool Changed = false;
430 
431   unsigned NumSlots = LS->getNumIntervals();
432   if (NumSlots == 0)
433     // Nothing to do!
434     return false;
435 
436   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
437   // coloring. The stack could be modified before the longjmp is executed,
438   // resulting in the wrong value being used afterwards. (See
439   // <rdar://problem/8007500>.)
440   if (MF.exposesReturnsTwice())
441     return false;
442 
443   // Gather spill slot references
444   ScanForSpillSlotRefs(MF);
445   InitializeSlots();
446   Changed = ColorSlots(MF);
447 
448   NextColor = -1;
449   SSIntervals.clear();
450   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
451     SSRefs[i].clear();
452   SSRefs.clear();
453   OrigAlignments.clear();
454   OrigSizes.clear();
455   AllColors.clear();
456   UsedColors.clear();
457   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
458     Assignments[i].clear();
459   Assignments.clear();
460 
461   return Changed;
462 }
463