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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 #include "llvm/CodeGen/Passes.h"
15 #include "llvm/ADT/BitVector.h"
16 #include "llvm/ADT/SmallVector.h"
17 #include "llvm/ADT/Statistic.h"
18 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
19 #include "llvm/CodeGen/LiveStackAnalysis.h"
20 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
21 #include "llvm/CodeGen/MachineFrameInfo.h"
22 #include "llvm/CodeGen/MachineInstrBuilder.h"
23 #include "llvm/CodeGen/MachineMemOperand.h"
24 #include "llvm/CodeGen/MachineRegisterInfo.h"
25 #include "llvm/CodeGen/PseudoSourceValue.h"
26 #include "llvm/IR/Module.h"
27 #include "llvm/Support/CommandLine.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Target/TargetInstrInfo.h"
31 #include "llvm/Target/TargetSubtargetInfo.h"
32 #include <vector>
33 using namespace llvm;
34 
35 #define DEBUG_TYPE "stackslotcoloring"
36 
37 static cl::opt<bool>
38 DisableSharing("no-stack-slot-sharing",
39              cl::init(false), cl::Hidden,
40              cl::desc("Suppress slot sharing during stack coloring"));
41 
42 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
43 
44 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
45 STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
46 
47 namespace {
48   class StackSlotColoring : public MachineFunctionPass {
49     LiveStacks* LS;
50     MachineFrameInfo *MFI;
51     const TargetInstrInfo  *TII;
52     const MachineBlockFrequencyInfo *MBFI;
53 
54     // SSIntervals - Spill slot intervals.
55     std::vector<LiveInterval*> SSIntervals;
56 
57     // SSRefs - Keep a list of MachineMemOperands for each spill slot.
58     // MachineMemOperands can be shared between instructions, so we need
59     // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
60     // become FI0 -> FI1 -> FI2.
61     SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
62 
63     // OrigAlignments - Alignments of stack objects before coloring.
64     SmallVector<unsigned, 16> OrigAlignments;
65 
66     // OrigSizes - Sizess of stack objects before coloring.
67     SmallVector<unsigned, 16> OrigSizes;
68 
69     // AllColors - If index is set, it's a spill slot, i.e. color.
70     // FIXME: This assumes PEI locate spill slot with smaller indices
71     // closest to stack pointer / frame pointer. Therefore, smaller
72     // index == better color.
73     BitVector AllColors;
74 
75     // NextColor - Next "color" that's not yet used.
76     int NextColor;
77 
78     // UsedColors - "Colors" that have been assigned.
79     BitVector UsedColors;
80 
81     // Assignments - Color to intervals mapping.
82     SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
83 
84   public:
85     static char ID; // Pass identification
StackSlotColoring()86     StackSlotColoring() :
87       MachineFunctionPass(ID), NextColor(-1) {
88         initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
89       }
90 
getAnalysisUsage(AnalysisUsage & AU) const91     void getAnalysisUsage(AnalysisUsage &AU) const override {
92       AU.setPreservesCFG();
93       AU.addRequired<SlotIndexes>();
94       AU.addPreserved<SlotIndexes>();
95       AU.addRequired<LiveStacks>();
96       AU.addRequired<MachineBlockFrequencyInfo>();
97       AU.addPreserved<MachineBlockFrequencyInfo>();
98       AU.addPreservedID(MachineDominatorsID);
99       MachineFunctionPass::getAnalysisUsage(AU);
100     }
101 
102     bool runOnMachineFunction(MachineFunction &MF) override;
103 
104   private:
105     void InitializeSlots();
106     void ScanForSpillSlotRefs(MachineFunction &MF);
107     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
108     int ColorSlot(LiveInterval *li);
109     bool ColorSlots(MachineFunction &MF);
110     void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping,
111                             MachineFunction &MF);
112     bool RemoveDeadStores(MachineBasicBlock* MBB);
113   };
114 } // end anonymous namespace
115 
116 char StackSlotColoring::ID = 0;
117 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
118 
119 INITIALIZE_PASS_BEGIN(StackSlotColoring, "stack-slot-coloring",
120                 "Stack Slot Coloring", false, false)
121 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
122 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
123 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
124 INITIALIZE_PASS_END(StackSlotColoring, "stack-slot-coloring",
125                 "Stack Slot Coloring", false, false)
126 
127 namespace {
128   // IntervalSorter - Comparison predicate that sort live intervals by
129   // their weight.
130   struct IntervalSorter {
operator ()__anon0707f7680211::IntervalSorter131     bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
132       return LHS->weight > RHS->weight;
133     }
134   };
135 }
136 
137 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
138 /// references and update spill slot weights.
ScanForSpillSlotRefs(MachineFunction & MF)139 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
140   SSRefs.resize(MFI->getObjectIndexEnd());
141 
142   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
143   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
144        MBBI != E; ++MBBI) {
145     MachineBasicBlock *MBB = &*MBBI;
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, MBFI, MI);
161       }
162       for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
163                                       EE = MI.memoperands_end();
164            MMOI != EE; ++MMOI) {
165         MachineMemOperand *MMO = *MMOI;
166         if (const FixedStackPseudoSourceValue *FSV =
167             dyn_cast_or_null<FixedStackPseudoSourceValue>(
168                 MMO->getPseudoValue())) {
169           int FI = FSV->getFrameIndex();
170           if (FI >= 0)
171             SSRefs[FI].push_back(MMO);
172         }
173       }
174     }
175   }
176 }
177 
178 /// InitializeSlots - Process all spill stack slot liveintervals and add them
179 /// to a sorted (by weight) list.
InitializeSlots()180 void 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   typedef std::iterator_traits<LiveStacks::iterator>::value_type Pair;
189   SmallVector<Pair *, 16> Intervals;
190   Intervals.reserve(LS->getNumIntervals());
191   for (auto &I : *LS)
192     Intervals.push_back(&I);
193   std::sort(Intervals.begin(), Intervals.end(),
194             [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
195 
196   // Gather all spill slots into a list.
197   DEBUG(dbgs() << "Spill slot intervals:\n");
198   for (auto *I : Intervals) {
199     LiveInterval &li = I->second;
200     DEBUG(li.dump());
201     int FI = TargetRegisterInfo::stackSlot2Index(li.reg);
202     if (MFI->isDeadObjectIndex(FI))
203       continue;
204     SSIntervals.push_back(&li);
205     OrigAlignments[FI] = MFI->getObjectAlignment(FI);
206     OrigSizes[FI]      = MFI->getObjectSize(FI);
207     AllColors.set(FI);
208   }
209   DEBUG(dbgs() << '\n');
210 
211   // Sort them by weight.
212   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
213 
214   // Get first "color".
215   NextColor = AllColors.find_first();
216 }
217 
218 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
219 /// LiveIntervals that have already been assigned to the specified color.
220 bool
OverlapWithAssignments(LiveInterval * li,int Color) const221 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
222   const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
223   for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
224     LiveInterval *OtherLI = OtherLIs[i];
225     if (OtherLI->overlaps(*li))
226       return true;
227   }
228   return false;
229 }
230 
231 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
232 ///
ColorSlot(LiveInterval * li)233 int StackSlotColoring::ColorSlot(LiveInterval *li) {
234   int Color = -1;
235   bool Share = false;
236   if (!DisableSharing) {
237     // Check if it's possible to reuse any of the used colors.
238     Color = UsedColors.find_first();
239     while (Color != -1) {
240       if (!OverlapWithAssignments(li, Color)) {
241         Share = true;
242         ++NumEliminated;
243         break;
244       }
245       Color = UsedColors.find_next(Color);
246     }
247   }
248 
249   // Assign it to the first available color (assumed to be the best) if it's
250   // not possible to share a used color with other objects.
251   if (!Share) {
252     assert(NextColor != -1 && "No more spill slots?");
253     Color = NextColor;
254     UsedColors.set(Color);
255     NextColor = AllColors.find_next(NextColor);
256   }
257 
258   // Record the assignment.
259   Assignments[Color].push_back(li);
260   int FI = TargetRegisterInfo::stackSlot2Index(li->reg);
261   DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
262 
263   // Change size and alignment of the allocated slot. If there are multiple
264   // objects sharing the same slot, then make sure the size and alignment
265   // are large enough for all.
266   unsigned Align = OrigAlignments[FI];
267   if (!Share || Align > MFI->getObjectAlignment(Color))
268     MFI->setObjectAlignment(Color, Align);
269   int64_t Size = OrigSizes[FI];
270   if (!Share || Size > MFI->getObjectSize(Color))
271     MFI->setObjectSize(Color, Size);
272   return Color;
273 }
274 
275 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
276 /// operands in the function.
ColorSlots(MachineFunction & MF)277 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
278   unsigned NumObjs = MFI->getObjectIndexEnd();
279   SmallVector<int, 16> SlotMapping(NumObjs, -1);
280   SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
281   SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
282   BitVector UsedColors(NumObjs);
283 
284   DEBUG(dbgs() << "Color spill slot intervals:\n");
285   bool Changed = false;
286   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
287     LiveInterval *li = SSIntervals[i];
288     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
289     int NewSS = ColorSlot(li);
290     assert(NewSS >= 0 && "Stack coloring failed?");
291     SlotMapping[SS] = NewSS;
292     RevMap[NewSS].push_back(SS);
293     SlotWeights[NewSS] += li->weight;
294     UsedColors.set(NewSS);
295     Changed |= (SS != NewSS);
296   }
297 
298   DEBUG(dbgs() << "\nSpill slots after coloring:\n");
299   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
300     LiveInterval *li = SSIntervals[i];
301     int SS = TargetRegisterInfo::stackSlot2Index(li->reg);
302     li->weight = SlotWeights[SS];
303   }
304   // Sort them by new weight.
305   std::stable_sort(SSIntervals.begin(), SSIntervals.end(), IntervalSorter());
306 
307 #ifndef NDEBUG
308   for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
309     DEBUG(SSIntervals[i]->dump());
310   DEBUG(dbgs() << '\n');
311 #endif
312 
313   if (!Changed)
314     return false;
315 
316   // Rewrite all MachineMemOperands.
317   for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
318     int NewFI = SlotMapping[SS];
319     if (NewFI == -1 || (NewFI == (int)SS))
320       continue;
321 
322     const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
323     SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
324     for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
325       RefMMOs[i]->setValue(NewSV);
326   }
327 
328   // Rewrite all MO_FrameIndex operands.  Look for dead stores.
329   for (MachineBasicBlock &MBB : MF) {
330     for (MachineInstr &MI : MBB)
331       RewriteInstruction(MI, SlotMapping, MF);
332     RemoveDeadStores(&MBB);
333   }
334 
335   // Delete unused stack slots.
336   while (NextColor != -1) {
337     DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
338     MFI->RemoveStackObject(NextColor);
339     NextColor = AllColors.find_next(NextColor);
340   }
341 
342   return true;
343 }
344 
345 /// RewriteInstruction - Rewrite specified instruction by replacing references
346 /// to old frame index with new one.
RewriteInstruction(MachineInstr & MI,SmallVectorImpl<int> & SlotMapping,MachineFunction & MF)347 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
348                                            SmallVectorImpl<int> &SlotMapping,
349                                            MachineFunction &MF) {
350   // Update the operands.
351   for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) {
352     MachineOperand &MO = MI.getOperand(i);
353     if (!MO.isFI())
354       continue;
355     int OldFI = MO.getIndex();
356     if (OldFI < 0)
357       continue;
358     int NewFI = SlotMapping[OldFI];
359     if (NewFI == -1 || NewFI == OldFI)
360       continue;
361     MO.setIndex(NewFI);
362   }
363 
364   // The MachineMemOperands have already been updated.
365 }
366 
367 
368 /// RemoveDeadStores - Scan through a basic block and look for loads followed
369 /// by stores.  If they're both using the same stack slot, then the store is
370 /// definitely dead.  This could obviously be much more aggressive (consider
371 /// pairs with instructions between them), but such extensions might have a
372 /// considerable compile time impact.
RemoveDeadStores(MachineBasicBlock * MBB)373 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
374   // FIXME: This could be much more aggressive, but we need to investigate
375   // the compile time impact of doing so.
376   bool changed = false;
377 
378   SmallVector<MachineInstr*, 4> toErase;
379 
380   for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
381        I != E; ++I) {
382     if (DCELimit != -1 && (int)NumDead >= DCELimit)
383       break;
384 
385     int FirstSS, SecondSS;
386     if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
387         FirstSS != -1) {
388       ++NumDead;
389       changed = true;
390       toErase.push_back(&*I);
391       continue;
392     }
393 
394     MachineBasicBlock::iterator NextMI = std::next(I);
395     if (NextMI == MBB->end()) continue;
396 
397     unsigned LoadReg = 0;
398     unsigned StoreReg = 0;
399     if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS)))
400       continue;
401     if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS)))
402       continue;
403     if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
404 
405     ++NumDead;
406     changed = true;
407 
408     if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
409       ++NumDead;
410       toErase.push_back(&*I);
411     }
412 
413     toErase.push_back(&*NextMI);
414     ++I;
415   }
416 
417   for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
418        E = toErase.end(); I != E; ++I)
419     (*I)->eraseFromParent();
420 
421   return changed;
422 }
423 
424 
runOnMachineFunction(MachineFunction & MF)425 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
426   DEBUG({
427       dbgs() << "********** Stack Slot Coloring **********\n"
428              << "********** Function: " << MF.getName() << '\n';
429     });
430 
431   MFI = MF.getFrameInfo();
432   TII = MF.getSubtarget().getInstrInfo();
433   LS = &getAnalysis<LiveStacks>();
434   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
435 
436   bool Changed = false;
437 
438   unsigned NumSlots = LS->getNumIntervals();
439   if (NumSlots == 0)
440     // Nothing to do!
441     return false;
442 
443   // If there are calls to setjmp or sigsetjmp, don't perform stack slot
444   // coloring. The stack could be modified before the longjmp is executed,
445   // resulting in the wrong value being used afterwards. (See
446   // <rdar://problem/8007500>.)
447   if (MF.exposesReturnsTwice())
448     return false;
449 
450   // Gather spill slot references
451   ScanForSpillSlotRefs(MF);
452   InitializeSlots();
453   Changed = ColorSlots(MF);
454 
455   NextColor = -1;
456   SSIntervals.clear();
457   for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
458     SSRefs[i].clear();
459   SSRefs.clear();
460   OrigAlignments.clear();
461   OrigSizes.clear();
462   AllColors.clear();
463   UsedColors.clear();
464   for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
465     Assignments[i].clear();
466   Assignments.clear();
467 
468   return Changed;
469 }
470