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/TargetMachine.h"
32 #include "llvm/Target/TargetSubtargetInfo.h"
33 #include <vector>
34 using namespace llvm;
35 
36 #define DEBUG_TYPE "stackslotcoloring"
37 
38 static cl::opt<bool>
39 DisableSharing("no-stack-slot-sharing",
40              cl::init(false), cl::Hidden,
41              cl::desc("Suppress slot sharing during stack coloring"));
42 
43 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
44 
45 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
46 STATISTIC(NumDead,       "Number of trivially dead stack accesses eliminated");
47 
48 namespace {
49   class StackSlotColoring : public MachineFunctionPass {
50     LiveStacks* LS;
51     MachineFrameInfo *MFI;
52     const TargetInstrInfo  *TII;
53     const MachineBlockFrequencyInfo *MBFI;
54 
55     // SSIntervals - Spill slot intervals.
56     std::vector<LiveInterval*> SSIntervals;
57 
58     // SSRefs - Keep a list of MachineMemOperands for each spill slot.
59     // MachineMemOperands can be shared between instructions, so we need
60     // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
61     // become FI0 -> FI1 -> FI2.
62     SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
63 
64     // OrigAlignments - Alignments of stack objects before coloring.
65     SmallVector<unsigned, 16> OrigAlignments;
66 
67     // OrigSizes - Sizess of stack objects before coloring.
68     SmallVector<unsigned, 16> OrigSizes;
69 
70     // AllColors - If index is set, it's a spill slot, i.e. color.
71     // FIXME: This assumes PEI locate spill slot with smaller indices
72     // closest to stack pointer / frame pointer. Therefore, smaller
73     // index == better color.
74     BitVector AllColors;
75 
76     // NextColor - Next "color" that's not yet used.
77     int NextColor;
78 
79     // UsedColors - "Colors" that have been assigned.
80     BitVector UsedColors;
81 
82     // Assignments - Color to intervals mapping.
83     SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
84 
85   public:
86     static char ID; // Pass identification
87     StackSlotColoring() :
88       MachineFunctionPass(ID), NextColor(-1) {
89         initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
90       }
91 
92     void getAnalysisUsage(AnalysisUsage &AU) const override {
93       AU.setPreservesCFG();
94       AU.addRequired<SlotIndexes>();
95       AU.addPreserved<SlotIndexes>();
96       AU.addRequired<LiveStacks>();
97       AU.addRequired<MachineBlockFrequencyInfo>();
98       AU.addPreserved<MachineBlockFrequencyInfo>();
99       AU.addPreservedID(MachineDominatorsID);
100       MachineFunctionPass::getAnalysisUsage(AU);
101     }
102 
103     bool runOnMachineFunction(MachineFunction &MF) override;
104 
105   private:
106     void InitializeSlots();
107     void ScanForSpillSlotRefs(MachineFunction &MF);
108     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
109     int ColorSlot(LiveInterval *li);
110     bool ColorSlots(MachineFunction &MF);
111     void RewriteInstruction(MachineInstr *MI, SmallVectorImpl<int> &SlotMapping,
112                             MachineFunction &MF);
113     bool RemoveDeadStores(MachineBasicBlock* MBB);
114   };
115 } // end anonymous namespace
116 
117 char StackSlotColoring::ID = 0;
118 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
119 
120 INITIALIZE_PASS_BEGIN(StackSlotColoring, "stack-slot-coloring",
121                 "Stack Slot Coloring", false, false)
122 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
123 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
124 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
125 INITIALIZE_PASS_END(StackSlotColoring, "stack-slot-coloring",
126                 "Stack Slot Coloring", false, false)
127 
128 namespace {
129   // IntervalSorter - Comparison predicate that sort live intervals by
130   // their weight.
131   struct IntervalSorter {
132     bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
133       return LHS->weight > RHS->weight;
134     }
135   };
136 }
137 
138 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
139 /// references and update spill slot weights.
140 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
141   SSRefs.resize(MFI->getObjectIndexEnd());
142 
143   // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
144   for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
145        MBBI != E; ++MBBI) {
146     MachineBasicBlock *MBB = &*MBBI;
147     for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
148          MII != EE; ++MII) {
149       MachineInstr *MI = &*MII;
150       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
151         MachineOperand &MO = MI->getOperand(i);
152         if (!MO.isFI())
153           continue;
154         int FI = MO.getIndex();
155         if (FI < 0)
156           continue;
157         if (!LS->hasInterval(FI))
158           continue;
159         LiveInterval &li = LS->getInterval(FI);
160         if (!MI->isDebugValue())
161           li.weight += LiveIntervals::getSpillWeight(false, true, MBFI, MI);
162       }
163       for (MachineInstr::mmo_iterator MMOI = MI->memoperands_begin(),
164            EE = MI->memoperands_end(); 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.
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   // 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.
212 bool
213 StackSlotColoring::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 ///
225 int 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.
269 bool 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 PseudoSourceValue *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.
342 void StackSlotColoring::RewriteInstruction(MachineInstr *MI,
343                                            SmallVectorImpl<int> &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.
368 bool 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 = std::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, nullptr) != -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 
419 bool 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.getSubtarget().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