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