1 //===- StackSlotColoring.cpp - Stack slot coloring pass. ------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements the stack slot coloring pass.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/ADT/BitVector.h"
14 #include "llvm/ADT/SmallVector.h"
15 #include "llvm/ADT/Statistic.h"
16 #include "llvm/CodeGen/LiveInterval.h"
17 #include "llvm/CodeGen/LiveIntervals.h"
18 #include "llvm/CodeGen/LiveStacks.h"
19 #include "llvm/CodeGen/MachineBasicBlock.h"
20 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
21 #include "llvm/CodeGen/MachineFrameInfo.h"
22 #include "llvm/CodeGen/MachineFunction.h"
23 #include "llvm/CodeGen/MachineFunctionPass.h"
24 #include "llvm/CodeGen/MachineInstr.h"
25 #include "llvm/CodeGen/MachineMemOperand.h"
26 #include "llvm/CodeGen/MachineOperand.h"
27 #include "llvm/CodeGen/Passes.h"
28 #include "llvm/CodeGen/PseudoSourceValue.h"
29 #include "llvm/CodeGen/SlotIndexes.h"
30 #include "llvm/CodeGen/TargetInstrInfo.h"
31 #include "llvm/CodeGen/TargetRegisterInfo.h"
32 #include "llvm/CodeGen/TargetSubtargetInfo.h"
33 #include "llvm/InitializePasses.h"
34 #include "llvm/Pass.h"
35 #include "llvm/Support/Casting.h"
36 #include "llvm/Support/CommandLine.h"
37 #include "llvm/Support/Debug.h"
38 #include "llvm/Support/raw_ostream.h"
39 #include <algorithm>
40 #include <cassert>
41 #include <cstdint>
42 #include <iterator>
43 #include <vector>
44
45 using namespace llvm;
46
47 #define DEBUG_TYPE "stack-slot-coloring"
48
49 static cl::opt<bool>
50 DisableSharing("no-stack-slot-sharing",
51 cl::init(false), cl::Hidden,
52 cl::desc("Suppress slot sharing during stack coloring"));
53
54 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
55
56 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
57 STATISTIC(NumDead, "Number of trivially dead stack accesses eliminated");
58
59 namespace {
60
61 class StackSlotColoring : public MachineFunctionPass {
62 LiveStacks* LS;
63 MachineFrameInfo *MFI;
64 const TargetInstrInfo *TII;
65 const MachineBlockFrequencyInfo *MBFI;
66
67 // SSIntervals - Spill slot intervals.
68 std::vector<LiveInterval*> SSIntervals;
69
70 // SSRefs - Keep a list of MachineMemOperands for each spill slot.
71 // MachineMemOperands can be shared between instructions, so we need
72 // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
73 // become FI0 -> FI1 -> FI2.
74 SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
75
76 // OrigAlignments - Alignments of stack objects before coloring.
77 SmallVector<Align, 16> OrigAlignments;
78
79 // OrigSizes - Sizess of stack objects before coloring.
80 SmallVector<unsigned, 16> OrigSizes;
81
82 // AllColors - If index is set, it's a spill slot, i.e. color.
83 // FIXME: This assumes PEI locate spill slot with smaller indices
84 // closest to stack pointer / frame pointer. Therefore, smaller
85 // index == better color. This is per stack ID.
86 SmallVector<BitVector, 2> AllColors;
87
88 // NextColor - Next "color" that's not yet used. This is per stack ID.
89 SmallVector<int, 2> NextColors = { -1 };
90
91 // UsedColors - "Colors" that have been assigned. This is per stack ID
92 SmallVector<BitVector, 2> UsedColors;
93
94 // Assignments - Color to intervals mapping.
95 SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments;
96
97 public:
98 static char ID; // Pass identification
99
StackSlotColoring()100 StackSlotColoring() : MachineFunctionPass(ID) {
101 initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
102 }
103
getAnalysisUsage(AnalysisUsage & AU) const104 void getAnalysisUsage(AnalysisUsage &AU) const override {
105 AU.setPreservesCFG();
106 AU.addRequired<SlotIndexes>();
107 AU.addPreserved<SlotIndexes>();
108 AU.addRequired<LiveStacks>();
109 AU.addRequired<MachineBlockFrequencyInfo>();
110 AU.addPreserved<MachineBlockFrequencyInfo>();
111 AU.addPreservedID(MachineDominatorsID);
112 MachineFunctionPass::getAnalysisUsage(AU);
113 }
114
115 bool runOnMachineFunction(MachineFunction &MF) override;
116
117 private:
118 void InitializeSlots();
119 void ScanForSpillSlotRefs(MachineFunction &MF);
120 bool OverlapWithAssignments(LiveInterval *li, int Color) const;
121 int ColorSlot(LiveInterval *li);
122 bool ColorSlots(MachineFunction &MF);
123 void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping,
124 MachineFunction &MF);
125 bool RemoveDeadStores(MachineBasicBlock* MBB);
126 };
127
128 } // end anonymous namespace
129
130 char StackSlotColoring::ID = 0;
131
132 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
133
134 INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE,
135 "Stack Slot Coloring", false, false)
136 INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
137 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
138 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
139 INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE,
140 "Stack Slot Coloring", false, false)
141
142 namespace {
143
144 // IntervalSorter - Comparison predicate that sort live intervals by
145 // their weight.
146 struct IntervalSorter {
operator ()__anon169487210211::IntervalSorter147 bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
148 return LHS->weight() > RHS->weight();
149 }
150 };
151
152 } // end anonymous namespace
153
154 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
155 /// references and update spill slot weights.
ScanForSpillSlotRefs(MachineFunction & MF)156 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
157 SSRefs.resize(MFI->getObjectIndexEnd());
158
159 // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
160 for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end();
161 MBBI != E; ++MBBI) {
162 MachineBasicBlock *MBB = &*MBBI;
163 for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end();
164 MII != EE; ++MII) {
165 MachineInstr &MI = *MII;
166 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
167 MachineOperand &MO = MI.getOperand(i);
168 if (!MO.isFI())
169 continue;
170 int FI = MO.getIndex();
171 if (FI < 0)
172 continue;
173 if (!LS->hasInterval(FI))
174 continue;
175 LiveInterval &li = LS->getInterval(FI);
176 if (!MI.isDebugValue())
177 li.incrementWeight(
178 LiveIntervals::getSpillWeight(false, true, MBFI, MI));
179 }
180 for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
181 EE = MI.memoperands_end();
182 MMOI != EE; ++MMOI) {
183 MachineMemOperand *MMO = *MMOI;
184 if (const FixedStackPseudoSourceValue *FSV =
185 dyn_cast_or_null<FixedStackPseudoSourceValue>(
186 MMO->getPseudoValue())) {
187 int FI = FSV->getFrameIndex();
188 if (FI >= 0)
189 SSRefs[FI].push_back(MMO);
190 }
191 }
192 }
193 }
194 }
195
196 /// InitializeSlots - Process all spill stack slot liveintervals and add them
197 /// to a sorted (by weight) list.
InitializeSlots()198 void StackSlotColoring::InitializeSlots() {
199 int LastFI = MFI->getObjectIndexEnd();
200
201 // There is always at least one stack ID.
202 AllColors.resize(1);
203 UsedColors.resize(1);
204
205 OrigAlignments.resize(LastFI);
206 OrigSizes.resize(LastFI);
207 AllColors[0].resize(LastFI);
208 UsedColors[0].resize(LastFI);
209 Assignments.resize(LastFI);
210
211 using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
212
213 SmallVector<Pair *, 16> Intervals;
214
215 Intervals.reserve(LS->getNumIntervals());
216 for (auto &I : *LS)
217 Intervals.push_back(&I);
218 llvm::sort(Intervals,
219 [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
220
221 // Gather all spill slots into a list.
222 LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
223 for (auto *I : Intervals) {
224 LiveInterval &li = I->second;
225 LLVM_DEBUG(li.dump());
226 int FI = Register::stackSlot2Index(li.reg());
227 if (MFI->isDeadObjectIndex(FI))
228 continue;
229
230 SSIntervals.push_back(&li);
231 OrigAlignments[FI] = MFI->getObjectAlign(FI);
232 OrigSizes[FI] = MFI->getObjectSize(FI);
233
234 auto StackID = MFI->getStackID(FI);
235 if (StackID != 0) {
236 AllColors.resize(StackID + 1);
237 UsedColors.resize(StackID + 1);
238 AllColors[StackID].resize(LastFI);
239 UsedColors[StackID].resize(LastFI);
240 }
241
242 AllColors[StackID].set(FI);
243 }
244 LLVM_DEBUG(dbgs() << '\n');
245
246 // Sort them by weight.
247 llvm::stable_sort(SSIntervals, IntervalSorter());
248
249 NextColors.resize(AllColors.size());
250
251 // Get first "color".
252 for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
253 NextColors[I] = AllColors[I].find_first();
254 }
255
256 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
257 /// LiveIntervals that have already been assigned to the specified color.
258 bool
OverlapWithAssignments(LiveInterval * li,int Color) const259 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
260 const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
261 for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
262 LiveInterval *OtherLI = OtherLIs[i];
263 if (OtherLI->overlaps(*li))
264 return true;
265 }
266 return false;
267 }
268
269 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
ColorSlot(LiveInterval * li)270 int StackSlotColoring::ColorSlot(LiveInterval *li) {
271 int Color = -1;
272 bool Share = false;
273 int FI = Register::stackSlot2Index(li->reg());
274 uint8_t StackID = MFI->getStackID(FI);
275
276 if (!DisableSharing) {
277
278 // Check if it's possible to reuse any of the used colors.
279 Color = UsedColors[StackID].find_first();
280 while (Color != -1) {
281 if (!OverlapWithAssignments(li, Color)) {
282 Share = true;
283 ++NumEliminated;
284 break;
285 }
286 Color = UsedColors[StackID].find_next(Color);
287 }
288 }
289
290 if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
291 LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
292 Share = false;
293 }
294
295 // Assign it to the first available color (assumed to be the best) if it's
296 // not possible to share a used color with other objects.
297 if (!Share) {
298 assert(NextColors[StackID] != -1 && "No more spill slots?");
299 Color = NextColors[StackID];
300 UsedColors[StackID].set(Color);
301 NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
302 }
303
304 assert(MFI->getStackID(Color) == MFI->getStackID(FI));
305
306 // Record the assignment.
307 Assignments[Color].push_back(li);
308 LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
309
310 // Change size and alignment of the allocated slot. If there are multiple
311 // objects sharing the same slot, then make sure the size and alignment
312 // are large enough for all.
313 Align Alignment = OrigAlignments[FI];
314 if (!Share || Alignment > MFI->getObjectAlign(Color))
315 MFI->setObjectAlignment(Color, Alignment);
316 int64_t Size = OrigSizes[FI];
317 if (!Share || Size > MFI->getObjectSize(Color))
318 MFI->setObjectSize(Color, Size);
319 return Color;
320 }
321
322 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
323 /// operands in the function.
ColorSlots(MachineFunction & MF)324 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
325 unsigned NumObjs = MFI->getObjectIndexEnd();
326 SmallVector<int, 16> SlotMapping(NumObjs, -1);
327 SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
328 SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
329 BitVector UsedColors(NumObjs);
330
331 LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
332 bool Changed = false;
333 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
334 LiveInterval *li = SSIntervals[i];
335 int SS = Register::stackSlot2Index(li->reg());
336 int NewSS = ColorSlot(li);
337 assert(NewSS >= 0 && "Stack coloring failed?");
338 SlotMapping[SS] = NewSS;
339 RevMap[NewSS].push_back(SS);
340 SlotWeights[NewSS] += li->weight();
341 UsedColors.set(NewSS);
342 Changed |= (SS != NewSS);
343 }
344
345 LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
346 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
347 LiveInterval *li = SSIntervals[i];
348 int SS = Register::stackSlot2Index(li->reg());
349 li->setWeight(SlotWeights[SS]);
350 }
351 // Sort them by new weight.
352 llvm::stable_sort(SSIntervals, IntervalSorter());
353
354 #ifndef NDEBUG
355 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
356 LLVM_DEBUG(SSIntervals[i]->dump());
357 LLVM_DEBUG(dbgs() << '\n');
358 #endif
359
360 if (!Changed)
361 return false;
362
363 // Rewrite all MachineMemOperands.
364 for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
365 int NewFI = SlotMapping[SS];
366 if (NewFI == -1 || (NewFI == (int)SS))
367 continue;
368
369 const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
370 SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
371 for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
372 RefMMOs[i]->setValue(NewSV);
373 }
374
375 // Rewrite all MO_FrameIndex operands. Look for dead stores.
376 for (MachineBasicBlock &MBB : MF) {
377 for (MachineInstr &MI : MBB)
378 RewriteInstruction(MI, SlotMapping, MF);
379 RemoveDeadStores(&MBB);
380 }
381
382 // Delete unused stack slots.
383 for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
384 int NextColor = NextColors[StackID];
385 while (NextColor != -1) {
386 LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
387 MFI->RemoveStackObject(NextColor);
388 NextColor = AllColors[StackID].find_next(NextColor);
389 }
390 }
391
392 return true;
393 }
394
395 /// RewriteInstruction - Rewrite specified instruction by replacing references
396 /// to old frame index with new one.
RewriteInstruction(MachineInstr & MI,SmallVectorImpl<int> & SlotMapping,MachineFunction & MF)397 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
398 SmallVectorImpl<int> &SlotMapping,
399 MachineFunction &MF) {
400 // Update the operands.
401 for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) {
402 MachineOperand &MO = MI.getOperand(i);
403 if (!MO.isFI())
404 continue;
405 int OldFI = MO.getIndex();
406 if (OldFI < 0)
407 continue;
408 int NewFI = SlotMapping[OldFI];
409 if (NewFI == -1 || NewFI == OldFI)
410 continue;
411
412 assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
413 MO.setIndex(NewFI);
414 }
415
416 // The MachineMemOperands have already been updated.
417 }
418
419 /// RemoveDeadStores - Scan through a basic block and look for loads followed
420 /// by stores. If they're both using the same stack slot, then the store is
421 /// definitely dead. This could obviously be much more aggressive (consider
422 /// pairs with instructions between them), but such extensions might have a
423 /// considerable compile time impact.
RemoveDeadStores(MachineBasicBlock * MBB)424 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
425 // FIXME: This could be much more aggressive, but we need to investigate
426 // the compile time impact of doing so.
427 bool changed = false;
428
429 SmallVector<MachineInstr*, 4> toErase;
430
431 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
432 I != E; ++I) {
433 if (DCELimit != -1 && (int)NumDead >= DCELimit)
434 break;
435 int FirstSS, SecondSS;
436 if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
437 FirstSS != -1) {
438 ++NumDead;
439 changed = true;
440 toErase.push_back(&*I);
441 continue;
442 }
443
444 MachineBasicBlock::iterator NextMI = std::next(I);
445 MachineBasicBlock::iterator ProbableLoadMI = I;
446
447 unsigned LoadReg = 0;
448 unsigned StoreReg = 0;
449 unsigned LoadSize = 0;
450 unsigned StoreSize = 0;
451 if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
452 continue;
453 // Skip the ...pseudo debugging... instructions between a load and store.
454 while ((NextMI != E) && NextMI->isDebugInstr()) {
455 ++NextMI;
456 ++I;
457 }
458 if (NextMI == E) continue;
459 if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
460 continue;
461 if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
462 LoadSize != StoreSize)
463 continue;
464
465 ++NumDead;
466 changed = true;
467
468 if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
469 ++NumDead;
470 toErase.push_back(&*ProbableLoadMI);
471 }
472
473 toErase.push_back(&*NextMI);
474 ++I;
475 }
476
477 for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(),
478 E = toErase.end(); I != E; ++I)
479 (*I)->eraseFromParent();
480
481 return changed;
482 }
483
runOnMachineFunction(MachineFunction & MF)484 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
485 LLVM_DEBUG({
486 dbgs() << "********** Stack Slot Coloring **********\n"
487 << "********** Function: " << MF.getName() << '\n';
488 });
489
490 if (skipFunction(MF.getFunction()))
491 return false;
492
493 MFI = &MF.getFrameInfo();
494 TII = MF.getSubtarget().getInstrInfo();
495 LS = &getAnalysis<LiveStacks>();
496 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
497
498 bool Changed = false;
499
500 unsigned NumSlots = LS->getNumIntervals();
501 if (NumSlots == 0)
502 // Nothing to do!
503 return false;
504
505 // If there are calls to setjmp or sigsetjmp, don't perform stack slot
506 // coloring. The stack could be modified before the longjmp is executed,
507 // resulting in the wrong value being used afterwards. (See
508 // <rdar://problem/8007500>.)
509 if (MF.exposesReturnsTwice())
510 return false;
511
512 // Gather spill slot references
513 ScanForSpillSlotRefs(MF);
514 InitializeSlots();
515 Changed = ColorSlots(MF);
516
517 for (int &Next : NextColors)
518 Next = -1;
519
520 SSIntervals.clear();
521 for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
522 SSRefs[i].clear();
523 SSRefs.clear();
524 OrigAlignments.clear();
525 OrigSizes.clear();
526 AllColors.clear();
527 UsedColors.clear();
528 for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
529 Assignments[i].clear();
530 Assignments.clear();
531
532 return Changed;
533 }
534