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/LiveDebugVariables.h"
17 #include "llvm/CodeGen/LiveInterval.h"
18 #include "llvm/CodeGen/LiveIntervalUnion.h"
19 #include "llvm/CodeGen/LiveIntervals.h"
20 #include "llvm/CodeGen/LiveStacks.h"
21 #include "llvm/CodeGen/MachineBasicBlock.h"
22 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
23 #include "llvm/CodeGen/MachineFrameInfo.h"
24 #include "llvm/CodeGen/MachineFunction.h"
25 #include "llvm/CodeGen/MachineFunctionPass.h"
26 #include "llvm/CodeGen/MachineInstr.h"
27 #include "llvm/CodeGen/MachineMemOperand.h"
28 #include "llvm/CodeGen/MachineOperand.h"
29 #include "llvm/CodeGen/Passes.h"
30 #include "llvm/CodeGen/PseudoSourceValue.h"
31 #include "llvm/CodeGen/PseudoSourceValueManager.h"
32 #include "llvm/CodeGen/SlotIndexes.h"
33 #include "llvm/CodeGen/TargetInstrInfo.h"
34 #include "llvm/CodeGen/TargetSubtargetInfo.h"
35 #include "llvm/InitializePasses.h"
36 #include "llvm/Pass.h"
37 #include "llvm/Support/Casting.h"
38 #include "llvm/Support/CommandLine.h"
39 #include "llvm/Support/Debug.h"
40 #include "llvm/Support/raw_ostream.h"
41 #include <algorithm>
42 #include <cassert>
43 #include <cstdint>
44 #include <iterator>
45 #include <vector>
46
47 using namespace llvm;
48
49 #define DEBUG_TYPE "stack-slot-coloring"
50
51 static cl::opt<bool>
52 DisableSharing("no-stack-slot-sharing",
53 cl::init(false), cl::Hidden,
54 cl::desc("Suppress slot sharing during stack coloring"));
55
56 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden);
57
58 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring");
59 STATISTIC(NumDead, "Number of trivially dead stack accesses eliminated");
60
61 namespace {
62
63 class StackSlotColoring : public MachineFunctionPass {
64 LiveStacks *LS = nullptr;
65 MachineFrameInfo *MFI = nullptr;
66 const TargetInstrInfo *TII = nullptr;
67 const MachineBlockFrequencyInfo *MBFI = nullptr;
68 SlotIndexes *Indexes = nullptr;
69
70 // SSIntervals - Spill slot intervals.
71 std::vector<LiveInterval*> SSIntervals;
72
73 // SSRefs - Keep a list of MachineMemOperands for each spill slot.
74 // MachineMemOperands can be shared between instructions, so we need
75 // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not
76 // become FI0 -> FI1 -> FI2.
77 SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs;
78
79 // OrigAlignments - Alignments of stack objects before coloring.
80 SmallVector<Align, 16> OrigAlignments;
81
82 // OrigSizes - Sizes of stack objects before coloring.
83 SmallVector<unsigned, 16> OrigSizes;
84
85 // AllColors - If index is set, it's a spill slot, i.e. color.
86 // FIXME: This assumes PEI locate spill slot with smaller indices
87 // closest to stack pointer / frame pointer. Therefore, smaller
88 // index == better color. This is per stack ID.
89 SmallVector<BitVector, 2> AllColors;
90
91 // NextColor - Next "color" that's not yet used. This is per stack ID.
92 SmallVector<int, 2> NextColors = { -1 };
93
94 // UsedColors - "Colors" that have been assigned. This is per stack ID
95 SmallVector<BitVector, 2> UsedColors;
96
97 // Join all intervals sharing one color into a single LiveIntervalUnion to
98 // speedup range overlap test.
99 class ColorAssignmentInfo {
100 // Single liverange (used to avoid creation of LiveIntervalUnion).
101 LiveInterval *SingleLI = nullptr;
102 // LiveIntervalUnion to perform overlap test.
103 LiveIntervalUnion *LIU = nullptr;
104 // LiveIntervalUnion has a parameter in its constructor so doing this
105 // dirty magic.
106 uint8_t LIUPad[sizeof(LiveIntervalUnion)];
107
108 public:
~ColorAssignmentInfo()109 ~ColorAssignmentInfo() {
110 if (LIU)
111 LIU->~LiveIntervalUnion(); // Dirty magic again.
112 }
113
114 // Return true if LiveInterval overlaps with any
115 // intervals that have already been assigned to this color.
overlaps(LiveInterval * LI) const116 bool overlaps(LiveInterval *LI) const {
117 if (LIU)
118 return LiveIntervalUnion::Query(*LI, *LIU).checkInterference();
119 return SingleLI ? SingleLI->overlaps(*LI) : false;
120 }
121
122 // Add new LiveInterval to this color.
add(LiveInterval * LI,LiveIntervalUnion::Allocator & Alloc)123 void add(LiveInterval *LI, LiveIntervalUnion::Allocator &Alloc) {
124 assert(!overlaps(LI));
125 if (LIU) {
126 LIU->unify(*LI, *LI);
127 } else if (SingleLI) {
128 LIU = new (LIUPad) LiveIntervalUnion(Alloc);
129 LIU->unify(*SingleLI, *SingleLI);
130 LIU->unify(*LI, *LI);
131 SingleLI = nullptr;
132 } else
133 SingleLI = LI;
134 }
135 };
136
137 LiveIntervalUnion::Allocator LIUAlloc;
138
139 // Assignments - Color to intervals mapping.
140 SmallVector<ColorAssignmentInfo, 16> Assignments;
141
142 public:
143 static char ID; // Pass identification
144
StackSlotColoring()145 StackSlotColoring() : MachineFunctionPass(ID) {
146 initializeStackSlotColoringPass(*PassRegistry::getPassRegistry());
147 }
148
getAnalysisUsage(AnalysisUsage & AU) const149 void getAnalysisUsage(AnalysisUsage &AU) const override {
150 AU.setPreservesCFG();
151 AU.addRequired<SlotIndexesWrapperPass>();
152 AU.addPreserved<SlotIndexesWrapperPass>();
153 AU.addRequired<LiveStacks>();
154 AU.addRequired<MachineBlockFrequencyInfoWrapperPass>();
155 AU.addPreserved<MachineBlockFrequencyInfoWrapperPass>();
156 AU.addPreservedID(MachineDominatorsID);
157
158 // In some Target's pipeline, register allocation (RA) might be
159 // split into multiple phases based on register class. So, this pass
160 // may be invoked multiple times requiring it to save these analyses to be
161 // used by RA later.
162 AU.addPreserved<LiveIntervalsWrapperPass>();
163 AU.addPreserved<LiveDebugVariables>();
164
165 MachineFunctionPass::getAnalysisUsage(AU);
166 }
167
168 bool runOnMachineFunction(MachineFunction &MF) override;
169
170 private:
171 void InitializeSlots();
172 void ScanForSpillSlotRefs(MachineFunction &MF);
173 int ColorSlot(LiveInterval *li);
174 bool ColorSlots(MachineFunction &MF);
175 void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping,
176 MachineFunction &MF);
177 bool RemoveDeadStores(MachineBasicBlock* MBB);
178 };
179
180 } // end anonymous namespace
181
182 char StackSlotColoring::ID = 0;
183
184 char &llvm::StackSlotColoringID = StackSlotColoring::ID;
185
186 INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE,
187 "Stack Slot Coloring", false, false)
188 INITIALIZE_PASS_DEPENDENCY(SlotIndexesWrapperPass)
189 INITIALIZE_PASS_DEPENDENCY(LiveStacks)
190 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfoWrapperPass)
191 INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE,
192 "Stack Slot Coloring", false, false)
193
194 namespace {
195
196 // IntervalSorter - Comparison predicate that sort live intervals by
197 // their weight.
198 struct IntervalSorter {
operator ()__anonb7b65c290211::IntervalSorter199 bool operator()(LiveInterval* LHS, LiveInterval* RHS) const {
200 return LHS->weight() > RHS->weight();
201 }
202 };
203
204 } // end anonymous namespace
205
206 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot
207 /// references and update spill slot weights.
ScanForSpillSlotRefs(MachineFunction & MF)208 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) {
209 SSRefs.resize(MFI->getObjectIndexEnd());
210
211 // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands.
212 for (MachineBasicBlock &MBB : MF) {
213 for (MachineInstr &MI : MBB) {
214 for (const MachineOperand &MO : MI.operands()) {
215 if (!MO.isFI())
216 continue;
217 int FI = MO.getIndex();
218 if (FI < 0)
219 continue;
220 if (!LS->hasInterval(FI))
221 continue;
222 LiveInterval &li = LS->getInterval(FI);
223 if (!MI.isDebugInstr())
224 li.incrementWeight(
225 LiveIntervals::getSpillWeight(false, true, MBFI, MI));
226 }
227 for (MachineMemOperand *MMO : MI.memoperands()) {
228 if (const FixedStackPseudoSourceValue *FSV =
229 dyn_cast_or_null<FixedStackPseudoSourceValue>(
230 MMO->getPseudoValue())) {
231 int FI = FSV->getFrameIndex();
232 if (FI >= 0)
233 SSRefs[FI].push_back(MMO);
234 }
235 }
236 }
237 }
238 }
239
240 /// InitializeSlots - Process all spill stack slot liveintervals and add them
241 /// to a sorted (by weight) list.
InitializeSlots()242 void StackSlotColoring::InitializeSlots() {
243 int LastFI = MFI->getObjectIndexEnd();
244
245 // There is always at least one stack ID.
246 AllColors.resize(1);
247 UsedColors.resize(1);
248
249 OrigAlignments.resize(LastFI);
250 OrigSizes.resize(LastFI);
251 AllColors[0].resize(LastFI);
252 UsedColors[0].resize(LastFI);
253 Assignments.resize(LastFI);
254
255 using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
256
257 SmallVector<Pair *, 16> Intervals;
258
259 Intervals.reserve(LS->getNumIntervals());
260 for (auto &I : *LS)
261 Intervals.push_back(&I);
262 llvm::sort(Intervals,
263 [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
264
265 // Gather all spill slots into a list.
266 LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
267 for (auto *I : Intervals) {
268 LiveInterval &li = I->second;
269 LLVM_DEBUG(li.dump());
270 int FI = Register::stackSlot2Index(li.reg());
271 if (MFI->isDeadObjectIndex(FI))
272 continue;
273
274 SSIntervals.push_back(&li);
275 OrigAlignments[FI] = MFI->getObjectAlign(FI);
276 OrigSizes[FI] = MFI->getObjectSize(FI);
277
278 auto StackID = MFI->getStackID(FI);
279 if (StackID != 0) {
280 AllColors.resize(StackID + 1);
281 UsedColors.resize(StackID + 1);
282 AllColors[StackID].resize(LastFI);
283 UsedColors[StackID].resize(LastFI);
284 }
285
286 AllColors[StackID].set(FI);
287 }
288 LLVM_DEBUG(dbgs() << '\n');
289
290 // Sort them by weight.
291 llvm::stable_sort(SSIntervals, IntervalSorter());
292
293 NextColors.resize(AllColors.size());
294
295 // Get first "color".
296 for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
297 NextColors[I] = AllColors[I].find_first();
298 }
299
300 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
ColorSlot(LiveInterval * li)301 int StackSlotColoring::ColorSlot(LiveInterval *li) {
302 int Color = -1;
303 bool Share = false;
304 int FI = Register::stackSlot2Index(li->reg());
305 uint8_t StackID = MFI->getStackID(FI);
306
307 if (!DisableSharing) {
308
309 // Check if it's possible to reuse any of the used colors.
310 Color = UsedColors[StackID].find_first();
311 while (Color != -1) {
312 if (!Assignments[Color].overlaps(li)) {
313 Share = true;
314 ++NumEliminated;
315 break;
316 }
317 Color = UsedColors[StackID].find_next(Color);
318 }
319 }
320
321 if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
322 LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
323 Share = false;
324 }
325
326 // Assign it to the first available color (assumed to be the best) if it's
327 // not possible to share a used color with other objects.
328 if (!Share) {
329 assert(NextColors[StackID] != -1 && "No more spill slots?");
330 Color = NextColors[StackID];
331 UsedColors[StackID].set(Color);
332 NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
333 }
334
335 assert(MFI->getStackID(Color) == MFI->getStackID(FI));
336
337 // Record the assignment.
338 Assignments[Color].add(li, LIUAlloc);
339 LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
340
341 // Change size and alignment of the allocated slot. If there are multiple
342 // objects sharing the same slot, then make sure the size and alignment
343 // are large enough for all.
344 Align Alignment = OrigAlignments[FI];
345 if (!Share || Alignment > MFI->getObjectAlign(Color))
346 MFI->setObjectAlignment(Color, Alignment);
347 int64_t Size = OrigSizes[FI];
348 if (!Share || Size > MFI->getObjectSize(Color))
349 MFI->setObjectSize(Color, Size);
350 return Color;
351 }
352
353 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
354 /// operands in the function.
ColorSlots(MachineFunction & MF)355 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
356 unsigned NumObjs = MFI->getObjectIndexEnd();
357 SmallVector<int, 16> SlotMapping(NumObjs, -1);
358 SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
359 SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
360 BitVector UsedColors(NumObjs);
361
362 LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
363 bool Changed = false;
364 for (LiveInterval *li : SSIntervals) {
365 int SS = Register::stackSlot2Index(li->reg());
366 int NewSS = ColorSlot(li);
367 assert(NewSS >= 0 && "Stack coloring failed?");
368 SlotMapping[SS] = NewSS;
369 RevMap[NewSS].push_back(SS);
370 SlotWeights[NewSS] += li->weight();
371 UsedColors.set(NewSS);
372 Changed |= (SS != NewSS);
373 }
374
375 LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
376 for (LiveInterval *li : SSIntervals) {
377 int SS = Register::stackSlot2Index(li->reg());
378 li->setWeight(SlotWeights[SS]);
379 }
380 // Sort them by new weight.
381 llvm::stable_sort(SSIntervals, IntervalSorter());
382
383 #ifndef NDEBUG
384 for (LiveInterval *li : SSIntervals)
385 LLVM_DEBUG(li->dump());
386 LLVM_DEBUG(dbgs() << '\n');
387 #endif
388
389 if (!Changed)
390 return false;
391
392 // Rewrite all MachineMemOperands.
393 for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
394 int NewFI = SlotMapping[SS];
395 if (NewFI == -1 || (NewFI == (int)SS))
396 continue;
397
398 const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
399 SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
400 for (MachineMemOperand *MMO : RefMMOs)
401 MMO->setValue(NewSV);
402 }
403
404 // Rewrite all MO_FrameIndex operands. Look for dead stores.
405 for (MachineBasicBlock &MBB : MF) {
406 for (MachineInstr &MI : MBB)
407 RewriteInstruction(MI, SlotMapping, MF);
408 RemoveDeadStores(&MBB);
409 }
410
411 // Delete unused stack slots.
412 for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
413 int NextColor = NextColors[StackID];
414 while (NextColor != -1) {
415 LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
416 MFI->RemoveStackObject(NextColor);
417 NextColor = AllColors[StackID].find_next(NextColor);
418 }
419 }
420
421 return true;
422 }
423
424 /// RewriteInstruction - Rewrite specified instruction by replacing references
425 /// to old frame index with new one.
RewriteInstruction(MachineInstr & MI,SmallVectorImpl<int> & SlotMapping,MachineFunction & MF)426 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
427 SmallVectorImpl<int> &SlotMapping,
428 MachineFunction &MF) {
429 // Update the operands.
430 for (MachineOperand &MO : MI.operands()) {
431 if (!MO.isFI())
432 continue;
433 int OldFI = MO.getIndex();
434 if (OldFI < 0)
435 continue;
436 int NewFI = SlotMapping[OldFI];
437 if (NewFI == -1 || NewFI == OldFI)
438 continue;
439
440 assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
441 MO.setIndex(NewFI);
442 }
443
444 // The MachineMemOperands have already been updated.
445 }
446
447 /// RemoveDeadStores - Scan through a basic block and look for loads followed
448 /// by stores. If they're both using the same stack slot, then the store is
449 /// definitely dead. This could obviously be much more aggressive (consider
450 /// pairs with instructions between them), but such extensions might have a
451 /// considerable compile time impact.
RemoveDeadStores(MachineBasicBlock * MBB)452 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
453 // FIXME: This could be much more aggressive, but we need to investigate
454 // the compile time impact of doing so.
455 bool changed = false;
456
457 SmallVector<MachineInstr*, 4> toErase;
458
459 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
460 I != E; ++I) {
461 if (DCELimit != -1 && (int)NumDead >= DCELimit)
462 break;
463 int FirstSS, SecondSS;
464 if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
465 FirstSS != -1) {
466 ++NumDead;
467 changed = true;
468 toErase.push_back(&*I);
469 continue;
470 }
471
472 MachineBasicBlock::iterator NextMI = std::next(I);
473 MachineBasicBlock::iterator ProbableLoadMI = I;
474
475 unsigned LoadReg = 0;
476 unsigned StoreReg = 0;
477 unsigned LoadSize = 0;
478 unsigned StoreSize = 0;
479 if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
480 continue;
481 // Skip the ...pseudo debugging... instructions between a load and store.
482 while ((NextMI != E) && NextMI->isDebugInstr()) {
483 ++NextMI;
484 ++I;
485 }
486 if (NextMI == E) continue;
487 if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
488 continue;
489 if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
490 LoadSize != StoreSize || !MFI->isSpillSlotObjectIndex(FirstSS))
491 continue;
492
493 ++NumDead;
494 changed = true;
495
496 if (NextMI->findRegisterUseOperandIdx(LoadReg, /*TRI=*/nullptr, true) !=
497 -1) {
498 ++NumDead;
499 toErase.push_back(&*ProbableLoadMI);
500 }
501
502 toErase.push_back(&*NextMI);
503 ++I;
504 }
505
506 for (MachineInstr *MI : toErase) {
507 if (Indexes)
508 Indexes->removeMachineInstrFromMaps(*MI);
509 MI->eraseFromParent();
510 }
511
512 return changed;
513 }
514
runOnMachineFunction(MachineFunction & MF)515 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
516 LLVM_DEBUG({
517 dbgs() << "********** Stack Slot Coloring **********\n"
518 << "********** Function: " << MF.getName() << '\n';
519 });
520
521 if (skipFunction(MF.getFunction()))
522 return false;
523
524 MFI = &MF.getFrameInfo();
525 TII = MF.getSubtarget().getInstrInfo();
526 LS = &getAnalysis<LiveStacks>();
527 MBFI = &getAnalysis<MachineBlockFrequencyInfoWrapperPass>().getMBFI();
528 Indexes = &getAnalysis<SlotIndexesWrapperPass>().getSI();
529
530 bool Changed = false;
531
532 unsigned NumSlots = LS->getNumIntervals();
533 if (NumSlots == 0)
534 // Nothing to do!
535 return false;
536
537 // If there are calls to setjmp or sigsetjmp, don't perform stack slot
538 // coloring. The stack could be modified before the longjmp is executed,
539 // resulting in the wrong value being used afterwards.
540 if (MF.exposesReturnsTwice())
541 return false;
542
543 // Gather spill slot references
544 ScanForSpillSlotRefs(MF);
545 InitializeSlots();
546 Changed = ColorSlots(MF);
547
548 for (int &Next : NextColors)
549 Next = -1;
550
551 SSIntervals.clear();
552 for (auto &RefMMOs : SSRefs)
553 RefMMOs.clear();
554 SSRefs.clear();
555 OrigAlignments.clear();
556 OrigSizes.clear();
557 AllColors.clear();
558 UsedColors.clear();
559 Assignments.clear();
560
561 return Changed;
562 }
563