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 - Sizes 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 ()__anonb3ae01450211::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 (MachineBasicBlock &MBB : MF) {
161 for (MachineInstr &MI : MBB) {
162 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
163 MachineOperand &MO = MI.getOperand(i);
164 if (!MO.isFI())
165 continue;
166 int FI = MO.getIndex();
167 if (FI < 0)
168 continue;
169 if (!LS->hasInterval(FI))
170 continue;
171 LiveInterval &li = LS->getInterval(FI);
172 if (!MI.isDebugValue())
173 li.incrementWeight(
174 LiveIntervals::getSpillWeight(false, true, MBFI, MI));
175 }
176 for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(),
177 EE = MI.memoperands_end();
178 MMOI != EE; ++MMOI) {
179 MachineMemOperand *MMO = *MMOI;
180 if (const FixedStackPseudoSourceValue *FSV =
181 dyn_cast_or_null<FixedStackPseudoSourceValue>(
182 MMO->getPseudoValue())) {
183 int FI = FSV->getFrameIndex();
184 if (FI >= 0)
185 SSRefs[FI].push_back(MMO);
186 }
187 }
188 }
189 }
190 }
191
192 /// InitializeSlots - Process all spill stack slot liveintervals and add them
193 /// to a sorted (by weight) list.
InitializeSlots()194 void StackSlotColoring::InitializeSlots() {
195 int LastFI = MFI->getObjectIndexEnd();
196
197 // There is always at least one stack ID.
198 AllColors.resize(1);
199 UsedColors.resize(1);
200
201 OrigAlignments.resize(LastFI);
202 OrigSizes.resize(LastFI);
203 AllColors[0].resize(LastFI);
204 UsedColors[0].resize(LastFI);
205 Assignments.resize(LastFI);
206
207 using Pair = std::iterator_traits<LiveStacks::iterator>::value_type;
208
209 SmallVector<Pair *, 16> Intervals;
210
211 Intervals.reserve(LS->getNumIntervals());
212 for (auto &I : *LS)
213 Intervals.push_back(&I);
214 llvm::sort(Intervals,
215 [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; });
216
217 // Gather all spill slots into a list.
218 LLVM_DEBUG(dbgs() << "Spill slot intervals:\n");
219 for (auto *I : Intervals) {
220 LiveInterval &li = I->second;
221 LLVM_DEBUG(li.dump());
222 int FI = Register::stackSlot2Index(li.reg());
223 if (MFI->isDeadObjectIndex(FI))
224 continue;
225
226 SSIntervals.push_back(&li);
227 OrigAlignments[FI] = MFI->getObjectAlign(FI);
228 OrigSizes[FI] = MFI->getObjectSize(FI);
229
230 auto StackID = MFI->getStackID(FI);
231 if (StackID != 0) {
232 AllColors.resize(StackID + 1);
233 UsedColors.resize(StackID + 1);
234 AllColors[StackID].resize(LastFI);
235 UsedColors[StackID].resize(LastFI);
236 }
237
238 AllColors[StackID].set(FI);
239 }
240 LLVM_DEBUG(dbgs() << '\n');
241
242 // Sort them by weight.
243 llvm::stable_sort(SSIntervals, IntervalSorter());
244
245 NextColors.resize(AllColors.size());
246
247 // Get first "color".
248 for (unsigned I = 0, E = AllColors.size(); I != E; ++I)
249 NextColors[I] = AllColors[I].find_first();
250 }
251
252 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any
253 /// LiveIntervals that have already been assigned to the specified color.
254 bool
OverlapWithAssignments(LiveInterval * li,int Color) const255 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const {
256 const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color];
257 for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) {
258 LiveInterval *OtherLI = OtherLIs[i];
259 if (OtherLI->overlaps(*li))
260 return true;
261 }
262 return false;
263 }
264
265 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot.
ColorSlot(LiveInterval * li)266 int StackSlotColoring::ColorSlot(LiveInterval *li) {
267 int Color = -1;
268 bool Share = false;
269 int FI = Register::stackSlot2Index(li->reg());
270 uint8_t StackID = MFI->getStackID(FI);
271
272 if (!DisableSharing) {
273
274 // Check if it's possible to reuse any of the used colors.
275 Color = UsedColors[StackID].find_first();
276 while (Color != -1) {
277 if (!OverlapWithAssignments(li, Color)) {
278 Share = true;
279 ++NumEliminated;
280 break;
281 }
282 Color = UsedColors[StackID].find_next(Color);
283 }
284 }
285
286 if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) {
287 LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n");
288 Share = false;
289 }
290
291 // Assign it to the first available color (assumed to be the best) if it's
292 // not possible to share a used color with other objects.
293 if (!Share) {
294 assert(NextColors[StackID] != -1 && "No more spill slots?");
295 Color = NextColors[StackID];
296 UsedColors[StackID].set(Color);
297 NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]);
298 }
299
300 assert(MFI->getStackID(Color) == MFI->getStackID(FI));
301
302 // Record the assignment.
303 Assignments[Color].push_back(li);
304 LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n");
305
306 // Change size and alignment of the allocated slot. If there are multiple
307 // objects sharing the same slot, then make sure the size and alignment
308 // are large enough for all.
309 Align Alignment = OrigAlignments[FI];
310 if (!Share || Alignment > MFI->getObjectAlign(Color))
311 MFI->setObjectAlignment(Color, Alignment);
312 int64_t Size = OrigSizes[FI];
313 if (!Share || Size > MFI->getObjectSize(Color))
314 MFI->setObjectSize(Color, Size);
315 return Color;
316 }
317
318 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine
319 /// operands in the function.
ColorSlots(MachineFunction & MF)320 bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
321 unsigned NumObjs = MFI->getObjectIndexEnd();
322 SmallVector<int, 16> SlotMapping(NumObjs, -1);
323 SmallVector<float, 16> SlotWeights(NumObjs, 0.0);
324 SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs);
325 BitVector UsedColors(NumObjs);
326
327 LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n");
328 bool Changed = false;
329 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
330 LiveInterval *li = SSIntervals[i];
331 int SS = Register::stackSlot2Index(li->reg());
332 int NewSS = ColorSlot(li);
333 assert(NewSS >= 0 && "Stack coloring failed?");
334 SlotMapping[SS] = NewSS;
335 RevMap[NewSS].push_back(SS);
336 SlotWeights[NewSS] += li->weight();
337 UsedColors.set(NewSS);
338 Changed |= (SS != NewSS);
339 }
340
341 LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n");
342 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) {
343 LiveInterval *li = SSIntervals[i];
344 int SS = Register::stackSlot2Index(li->reg());
345 li->setWeight(SlotWeights[SS]);
346 }
347 // Sort them by new weight.
348 llvm::stable_sort(SSIntervals, IntervalSorter());
349
350 #ifndef NDEBUG
351 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i)
352 LLVM_DEBUG(SSIntervals[i]->dump());
353 LLVM_DEBUG(dbgs() << '\n');
354 #endif
355
356 if (!Changed)
357 return false;
358
359 // Rewrite all MachineMemOperands.
360 for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) {
361 int NewFI = SlotMapping[SS];
362 if (NewFI == -1 || (NewFI == (int)SS))
363 continue;
364
365 const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI);
366 SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS];
367 for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i)
368 RefMMOs[i]->setValue(NewSV);
369 }
370
371 // Rewrite all MO_FrameIndex operands. Look for dead stores.
372 for (MachineBasicBlock &MBB : MF) {
373 for (MachineInstr &MI : MBB)
374 RewriteInstruction(MI, SlotMapping, MF);
375 RemoveDeadStores(&MBB);
376 }
377
378 // Delete unused stack slots.
379 for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) {
380 int NextColor = NextColors[StackID];
381 while (NextColor != -1) {
382 LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n");
383 MFI->RemoveStackObject(NextColor);
384 NextColor = AllColors[StackID].find_next(NextColor);
385 }
386 }
387
388 return true;
389 }
390
391 /// RewriteInstruction - Rewrite specified instruction by replacing references
392 /// to old frame index with new one.
RewriteInstruction(MachineInstr & MI,SmallVectorImpl<int> & SlotMapping,MachineFunction & MF)393 void StackSlotColoring::RewriteInstruction(MachineInstr &MI,
394 SmallVectorImpl<int> &SlotMapping,
395 MachineFunction &MF) {
396 // Update the operands.
397 for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) {
398 MachineOperand &MO = MI.getOperand(i);
399 if (!MO.isFI())
400 continue;
401 int OldFI = MO.getIndex();
402 if (OldFI < 0)
403 continue;
404 int NewFI = SlotMapping[OldFI];
405 if (NewFI == -1 || NewFI == OldFI)
406 continue;
407
408 assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI));
409 MO.setIndex(NewFI);
410 }
411
412 // The MachineMemOperands have already been updated.
413 }
414
415 /// RemoveDeadStores - Scan through a basic block and look for loads followed
416 /// by stores. If they're both using the same stack slot, then the store is
417 /// definitely dead. This could obviously be much more aggressive (consider
418 /// pairs with instructions between them), but such extensions might have a
419 /// considerable compile time impact.
RemoveDeadStores(MachineBasicBlock * MBB)420 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
421 // FIXME: This could be much more aggressive, but we need to investigate
422 // the compile time impact of doing so.
423 bool changed = false;
424
425 SmallVector<MachineInstr*, 4> toErase;
426
427 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
428 I != E; ++I) {
429 if (DCELimit != -1 && (int)NumDead >= DCELimit)
430 break;
431 int FirstSS, SecondSS;
432 if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS &&
433 FirstSS != -1) {
434 ++NumDead;
435 changed = true;
436 toErase.push_back(&*I);
437 continue;
438 }
439
440 MachineBasicBlock::iterator NextMI = std::next(I);
441 MachineBasicBlock::iterator ProbableLoadMI = I;
442
443 unsigned LoadReg = 0;
444 unsigned StoreReg = 0;
445 unsigned LoadSize = 0;
446 unsigned StoreSize = 0;
447 if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize)))
448 continue;
449 // Skip the ...pseudo debugging... instructions between a load and store.
450 while ((NextMI != E) && NextMI->isDebugInstr()) {
451 ++NextMI;
452 ++I;
453 }
454 if (NextMI == E) continue;
455 if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize)))
456 continue;
457 if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 ||
458 LoadSize != StoreSize)
459 continue;
460
461 ++NumDead;
462 changed = true;
463
464 if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) {
465 ++NumDead;
466 toErase.push_back(&*ProbableLoadMI);
467 }
468
469 toErase.push_back(&*NextMI);
470 ++I;
471 }
472
473 for (MachineInstr *MI : toErase)
474 MI->eraseFromParent();
475
476 return changed;
477 }
478
runOnMachineFunction(MachineFunction & MF)479 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
480 LLVM_DEBUG({
481 dbgs() << "********** Stack Slot Coloring **********\n"
482 << "********** Function: " << MF.getName() << '\n';
483 });
484
485 if (skipFunction(MF.getFunction()))
486 return false;
487
488 MFI = &MF.getFrameInfo();
489 TII = MF.getSubtarget().getInstrInfo();
490 LS = &getAnalysis<LiveStacks>();
491 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
492
493 bool Changed = false;
494
495 unsigned NumSlots = LS->getNumIntervals();
496 if (NumSlots == 0)
497 // Nothing to do!
498 return false;
499
500 // If there are calls to setjmp or sigsetjmp, don't perform stack slot
501 // coloring. The stack could be modified before the longjmp is executed,
502 // resulting in the wrong value being used afterwards. (See
503 // <rdar://problem/8007500>.)
504 if (MF.exposesReturnsTwice())
505 return false;
506
507 // Gather spill slot references
508 ScanForSpillSlotRefs(MF);
509 InitializeSlots();
510 Changed = ColorSlots(MF);
511
512 for (int &Next : NextColors)
513 Next = -1;
514
515 SSIntervals.clear();
516 for (unsigned i = 0, e = SSRefs.size(); i != e; ++i)
517 SSRefs[i].clear();
518 SSRefs.clear();
519 OrigAlignments.clear();
520 OrigSizes.clear();
521 AllColors.clear();
522 UsedColors.clear();
523 for (unsigned i = 0, e = Assignments.size(); i != e; ++i)
524 Assignments[i].clear();
525 Assignments.clear();
526
527 return Changed;
528 }
529