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/LiveIntervalUnion.h" 18 #include "llvm/CodeGen/LiveIntervals.h" 19 #include "llvm/CodeGen/LiveStacks.h" 20 #include "llvm/CodeGen/MachineBasicBlock.h" 21 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h" 22 #include "llvm/CodeGen/MachineFrameInfo.h" 23 #include "llvm/CodeGen/MachineFunction.h" 24 #include "llvm/CodeGen/MachineFunctionPass.h" 25 #include "llvm/CodeGen/MachineInstr.h" 26 #include "llvm/CodeGen/MachineMemOperand.h" 27 #include "llvm/CodeGen/MachineOperand.h" 28 #include "llvm/CodeGen/Passes.h" 29 #include "llvm/CodeGen/PseudoSourceValue.h" 30 #include "llvm/CodeGen/SlotIndexes.h" 31 #include "llvm/CodeGen/TargetInstrInfo.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 = nullptr; 63 MachineFrameInfo *MFI = nullptr; 64 const TargetInstrInfo *TII = nullptr; 65 const MachineBlockFrequencyInfo *MBFI = nullptr; 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 // Join all intervals sharing one color into a single LiveIntervalUnion to 95 // speedup range overlap test. 96 class ColorAssignmentInfo { 97 // Single liverange (used to avoid creation of LiveIntervalUnion). 98 LiveInterval *SingleLI = nullptr; 99 // LiveIntervalUnion to perform overlap test. 100 LiveIntervalUnion *LIU = nullptr; 101 // LiveIntervalUnion has a parameter in its constructor so doing this 102 // dirty magic. 103 uint8_t LIUPad[sizeof(LiveIntervalUnion)]; 104 105 public: 106 ~ColorAssignmentInfo() { 107 if (LIU) 108 LIU->~LiveIntervalUnion(); // Dirty magic again. 109 } 110 111 // Return true if LiveInterval overlaps with any 112 // intervals that have already been assigned to this color. 113 bool overlaps(LiveInterval *LI) const { 114 if (LIU) 115 return LiveIntervalUnion::Query(*LI, *LIU).checkInterference(); 116 return SingleLI ? SingleLI->overlaps(*LI) : false; 117 } 118 119 // Add new LiveInterval to this color. 120 void add(LiveInterval *LI, LiveIntervalUnion::Allocator &Alloc) { 121 assert(!overlaps(LI)); 122 if (LIU) { 123 LIU->unify(*LI, *LI); 124 } else if (SingleLI) { 125 LIU = new (LIUPad) LiveIntervalUnion(Alloc); 126 LIU->unify(*SingleLI, *SingleLI); 127 LIU->unify(*LI, *LI); 128 SingleLI = nullptr; 129 } else 130 SingleLI = LI; 131 } 132 }; 133 134 LiveIntervalUnion::Allocator LIUAlloc; 135 136 // Assignments - Color to intervals mapping. 137 SmallVector<ColorAssignmentInfo, 16> Assignments; 138 139 public: 140 static char ID; // Pass identification 141 142 StackSlotColoring() : MachineFunctionPass(ID) { 143 initializeStackSlotColoringPass(*PassRegistry::getPassRegistry()); 144 } 145 146 void getAnalysisUsage(AnalysisUsage &AU) const override { 147 AU.setPreservesCFG(); 148 AU.addRequired<SlotIndexes>(); 149 AU.addPreserved<SlotIndexes>(); 150 AU.addRequired<LiveStacks>(); 151 AU.addRequired<MachineBlockFrequencyInfo>(); 152 AU.addPreserved<MachineBlockFrequencyInfo>(); 153 AU.addPreservedID(MachineDominatorsID); 154 MachineFunctionPass::getAnalysisUsage(AU); 155 } 156 157 bool runOnMachineFunction(MachineFunction &MF) override; 158 159 private: 160 void InitializeSlots(); 161 void ScanForSpillSlotRefs(MachineFunction &MF); 162 int ColorSlot(LiveInterval *li); 163 bool ColorSlots(MachineFunction &MF); 164 void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping, 165 MachineFunction &MF); 166 bool RemoveDeadStores(MachineBasicBlock* MBB); 167 }; 168 169 } // end anonymous namespace 170 171 char StackSlotColoring::ID = 0; 172 173 char &llvm::StackSlotColoringID = StackSlotColoring::ID; 174 175 INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE, 176 "Stack Slot Coloring", false, false) 177 INITIALIZE_PASS_DEPENDENCY(SlotIndexes) 178 INITIALIZE_PASS_DEPENDENCY(LiveStacks) 179 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) 180 INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE, 181 "Stack Slot Coloring", false, false) 182 183 namespace { 184 185 // IntervalSorter - Comparison predicate that sort live intervals by 186 // their weight. 187 struct IntervalSorter { 188 bool operator()(LiveInterval* LHS, LiveInterval* RHS) const { 189 return LHS->weight() > RHS->weight(); 190 } 191 }; 192 193 } // end anonymous namespace 194 195 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot 196 /// references and update spill slot weights. 197 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) { 198 SSRefs.resize(MFI->getObjectIndexEnd()); 199 200 // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands. 201 for (MachineBasicBlock &MBB : MF) { 202 for (MachineInstr &MI : MBB) { 203 for (const MachineOperand &MO : MI.operands()) { 204 if (!MO.isFI()) 205 continue; 206 int FI = MO.getIndex(); 207 if (FI < 0) 208 continue; 209 if (!LS->hasInterval(FI)) 210 continue; 211 LiveInterval &li = LS->getInterval(FI); 212 if (!MI.isDebugInstr()) 213 li.incrementWeight( 214 LiveIntervals::getSpillWeight(false, true, MBFI, MI)); 215 } 216 for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(), 217 EE = MI.memoperands_end(); 218 MMOI != EE; ++MMOI) { 219 MachineMemOperand *MMO = *MMOI; 220 if (const FixedStackPseudoSourceValue *FSV = 221 dyn_cast_or_null<FixedStackPseudoSourceValue>( 222 MMO->getPseudoValue())) { 223 int FI = FSV->getFrameIndex(); 224 if (FI >= 0) 225 SSRefs[FI].push_back(MMO); 226 } 227 } 228 } 229 } 230 } 231 232 /// InitializeSlots - Process all spill stack slot liveintervals and add them 233 /// to a sorted (by weight) list. 234 void StackSlotColoring::InitializeSlots() { 235 int LastFI = MFI->getObjectIndexEnd(); 236 237 // There is always at least one stack ID. 238 AllColors.resize(1); 239 UsedColors.resize(1); 240 241 OrigAlignments.resize(LastFI); 242 OrigSizes.resize(LastFI); 243 AllColors[0].resize(LastFI); 244 UsedColors[0].resize(LastFI); 245 Assignments.resize(LastFI); 246 247 using Pair = std::iterator_traits<LiveStacks::iterator>::value_type; 248 249 SmallVector<Pair *, 16> Intervals; 250 251 Intervals.reserve(LS->getNumIntervals()); 252 for (auto &I : *LS) 253 Intervals.push_back(&I); 254 llvm::sort(Intervals, 255 [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; }); 256 257 // Gather all spill slots into a list. 258 LLVM_DEBUG(dbgs() << "Spill slot intervals:\n"); 259 for (auto *I : Intervals) { 260 LiveInterval &li = I->second; 261 LLVM_DEBUG(li.dump()); 262 int FI = Register::stackSlot2Index(li.reg()); 263 if (MFI->isDeadObjectIndex(FI)) 264 continue; 265 266 SSIntervals.push_back(&li); 267 OrigAlignments[FI] = MFI->getObjectAlign(FI); 268 OrigSizes[FI] = MFI->getObjectSize(FI); 269 270 auto StackID = MFI->getStackID(FI); 271 if (StackID != 0) { 272 AllColors.resize(StackID + 1); 273 UsedColors.resize(StackID + 1); 274 AllColors[StackID].resize(LastFI); 275 UsedColors[StackID].resize(LastFI); 276 } 277 278 AllColors[StackID].set(FI); 279 } 280 LLVM_DEBUG(dbgs() << '\n'); 281 282 // Sort them by weight. 283 llvm::stable_sort(SSIntervals, IntervalSorter()); 284 285 NextColors.resize(AllColors.size()); 286 287 // Get first "color". 288 for (unsigned I = 0, E = AllColors.size(); I != E; ++I) 289 NextColors[I] = AllColors[I].find_first(); 290 } 291 292 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot. 293 int StackSlotColoring::ColorSlot(LiveInterval *li) { 294 int Color = -1; 295 bool Share = false; 296 int FI = Register::stackSlot2Index(li->reg()); 297 uint8_t StackID = MFI->getStackID(FI); 298 299 if (!DisableSharing) { 300 301 // Check if it's possible to reuse any of the used colors. 302 Color = UsedColors[StackID].find_first(); 303 while (Color != -1) { 304 if (!Assignments[Color].overlaps(li)) { 305 Share = true; 306 ++NumEliminated; 307 break; 308 } 309 Color = UsedColors[StackID].find_next(Color); 310 } 311 } 312 313 if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) { 314 LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n"); 315 Share = false; 316 } 317 318 // Assign it to the first available color (assumed to be the best) if it's 319 // not possible to share a used color with other objects. 320 if (!Share) { 321 assert(NextColors[StackID] != -1 && "No more spill slots?"); 322 Color = NextColors[StackID]; 323 UsedColors[StackID].set(Color); 324 NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]); 325 } 326 327 assert(MFI->getStackID(Color) == MFI->getStackID(FI)); 328 329 // Record the assignment. 330 Assignments[Color].add(li, LIUAlloc); 331 LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n"); 332 333 // Change size and alignment of the allocated slot. If there are multiple 334 // objects sharing the same slot, then make sure the size and alignment 335 // are large enough for all. 336 Align Alignment = OrigAlignments[FI]; 337 if (!Share || Alignment > MFI->getObjectAlign(Color)) 338 MFI->setObjectAlignment(Color, Alignment); 339 int64_t Size = OrigSizes[FI]; 340 if (!Share || Size > MFI->getObjectSize(Color)) 341 MFI->setObjectSize(Color, Size); 342 return Color; 343 } 344 345 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine 346 /// operands in the function. 347 bool StackSlotColoring::ColorSlots(MachineFunction &MF) { 348 unsigned NumObjs = MFI->getObjectIndexEnd(); 349 SmallVector<int, 16> SlotMapping(NumObjs, -1); 350 SmallVector<float, 16> SlotWeights(NumObjs, 0.0); 351 SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs); 352 BitVector UsedColors(NumObjs); 353 354 LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n"); 355 bool Changed = false; 356 for (LiveInterval *li : SSIntervals) { 357 int SS = Register::stackSlot2Index(li->reg()); 358 int NewSS = ColorSlot(li); 359 assert(NewSS >= 0 && "Stack coloring failed?"); 360 SlotMapping[SS] = NewSS; 361 RevMap[NewSS].push_back(SS); 362 SlotWeights[NewSS] += li->weight(); 363 UsedColors.set(NewSS); 364 Changed |= (SS != NewSS); 365 } 366 367 LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n"); 368 for (LiveInterval *li : SSIntervals) { 369 int SS = Register::stackSlot2Index(li->reg()); 370 li->setWeight(SlotWeights[SS]); 371 } 372 // Sort them by new weight. 373 llvm::stable_sort(SSIntervals, IntervalSorter()); 374 375 #ifndef NDEBUG 376 for (LiveInterval *li : SSIntervals) 377 LLVM_DEBUG(li->dump()); 378 LLVM_DEBUG(dbgs() << '\n'); 379 #endif 380 381 if (!Changed) 382 return false; 383 384 // Rewrite all MachineMemOperands. 385 for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) { 386 int NewFI = SlotMapping[SS]; 387 if (NewFI == -1 || (NewFI == (int)SS)) 388 continue; 389 390 const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI); 391 SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS]; 392 for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i) 393 RefMMOs[i]->setValue(NewSV); 394 } 395 396 // Rewrite all MO_FrameIndex operands. Look for dead stores. 397 for (MachineBasicBlock &MBB : MF) { 398 for (MachineInstr &MI : MBB) 399 RewriteInstruction(MI, SlotMapping, MF); 400 RemoveDeadStores(&MBB); 401 } 402 403 // Delete unused stack slots. 404 for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) { 405 int NextColor = NextColors[StackID]; 406 while (NextColor != -1) { 407 LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n"); 408 MFI->RemoveStackObject(NextColor); 409 NextColor = AllColors[StackID].find_next(NextColor); 410 } 411 } 412 413 return true; 414 } 415 416 /// RewriteInstruction - Rewrite specified instruction by replacing references 417 /// to old frame index with new one. 418 void StackSlotColoring::RewriteInstruction(MachineInstr &MI, 419 SmallVectorImpl<int> &SlotMapping, 420 MachineFunction &MF) { 421 // Update the operands. 422 for (MachineOperand &MO : MI.operands()) { 423 if (!MO.isFI()) 424 continue; 425 int OldFI = MO.getIndex(); 426 if (OldFI < 0) 427 continue; 428 int NewFI = SlotMapping[OldFI]; 429 if (NewFI == -1 || NewFI == OldFI) 430 continue; 431 432 assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI)); 433 MO.setIndex(NewFI); 434 } 435 436 // The MachineMemOperands have already been updated. 437 } 438 439 /// RemoveDeadStores - Scan through a basic block and look for loads followed 440 /// by stores. If they're both using the same stack slot, then the store is 441 /// definitely dead. This could obviously be much more aggressive (consider 442 /// pairs with instructions between them), but such extensions might have a 443 /// considerable compile time impact. 444 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) { 445 // FIXME: This could be much more aggressive, but we need to investigate 446 // the compile time impact of doing so. 447 bool changed = false; 448 449 SmallVector<MachineInstr*, 4> toErase; 450 451 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 452 I != E; ++I) { 453 if (DCELimit != -1 && (int)NumDead >= DCELimit) 454 break; 455 int FirstSS, SecondSS; 456 if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS && 457 FirstSS != -1) { 458 ++NumDead; 459 changed = true; 460 toErase.push_back(&*I); 461 continue; 462 } 463 464 MachineBasicBlock::iterator NextMI = std::next(I); 465 MachineBasicBlock::iterator ProbableLoadMI = I; 466 467 unsigned LoadReg = 0; 468 unsigned StoreReg = 0; 469 unsigned LoadSize = 0; 470 unsigned StoreSize = 0; 471 if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize))) 472 continue; 473 // Skip the ...pseudo debugging... instructions between a load and store. 474 while ((NextMI != E) && NextMI->isDebugInstr()) { 475 ++NextMI; 476 ++I; 477 } 478 if (NextMI == E) continue; 479 if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize))) 480 continue; 481 if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 || 482 LoadSize != StoreSize) 483 continue; 484 485 ++NumDead; 486 changed = true; 487 488 if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) { 489 ++NumDead; 490 toErase.push_back(&*ProbableLoadMI); 491 } 492 493 toErase.push_back(&*NextMI); 494 ++I; 495 } 496 497 for (MachineInstr *MI : toErase) 498 MI->eraseFromParent(); 499 500 return changed; 501 } 502 503 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) { 504 LLVM_DEBUG({ 505 dbgs() << "********** Stack Slot Coloring **********\n" 506 << "********** Function: " << MF.getName() << '\n'; 507 }); 508 509 if (skipFunction(MF.getFunction())) 510 return false; 511 512 MFI = &MF.getFrameInfo(); 513 TII = MF.getSubtarget().getInstrInfo(); 514 LS = &getAnalysis<LiveStacks>(); 515 MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 516 517 bool Changed = false; 518 519 unsigned NumSlots = LS->getNumIntervals(); 520 if (NumSlots == 0) 521 // Nothing to do! 522 return false; 523 524 // If there are calls to setjmp or sigsetjmp, don't perform stack slot 525 // coloring. The stack could be modified before the longjmp is executed, 526 // resulting in the wrong value being used afterwards. (See 527 // <rdar://problem/8007500>.) 528 if (MF.exposesReturnsTwice()) 529 return false; 530 531 // Gather spill slot references 532 ScanForSpillSlotRefs(MF); 533 InitializeSlots(); 534 Changed = ColorSlots(MF); 535 536 for (int &Next : NextColors) 537 Next = -1; 538 539 SSIntervals.clear(); 540 for (unsigned i = 0, e = SSRefs.size(); i != e; ++i) 541 SSRefs[i].clear(); 542 SSRefs.clear(); 543 OrigAlignments.clear(); 544 OrigSizes.clear(); 545 AllColors.clear(); 546 UsedColors.clear(); 547 Assignments.clear(); 548 549 return Changed; 550 } 551