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/TargetSubtargetInfo.h" 32 #include "llvm/InitializePasses.h" 33 #include "llvm/Pass.h" 34 #include "llvm/Support/Casting.h" 35 #include "llvm/Support/CommandLine.h" 36 #include "llvm/Support/Debug.h" 37 #include "llvm/Support/raw_ostream.h" 38 #include <algorithm> 39 #include <cassert> 40 #include <cstdint> 41 #include <iterator> 42 #include <vector> 43 44 using namespace llvm; 45 46 #define DEBUG_TYPE "stack-slot-coloring" 47 48 static cl::opt<bool> 49 DisableSharing("no-stack-slot-sharing", 50 cl::init(false), cl::Hidden, 51 cl::desc("Suppress slot sharing during stack coloring")); 52 53 static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden); 54 55 STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring"); 56 STATISTIC(NumDead, "Number of trivially dead stack accesses eliminated"); 57 58 namespace { 59 60 class StackSlotColoring : public MachineFunctionPass { 61 LiveStacks* LS; 62 MachineFrameInfo *MFI; 63 const TargetInstrInfo *TII; 64 const MachineBlockFrequencyInfo *MBFI; 65 66 // SSIntervals - Spill slot intervals. 67 std::vector<LiveInterval*> SSIntervals; 68 69 // SSRefs - Keep a list of MachineMemOperands for each spill slot. 70 // MachineMemOperands can be shared between instructions, so we need 71 // to be careful that renames like [FI0, FI1] -> [FI1, FI2] do not 72 // become FI0 -> FI1 -> FI2. 73 SmallVector<SmallVector<MachineMemOperand *, 8>, 16> SSRefs; 74 75 // OrigAlignments - Alignments of stack objects before coloring. 76 SmallVector<Align, 16> OrigAlignments; 77 78 // OrigSizes - Sizes of stack objects before coloring. 79 SmallVector<unsigned, 16> OrigSizes; 80 81 // AllColors - If index is set, it's a spill slot, i.e. color. 82 // FIXME: This assumes PEI locate spill slot with smaller indices 83 // closest to stack pointer / frame pointer. Therefore, smaller 84 // index == better color. This is per stack ID. 85 SmallVector<BitVector, 2> AllColors; 86 87 // NextColor - Next "color" that's not yet used. This is per stack ID. 88 SmallVector<int, 2> NextColors = { -1 }; 89 90 // UsedColors - "Colors" that have been assigned. This is per stack ID 91 SmallVector<BitVector, 2> UsedColors; 92 93 // Assignments - Color to intervals mapping. 94 SmallVector<SmallVector<LiveInterval*,4>, 16> Assignments; 95 96 public: 97 static char ID; // Pass identification 98 99 StackSlotColoring() : MachineFunctionPass(ID) { 100 initializeStackSlotColoringPass(*PassRegistry::getPassRegistry()); 101 } 102 103 void getAnalysisUsage(AnalysisUsage &AU) const override { 104 AU.setPreservesCFG(); 105 AU.addRequired<SlotIndexes>(); 106 AU.addPreserved<SlotIndexes>(); 107 AU.addRequired<LiveStacks>(); 108 AU.addRequired<MachineBlockFrequencyInfo>(); 109 AU.addPreserved<MachineBlockFrequencyInfo>(); 110 AU.addPreservedID(MachineDominatorsID); 111 MachineFunctionPass::getAnalysisUsage(AU); 112 } 113 114 bool runOnMachineFunction(MachineFunction &MF) override; 115 116 private: 117 void InitializeSlots(); 118 void ScanForSpillSlotRefs(MachineFunction &MF); 119 bool OverlapWithAssignments(LiveInterval *li, int Color) const; 120 int ColorSlot(LiveInterval *li); 121 bool ColorSlots(MachineFunction &MF); 122 void RewriteInstruction(MachineInstr &MI, SmallVectorImpl<int> &SlotMapping, 123 MachineFunction &MF); 124 bool RemoveDeadStores(MachineBasicBlock* MBB); 125 }; 126 127 } // end anonymous namespace 128 129 char StackSlotColoring::ID = 0; 130 131 char &llvm::StackSlotColoringID = StackSlotColoring::ID; 132 133 INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE, 134 "Stack Slot Coloring", false, false) 135 INITIALIZE_PASS_DEPENDENCY(SlotIndexes) 136 INITIALIZE_PASS_DEPENDENCY(LiveStacks) 137 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) 138 INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE, 139 "Stack Slot Coloring", false, false) 140 141 namespace { 142 143 // IntervalSorter - Comparison predicate that sort live intervals by 144 // their weight. 145 struct IntervalSorter { 146 bool operator()(LiveInterval* LHS, LiveInterval* RHS) const { 147 return LHS->weight() > RHS->weight(); 148 } 149 }; 150 151 } // end anonymous namespace 152 153 /// ScanForSpillSlotRefs - Scan all the machine instructions for spill slot 154 /// references and update spill slot weights. 155 void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) { 156 SSRefs.resize(MFI->getObjectIndexEnd()); 157 158 // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands. 159 for (MachineBasicBlock &MBB : MF) { 160 for (MachineInstr &MI : MBB) { 161 for (const MachineOperand &MO : MI.operands()) { 162 if (!MO.isFI()) 163 continue; 164 int FI = MO.getIndex(); 165 if (FI < 0) 166 continue; 167 if (!LS->hasInterval(FI)) 168 continue; 169 LiveInterval &li = LS->getInterval(FI); 170 if (!MI.isDebugInstr()) 171 li.incrementWeight( 172 LiveIntervals::getSpillWeight(false, true, MBFI, MI)); 173 } 174 for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(), 175 EE = MI.memoperands_end(); 176 MMOI != EE; ++MMOI) { 177 MachineMemOperand *MMO = *MMOI; 178 if (const FixedStackPseudoSourceValue *FSV = 179 dyn_cast_or_null<FixedStackPseudoSourceValue>( 180 MMO->getPseudoValue())) { 181 int FI = FSV->getFrameIndex(); 182 if (FI >= 0) 183 SSRefs[FI].push_back(MMO); 184 } 185 } 186 } 187 } 188 } 189 190 /// InitializeSlots - Process all spill stack slot liveintervals and add them 191 /// to a sorted (by weight) list. 192 void StackSlotColoring::InitializeSlots() { 193 int LastFI = MFI->getObjectIndexEnd(); 194 195 // There is always at least one stack ID. 196 AllColors.resize(1); 197 UsedColors.resize(1); 198 199 OrigAlignments.resize(LastFI); 200 OrigSizes.resize(LastFI); 201 AllColors[0].resize(LastFI); 202 UsedColors[0].resize(LastFI); 203 Assignments.resize(LastFI); 204 205 using Pair = std::iterator_traits<LiveStacks::iterator>::value_type; 206 207 SmallVector<Pair *, 16> Intervals; 208 209 Intervals.reserve(LS->getNumIntervals()); 210 for (auto &I : *LS) 211 Intervals.push_back(&I); 212 llvm::sort(Intervals, 213 [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; }); 214 215 // Gather all spill slots into a list. 216 LLVM_DEBUG(dbgs() << "Spill slot intervals:\n"); 217 for (auto *I : Intervals) { 218 LiveInterval &li = I->second; 219 LLVM_DEBUG(li.dump()); 220 int FI = Register::stackSlot2Index(li.reg()); 221 if (MFI->isDeadObjectIndex(FI)) 222 continue; 223 224 SSIntervals.push_back(&li); 225 OrigAlignments[FI] = MFI->getObjectAlign(FI); 226 OrigSizes[FI] = MFI->getObjectSize(FI); 227 228 auto StackID = MFI->getStackID(FI); 229 if (StackID != 0) { 230 AllColors.resize(StackID + 1); 231 UsedColors.resize(StackID + 1); 232 AllColors[StackID].resize(LastFI); 233 UsedColors[StackID].resize(LastFI); 234 } 235 236 AllColors[StackID].set(FI); 237 } 238 LLVM_DEBUG(dbgs() << '\n'); 239 240 // Sort them by weight. 241 llvm::stable_sort(SSIntervals, IntervalSorter()); 242 243 NextColors.resize(AllColors.size()); 244 245 // Get first "color". 246 for (unsigned I = 0, E = AllColors.size(); I != E; ++I) 247 NextColors[I] = AllColors[I].find_first(); 248 } 249 250 /// OverlapWithAssignments - Return true if LiveInterval overlaps with any 251 /// LiveIntervals that have already been assigned to the specified color. 252 bool 253 StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const { 254 const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color]; 255 for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) { 256 LiveInterval *OtherLI = OtherLIs[i]; 257 if (OtherLI->overlaps(*li)) 258 return true; 259 } 260 return false; 261 } 262 263 /// ColorSlot - Assign a "color" (stack slot) to the specified stack slot. 264 int StackSlotColoring::ColorSlot(LiveInterval *li) { 265 int Color = -1; 266 bool Share = false; 267 int FI = Register::stackSlot2Index(li->reg()); 268 uint8_t StackID = MFI->getStackID(FI); 269 270 if (!DisableSharing) { 271 272 // Check if it's possible to reuse any of the used colors. 273 Color = UsedColors[StackID].find_first(); 274 while (Color != -1) { 275 if (!OverlapWithAssignments(li, Color)) { 276 Share = true; 277 ++NumEliminated; 278 break; 279 } 280 Color = UsedColors[StackID].find_next(Color); 281 } 282 } 283 284 if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) { 285 LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n"); 286 Share = false; 287 } 288 289 // Assign it to the first available color (assumed to be the best) if it's 290 // not possible to share a used color with other objects. 291 if (!Share) { 292 assert(NextColors[StackID] != -1 && "No more spill slots?"); 293 Color = NextColors[StackID]; 294 UsedColors[StackID].set(Color); 295 NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]); 296 } 297 298 assert(MFI->getStackID(Color) == MFI->getStackID(FI)); 299 300 // Record the assignment. 301 Assignments[Color].push_back(li); 302 LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n"); 303 304 // Change size and alignment of the allocated slot. If there are multiple 305 // objects sharing the same slot, then make sure the size and alignment 306 // are large enough for all. 307 Align Alignment = OrigAlignments[FI]; 308 if (!Share || Alignment > MFI->getObjectAlign(Color)) 309 MFI->setObjectAlignment(Color, Alignment); 310 int64_t Size = OrigSizes[FI]; 311 if (!Share || Size > MFI->getObjectSize(Color)) 312 MFI->setObjectSize(Color, Size); 313 return Color; 314 } 315 316 /// Colorslots - Color all spill stack slots and rewrite all frameindex machine 317 /// operands in the function. 318 bool StackSlotColoring::ColorSlots(MachineFunction &MF) { 319 unsigned NumObjs = MFI->getObjectIndexEnd(); 320 SmallVector<int, 16> SlotMapping(NumObjs, -1); 321 SmallVector<float, 16> SlotWeights(NumObjs, 0.0); 322 SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs); 323 BitVector UsedColors(NumObjs); 324 325 LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n"); 326 bool Changed = false; 327 for (LiveInterval *li : SSIntervals) { 328 int SS = Register::stackSlot2Index(li->reg()); 329 int NewSS = ColorSlot(li); 330 assert(NewSS >= 0 && "Stack coloring failed?"); 331 SlotMapping[SS] = NewSS; 332 RevMap[NewSS].push_back(SS); 333 SlotWeights[NewSS] += li->weight(); 334 UsedColors.set(NewSS); 335 Changed |= (SS != NewSS); 336 } 337 338 LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n"); 339 for (LiveInterval *li : SSIntervals) { 340 int SS = Register::stackSlot2Index(li->reg()); 341 li->setWeight(SlotWeights[SS]); 342 } 343 // Sort them by new weight. 344 llvm::stable_sort(SSIntervals, IntervalSorter()); 345 346 #ifndef NDEBUG 347 for (LiveInterval *li : SSIntervals) 348 LLVM_DEBUG(li->dump()); 349 LLVM_DEBUG(dbgs() << '\n'); 350 #endif 351 352 if (!Changed) 353 return false; 354 355 // Rewrite all MachineMemOperands. 356 for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) { 357 int NewFI = SlotMapping[SS]; 358 if (NewFI == -1 || (NewFI == (int)SS)) 359 continue; 360 361 const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI); 362 SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS]; 363 for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i) 364 RefMMOs[i]->setValue(NewSV); 365 } 366 367 // Rewrite all MO_FrameIndex operands. Look for dead stores. 368 for (MachineBasicBlock &MBB : MF) { 369 for (MachineInstr &MI : MBB) 370 RewriteInstruction(MI, SlotMapping, MF); 371 RemoveDeadStores(&MBB); 372 } 373 374 // Delete unused stack slots. 375 for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) { 376 int NextColor = NextColors[StackID]; 377 while (NextColor != -1) { 378 LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n"); 379 MFI->RemoveStackObject(NextColor); 380 NextColor = AllColors[StackID].find_next(NextColor); 381 } 382 } 383 384 return true; 385 } 386 387 /// RewriteInstruction - Rewrite specified instruction by replacing references 388 /// to old frame index with new one. 389 void StackSlotColoring::RewriteInstruction(MachineInstr &MI, 390 SmallVectorImpl<int> &SlotMapping, 391 MachineFunction &MF) { 392 // Update the operands. 393 for (MachineOperand &MO : MI.operands()) { 394 if (!MO.isFI()) 395 continue; 396 int OldFI = MO.getIndex(); 397 if (OldFI < 0) 398 continue; 399 int NewFI = SlotMapping[OldFI]; 400 if (NewFI == -1 || NewFI == OldFI) 401 continue; 402 403 assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI)); 404 MO.setIndex(NewFI); 405 } 406 407 // The MachineMemOperands have already been updated. 408 } 409 410 /// RemoveDeadStores - Scan through a basic block and look for loads followed 411 /// by stores. If they're both using the same stack slot, then the store is 412 /// definitely dead. This could obviously be much more aggressive (consider 413 /// pairs with instructions between them), but such extensions might have a 414 /// considerable compile time impact. 415 bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) { 416 // FIXME: This could be much more aggressive, but we need to investigate 417 // the compile time impact of doing so. 418 bool changed = false; 419 420 SmallVector<MachineInstr*, 4> toErase; 421 422 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 423 I != E; ++I) { 424 if (DCELimit != -1 && (int)NumDead >= DCELimit) 425 break; 426 int FirstSS, SecondSS; 427 if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS && 428 FirstSS != -1) { 429 ++NumDead; 430 changed = true; 431 toErase.push_back(&*I); 432 continue; 433 } 434 435 MachineBasicBlock::iterator NextMI = std::next(I); 436 MachineBasicBlock::iterator ProbableLoadMI = I; 437 438 unsigned LoadReg = 0; 439 unsigned StoreReg = 0; 440 unsigned LoadSize = 0; 441 unsigned StoreSize = 0; 442 if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize))) 443 continue; 444 // Skip the ...pseudo debugging... instructions between a load and store. 445 while ((NextMI != E) && NextMI->isDebugInstr()) { 446 ++NextMI; 447 ++I; 448 } 449 if (NextMI == E) continue; 450 if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize))) 451 continue; 452 if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 || 453 LoadSize != StoreSize) 454 continue; 455 456 ++NumDead; 457 changed = true; 458 459 if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) { 460 ++NumDead; 461 toErase.push_back(&*ProbableLoadMI); 462 } 463 464 toErase.push_back(&*NextMI); 465 ++I; 466 } 467 468 for (MachineInstr *MI : toErase) 469 MI->eraseFromParent(); 470 471 return changed; 472 } 473 474 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) { 475 LLVM_DEBUG({ 476 dbgs() << "********** Stack Slot Coloring **********\n" 477 << "********** Function: " << MF.getName() << '\n'; 478 }); 479 480 if (skipFunction(MF.getFunction())) 481 return false; 482 483 MFI = &MF.getFrameInfo(); 484 TII = MF.getSubtarget().getInstrInfo(); 485 LS = &getAnalysis<LiveStacks>(); 486 MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 487 488 bool Changed = false; 489 490 unsigned NumSlots = LS->getNumIntervals(); 491 if (NumSlots == 0) 492 // Nothing to do! 493 return false; 494 495 // If there are calls to setjmp or sigsetjmp, don't perform stack slot 496 // coloring. The stack could be modified before the longjmp is executed, 497 // resulting in the wrong value being used afterwards. (See 498 // <rdar://problem/8007500>.) 499 if (MF.exposesReturnsTwice()) 500 return false; 501 502 // Gather spill slot references 503 ScanForSpillSlotRefs(MF); 504 InitializeSlots(); 505 Changed = ColorSlots(MF); 506 507 for (int &Next : NextColors) 508 Next = -1; 509 510 SSIntervals.clear(); 511 for (unsigned i = 0, e = SSRefs.size(); i != e; ++i) 512 SSRefs[i].clear(); 513 SSRefs.clear(); 514 OrigAlignments.clear(); 515 OrigSizes.clear(); 516 AllColors.clear(); 517 UsedColors.clear(); 518 for (unsigned i = 0, e = Assignments.size(); i != e; ++i) 519 Assignments[i].clear(); 520 Assignments.clear(); 521 522 return Changed; 523 } 524