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 99 100 StackSlotColoring() : MachineFunctionPass(ID) { 101 initializeStackSlotColoringPass(*PassRegistry::getPassRegistry()); 102 } 103 104 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 { 147 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. 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. 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 255 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. 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. 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. 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. 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 } 478 479 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