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 45using namespace llvm; 46 47#define DEBUG_TYPE "stack-slot-coloring" 48 49static cl::opt<bool> 50DisableSharing("no-stack-slot-sharing", 51 cl::init(false), cl::Hidden, 52 cl::desc("Suppress slot sharing during stack coloring")); 53 54static cl::opt<int> DCELimit("ssc-dce-limit", cl::init(-1), cl::Hidden); 55 56STATISTIC(NumEliminated, "Number of stack slots eliminated due to coloring"); 57STATISTIC(NumDead, "Number of trivially dead stack accesses eliminated"); 58 59namespace { 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 - Sizess 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 130char StackSlotColoring::ID = 0; 131 132char &llvm::StackSlotColoringID = StackSlotColoring::ID; 133 134INITIALIZE_PASS_BEGIN(StackSlotColoring, DEBUG_TYPE, 135 "Stack Slot Coloring", false, false) 136INITIALIZE_PASS_DEPENDENCY(SlotIndexes) 137INITIALIZE_PASS_DEPENDENCY(LiveStacks) 138INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo) 139INITIALIZE_PASS_END(StackSlotColoring, DEBUG_TYPE, 140 "Stack Slot Coloring", false, false) 141 142namespace { 143 144// IntervalSorter - Comparison predicate that sort live intervals by 145// their weight. 146struct 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. 156void StackSlotColoring::ScanForSpillSlotRefs(MachineFunction &MF) { 157 SSRefs.resize(MFI->getObjectIndexEnd()); 158 159 // FIXME: Need the equivalent of MachineRegisterInfo for frameindex operands. 160 for (MachineFunction::iterator MBBI = MF.begin(), E = MF.end(); 161 MBBI != E; ++MBBI) { 162 MachineBasicBlock *MBB = &*MBBI; 163 for (MachineBasicBlock::iterator MII = MBB->begin(), EE = MBB->end(); 164 MII != EE; ++MII) { 165 MachineInstr &MI = *MII; 166 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { 167 MachineOperand &MO = MI.getOperand(i); 168 if (!MO.isFI()) 169 continue; 170 int FI = MO.getIndex(); 171 if (FI < 0) 172 continue; 173 if (!LS->hasInterval(FI)) 174 continue; 175 LiveInterval &li = LS->getInterval(FI); 176 if (!MI.isDebugValue()) 177 li.weight += LiveIntervals::getSpillWeight(false, true, MBFI, MI); 178 } 179 for (MachineInstr::mmo_iterator MMOI = MI.memoperands_begin(), 180 EE = MI.memoperands_end(); 181 MMOI != EE; ++MMOI) { 182 MachineMemOperand *MMO = *MMOI; 183 if (const FixedStackPseudoSourceValue *FSV = 184 dyn_cast_or_null<FixedStackPseudoSourceValue>( 185 MMO->getPseudoValue())) { 186 int FI = FSV->getFrameIndex(); 187 if (FI >= 0) 188 SSRefs[FI].push_back(MMO); 189 } 190 } 191 } 192 } 193} 194 195/// InitializeSlots - Process all spill stack slot liveintervals and add them 196/// to a sorted (by weight) list. 197void StackSlotColoring::InitializeSlots() { 198 int LastFI = MFI->getObjectIndexEnd(); 199 200 // There is always at least one stack ID. 201 AllColors.resize(1); 202 UsedColors.resize(1); 203 204 OrigAlignments.resize(LastFI); 205 OrigSizes.resize(LastFI); 206 AllColors[0].resize(LastFI); 207 UsedColors[0].resize(LastFI); 208 Assignments.resize(LastFI); 209 210 using Pair = std::iterator_traits<LiveStacks::iterator>::value_type; 211 212 SmallVector<Pair *, 16> Intervals; 213 214 Intervals.reserve(LS->getNumIntervals()); 215 for (auto &I : *LS) 216 Intervals.push_back(&I); 217 llvm::sort(Intervals, 218 [](Pair *LHS, Pair *RHS) { return LHS->first < RHS->first; }); 219 220 // Gather all spill slots into a list. 221 LLVM_DEBUG(dbgs() << "Spill slot intervals:\n"); 222 for (auto *I : Intervals) { 223 LiveInterval &li = I->second; 224 LLVM_DEBUG(li.dump()); 225 int FI = Register::stackSlot2Index(li.reg); 226 if (MFI->isDeadObjectIndex(FI)) 227 continue; 228 229 SSIntervals.push_back(&li); 230 OrigAlignments[FI] = MFI->getObjectAlign(FI); 231 OrigSizes[FI] = MFI->getObjectSize(FI); 232 233 auto StackID = MFI->getStackID(FI); 234 if (StackID != 0) { 235 AllColors.resize(StackID + 1); 236 UsedColors.resize(StackID + 1); 237 AllColors[StackID].resize(LastFI); 238 UsedColors[StackID].resize(LastFI); 239 } 240 241 AllColors[StackID].set(FI); 242 } 243 LLVM_DEBUG(dbgs() << '\n'); 244 245 // Sort them by weight. 246 llvm::stable_sort(SSIntervals, IntervalSorter()); 247 248 NextColors.resize(AllColors.size()); 249 250 // Get first "color". 251 for (unsigned I = 0, E = AllColors.size(); I != E; ++I) 252 NextColors[I] = AllColors[I].find_first(); 253} 254 255/// OverlapWithAssignments - Return true if LiveInterval overlaps with any 256/// LiveIntervals that have already been assigned to the specified color. 257bool 258StackSlotColoring::OverlapWithAssignments(LiveInterval *li, int Color) const { 259 const SmallVectorImpl<LiveInterval *> &OtherLIs = Assignments[Color]; 260 for (unsigned i = 0, e = OtherLIs.size(); i != e; ++i) { 261 LiveInterval *OtherLI = OtherLIs[i]; 262 if (OtherLI->overlaps(*li)) 263 return true; 264 } 265 return false; 266} 267 268/// ColorSlot - Assign a "color" (stack slot) to the specified stack slot. 269int StackSlotColoring::ColorSlot(LiveInterval *li) { 270 int Color = -1; 271 bool Share = false; 272 int FI = Register::stackSlot2Index(li->reg); 273 uint8_t StackID = MFI->getStackID(FI); 274 275 if (!DisableSharing) { 276 277 // Check if it's possible to reuse any of the used colors. 278 Color = UsedColors[StackID].find_first(); 279 while (Color != -1) { 280 if (!OverlapWithAssignments(li, Color)) { 281 Share = true; 282 ++NumEliminated; 283 break; 284 } 285 Color = UsedColors[StackID].find_next(Color); 286 } 287 } 288 289 if (Color != -1 && MFI->getStackID(Color) != MFI->getStackID(FI)) { 290 LLVM_DEBUG(dbgs() << "cannot share FIs with different stack IDs\n"); 291 Share = false; 292 } 293 294 // Assign it to the first available color (assumed to be the best) if it's 295 // not possible to share a used color with other objects. 296 if (!Share) { 297 assert(NextColors[StackID] != -1 && "No more spill slots?"); 298 Color = NextColors[StackID]; 299 UsedColors[StackID].set(Color); 300 NextColors[StackID] = AllColors[StackID].find_next(NextColors[StackID]); 301 } 302 303 assert(MFI->getStackID(Color) == MFI->getStackID(FI)); 304 305 // Record the assignment. 306 Assignments[Color].push_back(li); 307 LLVM_DEBUG(dbgs() << "Assigning fi#" << FI << " to fi#" << Color << "\n"); 308 309 // Change size and alignment of the allocated slot. If there are multiple 310 // objects sharing the same slot, then make sure the size and alignment 311 // are large enough for all. 312 Align Alignment = OrigAlignments[FI]; 313 if (!Share || Alignment > MFI->getObjectAlign(Color)) 314 MFI->setObjectAlignment(Color, Alignment); 315 int64_t Size = OrigSizes[FI]; 316 if (!Share || Size > MFI->getObjectSize(Color)) 317 MFI->setObjectSize(Color, Size); 318 return Color; 319} 320 321/// Colorslots - Color all spill stack slots and rewrite all frameindex machine 322/// operands in the function. 323bool StackSlotColoring::ColorSlots(MachineFunction &MF) { 324 unsigned NumObjs = MFI->getObjectIndexEnd(); 325 SmallVector<int, 16> SlotMapping(NumObjs, -1); 326 SmallVector<float, 16> SlotWeights(NumObjs, 0.0); 327 SmallVector<SmallVector<int, 4>, 16> RevMap(NumObjs); 328 BitVector UsedColors(NumObjs); 329 330 LLVM_DEBUG(dbgs() << "Color spill slot intervals:\n"); 331 bool Changed = false; 332 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) { 333 LiveInterval *li = SSIntervals[i]; 334 int SS = Register::stackSlot2Index(li->reg); 335 int NewSS = ColorSlot(li); 336 assert(NewSS >= 0 && "Stack coloring failed?"); 337 SlotMapping[SS] = NewSS; 338 RevMap[NewSS].push_back(SS); 339 SlotWeights[NewSS] += li->weight; 340 UsedColors.set(NewSS); 341 Changed |= (SS != NewSS); 342 } 343 344 LLVM_DEBUG(dbgs() << "\nSpill slots after coloring:\n"); 345 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) { 346 LiveInterval *li = SSIntervals[i]; 347 int SS = Register::stackSlot2Index(li->reg); 348 li->weight = SlotWeights[SS]; 349 } 350 // Sort them by new weight. 351 llvm::stable_sort(SSIntervals, IntervalSorter()); 352 353#ifndef NDEBUG 354 for (unsigned i = 0, e = SSIntervals.size(); i != e; ++i) 355 LLVM_DEBUG(SSIntervals[i]->dump()); 356 LLVM_DEBUG(dbgs() << '\n'); 357#endif 358 359 if (!Changed) 360 return false; 361 362 // Rewrite all MachineMemOperands. 363 for (unsigned SS = 0, SE = SSRefs.size(); SS != SE; ++SS) { 364 int NewFI = SlotMapping[SS]; 365 if (NewFI == -1 || (NewFI == (int)SS)) 366 continue; 367 368 const PseudoSourceValue *NewSV = MF.getPSVManager().getFixedStack(NewFI); 369 SmallVectorImpl<MachineMemOperand *> &RefMMOs = SSRefs[SS]; 370 for (unsigned i = 0, e = RefMMOs.size(); i != e; ++i) 371 RefMMOs[i]->setValue(NewSV); 372 } 373 374 // Rewrite all MO_FrameIndex operands. Look for dead stores. 375 for (MachineBasicBlock &MBB : MF) { 376 for (MachineInstr &MI : MBB) 377 RewriteInstruction(MI, SlotMapping, MF); 378 RemoveDeadStores(&MBB); 379 } 380 381 // Delete unused stack slots. 382 for (int StackID = 0, E = AllColors.size(); StackID != E; ++StackID) { 383 int NextColor = NextColors[StackID]; 384 while (NextColor != -1) { 385 LLVM_DEBUG(dbgs() << "Removing unused stack object fi#" << NextColor << "\n"); 386 MFI->RemoveStackObject(NextColor); 387 NextColor = AllColors[StackID].find_next(NextColor); 388 } 389 } 390 391 return true; 392} 393 394/// RewriteInstruction - Rewrite specified instruction by replacing references 395/// to old frame index with new one. 396void StackSlotColoring::RewriteInstruction(MachineInstr &MI, 397 SmallVectorImpl<int> &SlotMapping, 398 MachineFunction &MF) { 399 // Update the operands. 400 for (unsigned i = 0, ee = MI.getNumOperands(); i != ee; ++i) { 401 MachineOperand &MO = MI.getOperand(i); 402 if (!MO.isFI()) 403 continue; 404 int OldFI = MO.getIndex(); 405 if (OldFI < 0) 406 continue; 407 int NewFI = SlotMapping[OldFI]; 408 if (NewFI == -1 || NewFI == OldFI) 409 continue; 410 411 assert(MFI->getStackID(OldFI) == MFI->getStackID(NewFI)); 412 MO.setIndex(NewFI); 413 } 414 415 // The MachineMemOperands have already been updated. 416} 417 418/// RemoveDeadStores - Scan through a basic block and look for loads followed 419/// by stores. If they're both using the same stack slot, then the store is 420/// definitely dead. This could obviously be much more aggressive (consider 421/// pairs with instructions between them), but such extensions might have a 422/// considerable compile time impact. 423bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) { 424 // FIXME: This could be much more aggressive, but we need to investigate 425 // the compile time impact of doing so. 426 bool changed = false; 427 428 SmallVector<MachineInstr*, 4> toErase; 429 430 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 431 I != E; ++I) { 432 if (DCELimit != -1 && (int)NumDead >= DCELimit) 433 break; 434 int FirstSS, SecondSS; 435 if (TII->isStackSlotCopy(*I, FirstSS, SecondSS) && FirstSS == SecondSS && 436 FirstSS != -1) { 437 ++NumDead; 438 changed = true; 439 toErase.push_back(&*I); 440 continue; 441 } 442 443 MachineBasicBlock::iterator NextMI = std::next(I); 444 MachineBasicBlock::iterator ProbableLoadMI = I; 445 446 unsigned LoadReg = 0; 447 unsigned StoreReg = 0; 448 unsigned LoadSize = 0; 449 unsigned StoreSize = 0; 450 if (!(LoadReg = TII->isLoadFromStackSlot(*I, FirstSS, LoadSize))) 451 continue; 452 // Skip the ...pseudo debugging... instructions between a load and store. 453 while ((NextMI != E) && NextMI->isDebugInstr()) { 454 ++NextMI; 455 ++I; 456 } 457 if (NextMI == E) continue; 458 if (!(StoreReg = TII->isStoreToStackSlot(*NextMI, SecondSS, StoreSize))) 459 continue; 460 if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1 || 461 LoadSize != StoreSize) 462 continue; 463 464 ++NumDead; 465 changed = true; 466 467 if (NextMI->findRegisterUseOperandIdx(LoadReg, true, nullptr) != -1) { 468 ++NumDead; 469 toErase.push_back(&*ProbableLoadMI); 470 } 471 472 toErase.push_back(&*NextMI); 473 ++I; 474 } 475 476 for (SmallVectorImpl<MachineInstr *>::iterator I = toErase.begin(), 477 E = toErase.end(); I != E; ++I) 478 (*I)->eraseFromParent(); 479 480 return changed; 481} 482 483bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) { 484 LLVM_DEBUG({ 485 dbgs() << "********** Stack Slot Coloring **********\n" 486 << "********** Function: " << MF.getName() << '\n'; 487 }); 488 489 if (skipFunction(MF.getFunction())) 490 return false; 491 492 MFI = &MF.getFrameInfo(); 493 TII = MF.getSubtarget().getInstrInfo(); 494 LS = &getAnalysis<LiveStacks>(); 495 MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 496 497 bool Changed = false; 498 499 unsigned NumSlots = LS->getNumIntervals(); 500 if (NumSlots == 0) 501 // Nothing to do! 502 return false; 503 504 // If there are calls to setjmp or sigsetjmp, don't perform stack slot 505 // coloring. The stack could be modified before the longjmp is executed, 506 // resulting in the wrong value being used afterwards. (See 507 // <rdar://problem/8007500>.) 508 if (MF.exposesReturnsTwice()) 509 return false; 510 511 // Gather spill slot references 512 ScanForSpillSlotRefs(MF); 513 InitializeSlots(); 514 Changed = ColorSlots(MF); 515 516 for (int &Next : NextColors) 517 Next = -1; 518 519 SSIntervals.clear(); 520 for (unsigned i = 0, e = SSRefs.size(); i != e; ++i) 521 SSRefs[i].clear(); 522 SSRefs.clear(); 523 OrigAlignments.clear(); 524 OrigSizes.clear(); 525 AllColors.clear(); 526 UsedColors.clear(); 527 for (unsigned i = 0, e = Assignments.size(); i != e; ++i) 528 Assignments[i].clear(); 529 Assignments.clear(); 530 531 return Changed; 532} 533