CriticalAntiDepBreaker.cpp revision 4de099d8ca651e00fa5fac22bace4f4dba2d0292
1//===----- CriticalAntiDepBreaker.cpp - Anti-dep breaker -------- ---------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file implements the CriticalAntiDepBreaker class, which 11// implements register anti-dependence breaking along a blocks 12// critical path during post-RA scheduler. 13// 14//===----------------------------------------------------------------------===// 15 16#define DEBUG_TYPE "post-RA-sched" 17#include "CriticalAntiDepBreaker.h" 18#include "llvm/CodeGen/MachineBasicBlock.h" 19#include "llvm/CodeGen/MachineFrameInfo.h" 20#include "llvm/Target/TargetMachine.h" 21#include "llvm/Target/TargetRegisterInfo.h" 22#include "llvm/Support/Debug.h" 23#include "llvm/Support/ErrorHandling.h" 24#include "llvm/Support/raw_ostream.h" 25 26using namespace llvm; 27 28CriticalAntiDepBreaker:: 29CriticalAntiDepBreaker(MachineFunction& MFi) : 30 AntiDepBreaker(), MF(MFi), 31 MRI(MF.getRegInfo()), 32 TRI(MF.getTarget().getRegisterInfo()), 33 AllocatableSet(TRI->getAllocatableSet(MF)) 34{ 35} 36 37CriticalAntiDepBreaker::~CriticalAntiDepBreaker() { 38} 39 40void CriticalAntiDepBreaker::StartBlock(MachineBasicBlock *BB) { 41 // Clear out the register class data. 42 std::fill(Classes, array_endof(Classes), 43 static_cast<const TargetRegisterClass *>(0)); 44 45 // Initialize the indices to indicate that no registers are live. 46 std::fill(KillIndices, array_endof(KillIndices), ~0u); 47 std::fill(DefIndices, array_endof(DefIndices), BB->size()); 48 49 // Clear "do not change" set. 50 KeepRegs.clear(); 51 52 bool IsReturnBlock = (!BB->empty() && BB->back().getDesc().isReturn()); 53 54 // Determine the live-out physregs for this block. 55 if (IsReturnBlock) { 56 // In a return block, examine the function live-out regs. 57 for (MachineRegisterInfo::liveout_iterator I = MRI.liveout_begin(), 58 E = MRI.liveout_end(); I != E; ++I) { 59 unsigned Reg = *I; 60 Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); 61 KillIndices[Reg] = BB->size(); 62 DefIndices[Reg] = ~0u; 63 // Repeat, for all aliases. 64 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) { 65 unsigned AliasReg = *Alias; 66 Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1); 67 KillIndices[AliasReg] = BB->size(); 68 DefIndices[AliasReg] = ~0u; 69 } 70 } 71 } else { 72 // In a non-return block, examine the live-in regs of all successors. 73 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(), 74 SE = BB->succ_end(); SI != SE; ++SI) 75 for (MachineBasicBlock::livein_iterator I = (*SI)->livein_begin(), 76 E = (*SI)->livein_end(); I != E; ++I) { 77 unsigned Reg = *I; 78 Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); 79 KillIndices[Reg] = BB->size(); 80 DefIndices[Reg] = ~0u; 81 // Repeat, for all aliases. 82 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) { 83 unsigned AliasReg = *Alias; 84 Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1); 85 KillIndices[AliasReg] = BB->size(); 86 DefIndices[AliasReg] = ~0u; 87 } 88 } 89 } 90 91 // Mark live-out callee-saved registers. In a return block this is 92 // all callee-saved registers. In non-return this is any 93 // callee-saved register that is not saved in the prolog. 94 const MachineFrameInfo *MFI = MF.getFrameInfo(); 95 BitVector Pristine = MFI->getPristineRegs(BB); 96 for (const unsigned *I = TRI->getCalleeSavedRegs(); *I; ++I) { 97 unsigned Reg = *I; 98 if (!IsReturnBlock && !Pristine.test(Reg)) continue; 99 Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); 100 KillIndices[Reg] = BB->size(); 101 DefIndices[Reg] = ~0u; 102 // Repeat, for all aliases. 103 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) { 104 unsigned AliasReg = *Alias; 105 Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1); 106 KillIndices[AliasReg] = BB->size(); 107 DefIndices[AliasReg] = ~0u; 108 } 109 } 110} 111 112void CriticalAntiDepBreaker::FinishBlock() { 113 RegRefs.clear(); 114 KeepRegs.clear(); 115} 116 117void CriticalAntiDepBreaker::Observe(MachineInstr *MI, unsigned Count, 118 unsigned InsertPosIndex) { 119 assert(Count < InsertPosIndex && "Instruction index out of expected range!"); 120 121 // Any register which was defined within the previous scheduling region 122 // may have been rescheduled and its lifetime may overlap with registers 123 // in ways not reflected in our current liveness state. For each such 124 // register, adjust the liveness state to be conservatively correct. 125 for (unsigned Reg = 0; Reg != TargetRegisterInfo::FirstVirtualRegister; ++Reg) 126 if (DefIndices[Reg] < InsertPosIndex && DefIndices[Reg] >= Count) { 127 assert(KillIndices[Reg] == ~0u && "Clobbered register is live!"); 128 // Mark this register to be non-renamable. 129 Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); 130 // Move the def index to the end of the previous region, to reflect 131 // that the def could theoretically have been scheduled at the end. 132 DefIndices[Reg] = InsertPosIndex; 133 } 134 135 PrescanInstruction(MI); 136 ScanInstruction(MI, Count); 137} 138 139/// CriticalPathStep - Return the next SUnit after SU on the bottom-up 140/// critical path. 141static SDep *CriticalPathStep(SUnit *SU) { 142 SDep *Next = 0; 143 unsigned NextDepth = 0; 144 // Find the predecessor edge with the greatest depth. 145 for (SUnit::pred_iterator P = SU->Preds.begin(), PE = SU->Preds.end(); 146 P != PE; ++P) { 147 SUnit *PredSU = P->getSUnit(); 148 unsigned PredLatency = P->getLatency(); 149 unsigned PredTotalLatency = PredSU->getDepth() + PredLatency; 150 // In the case of a latency tie, prefer an anti-dependency edge over 151 // other types of edges. 152 if (NextDepth < PredTotalLatency || 153 (NextDepth == PredTotalLatency && P->getKind() == SDep::Anti)) { 154 NextDepth = PredTotalLatency; 155 Next = &*P; 156 } 157 } 158 return Next; 159} 160 161void CriticalAntiDepBreaker::PrescanInstruction(MachineInstr *MI) { 162 // Scan the register operands for this instruction and update 163 // Classes and RegRefs. 164 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 165 MachineOperand &MO = MI->getOperand(i); 166 if (!MO.isReg()) continue; 167 unsigned Reg = MO.getReg(); 168 if (Reg == 0) continue; 169 const TargetRegisterClass *NewRC = 0; 170 171 if (i < MI->getDesc().getNumOperands()) 172 NewRC = MI->getDesc().OpInfo[i].getRegClass(TRI); 173 174 // For now, only allow the register to be changed if its register 175 // class is consistent across all uses. 176 if (!Classes[Reg] && NewRC) 177 Classes[Reg] = NewRC; 178 else if (!NewRC || Classes[Reg] != NewRC) 179 Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); 180 181 // Now check for aliases. 182 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) { 183 // If an alias of the reg is used during the live range, give up. 184 // Note that this allows us to skip checking if AntiDepReg 185 // overlaps with any of the aliases, among other things. 186 unsigned AliasReg = *Alias; 187 if (Classes[AliasReg]) { 188 Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1); 189 Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); 190 } 191 } 192 193 // If we're still willing to consider this register, note the reference. 194 if (Classes[Reg] != reinterpret_cast<TargetRegisterClass *>(-1)) 195 RegRefs.insert(std::make_pair(Reg, &MO)); 196 197 // It's not safe to change register allocation for source operands of 198 // that have special allocation requirements. 199 if (MO.isUse() && MI->getDesc().hasExtraSrcRegAllocReq()) { 200 if (KeepRegs.insert(Reg)) { 201 for (const unsigned *Subreg = TRI->getSubRegisters(Reg); 202 *Subreg; ++Subreg) 203 KeepRegs.insert(*Subreg); 204 } 205 } 206 } 207} 208 209void CriticalAntiDepBreaker::ScanInstruction(MachineInstr *MI, 210 unsigned Count) { 211 // Update liveness. 212 // Proceding upwards, registers that are defed but not used in this 213 // instruction are now dead. 214 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 215 MachineOperand &MO = MI->getOperand(i); 216 if (!MO.isReg()) continue; 217 unsigned Reg = MO.getReg(); 218 if (Reg == 0) continue; 219 if (!MO.isDef()) continue; 220 // Ignore two-addr defs. 221 if (MI->isRegTiedToUseOperand(i)) continue; 222 223 DefIndices[Reg] = Count; 224 KillIndices[Reg] = ~0u; 225 assert(((KillIndices[Reg] == ~0u) != 226 (DefIndices[Reg] == ~0u)) && 227 "Kill and Def maps aren't consistent for Reg!"); 228 KeepRegs.erase(Reg); 229 Classes[Reg] = 0; 230 RegRefs.erase(Reg); 231 // Repeat, for all subregs. 232 for (const unsigned *Subreg = TRI->getSubRegisters(Reg); 233 *Subreg; ++Subreg) { 234 unsigned SubregReg = *Subreg; 235 DefIndices[SubregReg] = Count; 236 KillIndices[SubregReg] = ~0u; 237 KeepRegs.erase(SubregReg); 238 Classes[SubregReg] = 0; 239 RegRefs.erase(SubregReg); 240 } 241 // Conservatively mark super-registers as unusable. 242 for (const unsigned *Super = TRI->getSuperRegisters(Reg); 243 *Super; ++Super) { 244 unsigned SuperReg = *Super; 245 Classes[SuperReg] = reinterpret_cast<TargetRegisterClass *>(-1); 246 } 247 } 248 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 249 MachineOperand &MO = MI->getOperand(i); 250 if (!MO.isReg()) continue; 251 unsigned Reg = MO.getReg(); 252 if (Reg == 0) continue; 253 if (!MO.isUse()) continue; 254 255 const TargetRegisterClass *NewRC = 0; 256 if (i < MI->getDesc().getNumOperands()) 257 NewRC = MI->getDesc().OpInfo[i].getRegClass(TRI); 258 259 // For now, only allow the register to be changed if its register 260 // class is consistent across all uses. 261 if (!Classes[Reg] && NewRC) 262 Classes[Reg] = NewRC; 263 else if (!NewRC || Classes[Reg] != NewRC) 264 Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); 265 266 RegRefs.insert(std::make_pair(Reg, &MO)); 267 268 // It wasn't previously live but now it is, this is a kill. 269 if (KillIndices[Reg] == ~0u) { 270 KillIndices[Reg] = Count; 271 DefIndices[Reg] = ~0u; 272 assert(((KillIndices[Reg] == ~0u) != 273 (DefIndices[Reg] == ~0u)) && 274 "Kill and Def maps aren't consistent for Reg!"); 275 } 276 // Repeat, for all aliases. 277 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) { 278 unsigned AliasReg = *Alias; 279 if (KillIndices[AliasReg] == ~0u) { 280 KillIndices[AliasReg] = Count; 281 DefIndices[AliasReg] = ~0u; 282 } 283 } 284 } 285} 286 287unsigned 288CriticalAntiDepBreaker::findSuitableFreeRegister(unsigned AntiDepReg, 289 unsigned LastNewReg, 290 const TargetRegisterClass *RC) { 291 for (TargetRegisterClass::iterator R = RC->allocation_order_begin(MF), 292 RE = RC->allocation_order_end(MF); R != RE; ++R) { 293 unsigned NewReg = *R; 294 // Don't replace a register with itself. 295 if (NewReg == AntiDepReg) continue; 296 // Don't replace a register with one that was recently used to repair 297 // an anti-dependence with this AntiDepReg, because that would 298 // re-introduce that anti-dependence. 299 if (NewReg == LastNewReg) continue; 300 // If NewReg is dead and NewReg's most recent def is not before 301 // AntiDepReg's kill, it's safe to replace AntiDepReg with NewReg. 302 assert(((KillIndices[AntiDepReg] == ~0u) != (DefIndices[AntiDepReg] == ~0u)) && 303 "Kill and Def maps aren't consistent for AntiDepReg!"); 304 assert(((KillIndices[NewReg] == ~0u) != (DefIndices[NewReg] == ~0u)) && 305 "Kill and Def maps aren't consistent for NewReg!"); 306 if (KillIndices[NewReg] != ~0u || 307 Classes[NewReg] == reinterpret_cast<TargetRegisterClass *>(-1) || 308 KillIndices[AntiDepReg] > DefIndices[NewReg]) 309 continue; 310 return NewReg; 311 } 312 313 // No registers are free and available! 314 return 0; 315} 316 317unsigned CriticalAntiDepBreaker:: 318BreakAntiDependencies(std::vector<SUnit>& SUnits, 319 CandidateMap& Candidates, 320 MachineBasicBlock::iterator& Begin, 321 MachineBasicBlock::iterator& End, 322 unsigned InsertPosIndex) { 323 // The code below assumes that there is at least one instruction, 324 // so just duck out immediately if the block is empty. 325 if (SUnits.empty()) return 0; 326 327 // Find the node at the bottom of the critical path. 328 SUnit *Max = 0; 329 for (unsigned i = 0, e = SUnits.size(); i != e; ++i) { 330 SUnit *SU = &SUnits[i]; 331 if (!Max || SU->getDepth() + SU->Latency > Max->getDepth() + Max->Latency) 332 Max = SU; 333 } 334 335#ifndef NDEBUG 336 { 337 DEBUG(errs() << "Critical path has total latency " 338 << (Max->getDepth() + Max->Latency) << "\n"); 339 DEBUG(errs() << "Available regs:"); 340 for (unsigned Reg = 0; Reg < TRI->getNumRegs(); ++Reg) { 341 if (KillIndices[Reg] == ~0u) 342 DEBUG(errs() << " " << TRI->getName(Reg)); 343 } 344 DEBUG(errs() << '\n'); 345 } 346#endif 347 348 // Track progress along the critical path through the SUnit graph as we walk 349 // the instructions. 350 SUnit *CriticalPathSU = Max; 351 MachineInstr *CriticalPathMI = CriticalPathSU->getInstr(); 352 353 // Consider this pattern: 354 // A = ... 355 // ... = A 356 // A = ... 357 // ... = A 358 // A = ... 359 // ... = A 360 // A = ... 361 // ... = A 362 // There are three anti-dependencies here, and without special care, 363 // we'd break all of them using the same register: 364 // A = ... 365 // ... = A 366 // B = ... 367 // ... = B 368 // B = ... 369 // ... = B 370 // B = ... 371 // ... = B 372 // because at each anti-dependence, B is the first register that 373 // isn't A which is free. This re-introduces anti-dependencies 374 // at all but one of the original anti-dependencies that we were 375 // trying to break. To avoid this, keep track of the most recent 376 // register that each register was replaced with, avoid 377 // using it to repair an anti-dependence on the same register. 378 // This lets us produce this: 379 // A = ... 380 // ... = A 381 // B = ... 382 // ... = B 383 // C = ... 384 // ... = C 385 // B = ... 386 // ... = B 387 // This still has an anti-dependence on B, but at least it isn't on the 388 // original critical path. 389 // 390 // TODO: If we tracked more than one register here, we could potentially 391 // fix that remaining critical edge too. This is a little more involved, 392 // because unlike the most recent register, less recent registers should 393 // still be considered, though only if no other registers are available. 394 unsigned LastNewReg[TargetRegisterInfo::FirstVirtualRegister] = {}; 395 396 // Attempt to break anti-dependence edges on the critical path. Walk the 397 // instructions from the bottom up, tracking information about liveness 398 // as we go to help determine which registers are available. 399 unsigned Broken = 0; 400 unsigned Count = InsertPosIndex - 1; 401 for (MachineBasicBlock::iterator I = End, E = Begin; 402 I != E; --Count) { 403 MachineInstr *MI = --I; 404 405 // Check if this instruction has a dependence on the critical path that 406 // is an anti-dependence that we may be able to break. If it is, set 407 // AntiDepReg to the non-zero register associated with the anti-dependence. 408 // 409 // We limit our attention to the critical path as a heuristic to avoid 410 // breaking anti-dependence edges that aren't going to significantly 411 // impact the overall schedule. There are a limited number of registers 412 // and we want to save them for the important edges. 413 // 414 // TODO: Instructions with multiple defs could have multiple 415 // anti-dependencies. The current code here only knows how to break one 416 // edge per instruction. Note that we'd have to be able to break all of 417 // the anti-dependencies in an instruction in order to be effective. 418 unsigned AntiDepReg = 0; 419 if (MI == CriticalPathMI) { 420 if (SDep *Edge = CriticalPathStep(CriticalPathSU)) { 421 SUnit *NextSU = Edge->getSUnit(); 422 423 // Only consider anti-dependence edges. 424 if (Edge->getKind() == SDep::Anti) { 425 AntiDepReg = Edge->getReg(); 426 assert(AntiDepReg != 0 && "Anti-dependence on reg0?"); 427 if (!AllocatableSet.test(AntiDepReg)) 428 // Don't break anti-dependencies on non-allocatable registers. 429 AntiDepReg = 0; 430 else if (KeepRegs.count(AntiDepReg)) 431 // Don't break anti-dependencies if an use down below requires 432 // this exact register. 433 AntiDepReg = 0; 434 else { 435 // If the SUnit has other dependencies on the SUnit that it 436 // anti-depends on, don't bother breaking the anti-dependency 437 // since those edges would prevent such units from being 438 // scheduled past each other regardless. 439 // 440 // Also, if there are dependencies on other SUnits with the 441 // same register as the anti-dependency, don't attempt to 442 // break it. 443 for (SUnit::pred_iterator P = CriticalPathSU->Preds.begin(), 444 PE = CriticalPathSU->Preds.end(); P != PE; ++P) 445 if (P->getSUnit() == NextSU ? 446 (P->getKind() != SDep::Anti || P->getReg() != AntiDepReg) : 447 (P->getKind() == SDep::Data && P->getReg() == AntiDepReg)) { 448 AntiDepReg = 0; 449 break; 450 } 451 } 452 } 453 CriticalPathSU = NextSU; 454 CriticalPathMI = CriticalPathSU->getInstr(); 455 } else { 456 // We've reached the end of the critical path. 457 CriticalPathSU = 0; 458 CriticalPathMI = 0; 459 } 460 } 461 462 PrescanInstruction(MI); 463 464 if (MI->getDesc().hasExtraDefRegAllocReq()) 465 // If this instruction's defs have special allocation requirement, don't 466 // break this anti-dependency. 467 AntiDepReg = 0; 468 else if (AntiDepReg) { 469 // If this instruction has a use of AntiDepReg, breaking it 470 // is invalid. 471 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 472 MachineOperand &MO = MI->getOperand(i); 473 if (!MO.isReg()) continue; 474 unsigned Reg = MO.getReg(); 475 if (Reg == 0) continue; 476 if (MO.isUse() && AntiDepReg == Reg) { 477 AntiDepReg = 0; 478 break; 479 } 480 } 481 } 482 483 // Determine AntiDepReg's register class, if it is live and is 484 // consistently used within a single class. 485 const TargetRegisterClass *RC = AntiDepReg != 0 ? Classes[AntiDepReg] : 0; 486 assert((AntiDepReg == 0 || RC != NULL) && 487 "Register should be live if it's causing an anti-dependence!"); 488 if (RC == reinterpret_cast<TargetRegisterClass *>(-1)) 489 AntiDepReg = 0; 490 491 // Look for a suitable register to use to break the anti-depenence. 492 // 493 // TODO: Instead of picking the first free register, consider which might 494 // be the best. 495 if (AntiDepReg != 0) { 496 if (unsigned NewReg = findSuitableFreeRegister(AntiDepReg, 497 LastNewReg[AntiDepReg], 498 RC)) { 499 DEBUG(errs() << "Breaking anti-dependence edge on " 500 << TRI->getName(AntiDepReg) 501 << " with " << RegRefs.count(AntiDepReg) << " references" 502 << " using " << TRI->getName(NewReg) << "!\n"); 503 504 // Update the references to the old register to refer to the new 505 // register. 506 std::pair<std::multimap<unsigned, MachineOperand *>::iterator, 507 std::multimap<unsigned, MachineOperand *>::iterator> 508 Range = RegRefs.equal_range(AntiDepReg); 509 for (std::multimap<unsigned, MachineOperand *>::iterator 510 Q = Range.first, QE = Range.second; Q != QE; ++Q) 511 Q->second->setReg(NewReg); 512 513 // We just went back in time and modified history; the 514 // liveness information for the anti-depenence reg is now 515 // inconsistent. Set the state as if it were dead. 516 Classes[NewReg] = Classes[AntiDepReg]; 517 DefIndices[NewReg] = DefIndices[AntiDepReg]; 518 KillIndices[NewReg] = KillIndices[AntiDepReg]; 519 assert(((KillIndices[NewReg] == ~0u) != 520 (DefIndices[NewReg] == ~0u)) && 521 "Kill and Def maps aren't consistent for NewReg!"); 522 523 Classes[AntiDepReg] = 0; 524 DefIndices[AntiDepReg] = KillIndices[AntiDepReg]; 525 KillIndices[AntiDepReg] = ~0u; 526 assert(((KillIndices[AntiDepReg] == ~0u) != 527 (DefIndices[AntiDepReg] == ~0u)) && 528 "Kill and Def maps aren't consistent for AntiDepReg!"); 529 530 RegRefs.erase(AntiDepReg); 531 LastNewReg[AntiDepReg] = NewReg; 532 ++Broken; 533 } 534 } 535 536 ScanInstruction(MI, Count); 537 } 538 539 return Broken; 540} 541