TwoAddressInstructionPass.cpp revision c6959726ab99e740101d8b5b663aee5fa1332791
1//===-- TwoAddressInstructionPass.cpp - Two-Address instruction pass ------===// 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 TwoAddress instruction pass which is used 11// by most register allocators. Two-Address instructions are rewritten 12// from: 13// 14// A = B op C 15// 16// to: 17// 18// A = B 19// A op= C 20// 21// Note that if a register allocator chooses to use this pass, that it 22// has to be capable of handling the non-SSA nature of these rewritten 23// virtual registers. 24// 25// It is also worth noting that the duplicate operand of the two 26// address instruction is removed. 27// 28//===----------------------------------------------------------------------===// 29 30#define DEBUG_TYPE "twoaddrinstr" 31#include "llvm/CodeGen/Passes.h" 32#include "llvm/Function.h" 33#include "llvm/CodeGen/LiveVariables.h" 34#include "llvm/CodeGen/MachineFunctionPass.h" 35#include "llvm/CodeGen/MachineInstr.h" 36#include "llvm/CodeGen/MachineRegisterInfo.h" 37#include "llvm/Target/TargetRegisterInfo.h" 38#include "llvm/Target/TargetInstrInfo.h" 39#include "llvm/Target/TargetMachine.h" 40#include "llvm/Support/Compiler.h" 41#include "llvm/Support/Debug.h" 42#include "llvm/ADT/BitVector.h" 43#include "llvm/ADT/DenseMap.h" 44#include "llvm/ADT/SmallPtrSet.h" 45#include "llvm/ADT/Statistic.h" 46#include "llvm/ADT/STLExtras.h" 47using namespace llvm; 48 49STATISTIC(NumTwoAddressInstrs, "Number of two-address instructions"); 50STATISTIC(NumCommuted , "Number of instructions commuted to coalesce"); 51STATISTIC(NumConvertedTo3Addr, "Number of instructions promoted to 3-address"); 52STATISTIC(Num3AddrSunk, "Number of 3-address instructions sunk"); 53STATISTIC(NumReMats, "Number of instructions re-materialized"); 54 55namespace { 56 class VISIBILITY_HIDDEN TwoAddressInstructionPass 57 : public MachineFunctionPass { 58 const TargetInstrInfo *TII; 59 const TargetRegisterInfo *TRI; 60 MachineRegisterInfo *MRI; 61 LiveVariables *LV; 62 63 bool Sink3AddrInstruction(MachineBasicBlock *MBB, MachineInstr *MI, 64 unsigned Reg, 65 MachineBasicBlock::iterator OldPos); 66 67 bool isSafeToReMat(unsigned DstReg, MachineInstr *MI); 68 bool isProfitableToReMat(unsigned Reg, const TargetRegisterClass *RC, 69 MachineInstr *MI, MachineInstr *DefMI, 70 MachineBasicBlock *MBB, unsigned Loc, 71 DenseMap<MachineInstr*, unsigned> &DistanceMap); 72 public: 73 static char ID; // Pass identification, replacement for typeid 74 TwoAddressInstructionPass() : MachineFunctionPass((intptr_t)&ID) {} 75 76 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 77 AU.addPreserved<LiveVariables>(); 78 AU.addPreservedID(MachineLoopInfoID); 79 AU.addPreservedID(MachineDominatorsID); 80 AU.addPreservedID(PHIEliminationID); 81 MachineFunctionPass::getAnalysisUsage(AU); 82 } 83 84 /// runOnMachineFunction - Pass entry point. 85 bool runOnMachineFunction(MachineFunction&); 86 }; 87} 88 89char TwoAddressInstructionPass::ID = 0; 90static RegisterPass<TwoAddressInstructionPass> 91X("twoaddressinstruction", "Two-Address instruction pass"); 92 93const PassInfo *const llvm::TwoAddressInstructionPassID = &X; 94 95/// Sink3AddrInstruction - A two-address instruction has been converted to a 96/// three-address instruction to avoid clobbering a register. Try to sink it 97/// past the instruction that would kill the above mentioned register to reduce 98/// register pressure. 99bool TwoAddressInstructionPass::Sink3AddrInstruction(MachineBasicBlock *MBB, 100 MachineInstr *MI, unsigned SavedReg, 101 MachineBasicBlock::iterator OldPos) { 102 // Check if it's safe to move this instruction. 103 bool SeenStore = true; // Be conservative. 104 if (!MI->isSafeToMove(TII, SeenStore)) 105 return false; 106 107 unsigned DefReg = 0; 108 SmallSet<unsigned, 4> UseRegs; 109 110 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 111 const MachineOperand &MO = MI->getOperand(i); 112 if (!MO.isRegister()) 113 continue; 114 unsigned MOReg = MO.getReg(); 115 if (!MOReg) 116 continue; 117 if (MO.isUse() && MOReg != SavedReg) 118 UseRegs.insert(MO.getReg()); 119 if (!MO.isDef()) 120 continue; 121 if (MO.isImplicit()) 122 // Don't try to move it if it implicitly defines a register. 123 return false; 124 if (DefReg) 125 // For now, don't move any instructions that define multiple registers. 126 return false; 127 DefReg = MO.getReg(); 128 } 129 130 // Find the instruction that kills SavedReg. 131 MachineInstr *KillMI = NULL; 132 for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(SavedReg), 133 UE = MRI->use_end(); UI != UE; ++UI) { 134 MachineOperand &UseMO = UI.getOperand(); 135 if (!UseMO.isKill()) 136 continue; 137 KillMI = UseMO.getParent(); 138 break; 139 } 140 141 if (!KillMI || KillMI->getParent() != MBB) 142 return false; 143 144 // If any of the definitions are used by another instruction between the 145 // position and the kill use, then it's not safe to sink it. 146 // 147 // FIXME: This can be sped up if there is an easy way to query whether an 148 // instruction is before or after another instruction. Then we can use 149 // MachineRegisterInfo def / use instead. 150 MachineOperand *KillMO = NULL; 151 MachineBasicBlock::iterator KillPos = KillMI; 152 ++KillPos; 153 154 unsigned NumVisited = 0; 155 for (MachineBasicBlock::iterator I = next(OldPos); I != KillPos; ++I) { 156 MachineInstr *OtherMI = I; 157 if (NumVisited > 30) // FIXME: Arbitrary limit to reduce compile time cost. 158 return false; 159 ++NumVisited; 160 for (unsigned i = 0, e = OtherMI->getNumOperands(); i != e; ++i) { 161 MachineOperand &MO = OtherMI->getOperand(i); 162 if (!MO.isRegister()) 163 continue; 164 unsigned MOReg = MO.getReg(); 165 if (!MOReg) 166 continue; 167 if (DefReg == MOReg) 168 return false; 169 170 if (MO.isKill()) { 171 if (OtherMI == KillMI && MOReg == SavedReg) 172 // Save the operand that kills the register. We want to unset the kill 173 // marker if we can sink MI past it. 174 KillMO = &MO; 175 else if (UseRegs.count(MOReg)) 176 // One of the uses is killed before the destination. 177 return false; 178 } 179 } 180 } 181 182 // Update kill and LV information. 183 KillMO->setIsKill(false); 184 KillMO = MI->findRegisterUseOperand(SavedReg, false, TRI); 185 KillMO->setIsKill(true); 186 187 if (LV) { 188 LiveVariables::VarInfo& VarInfo = LV->getVarInfo(SavedReg); 189 VarInfo.removeKill(KillMI); 190 VarInfo.Kills.push_back(MI); 191 } 192 193 // Move instruction to its destination. 194 MBB->remove(MI); 195 MBB->insert(KillPos, MI); 196 197 ++Num3AddrSunk; 198 return true; 199} 200 201/// isSafeToReMat - Return true if it's safe to rematerialize the specified 202/// instruction which defined the specified register instead of copying it. 203bool 204TwoAddressInstructionPass::isSafeToReMat(unsigned DstReg, MachineInstr *MI) { 205 const TargetInstrDesc &TID = MI->getDesc(); 206 if (!TID.isAsCheapAsAMove()) 207 return false; 208 bool SawStore = false; 209 if (!MI->isSafeToMove(TII, SawStore)) 210 return false; 211 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 212 MachineOperand &MO = MI->getOperand(i); 213 if (!MO.isRegister()) 214 continue; 215 // FIXME: For now, do not remat any instruction with register operands. 216 // Later on, we can loosen the restriction is the register operands have 217 // not been modified between the def and use. Note, this is different from 218 // MachineSink because the code in no longer in two-address form (at least 219 // partially). 220 if (MO.isUse()) 221 return false; 222 else if (!MO.isDead() && MO.getReg() != DstReg) 223 return false; 224 } 225 return true; 226} 227 228/// isTwoAddrUse - Return true if the specified MI is using the specified 229/// register as a two-address operand. 230static bool isTwoAddrUse(MachineInstr *UseMI, unsigned Reg) { 231 const TargetInstrDesc &TID = UseMI->getDesc(); 232 for (unsigned i = 0, e = TID.getNumOperands(); i != e; ++i) { 233 MachineOperand &MO = UseMI->getOperand(i); 234 if (MO.isRegister() && MO.getReg() == Reg && 235 (MO.isDef() || TID.getOperandConstraint(i, TOI::TIED_TO) != -1)) 236 // Earlier use is a two-address one. 237 return true; 238 } 239 return false; 240} 241 242/// isProfitableToReMat - Return true if the heuristics determines it is likely 243/// to be profitable to re-materialize the definition of Reg rather than copy 244/// the register. 245bool 246TwoAddressInstructionPass::isProfitableToReMat(unsigned Reg, 247 const TargetRegisterClass *RC, 248 MachineInstr *MI, MachineInstr *DefMI, 249 MachineBasicBlock *MBB, unsigned Loc, 250 DenseMap<MachineInstr*, unsigned> &DistanceMap){ 251 bool OtherUse = false; 252 for (MachineRegisterInfo::use_iterator UI = MRI->use_begin(Reg), 253 UE = MRI->use_end(); UI != UE; ++UI) { 254 MachineOperand &UseMO = UI.getOperand(); 255 if (!UseMO.isUse()) 256 continue; 257 MachineInstr *UseMI = UseMO.getParent(); 258 MachineBasicBlock *UseMBB = UseMI->getParent(); 259 if (UseMBB == MBB) { 260 DenseMap<MachineInstr*, unsigned>::iterator DI = DistanceMap.find(UseMI); 261 if (DI != DistanceMap.end() && DI->second == Loc) 262 continue; // Current use. 263 OtherUse = true; 264 // There is at least one other use in the MBB that will clobber the 265 // register. 266 if (isTwoAddrUse(UseMI, Reg)) 267 return true; 268 } 269 } 270 271 // If other uses in MBB are not two-address uses, then don't remat. 272 if (OtherUse) 273 return false; 274 275 // No other uses in the same block, remat if it's defined in the same 276 // block so it does not unnecessarily extend the live range. 277 return MBB == DefMI->getParent(); 278} 279 280/// runOnMachineFunction - Reduce two-address instructions to two operands. 281/// 282bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) { 283 DOUT << "Machine Function\n"; 284 const TargetMachine &TM = MF.getTarget(); 285 MRI = &MF.getRegInfo(); 286 TII = TM.getInstrInfo(); 287 TRI = TM.getRegisterInfo(); 288 LV = getAnalysisToUpdate<LiveVariables>(); 289 290 bool MadeChange = false; 291 292 DOUT << "********** REWRITING TWO-ADDR INSTRS **********\n"; 293 DOUT << "********** Function: " << MF.getFunction()->getName() << '\n'; 294 295 // ReMatRegs - Keep track of the registers whose def's are remat'ed. 296 BitVector ReMatRegs; 297 ReMatRegs.resize(MRI->getLastVirtReg()+1); 298 299 // DistanceMap - Keep track the distance of a MI from the start of the 300 // current basic block. 301 DenseMap<MachineInstr*, unsigned> DistanceMap; 302 303 for (MachineFunction::iterator mbbi = MF.begin(), mbbe = MF.end(); 304 mbbi != mbbe; ++mbbi) { 305 unsigned Dist = 0; 306 DistanceMap.clear(); 307 for (MachineBasicBlock::iterator mi = mbbi->begin(), me = mbbi->end(); 308 mi != me; ) { 309 MachineBasicBlock::iterator nmi = next(mi); 310 const TargetInstrDesc &TID = mi->getDesc(); 311 bool FirstTied = true; 312 313 DistanceMap.insert(std::make_pair(mi, ++Dist)); 314 for (unsigned si = 1, e = TID.getNumOperands(); si < e; ++si) { 315 int ti = TID.getOperandConstraint(si, TOI::TIED_TO); 316 if (ti == -1) 317 continue; 318 319 if (FirstTied) { 320 ++NumTwoAddressInstrs; 321 DOUT << '\t'; DEBUG(mi->print(*cerr.stream(), &TM)); 322 } 323 324 FirstTied = false; 325 326 assert(mi->getOperand(si).isRegister() && mi->getOperand(si).getReg() && 327 mi->getOperand(si).isUse() && "two address instruction invalid"); 328 329 // If the two operands are the same we just remove the use 330 // and mark the def as def&use, otherwise we have to insert a copy. 331 if (mi->getOperand(ti).getReg() != mi->getOperand(si).getReg()) { 332 // Rewrite: 333 // a = b op c 334 // to: 335 // a = b 336 // a = a op c 337 unsigned regA = mi->getOperand(ti).getReg(); 338 unsigned regB = mi->getOperand(si).getReg(); 339 340 assert(TargetRegisterInfo::isVirtualRegister(regA) && 341 TargetRegisterInfo::isVirtualRegister(regB) && 342 "cannot update physical register live information"); 343 344#ifndef NDEBUG 345 // First, verify that we don't have a use of a in the instruction (a = 346 // b + a for example) because our transformation will not work. This 347 // should never occur because we are in SSA form. 348 for (unsigned i = 0; i != mi->getNumOperands(); ++i) 349 assert((int)i == ti || 350 !mi->getOperand(i).isRegister() || 351 mi->getOperand(i).getReg() != regA); 352#endif 353 354 // If this instruction is not the killing user of B, see if we can 355 // rearrange the code to make it so. Making it the killing user will 356 // allow us to coalesce A and B together, eliminating the copy we are 357 // about to insert. 358 if (!mi->killsRegister(regB)) { 359 // If this instruction is commutative, check to see if C dies. If 360 // so, swap the B and C operands. This makes the live ranges of A 361 // and C joinable. 362 // FIXME: This code also works for A := B op C instructions. 363 if (TID.isCommutable() && mi->getNumOperands() >= 3) { 364 assert(mi->getOperand(3-si).isRegister() && 365 "Not a proper commutative instruction!"); 366 unsigned regC = mi->getOperand(3-si).getReg(); 367 368 if (mi->killsRegister(regC)) { 369 DOUT << "2addr: COMMUTING : " << *mi; 370 MachineInstr *NewMI = TII->commuteInstruction(mi); 371 372 if (NewMI == 0) { 373 DOUT << "2addr: COMMUTING FAILED!\n"; 374 } else { 375 DOUT << "2addr: COMMUTED TO: " << *NewMI; 376 // If the instruction changed to commute it, update livevar. 377 if (NewMI != mi) { 378 if (LV) { 379 // Update live variables 380 LV->instructionChanged(mi, NewMI); 381 } 382 383 mbbi->insert(mi, NewMI); // Insert the new inst 384 mbbi->erase(mi); // Nuke the old inst. 385 mi = NewMI; 386 DistanceMap.insert(std::make_pair(NewMI, Dist)); 387 } 388 389 ++NumCommuted; 390 regB = regC; 391 goto InstructionRearranged; 392 } 393 } 394 } 395 396 // If this instruction is potentially convertible to a true 397 // three-address instruction, 398 if (TID.isConvertibleTo3Addr()) { 399 // FIXME: This assumes there are no more operands which are tied 400 // to another register. 401#ifndef NDEBUG 402 for (unsigned i = si + 1, e = TID.getNumOperands(); i < e; ++i) 403 assert(TID.getOperandConstraint(i, TOI::TIED_TO) == -1); 404#endif 405 406 MachineInstr *NewMI = TII->convertToThreeAddress(mbbi, mi, LV); 407 if (NewMI) { 408 DOUT << "2addr: CONVERTING 2-ADDR: " << *mi; 409 DOUT << "2addr: TO 3-ADDR: " << *NewMI; 410 bool Sunk = false; 411 412 if (NewMI->findRegisterUseOperand(regB, false, TRI)) 413 // FIXME: Temporary workaround. If the new instruction doesn't 414 // uses regB, convertToThreeAddress must have created more 415 // then one instruction. 416 Sunk = Sink3AddrInstruction(mbbi, NewMI, regB, mi); 417 418 mbbi->erase(mi); // Nuke the old inst. 419 420 if (!Sunk) { 421 DistanceMap.insert(std::make_pair(NewMI, Dist)); 422 mi = NewMI; 423 nmi = next(mi); 424 } 425 426 ++NumConvertedTo3Addr; 427 break; // Done with this instruction. 428 } 429 } 430 } 431 432 InstructionRearranged: 433 const TargetRegisterClass* rc = MRI->getRegClass(regA); 434 MachineInstr *DefMI = MRI->getVRegDef(regB); 435 // If it's safe and profitable, remat the definition instead of 436 // copying it. 437 if (DefMI && 438 isSafeToReMat(regB, DefMI) && 439 isProfitableToReMat(regB, rc, mi, DefMI, mbbi, Dist,DistanceMap)){ 440 DEBUG(cerr << "2addr: REMATTING : " << *DefMI << "\n"); 441 TII->reMaterialize(*mbbi, mi, regA, DefMI); 442 ReMatRegs.set(regB); 443 ++NumReMats; 444 } else { 445 TII->copyRegToReg(*mbbi, mi, regA, regB, rc, rc); 446 } 447 448 MachineBasicBlock::iterator prevMi = prior(mi); 449 DOUT << "\t\tprepend:\t"; DEBUG(prevMi->print(*cerr.stream(), &TM)); 450 451 // Update live variables for regB. 452 if (LV) { 453 LiveVariables::VarInfo& varInfoB = LV->getVarInfo(regB); 454 455 // regB is used in this BB. 456 varInfoB.UsedBlocks[mbbi->getNumber()] = true; 457 458 if (LV->removeVirtualRegisterKilled(regB, mbbi, mi)) 459 LV->addVirtualRegisterKilled(regB, prevMi); 460 461 if (LV->removeVirtualRegisterDead(regB, mbbi, mi)) 462 LV->addVirtualRegisterDead(regB, prevMi); 463 } 464 465 // Replace all occurences of regB with regA. 466 for (unsigned i = 0, e = mi->getNumOperands(); i != e; ++i) { 467 if (mi->getOperand(i).isRegister() && 468 mi->getOperand(i).getReg() == regB) 469 mi->getOperand(i).setReg(regA); 470 } 471 } 472 473 assert(mi->getOperand(ti).isDef() && mi->getOperand(si).isUse()); 474 mi->getOperand(ti).setReg(mi->getOperand(si).getReg()); 475 MadeChange = true; 476 477 DOUT << "\t\trewrite to:\t"; DEBUG(mi->print(*cerr.stream(), &TM)); 478 } 479 480 mi = nmi; 481 } 482 } 483 484 // Some remat'ed instructions are dead. 485 int VReg = ReMatRegs.find_first(); 486 while (VReg != -1) { 487 if (MRI->use_empty(VReg)) { 488 MachineInstr *DefMI = MRI->getVRegDef(VReg); 489 DefMI->eraseFromParent(); 490 } 491 VReg = ReMatRegs.find_next(VReg); 492 } 493 494 return MadeChange; 495} 496