PHIElimination.cpp revision 28428cd6f31c1ea3cf4cea03e64189a4c32b84a3
1//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===// 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 pass eliminates machine instruction PHI nodes by inserting copy 11// instructions. This destroys SSA information, but is the desired input for 12// some register allocators. 13// 14//===----------------------------------------------------------------------===// 15 16#define DEBUG_TYPE "phielim" 17#include "PHIElimination.h" 18#include "llvm/CodeGen/LiveVariables.h" 19#include "llvm/CodeGen/Passes.h" 20#include "llvm/CodeGen/MachineDominators.h" 21#include "llvm/CodeGen/MachineInstr.h" 22#include "llvm/CodeGen/MachineInstrBuilder.h" 23#include "llvm/CodeGen/MachineRegisterInfo.h" 24#include "llvm/Target/TargetInstrInfo.h" 25#include "llvm/Function.h" 26#include "llvm/Target/TargetMachine.h" 27#include "llvm/ADT/SmallPtrSet.h" 28#include "llvm/ADT/STLExtras.h" 29#include "llvm/ADT/Statistic.h" 30#include "llvm/Support/CommandLine.h" 31#include "llvm/Support/Compiler.h" 32#include "llvm/Support/Debug.h" 33#include <algorithm> 34#include <map> 35using namespace llvm; 36 37STATISTIC(NumAtomic, "Number of atomic phis lowered"); 38STATISTIC(NumSplits, "Number of critical edges split on demand"); 39STATISTIC(NumReused, "Number of reused lowered phis"); 40 41char PHIElimination::ID = 0; 42static RegisterPass<PHIElimination> 43X("phi-node-elimination", "Eliminate PHI nodes for register allocation"); 44 45const PassInfo *const llvm::PHIEliminationID = &X; 46 47void llvm::PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const { 48 AU.addPreserved<LiveVariables>(); 49 AU.addPreserved<MachineDominatorTree>(); 50 // rdar://7401784 This would be nice: 51 // AU.addPreservedID(MachineLoopInfoID); 52 MachineFunctionPass::getAnalysisUsage(AU); 53} 54 55bool llvm::PHIElimination::runOnMachineFunction(MachineFunction &MF) { 56 MRI = &MF.getRegInfo(); 57 58 bool Changed = false; 59 60 // Split critical edges to help the coalescer 61 if (LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>()) 62 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) 63 Changed |= SplitPHIEdges(MF, *I, *LV); 64 65 // Populate VRegPHIUseCount 66 analyzePHINodes(MF); 67 68 // Eliminate PHI instructions by inserting copies into predecessor blocks. 69 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) 70 Changed |= EliminatePHINodes(MF, *I); 71 72 // Remove dead IMPLICIT_DEF instructions. 73 for (SmallPtrSet<MachineInstr*, 4>::iterator I = ImpDefs.begin(), 74 E = ImpDefs.end(); I != E; ++I) { 75 MachineInstr *DefMI = *I; 76 unsigned DefReg = DefMI->getOperand(0).getReg(); 77 if (MRI->use_nodbg_empty(DefReg)) 78 DefMI->eraseFromParent(); 79 } 80 81 // Clean up the lowered PHI instructions. 82 for (LoweredPHIMap::iterator I = LoweredPHIs.begin(), E = LoweredPHIs.end(); 83 I != E; ++I) 84 MF.DeleteMachineInstr(I->first); 85 86 LoweredPHIs.clear(); 87 ImpDefs.clear(); 88 VRegPHIUseCount.clear(); 89 90 return Changed; 91} 92 93/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in 94/// predecessor basic blocks. 95/// 96bool llvm::PHIElimination::EliminatePHINodes(MachineFunction &MF, 97 MachineBasicBlock &MBB) { 98 if (MBB.empty() || !MBB.front().isPHI()) 99 return false; // Quick exit for basic blocks without PHIs. 100 101 // Get an iterator to the first instruction after the last PHI node (this may 102 // also be the end of the basic block). 103 MachineBasicBlock::iterator AfterPHIsIt = SkipPHIsAndLabels(MBB, MBB.begin()); 104 105 while (MBB.front().isPHI()) 106 LowerAtomicPHINode(MBB, AfterPHIsIt); 107 108 return true; 109} 110 111/// isSourceDefinedByImplicitDef - Return true if all sources of the phi node 112/// are implicit_def's. 113static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi, 114 const MachineRegisterInfo *MRI) { 115 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) { 116 unsigned SrcReg = MPhi->getOperand(i).getReg(); 117 const MachineInstr *DefMI = MRI->getVRegDef(SrcReg); 118 if (!DefMI || !DefMI->isImplicitDef()) 119 return false; 120 } 121 return true; 122} 123 124// FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg 125// when following the CFG edge to SuccMBB. This needs to be after any def of 126// SrcReg, but before any subsequent point where control flow might jump out of 127// the basic block. 128MachineBasicBlock::iterator 129llvm::PHIElimination::FindCopyInsertPoint(MachineBasicBlock &MBB, 130 MachineBasicBlock &SuccMBB, 131 unsigned SrcReg) { 132 // Handle the trivial case trivially. 133 if (MBB.empty()) 134 return MBB.begin(); 135 136 // Usually, we just want to insert the copy before the first terminator 137 // instruction. However, for the edge going to a landing pad, we must insert 138 // the copy before the call/invoke instruction. 139 if (!SuccMBB.isLandingPad()) 140 return MBB.getFirstTerminator(); 141 142 // Discover any defs/uses in this basic block. 143 SmallPtrSet<MachineInstr*, 8> DefUsesInMBB; 144 for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg), 145 RE = MRI->reg_end(); RI != RE; ++RI) { 146 MachineInstr *DefUseMI = &*RI; 147 if (DefUseMI->getParent() == &MBB) 148 DefUsesInMBB.insert(DefUseMI); 149 } 150 151 MachineBasicBlock::iterator InsertPoint; 152 if (DefUsesInMBB.empty()) { 153 // No defs. Insert the copy at the start of the basic block. 154 InsertPoint = MBB.begin(); 155 } else if (DefUsesInMBB.size() == 1) { 156 // Insert the copy immediately after the def/use. 157 InsertPoint = *DefUsesInMBB.begin(); 158 ++InsertPoint; 159 } else { 160 // Insert the copy immediately after the last def/use. 161 InsertPoint = MBB.end(); 162 while (!DefUsesInMBB.count(&*--InsertPoint)) {} 163 ++InsertPoint; 164 } 165 166 // Make sure the copy goes after any phi nodes however. 167 return SkipPHIsAndLabels(MBB, InsertPoint); 168} 169 170/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block, 171/// under the assuption that it needs to be lowered in a way that supports 172/// atomic execution of PHIs. This lowering method is always correct all of the 173/// time. 174/// 175void llvm::PHIElimination::LowerAtomicPHINode( 176 MachineBasicBlock &MBB, 177 MachineBasicBlock::iterator AfterPHIsIt) { 178 ++NumAtomic; 179 // Unlink the PHI node from the basic block, but don't delete the PHI yet. 180 MachineInstr *MPhi = MBB.remove(MBB.begin()); 181 182 unsigned NumSrcs = (MPhi->getNumOperands() - 1) / 2; 183 unsigned DestReg = MPhi->getOperand(0).getReg(); 184 bool isDead = MPhi->getOperand(0).isDead(); 185 186 // Create a new register for the incoming PHI arguments. 187 MachineFunction &MF = *MBB.getParent(); 188 const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg); 189 unsigned IncomingReg = 0; 190 bool reusedIncoming = false; // Is IncomingReg reused from an earlier PHI? 191 192 // Insert a register to register copy at the top of the current block (but 193 // after any remaining phi nodes) which copies the new incoming register 194 // into the phi node destination. 195 const TargetInstrInfo *TII = MF.getTarget().getInstrInfo(); 196 if (isSourceDefinedByImplicitDef(MPhi, MRI)) 197 // If all sources of a PHI node are implicit_def, just emit an 198 // implicit_def instead of a copy. 199 BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(), 200 TII->get(TargetOpcode::IMPLICIT_DEF), DestReg); 201 else { 202 // Can we reuse an earlier PHI node? This only happens for critical edges, 203 // typically those created by tail duplication. 204 unsigned &entry = LoweredPHIs[MPhi]; 205 if (entry) { 206 // An identical PHI node was already lowered. Reuse the incoming register. 207 IncomingReg = entry; 208 reusedIncoming = true; 209 ++NumReused; 210 DEBUG(dbgs() << "Reusing %reg" << IncomingReg << " for " << *MPhi); 211 } else { 212 entry = IncomingReg = MF.getRegInfo().createVirtualRegister(RC); 213 } 214 TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC); 215 } 216 217 // Update live variable information if there is any. 218 LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>(); 219 if (LV) { 220 MachineInstr *PHICopy = prior(AfterPHIsIt); 221 222 if (IncomingReg) { 223 LiveVariables::VarInfo &VI = LV->getVarInfo(IncomingReg); 224 225 // Increment use count of the newly created virtual register. 226 VI.NumUses++; 227 LV->setPHIJoin(IncomingReg); 228 229 // When we are reusing the incoming register, it may already have been 230 // killed in this block. The old kill will also have been inserted at 231 // AfterPHIsIt, so it appears before the current PHICopy. 232 if (reusedIncoming) 233 if (MachineInstr *OldKill = VI.findKill(&MBB)) { 234 DEBUG(dbgs() << "Remove old kill from " << *OldKill); 235 LV->removeVirtualRegisterKilled(IncomingReg, OldKill); 236 DEBUG(MBB.dump()); 237 } 238 239 // Add information to LiveVariables to know that the incoming value is 240 // killed. Note that because the value is defined in several places (once 241 // each for each incoming block), the "def" block and instruction fields 242 // for the VarInfo is not filled in. 243 LV->addVirtualRegisterKilled(IncomingReg, PHICopy); 244 } 245 246 // Since we are going to be deleting the PHI node, if it is the last use of 247 // any registers, or if the value itself is dead, we need to move this 248 // information over to the new copy we just inserted. 249 LV->removeVirtualRegistersKilled(MPhi); 250 251 // If the result is dead, update LV. 252 if (isDead) { 253 LV->addVirtualRegisterDead(DestReg, PHICopy); 254 LV->removeVirtualRegisterDead(DestReg, MPhi); 255 } 256 } 257 258 // Adjust the VRegPHIUseCount map to account for the removal of this PHI node. 259 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) 260 --VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i+1).getMBB()->getNumber(), 261 MPhi->getOperand(i).getReg())]; 262 263 // Now loop over all of the incoming arguments, changing them to copy into the 264 // IncomingReg register in the corresponding predecessor basic block. 265 SmallPtrSet<MachineBasicBlock*, 8> MBBsInsertedInto; 266 for (int i = NumSrcs - 1; i >= 0; --i) { 267 unsigned SrcReg = MPhi->getOperand(i*2+1).getReg(); 268 assert(TargetRegisterInfo::isVirtualRegister(SrcReg) && 269 "Machine PHI Operands must all be virtual registers!"); 270 271 // Get the MachineBasicBlock equivalent of the BasicBlock that is the source 272 // path the PHI. 273 MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB(); 274 275 // If source is defined by an implicit def, there is no need to insert a 276 // copy. 277 MachineInstr *DefMI = MRI->getVRegDef(SrcReg); 278 if (DefMI->isImplicitDef()) { 279 ImpDefs.insert(DefMI); 280 continue; 281 } 282 283 // Check to make sure we haven't already emitted the copy for this block. 284 // This can happen because PHI nodes may have multiple entries for the same 285 // basic block. 286 if (!MBBsInsertedInto.insert(&opBlock)) 287 continue; // If the copy has already been emitted, we're done. 288 289 // Find a safe location to insert the copy, this may be the first terminator 290 // in the block (or end()). 291 MachineBasicBlock::iterator InsertPos = 292 FindCopyInsertPoint(opBlock, MBB, SrcReg); 293 294 // Insert the copy. 295 if (!reusedIncoming && IncomingReg) 296 TII->copyRegToReg(opBlock, InsertPos, IncomingReg, SrcReg, RC, RC); 297 298 // Now update live variable information if we have it. Otherwise we're done 299 if (!LV) continue; 300 301 // We want to be able to insert a kill of the register if this PHI (aka, the 302 // copy we just inserted) is the last use of the source value. Live 303 // variable analysis conservatively handles this by saying that the value is 304 // live until the end of the block the PHI entry lives in. If the value 305 // really is dead at the PHI copy, there will be no successor blocks which 306 // have the value live-in. 307 308 // Also check to see if this register is in use by another PHI node which 309 // has not yet been eliminated. If so, it will be killed at an appropriate 310 // point later. 311 312 // Is it used by any PHI instructions in this block? 313 bool ValueIsUsed = VRegPHIUseCount[BBVRegPair(opBlock.getNumber(), SrcReg)]; 314 315 // Okay, if we now know that the value is not live out of the block, we can 316 // add a kill marker in this block saying that it kills the incoming value! 317 if (!ValueIsUsed && !LV->isLiveOut(SrcReg, opBlock)) { 318 // In our final twist, we have to decide which instruction kills the 319 // register. In most cases this is the copy, however, the first 320 // terminator instruction at the end of the block may also use the value. 321 // In this case, we should mark *it* as being the killing block, not the 322 // copy. 323 MachineBasicBlock::iterator KillInst; 324 MachineBasicBlock::iterator Term = opBlock.getFirstTerminator(); 325 if (Term != opBlock.end() && Term->readsRegister(SrcReg)) { 326 KillInst = Term; 327 328 // Check that no other terminators use values. 329#ifndef NDEBUG 330 for (MachineBasicBlock::iterator TI = llvm::next(Term); 331 TI != opBlock.end(); ++TI) { 332 assert(!TI->readsRegister(SrcReg) && 333 "Terminator instructions cannot use virtual registers unless" 334 "they are the first terminator in a block!"); 335 } 336#endif 337 } else if (reusedIncoming || !IncomingReg) { 338 // We may have to rewind a bit if we didn't insert a copy this time. 339 KillInst = Term; 340 while (KillInst != opBlock.begin()) 341 if ((--KillInst)->readsRegister(SrcReg)) 342 break; 343 } else { 344 // We just inserted this copy. 345 KillInst = prior(InsertPos); 346 } 347 assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction"); 348 349 // Finally, mark it killed. 350 LV->addVirtualRegisterKilled(SrcReg, KillInst); 351 352 // This vreg no longer lives all of the way through opBlock. 353 unsigned opBlockNum = opBlock.getNumber(); 354 LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum); 355 } 356 } 357 358 // Really delete the PHI instruction now, if it is not in the LoweredPHIs map. 359 if (reusedIncoming || !IncomingReg) 360 MF.DeleteMachineInstr(MPhi); 361} 362 363/// analyzePHINodes - Gather information about the PHI nodes in here. In 364/// particular, we want to map the number of uses of a virtual register which is 365/// used in a PHI node. We map that to the BB the vreg is coming from. This is 366/// used later to determine when the vreg is killed in the BB. 367/// 368void llvm::PHIElimination::analyzePHINodes(const MachineFunction& MF) { 369 for (MachineFunction::const_iterator I = MF.begin(), E = MF.end(); 370 I != E; ++I) 371 for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end(); 372 BBI != BBE && BBI->isPHI(); ++BBI) 373 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) 374 ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i+1).getMBB()->getNumber(), 375 BBI->getOperand(i).getReg())]; 376} 377 378bool llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF, 379 MachineBasicBlock &MBB, 380 LiveVariables &LV) { 381 if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad()) 382 return false; // Quick exit for basic blocks without PHIs. 383 384 for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end(); 385 BBI != BBE && BBI->isPHI(); ++BBI) { 386 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) { 387 unsigned Reg = BBI->getOperand(i).getReg(); 388 MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB(); 389 // We break edges when registers are live out from the predecessor block 390 // (not considering PHI nodes). If the register is live in to this block 391 // anyway, we would gain nothing from splitting. 392 if (!LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB)) 393 SplitCriticalEdge(PreMBB, &MBB); 394 } 395 } 396 return true; 397} 398 399MachineBasicBlock *PHIElimination::SplitCriticalEdge(MachineBasicBlock *A, 400 MachineBasicBlock *B) { 401 assert(A && B && "Missing MBB end point"); 402 403 MachineFunction *MF = A->getParent(); 404 405 // We may need to update A's terminator, but we can't do that if AnalyzeBranch 406 // fails. If A uses a jump table, we won't touch it. 407 const TargetInstrInfo *TII = MF->getTarget().getInstrInfo(); 408 MachineBasicBlock *TBB = 0, *FBB = 0; 409 SmallVector<MachineOperand, 4> Cond; 410 if (TII->AnalyzeBranch(*A, TBB, FBB, Cond)) 411 return NULL; 412 413 ++NumSplits; 414 415 MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock(); 416 MF->insert(llvm::next(MachineFunction::iterator(A)), NMBB); 417 DEBUG(dbgs() << "PHIElimination splitting critical edge:" 418 " BB#" << A->getNumber() 419 << " -- BB#" << NMBB->getNumber() 420 << " -- BB#" << B->getNumber() << '\n'); 421 422 A->ReplaceUsesOfBlockWith(B, NMBB); 423 A->updateTerminator(); 424 425 // Insert unconditional "jump B" instruction in NMBB if necessary. 426 NMBB->addSuccessor(B); 427 if (!NMBB->isLayoutSuccessor(B)) { 428 Cond.clear(); 429 MF->getTarget().getInstrInfo()->InsertBranch(*NMBB, B, NULL, Cond); 430 } 431 432 // Fix PHI nodes in B so they refer to NMBB instead of A 433 for (MachineBasicBlock::iterator i = B->begin(), e = B->end(); 434 i != e && i->isPHI(); ++i) 435 for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2) 436 if (i->getOperand(ni+1).getMBB() == A) 437 i->getOperand(ni+1).setMBB(NMBB); 438 439 if (LiveVariables *LV=getAnalysisIfAvailable<LiveVariables>()) 440 LV->addNewBlock(NMBB, A, B); 441 442 if (MachineDominatorTree *MDT=getAnalysisIfAvailable<MachineDominatorTree>()) 443 MDT->addNewBlock(NMBB, A); 444 445 return NMBB; 446} 447