PHIElimination.cpp revision 5052c1547ef75f401fd397084831e0bb15311b09
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/CodeGen/RegAllocRegistry.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"); 39 40static cl::opt<bool> 41SplitEdges("split-phi-edges", 42 cl::desc("Split critical edges during phi elimination"), 43 cl::init(false), cl::Hidden); 44 45char PHIElimination::ID = 0; 46static RegisterPass<PHIElimination> 47X("phi-node-elimination", "Eliminate PHI nodes for register allocation"); 48 49const PassInfo *const llvm::PHIEliminationID = &X; 50 51namespace llvm { FunctionPass *createLocalRegisterAllocator(); } 52 53// Should we run edge splitting? 54static bool shouldSplitEdges() { 55 // Edge splitting breaks the local register allocator. It cannot tolerate 56 // LiveVariables being run. 57 if (RegisterRegAlloc::getDefault() == createLocalRegisterAllocator) 58 return false; 59 return SplitEdges; 60} 61 62void llvm::PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const { 63 AU.addPreserved<LiveVariables>(); 64 AU.addPreserved<MachineDominatorTree>(); 65 if (shouldSplitEdges()) { 66 AU.addRequired<LiveVariables>(); 67 } else { 68 AU.setPreservesCFG(); 69 AU.addPreservedID(MachineLoopInfoID); 70 } 71 MachineFunctionPass::getAnalysisUsage(AU); 72} 73 74bool llvm::PHIElimination::runOnMachineFunction(MachineFunction &Fn) { 75 MRI = &Fn.getRegInfo(); 76 77 PHIDefs.clear(); 78 PHIKills.clear(); 79 bool Changed = false; 80 81 // Split critical edges to help the coalescer 82 if (shouldSplitEdges()) 83 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) 84 Changed |= SplitPHIEdges(Fn, *I); 85 86 // Populate VRegPHIUseCount 87 analyzePHINodes(Fn); 88 89 // Eliminate PHI instructions by inserting copies into predecessor blocks. 90 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) 91 Changed |= EliminatePHINodes(Fn, *I); 92 93 // Remove dead IMPLICIT_DEF instructions. 94 for (SmallPtrSet<MachineInstr*,4>::iterator I = ImpDefs.begin(), 95 E = ImpDefs.end(); I != E; ++I) { 96 MachineInstr *DefMI = *I; 97 unsigned DefReg = DefMI->getOperand(0).getReg(); 98 if (MRI->use_empty(DefReg)) 99 DefMI->eraseFromParent(); 100 } 101 102 ImpDefs.clear(); 103 VRegPHIUseCount.clear(); 104 return Changed; 105} 106 107/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in 108/// predecessor basic blocks. 109/// 110bool llvm::PHIElimination::EliminatePHINodes(MachineFunction &MF, 111 MachineBasicBlock &MBB) { 112 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI) 113 return false; // Quick exit for basic blocks without PHIs. 114 115 // Get an iterator to the first instruction after the last PHI node (this may 116 // also be the end of the basic block). 117 MachineBasicBlock::iterator AfterPHIsIt = SkipPHIsAndLabels(MBB, MBB.begin()); 118 119 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) 120 LowerAtomicPHINode(MBB, AfterPHIsIt); 121 122 return true; 123} 124 125/// isSourceDefinedByImplicitDef - Return true if all sources of the phi node 126/// are implicit_def's. 127static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi, 128 const MachineRegisterInfo *MRI) { 129 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) { 130 unsigned SrcReg = MPhi->getOperand(i).getReg(); 131 const MachineInstr *DefMI = MRI->getVRegDef(SrcReg); 132 if (!DefMI || DefMI->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) 133 return false; 134 } 135 return true; 136} 137 138// FindCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg 139// when following the CFG edge to SuccMBB. This needs to be after any def of 140// SrcReg, but before any subsequent point where control flow might jump out of 141// the basic block. 142MachineBasicBlock::iterator 143llvm::PHIElimination::FindCopyInsertPoint(MachineBasicBlock &MBB, 144 MachineBasicBlock &SuccMBB, 145 unsigned SrcReg) { 146 // Handle the trivial case trivially. 147 if (MBB.empty()) 148 return MBB.begin(); 149 150 // Usually, we just want to insert the copy before the first terminator 151 // instruction. However, for the edge going to a landing pad, we must insert 152 // the copy before the call/invoke instruction. 153 if (!SuccMBB.isLandingPad()) 154 return MBB.getFirstTerminator(); 155 156 // Discover any defs/uses in this basic block. 157 SmallPtrSet<MachineInstr*, 8> DefUsesInMBB; 158 for (MachineRegisterInfo::reg_iterator RI = MRI->reg_begin(SrcReg), 159 RE = MRI->reg_end(); RI != RE; ++RI) { 160 MachineInstr *DefUseMI = &*RI; 161 if (DefUseMI->getParent() == &MBB) 162 DefUsesInMBB.insert(DefUseMI); 163 } 164 165 MachineBasicBlock::iterator InsertPoint; 166 if (DefUsesInMBB.empty()) { 167 // No defs. Insert the copy at the start of the basic block. 168 InsertPoint = MBB.begin(); 169 } else if (DefUsesInMBB.size() == 1) { 170 // Insert the copy immediately after the def/use. 171 InsertPoint = *DefUsesInMBB.begin(); 172 ++InsertPoint; 173 } else { 174 // Insert the copy immediately after the last def/use. 175 InsertPoint = MBB.end(); 176 while (!DefUsesInMBB.count(&*--InsertPoint)) {} 177 ++InsertPoint; 178 } 179 180 // Make sure the copy goes after any phi nodes however. 181 return SkipPHIsAndLabels(MBB, InsertPoint); 182} 183 184/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block, 185/// under the assuption that it needs to be lowered in a way that supports 186/// atomic execution of PHIs. This lowering method is always correct all of the 187/// time. 188/// 189void llvm::PHIElimination::LowerAtomicPHINode( 190 MachineBasicBlock &MBB, 191 MachineBasicBlock::iterator AfterPHIsIt) { 192 // Unlink the PHI node from the basic block, but don't delete the PHI yet. 193 MachineInstr *MPhi = MBB.remove(MBB.begin()); 194 195 unsigned NumSrcs = (MPhi->getNumOperands() - 1) / 2; 196 unsigned DestReg = MPhi->getOperand(0).getReg(); 197 bool isDead = MPhi->getOperand(0).isDead(); 198 199 // Create a new register for the incoming PHI arguments. 200 MachineFunction &MF = *MBB.getParent(); 201 const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg); 202 unsigned IncomingReg = 0; 203 204 // Insert a register to register copy at the top of the current block (but 205 // after any remaining phi nodes) which copies the new incoming register 206 // into the phi node destination. 207 const TargetInstrInfo *TII = MF.getTarget().getInstrInfo(); 208 if (isSourceDefinedByImplicitDef(MPhi, MRI)) 209 // If all sources of a PHI node are implicit_def, just emit an 210 // implicit_def instead of a copy. 211 BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(), 212 TII->get(TargetInstrInfo::IMPLICIT_DEF), DestReg); 213 else { 214 IncomingReg = MF.getRegInfo().createVirtualRegister(RC); 215 TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC); 216 } 217 218 // Record PHI def. 219 assert(!hasPHIDef(DestReg) && "Vreg has multiple phi-defs?"); 220 PHIDefs[DestReg] = &MBB; 221 222 // Update live variable information if there is any. 223 LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>(); 224 if (LV) { 225 MachineInstr *PHICopy = prior(AfterPHIsIt); 226 227 if (IncomingReg) { 228 // Increment use count of the newly created virtual register. 229 LV->getVarInfo(IncomingReg).NumUses++; 230 231 // Add information to LiveVariables to know that the incoming value is 232 // killed. Note that because the value is defined in several places (once 233 // each for each incoming block), the "def" block and instruction fields 234 // for the VarInfo is not filled in. 235 LV->addVirtualRegisterKilled(IncomingReg, PHICopy); 236 } 237 238 // Since we are going to be deleting the PHI node, if it is the last use of 239 // any registers, or if the value itself is dead, we need to move this 240 // information over to the new copy we just inserted. 241 LV->removeVirtualRegistersKilled(MPhi); 242 243 // If the result is dead, update LV. 244 if (isDead) { 245 LV->addVirtualRegisterDead(DestReg, PHICopy); 246 LV->removeVirtualRegisterDead(DestReg, MPhi); 247 } 248 } 249 250 // Adjust the VRegPHIUseCount map to account for the removal of this PHI node. 251 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) 252 --VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i + 1).getMBB(), 253 MPhi->getOperand(i).getReg())]; 254 255 // Now loop over all of the incoming arguments, changing them to copy into the 256 // IncomingReg register in the corresponding predecessor basic block. 257 SmallPtrSet<MachineBasicBlock*, 8> MBBsInsertedInto; 258 for (int i = NumSrcs - 1; i >= 0; --i) { 259 unsigned SrcReg = MPhi->getOperand(i*2+1).getReg(); 260 assert(TargetRegisterInfo::isVirtualRegister(SrcReg) && 261 "Machine PHI Operands must all be virtual registers!"); 262 263 // Get the MachineBasicBlock equivalent of the BasicBlock that is the source 264 // path the PHI. 265 MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB(); 266 267 // Record the kill. 268 PHIKills[SrcReg].insert(&opBlock); 269 270 // If source is defined by an implicit def, there is no need to insert a 271 // copy. 272 MachineInstr *DefMI = MRI->getVRegDef(SrcReg); 273 if (DefMI->getOpcode() == TargetInstrInfo::IMPLICIT_DEF) { 274 ImpDefs.insert(DefMI); 275 continue; 276 } 277 278 // Check to make sure we haven't already emitted the copy for this block. 279 // This can happen because PHI nodes may have multiple entries for the same 280 // basic block. 281 if (!MBBsInsertedInto.insert(&opBlock)) 282 continue; // If the copy has already been emitted, we're done. 283 284 // Find a safe location to insert the copy, this may be the first terminator 285 // in the block (or end()). 286 MachineBasicBlock::iterator InsertPos = 287 FindCopyInsertPoint(opBlock, MBB, SrcReg); 288 289 // Insert the copy. 290 TII->copyRegToReg(opBlock, InsertPos, IncomingReg, SrcReg, RC, RC); 291 292 // Now update live variable information if we have it. Otherwise we're done 293 if (!LV) continue; 294 295 // We want to be able to insert a kill of the register if this PHI (aka, the 296 // copy we just inserted) is the last use of the source value. Live 297 // variable analysis conservatively handles this by saying that the value is 298 // live until the end of the block the PHI entry lives in. If the value 299 // really is dead at the PHI copy, there will be no successor blocks which 300 // have the value live-in. 301 302 // Also check to see if this register is in use by another PHI node which 303 // has not yet been eliminated. If so, it will be killed at an appropriate 304 // point later. 305 306 // Is it used by any PHI instructions in this block? 307 bool ValueIsUsed = VRegPHIUseCount[BBVRegPair(&opBlock, SrcReg)] != 0; 308 309 // Okay, if we now know that the value is not live out of the block, we can 310 // add a kill marker in this block saying that it kills the incoming value! 311 if (!ValueIsUsed && !isLiveOut(SrcReg, opBlock, *LV)) { 312 // In our final twist, we have to decide which instruction kills the 313 // register. In most cases this is the copy, however, the first 314 // terminator instruction at the end of the block may also use the value. 315 // In this case, we should mark *it* as being the killing block, not the 316 // copy. 317 MachineBasicBlock::iterator KillInst = prior(InsertPos); 318 MachineBasicBlock::iterator Term = opBlock.getFirstTerminator(); 319 if (Term != opBlock.end()) { 320 if (Term->readsRegister(SrcReg)) 321 KillInst = Term; 322 323 // Check that no other terminators use values. 324#ifndef NDEBUG 325 for (MachineBasicBlock::iterator TI = next(Term); TI != opBlock.end(); 326 ++TI) { 327 assert(!TI->readsRegister(SrcReg) && 328 "Terminator instructions cannot use virtual registers unless" 329 "they are the first terminator in a block!"); 330 } 331#endif 332 } 333 334 // Finally, mark it killed. 335 LV->addVirtualRegisterKilled(SrcReg, KillInst); 336 337 // This vreg no longer lives all of the way through opBlock. 338 unsigned opBlockNum = opBlock.getNumber(); 339 LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum); 340 } 341 } 342 343 // Really delete the PHI instruction now! 344 MF.DeleteMachineInstr(MPhi); 345 ++NumAtomic; 346} 347 348/// analyzePHINodes - Gather information about the PHI nodes in here. In 349/// particular, we want to map the number of uses of a virtual register which is 350/// used in a PHI node. We map that to the BB the vreg is coming from. This is 351/// used later to determine when the vreg is killed in the BB. 352/// 353void llvm::PHIElimination::analyzePHINodes(const MachineFunction& Fn) { 354 for (MachineFunction::const_iterator I = Fn.begin(), E = Fn.end(); 355 I != E; ++I) 356 for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end(); 357 BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI) 358 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) 359 ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i + 1).getMBB(), 360 BBI->getOperand(i).getReg())]; 361} 362 363bool llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF, 364 MachineBasicBlock &MBB) { 365 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI) 366 return false; // Quick exit for basic blocks without PHIs. 367 LiveVariables &LV = getAnalysis<LiveVariables>(); 368 for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end(); 369 BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI) { 370 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) { 371 unsigned Reg = BBI->getOperand(i).getReg(); 372 MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB(); 373 // We break edges when registers are live out from the predecessor block 374 // (not considering PHI nodes). If the register is live in to this block 375 // anyway, we would gain nothing from splitting. 376 if (isLiveOut(Reg, *PreMBB, LV) && !isLiveIn(Reg, MBB, LV)) 377 SplitCriticalEdge(PreMBB, &MBB); 378 } 379 } 380 return true; 381} 382 383bool llvm::PHIElimination::isLiveOut(unsigned Reg, const MachineBasicBlock &MBB, 384 LiveVariables &LV) { 385 LiveVariables::VarInfo &VI = LV.getVarInfo(Reg); 386 387 // Loop over all of the successors of the basic block, checking to see if 388 // the value is either live in the block, or if it is killed in the block. 389 std::vector<MachineBasicBlock*> OpSuccBlocks; 390 for (MachineBasicBlock::const_succ_iterator SI = MBB.succ_begin(), 391 E = MBB.succ_end(); SI != E; ++SI) { 392 MachineBasicBlock *SuccMBB = *SI; 393 394 // Is it alive in this successor? 395 unsigned SuccIdx = SuccMBB->getNumber(); 396 if (VI.AliveBlocks.test(SuccIdx)) 397 return true; 398 OpSuccBlocks.push_back(SuccMBB); 399 } 400 401 // Check to see if this value is live because there is a use in a successor 402 // that kills it. 403 switch (OpSuccBlocks.size()) { 404 case 1: { 405 MachineBasicBlock *SuccMBB = OpSuccBlocks[0]; 406 for (unsigned i = 0, e = VI.Kills.size(); i != e; ++i) 407 if (VI.Kills[i]->getParent() == SuccMBB) 408 return true; 409 break; 410 } 411 case 2: { 412 MachineBasicBlock *SuccMBB1 = OpSuccBlocks[0], *SuccMBB2 = OpSuccBlocks[1]; 413 for (unsigned i = 0, e = VI.Kills.size(); i != e; ++i) 414 if (VI.Kills[i]->getParent() == SuccMBB1 || 415 VI.Kills[i]->getParent() == SuccMBB2) 416 return true; 417 break; 418 } 419 default: 420 std::sort(OpSuccBlocks.begin(), OpSuccBlocks.end()); 421 for (unsigned i = 0, e = VI.Kills.size(); i != e; ++i) 422 if (std::binary_search(OpSuccBlocks.begin(), OpSuccBlocks.end(), 423 VI.Kills[i]->getParent())) 424 return true; 425 } 426 return false; 427} 428 429bool llvm::PHIElimination::isLiveIn(unsigned Reg, const MachineBasicBlock &MBB, 430 LiveVariables &LV) { 431 LiveVariables::VarInfo &VI = LV.getVarInfo(Reg); 432 433 if (VI.AliveBlocks.test(MBB.getNumber())) 434 return true; 435 436 // defined in MBB? 437 const MachineInstr *Def = MRI->getVRegDef(Reg); 438 if (Def && Def->getParent() == &MBB) 439 return false; 440 441 // killed in MBB? 442 return VI.findKill(&MBB); 443} 444 445MachineBasicBlock *PHIElimination::SplitCriticalEdge(MachineBasicBlock *A, 446 MachineBasicBlock *B) { 447 assert(A && B && "Missing MBB end point"); 448 449 MachineFunction *MF = A->getParent(); 450 451 // We may need to update A's terminator, but we can't do that if AnalyzeBranch 452 // fails. If A uses a jump table, we won't touch it. 453 const TargetInstrInfo *TII = MF->getTarget().getInstrInfo(); 454 MachineBasicBlock *TBB = 0, *FBB = 0; 455 SmallVector<MachineOperand, 4> Cond; 456 if (TII->AnalyzeBranch(*A, TBB, FBB, Cond)) 457 return NULL; 458 459 ++NumSplits; 460 461 MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock(); 462 MF->push_back(NMBB); 463 DEBUG(errs() << "PHIElimination splitting critical edge:" 464 " BB#" << A->getNumber() 465 << " -- BB#" << NMBB->getNumber() 466 << " -- BB#" << B->getNumber() << '\n'); 467 468 A->ReplaceUsesOfBlockWith(B, NMBB); 469 // If A may fall through to B, we may have to insert a branch. 470 if (A->isLayoutSuccessor(B)) 471 A->updateTerminator(); 472 473 // Insert unconditional "jump B" instruction in NMBB. 474 NMBB->addSuccessor(B); 475 Cond.clear(); 476 MF->getTarget().getInstrInfo()->InsertBranch(*NMBB, B, NULL, Cond); 477 478 // Fix PHI nodes in B so they refer to NMBB instead of A 479 for (MachineBasicBlock::iterator i = B->begin(), e = B->end(); 480 i != e && i->getOpcode() == TargetInstrInfo::PHI; ++i) 481 for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2) 482 if (i->getOperand(ni+1).getMBB() == A) 483 i->getOperand(ni+1).setMBB(NMBB); 484 485 if (LiveVariables *LV=getAnalysisIfAvailable<LiveVariables>()) 486 LV->addNewBlock(NMBB, A); 487 488 if (MachineDominatorTree *MDT=getAnalysisIfAvailable<MachineDominatorTree>()) 489 MDT->addNewBlock(NMBB, A); 490 491 return NMBB; 492} 493