PHIElimination.cpp revision ce665bd2e2b581ab0858d1afe359192bac96b868
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/MachineLoopInfo.h" 24#include "llvm/CodeGen/MachineRegisterInfo.h" 25#include "llvm/Target/TargetInstrInfo.h" 26#include "llvm/Function.h" 27#include "llvm/Target/TargetMachine.h" 28#include "llvm/ADT/SmallPtrSet.h" 29#include "llvm/ADT/STLExtras.h" 30#include "llvm/ADT/Statistic.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(NumReused, "Number of reused lowered phis"); 39 40char PHIElimination::ID = 0; 41INITIALIZE_PASS(PHIElimination, "phi-node-elimination", 42 "Eliminate PHI nodes for register allocation", false, false) 43 44char &llvm::PHIEliminationID = PHIElimination::ID; 45 46void llvm::PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const { 47 AU.addPreserved<LiveVariables>(); 48 AU.addPreserved<MachineDominatorTree>(); 49 AU.addPreserved<MachineLoopInfo>(); 50 MachineFunctionPass::getAnalysisUsage(AU); 51} 52 53bool llvm::PHIElimination::runOnMachineFunction(MachineFunction &MF) { 54 MRI = &MF.getRegInfo(); 55 56 bool Changed = false; 57 58 // Split critical edges to help the coalescer 59 if (LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>()) { 60 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>(); 61 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) 62 Changed |= SplitPHIEdges(MF, *I, *LV, MLI); 63 } 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 assert(MPhi->getOperand(0).getSubReg() == 0 && "Can't handle sub-reg PHIs"); 185 bool isDead = MPhi->getOperand(0).isDead(); 186 187 // Create a new register for the incoming PHI arguments. 188 MachineFunction &MF = *MBB.getParent(); 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 const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg); 213 entry = IncomingReg = MF.getRegInfo().createVirtualRegister(RC); 214 } 215 BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(), 216 TII->get(TargetOpcode::COPY), DestReg) 217 .addReg(IncomingReg); 218 } 219 220 // Update live variable information if there is any. 221 LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>(); 222 if (LV) { 223 MachineInstr *PHICopy = prior(AfterPHIsIt); 224 225 if (IncomingReg) { 226 LiveVariables::VarInfo &VI = LV->getVarInfo(IncomingReg); 227 228 // Increment use count of the newly created virtual register. 229 VI.NumUses++; 230 LV->setPHIJoin(IncomingReg); 231 232 // When we are reusing the incoming register, it may already have been 233 // killed in this block. The old kill will also have been inserted at 234 // AfterPHIsIt, so it appears before the current PHICopy. 235 if (reusedIncoming) 236 if (MachineInstr *OldKill = VI.findKill(&MBB)) { 237 DEBUG(dbgs() << "Remove old kill from " << *OldKill); 238 LV->removeVirtualRegisterKilled(IncomingReg, OldKill); 239 DEBUG(MBB.dump()); 240 } 241 242 // Add information to LiveVariables to know that the incoming value is 243 // killed. Note that because the value is defined in several places (once 244 // each for each incoming block), the "def" block and instruction fields 245 // for the VarInfo is not filled in. 246 LV->addVirtualRegisterKilled(IncomingReg, PHICopy); 247 } 248 249 // Since we are going to be deleting the PHI node, if it is the last use of 250 // any registers, or if the value itself is dead, we need to move this 251 // information over to the new copy we just inserted. 252 LV->removeVirtualRegistersKilled(MPhi); 253 254 // If the result is dead, update LV. 255 if (isDead) { 256 LV->addVirtualRegisterDead(DestReg, PHICopy); 257 LV->removeVirtualRegisterDead(DestReg, MPhi); 258 } 259 } 260 261 // Adjust the VRegPHIUseCount map to account for the removal of this PHI node. 262 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) 263 --VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i+1).getMBB()->getNumber(), 264 MPhi->getOperand(i).getReg())]; 265 266 // Now loop over all of the incoming arguments, changing them to copy into the 267 // IncomingReg register in the corresponding predecessor basic block. 268 SmallPtrSet<MachineBasicBlock*, 8> MBBsInsertedInto; 269 for (int i = NumSrcs - 1; i >= 0; --i) { 270 unsigned SrcReg = MPhi->getOperand(i*2+1).getReg(); 271 unsigned SrcSubReg = MPhi->getOperand(i*2+1).getSubReg(); 272 273 assert(TargetRegisterInfo::isVirtualRegister(SrcReg) && 274 "Machine PHI Operands must all be virtual registers!"); 275 276 // Get the MachineBasicBlock equivalent of the BasicBlock that is the source 277 // path the PHI. 278 MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB(); 279 280 // If source is defined by an implicit def, there is no need to insert a 281 // copy. 282 MachineInstr *DefMI = MRI->getVRegDef(SrcReg); 283 if (DefMI->isImplicitDef()) { 284 ImpDefs.insert(DefMI); 285 continue; 286 } 287 288 // Check to make sure we haven't already emitted the copy for this block. 289 // This can happen because PHI nodes may have multiple entries for the same 290 // basic block. 291 if (!MBBsInsertedInto.insert(&opBlock)) 292 continue; // If the copy has already been emitted, we're done. 293 294 // Find a safe location to insert the copy, this may be the first terminator 295 // in the block (or end()). 296 MachineBasicBlock::iterator InsertPos = 297 FindCopyInsertPoint(opBlock, MBB, SrcReg); 298 299 // Insert the copy. 300 if (!reusedIncoming && IncomingReg) 301 BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(), 302 TII->get(TargetOpcode::COPY), IncomingReg).addReg(SrcReg, 0, SrcSubReg); 303 304 // Now update live variable information if we have it. Otherwise we're done 305 if (!LV) continue; 306 307 // We want to be able to insert a kill of the register if this PHI (aka, the 308 // copy we just inserted) is the last use of the source value. Live 309 // variable analysis conservatively handles this by saying that the value is 310 // live until the end of the block the PHI entry lives in. If the value 311 // really is dead at the PHI copy, there will be no successor blocks which 312 // have the value live-in. 313 314 // Also check to see if this register is in use by another PHI node which 315 // has not yet been eliminated. If so, it will be killed at an appropriate 316 // point later. 317 318 // Is it used by any PHI instructions in this block? 319 bool ValueIsUsed = VRegPHIUseCount[BBVRegPair(opBlock.getNumber(), SrcReg)]; 320 321 // Okay, if we now know that the value is not live out of the block, we can 322 // add a kill marker in this block saying that it kills the incoming value! 323 if (!ValueIsUsed && !LV->isLiveOut(SrcReg, opBlock)) { 324 // In our final twist, we have to decide which instruction kills the 325 // register. In most cases this is the copy, however, the first 326 // terminator instruction at the end of the block may also use the value. 327 // In this case, we should mark *it* as being the killing block, not the 328 // copy. 329 MachineBasicBlock::iterator KillInst; 330 MachineBasicBlock::iterator Term = opBlock.getFirstTerminator(); 331 if (Term != opBlock.end() && Term->readsRegister(SrcReg)) { 332 KillInst = Term; 333 334 // Check that no other terminators use values. 335#ifndef NDEBUG 336 for (MachineBasicBlock::iterator TI = llvm::next(Term); 337 TI != opBlock.end(); ++TI) { 338 assert(!TI->readsRegister(SrcReg) && 339 "Terminator instructions cannot use virtual registers unless" 340 "they are the first terminator in a block!"); 341 } 342#endif 343 } else if (reusedIncoming || !IncomingReg) { 344 // We may have to rewind a bit if we didn't insert a copy this time. 345 KillInst = Term; 346 while (KillInst != opBlock.begin()) 347 if ((--KillInst)->readsRegister(SrcReg)) 348 break; 349 } else { 350 // We just inserted this copy. 351 KillInst = prior(InsertPos); 352 } 353 assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction"); 354 355 // Finally, mark it killed. 356 LV->addVirtualRegisterKilled(SrcReg, KillInst); 357 358 // This vreg no longer lives all of the way through opBlock. 359 unsigned opBlockNum = opBlock.getNumber(); 360 LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum); 361 } 362 } 363 364 // Really delete the PHI instruction now, if it is not in the LoweredPHIs map. 365 if (reusedIncoming || !IncomingReg) 366 MF.DeleteMachineInstr(MPhi); 367} 368 369/// analyzePHINodes - Gather information about the PHI nodes in here. In 370/// particular, we want to map the number of uses of a virtual register which is 371/// used in a PHI node. We map that to the BB the vreg is coming from. This is 372/// used later to determine when the vreg is killed in the BB. 373/// 374void llvm::PHIElimination::analyzePHINodes(const MachineFunction& MF) { 375 for (MachineFunction::const_iterator I = MF.begin(), E = MF.end(); 376 I != E; ++I) 377 for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end(); 378 BBI != BBE && BBI->isPHI(); ++BBI) 379 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) 380 ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i+1).getMBB()->getNumber(), 381 BBI->getOperand(i).getReg())]; 382} 383 384bool llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF, 385 MachineBasicBlock &MBB, 386 LiveVariables &LV, 387 MachineLoopInfo *MLI) { 388 if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad()) 389 return false; // Quick exit for basic blocks without PHIs. 390 391 bool Changed = false; 392 for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end(); 393 BBI != BBE && BBI->isPHI(); ++BBI) { 394 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) { 395 unsigned Reg = BBI->getOperand(i).getReg(); 396 MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB(); 397 // We break edges when registers are live out from the predecessor block 398 // (not considering PHI nodes). If the register is live in to this block 399 // anyway, we would gain nothing from splitting. 400 // Avoid splitting backedges of loops. It would introduce small 401 // out-of-line blocks into the loop which is very bad for code placement. 402 if (PreMBB != &MBB && 403 !LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB)) { 404 if (!MLI || 405 !(MLI->getLoopFor(PreMBB) == MLI->getLoopFor(&MBB) && 406 MLI->isLoopHeader(&MBB))) 407 Changed |= PreMBB->SplitCriticalEdge(&MBB, this) != 0; 408 } 409 } 410 } 411 return true; 412} 413