LiveRangeCalc.cpp revision 261991
1//===---- LiveRangeCalc.cpp - Calculate live ranges -----------------------===// 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// Implementation of the LiveRangeCalc class. 11// 12//===----------------------------------------------------------------------===// 13 14#define DEBUG_TYPE "regalloc" 15#include "LiveRangeCalc.h" 16#include "llvm/CodeGen/MachineDominators.h" 17#include "llvm/CodeGen/MachineRegisterInfo.h" 18 19using namespace llvm; 20 21void LiveRangeCalc::reset(const MachineFunction *mf, 22 SlotIndexes *SI, 23 MachineDominatorTree *MDT, 24 VNInfo::Allocator *VNIA) { 25 MF = mf; 26 MRI = &MF->getRegInfo(); 27 Indexes = SI; 28 DomTree = MDT; 29 Alloc = VNIA; 30 31 unsigned N = MF->getNumBlockIDs(); 32 Seen.clear(); 33 Seen.resize(N); 34 LiveOut.resize(N); 35 LiveIn.clear(); 36} 37 38 39void LiveRangeCalc::createDeadDefs(LiveRange &LR, unsigned Reg) { 40 assert(MRI && Indexes && "call reset() first"); 41 42 // Visit all def operands. If the same instruction has multiple defs of Reg, 43 // LR.createDeadDef() will deduplicate. 44 for (MachineRegisterInfo::def_iterator 45 I = MRI->def_begin(Reg), E = MRI->def_end(); I != E; ++I) { 46 const MachineInstr *MI = &*I; 47 // Find the corresponding slot index. 48 SlotIndex Idx; 49 if (MI->isPHI()) 50 // PHI defs begin at the basic block start index. 51 Idx = Indexes->getMBBStartIdx(MI->getParent()); 52 else 53 // Instructions are either normal 'r', or early clobber 'e'. 54 Idx = Indexes->getInstructionIndex(MI) 55 .getRegSlot(I.getOperand().isEarlyClobber()); 56 57 // Create the def in LR. This may find an existing def. 58 LR.createDeadDef(Idx, *Alloc); 59 } 60} 61 62 63void LiveRangeCalc::extendToUses(LiveRange &LR, unsigned Reg) { 64 assert(MRI && Indexes && "call reset() first"); 65 66 // Visit all operands that read Reg. This may include partial defs. 67 for (MachineRegisterInfo::reg_nodbg_iterator I = MRI->reg_nodbg_begin(Reg), 68 E = MRI->reg_nodbg_end(); I != E; ++I) { 69 MachineOperand &MO = I.getOperand(); 70 // Clear all kill flags. They will be reinserted after register allocation 71 // by LiveIntervalAnalysis::addKillFlags(). 72 if (MO.isUse()) 73 MO.setIsKill(false); 74 if (!MO.readsReg()) 75 continue; 76 // MI is reading Reg. We may have visited MI before if it happens to be 77 // reading Reg multiple times. That is OK, extend() is idempotent. 78 const MachineInstr *MI = &*I; 79 80 // Find the SlotIndex being read. 81 SlotIndex Idx; 82 if (MI->isPHI()) { 83 assert(!MO.isDef() && "Cannot handle PHI def of partial register."); 84 // PHI operands are paired: (Reg, PredMBB). 85 // Extend the live range to be live-out from PredMBB. 86 Idx = Indexes->getMBBEndIdx(MI->getOperand(I.getOperandNo()+1).getMBB()); 87 } else { 88 // This is a normal instruction. 89 Idx = Indexes->getInstructionIndex(MI).getRegSlot(); 90 // Check for early-clobber redefs. 91 unsigned DefIdx; 92 if (MO.isDef()) { 93 if (MO.isEarlyClobber()) 94 Idx = Idx.getRegSlot(true); 95 } else if (MI->isRegTiedToDefOperand(I.getOperandNo(), &DefIdx)) { 96 // FIXME: This would be a lot easier if tied early-clobber uses also 97 // had an early-clobber flag. 98 if (MI->getOperand(DefIdx).isEarlyClobber()) 99 Idx = Idx.getRegSlot(true); 100 } 101 } 102 extend(LR, Idx, Reg); 103 } 104} 105 106 107// Transfer information from the LiveIn vector to the live ranges. 108void LiveRangeCalc::updateLiveIns() { 109 LiveRangeUpdater Updater; 110 for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(), 111 E = LiveIn.end(); I != E; ++I) { 112 if (!I->DomNode) 113 continue; 114 MachineBasicBlock *MBB = I->DomNode->getBlock(); 115 assert(I->Value && "No live-in value found"); 116 SlotIndex Start, End; 117 tie(Start, End) = Indexes->getMBBRange(MBB); 118 119 if (I->Kill.isValid()) 120 // Value is killed inside this block. 121 End = I->Kill; 122 else { 123 // The value is live-through, update LiveOut as well. 124 // Defer the Domtree lookup until it is needed. 125 assert(Seen.test(MBB->getNumber())); 126 LiveOut[MBB] = LiveOutPair(I->Value, (MachineDomTreeNode *)0); 127 } 128 Updater.setDest(&I->LR); 129 Updater.add(Start, End, I->Value); 130 } 131 LiveIn.clear(); 132} 133 134 135void LiveRangeCalc::extend(LiveRange &LR, SlotIndex Kill, unsigned PhysReg) { 136 assert(Kill.isValid() && "Invalid SlotIndex"); 137 assert(Indexes && "Missing SlotIndexes"); 138 assert(DomTree && "Missing dominator tree"); 139 140 MachineBasicBlock *KillMBB = Indexes->getMBBFromIndex(Kill.getPrevSlot()); 141 assert(KillMBB && "No MBB at Kill"); 142 143 // Is there a def in the same MBB we can extend? 144 if (LR.extendInBlock(Indexes->getMBBStartIdx(KillMBB), Kill)) 145 return; 146 147 // Find the single reaching def, or determine if Kill is jointly dominated by 148 // multiple values, and we may need to create even more phi-defs to preserve 149 // VNInfo SSA form. Perform a search for all predecessor blocks where we 150 // know the dominating VNInfo. 151 if (findReachingDefs(LR, *KillMBB, Kill, PhysReg)) 152 return; 153 154 // When there were multiple different values, we may need new PHIs. 155 calculateValues(); 156} 157 158 159// This function is called by a client after using the low-level API to add 160// live-out and live-in blocks. The unique value optimization is not 161// available, SplitEditor::transferValues handles that case directly anyway. 162void LiveRangeCalc::calculateValues() { 163 assert(Indexes && "Missing SlotIndexes"); 164 assert(DomTree && "Missing dominator tree"); 165 updateSSA(); 166 updateLiveIns(); 167} 168 169 170bool LiveRangeCalc::findReachingDefs(LiveRange &LR, MachineBasicBlock &KillMBB, 171 SlotIndex Kill, unsigned PhysReg) { 172 unsigned KillMBBNum = KillMBB.getNumber(); 173 174 // Block numbers where LR should be live-in. 175 SmallVector<unsigned, 16> WorkList(1, KillMBBNum); 176 177 // Remember if we have seen more than one value. 178 bool UniqueVNI = true; 179 VNInfo *TheVNI = 0; 180 181 // Using Seen as a visited set, perform a BFS for all reaching defs. 182 for (unsigned i = 0; i != WorkList.size(); ++i) { 183 MachineBasicBlock *MBB = MF->getBlockNumbered(WorkList[i]); 184 185#ifndef NDEBUG 186 if (MBB->pred_empty()) { 187 MBB->getParent()->verify(); 188 llvm_unreachable("Use not jointly dominated by defs."); 189 } 190 191 if (TargetRegisterInfo::isPhysicalRegister(PhysReg) && 192 !MBB->isLiveIn(PhysReg)) { 193 MBB->getParent()->verify(); 194 errs() << "The register needs to be live in to BB#" << MBB->getNumber() 195 << ", but is missing from the live-in list.\n"; 196 llvm_unreachable("Invalid global physical register"); 197 } 198#endif 199 200 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 201 PE = MBB->pred_end(); PI != PE; ++PI) { 202 MachineBasicBlock *Pred = *PI; 203 204 // Is this a known live-out block? 205 if (Seen.test(Pred->getNumber())) { 206 if (VNInfo *VNI = LiveOut[Pred].first) { 207 if (TheVNI && TheVNI != VNI) 208 UniqueVNI = false; 209 TheVNI = VNI; 210 } 211 continue; 212 } 213 214 SlotIndex Start, End; 215 tie(Start, End) = Indexes->getMBBRange(Pred); 216 217 // First time we see Pred. Try to determine the live-out value, but set 218 // it as null if Pred is live-through with an unknown value. 219 VNInfo *VNI = LR.extendInBlock(Start, End); 220 setLiveOutValue(Pred, VNI); 221 if (VNI) { 222 if (TheVNI && TheVNI != VNI) 223 UniqueVNI = false; 224 TheVNI = VNI; 225 continue; 226 } 227 228 // No, we need a live-in value for Pred as well 229 if (Pred != &KillMBB) 230 WorkList.push_back(Pred->getNumber()); 231 else 232 // Loopback to KillMBB, so value is really live through. 233 Kill = SlotIndex(); 234 } 235 } 236 237 LiveIn.clear(); 238 239 // Both updateSSA() and LiveRangeUpdater benefit from ordered blocks, but 240 // neither require it. Skip the sorting overhead for small updates. 241 if (WorkList.size() > 4) 242 array_pod_sort(WorkList.begin(), WorkList.end()); 243 244 // If a unique reaching def was found, blit in the live ranges immediately. 245 if (UniqueVNI) { 246 LiveRangeUpdater Updater(&LR); 247 for (SmallVectorImpl<unsigned>::const_iterator I = WorkList.begin(), 248 E = WorkList.end(); I != E; ++I) { 249 SlotIndex Start, End; 250 tie(Start, End) = Indexes->getMBBRange(*I); 251 // Trim the live range in KillMBB. 252 if (*I == KillMBBNum && Kill.isValid()) 253 End = Kill; 254 else 255 LiveOut[MF->getBlockNumbered(*I)] = 256 LiveOutPair(TheVNI, (MachineDomTreeNode *)0); 257 Updater.add(Start, End, TheVNI); 258 } 259 return true; 260 } 261 262 // Multiple values were found, so transfer the work list to the LiveIn array 263 // where UpdateSSA will use it as a work list. 264 LiveIn.reserve(WorkList.size()); 265 for (SmallVectorImpl<unsigned>::const_iterator 266 I = WorkList.begin(), E = WorkList.end(); I != E; ++I) { 267 MachineBasicBlock *MBB = MF->getBlockNumbered(*I); 268 addLiveInBlock(LR, DomTree->getNode(MBB)); 269 if (MBB == &KillMBB) 270 LiveIn.back().Kill = Kill; 271 } 272 273 return false; 274} 275 276 277// This is essentially the same iterative algorithm that SSAUpdater uses, 278// except we already have a dominator tree, so we don't have to recompute it. 279void LiveRangeCalc::updateSSA() { 280 assert(Indexes && "Missing SlotIndexes"); 281 assert(DomTree && "Missing dominator tree"); 282 283 // Interate until convergence. 284 unsigned Changes; 285 do { 286 Changes = 0; 287 // Propagate live-out values down the dominator tree, inserting phi-defs 288 // when necessary. 289 for (SmallVectorImpl<LiveInBlock>::iterator I = LiveIn.begin(), 290 E = LiveIn.end(); I != E; ++I) { 291 MachineDomTreeNode *Node = I->DomNode; 292 // Skip block if the live-in value has already been determined. 293 if (!Node) 294 continue; 295 MachineBasicBlock *MBB = Node->getBlock(); 296 MachineDomTreeNode *IDom = Node->getIDom(); 297 LiveOutPair IDomValue; 298 299 // We need a live-in value to a block with no immediate dominator? 300 // This is probably an unreachable block that has survived somehow. 301 bool needPHI = !IDom || !Seen.test(IDom->getBlock()->getNumber()); 302 303 // IDom dominates all of our predecessors, but it may not be their 304 // immediate dominator. Check if any of them have live-out values that are 305 // properly dominated by IDom. If so, we need a phi-def here. 306 if (!needPHI) { 307 IDomValue = LiveOut[IDom->getBlock()]; 308 309 // Cache the DomTree node that defined the value. 310 if (IDomValue.first && !IDomValue.second) 311 LiveOut[IDom->getBlock()].second = IDomValue.second = 312 DomTree->getNode(Indexes->getMBBFromIndex(IDomValue.first->def)); 313 314 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 315 PE = MBB->pred_end(); PI != PE; ++PI) { 316 LiveOutPair &Value = LiveOut[*PI]; 317 if (!Value.first || Value.first == IDomValue.first) 318 continue; 319 320 // Cache the DomTree node that defined the value. 321 if (!Value.second) 322 Value.second = 323 DomTree->getNode(Indexes->getMBBFromIndex(Value.first->def)); 324 325 // This predecessor is carrying something other than IDomValue. 326 // It could be because IDomValue hasn't propagated yet, or it could be 327 // because MBB is in the dominance frontier of that value. 328 if (DomTree->dominates(IDom, Value.second)) { 329 needPHI = true; 330 break; 331 } 332 } 333 } 334 335 // The value may be live-through even if Kill is set, as can happen when 336 // we are called from extendRange. In that case LiveOutSeen is true, and 337 // LiveOut indicates a foreign or missing value. 338 LiveOutPair &LOP = LiveOut[MBB]; 339 340 // Create a phi-def if required. 341 if (needPHI) { 342 ++Changes; 343 assert(Alloc && "Need VNInfo allocator to create PHI-defs"); 344 SlotIndex Start, End; 345 tie(Start, End) = Indexes->getMBBRange(MBB); 346 LiveRange &LR = I->LR; 347 VNInfo *VNI = LR.getNextValue(Start, *Alloc); 348 I->Value = VNI; 349 // This block is done, we know the final value. 350 I->DomNode = 0; 351 352 // Add liveness since updateLiveIns now skips this node. 353 if (I->Kill.isValid()) 354 LR.addSegment(LiveInterval::Segment(Start, I->Kill, VNI)); 355 else { 356 LR.addSegment(LiveInterval::Segment(Start, End, VNI)); 357 LOP = LiveOutPair(VNI, Node); 358 } 359 } else if (IDomValue.first) { 360 // No phi-def here. Remember incoming value. 361 I->Value = IDomValue.first; 362 363 // If the IDomValue is killed in the block, don't propagate through. 364 if (I->Kill.isValid()) 365 continue; 366 367 // Propagate IDomValue if it isn't killed: 368 // MBB is live-out and doesn't define its own value. 369 if (LOP.first == IDomValue.first) 370 continue; 371 ++Changes; 372 LOP = IDomValue; 373 } 374 } 375 } while (Changes); 376} 377