1 //===- SSAUpdaterImpl.h - SSA Updater Implementation ------------*- C++ -*-===// 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 provides a template that implements the core algorithm for the 11 // SSAUpdater and MachineSSAUpdater. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H 16 #define LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H 17 18 #include "llvm/ADT/DenseMap.h" 19 #include "llvm/ADT/SmallVector.h" 20 #include "llvm/Support/Allocator.h" 21 #include "llvm/Support/Debug.h" 22 #include "llvm/Support/raw_ostream.h" 23 24 #define DEBUG_TYPE "ssaupdater" 25 26 namespace llvm { 27 28 template<typename T> class SSAUpdaterTraits; 29 30 template<typename UpdaterT> 31 class SSAUpdaterImpl { 32 private: 33 UpdaterT *Updater; 34 35 using Traits = SSAUpdaterTraits<UpdaterT>; 36 using BlkT = typename Traits::BlkT; 37 using ValT = typename Traits::ValT; 38 using PhiT = typename Traits::PhiT; 39 40 /// BBInfo - Per-basic block information used internally by SSAUpdaterImpl. 41 /// The predecessors of each block are cached here since pred_iterator is 42 /// slow and we need to iterate over the blocks at least a few times. 43 class BBInfo { 44 public: 45 // Back-pointer to the corresponding block. 46 BlkT *BB; 47 48 // Value to use in this block. 49 ValT AvailableVal; 50 51 // Block that defines the available value. 52 BBInfo *DefBB; 53 54 // Postorder number. 55 int BlkNum = 0; 56 57 // Immediate dominator. 58 BBInfo *IDom = nullptr; 59 60 // Number of predecessor blocks. 61 unsigned NumPreds = 0; 62 63 // Array[NumPreds] of predecessor blocks. 64 BBInfo **Preds = nullptr; 65 66 // Marker for existing PHIs that match. 67 PhiT *PHITag = nullptr; 68 69 BBInfo(BlkT *ThisBB, ValT V) 70 : BB(ThisBB), AvailableVal(V), DefBB(V ? this : nullptr) {} 71 }; 72 73 using AvailableValsTy = DenseMap<BlkT *, ValT>; 74 75 AvailableValsTy *AvailableVals; 76 77 SmallVectorImpl<PhiT *> *InsertedPHIs; 78 79 using BlockListTy = SmallVectorImpl<BBInfo *>; 80 using BBMapTy = DenseMap<BlkT *, BBInfo *>; 81 82 BBMapTy BBMap; 83 BumpPtrAllocator Allocator; 84 85 public: 86 explicit SSAUpdaterImpl(UpdaterT *U, AvailableValsTy *A, 87 SmallVectorImpl<PhiT *> *Ins) : 88 Updater(U), AvailableVals(A), InsertedPHIs(Ins) {} 89 90 /// GetValue - Check to see if AvailableVals has an entry for the specified 91 /// BB and if so, return it. If not, construct SSA form by first 92 /// calculating the required placement of PHIs and then inserting new PHIs 93 /// where needed. 94 ValT GetValue(BlkT *BB) { 95 SmallVector<BBInfo *, 100> BlockList; 96 BBInfo *PseudoEntry = BuildBlockList(BB, &BlockList); 97 98 // Special case: bail out if BB is unreachable. 99 if (BlockList.size() == 0) { 100 ValT V = Traits::GetUndefVal(BB, Updater); 101 (*AvailableVals)[BB] = V; 102 return V; 103 } 104 105 FindDominators(&BlockList, PseudoEntry); 106 FindPHIPlacement(&BlockList); 107 FindAvailableVals(&BlockList); 108 109 return BBMap[BB]->DefBB->AvailableVal; 110 } 111 112 /// BuildBlockList - Starting from the specified basic block, traverse back 113 /// through its predecessors until reaching blocks with known values. 114 /// Create BBInfo structures for the blocks and append them to the block 115 /// list. 116 BBInfo *BuildBlockList(BlkT *BB, BlockListTy *BlockList) { 117 SmallVector<BBInfo *, 10> RootList; 118 SmallVector<BBInfo *, 64> WorkList; 119 120 BBInfo *Info = new (Allocator) BBInfo(BB, 0); 121 BBMap[BB] = Info; 122 WorkList.push_back(Info); 123 124 // Search backward from BB, creating BBInfos along the way and stopping 125 // when reaching blocks that define the value. Record those defining 126 // blocks on the RootList. 127 SmallVector<BlkT *, 10> Preds; 128 while (!WorkList.empty()) { 129 Info = WorkList.pop_back_val(); 130 Preds.clear(); 131 Traits::FindPredecessorBlocks(Info->BB, &Preds); 132 Info->NumPreds = Preds.size(); 133 if (Info->NumPreds == 0) 134 Info->Preds = nullptr; 135 else 136 Info->Preds = static_cast<BBInfo **>(Allocator.Allocate( 137 Info->NumPreds * sizeof(BBInfo *), alignof(BBInfo *))); 138 139 for (unsigned p = 0; p != Info->NumPreds; ++p) { 140 BlkT *Pred = Preds[p]; 141 // Check if BBMap already has a BBInfo for the predecessor block. 142 typename BBMapTy::value_type &BBMapBucket = 143 BBMap.FindAndConstruct(Pred); 144 if (BBMapBucket.second) { 145 Info->Preds[p] = BBMapBucket.second; 146 continue; 147 } 148 149 // Create a new BBInfo for the predecessor. 150 ValT PredVal = AvailableVals->lookup(Pred); 151 BBInfo *PredInfo = new (Allocator) BBInfo(Pred, PredVal); 152 BBMapBucket.second = PredInfo; 153 Info->Preds[p] = PredInfo; 154 155 if (PredInfo->AvailableVal) { 156 RootList.push_back(PredInfo); 157 continue; 158 } 159 WorkList.push_back(PredInfo); 160 } 161 } 162 163 // Now that we know what blocks are backwards-reachable from the starting 164 // block, do a forward depth-first traversal to assign postorder numbers 165 // to those blocks. 166 BBInfo *PseudoEntry = new (Allocator) BBInfo(nullptr, 0); 167 unsigned BlkNum = 1; 168 169 // Initialize the worklist with the roots from the backward traversal. 170 while (!RootList.empty()) { 171 Info = RootList.pop_back_val(); 172 Info->IDom = PseudoEntry; 173 Info->BlkNum = -1; 174 WorkList.push_back(Info); 175 } 176 177 while (!WorkList.empty()) { 178 Info = WorkList.back(); 179 180 if (Info->BlkNum == -2) { 181 // All the successors have been handled; assign the postorder number. 182 Info->BlkNum = BlkNum++; 183 // If not a root, put it on the BlockList. 184 if (!Info->AvailableVal) 185 BlockList->push_back(Info); 186 WorkList.pop_back(); 187 continue; 188 } 189 190 // Leave this entry on the worklist, but set its BlkNum to mark that its 191 // successors have been put on the worklist. When it returns to the top 192 // the list, after handling its successors, it will be assigned a 193 // number. 194 Info->BlkNum = -2; 195 196 // Add unvisited successors to the work list. 197 for (typename Traits::BlkSucc_iterator SI = 198 Traits::BlkSucc_begin(Info->BB), 199 E = Traits::BlkSucc_end(Info->BB); SI != E; ++SI) { 200 BBInfo *SuccInfo = BBMap[*SI]; 201 if (!SuccInfo || SuccInfo->BlkNum) 202 continue; 203 SuccInfo->BlkNum = -1; 204 WorkList.push_back(SuccInfo); 205 } 206 } 207 PseudoEntry->BlkNum = BlkNum; 208 return PseudoEntry; 209 } 210 211 /// IntersectDominators - This is the dataflow lattice "meet" operation for 212 /// finding dominators. Given two basic blocks, it walks up the dominator 213 /// tree until it finds a common dominator of both. It uses the postorder 214 /// number of the blocks to determine how to do that. 215 BBInfo *IntersectDominators(BBInfo *Blk1, BBInfo *Blk2) { 216 while (Blk1 != Blk2) { 217 while (Blk1->BlkNum < Blk2->BlkNum) { 218 Blk1 = Blk1->IDom; 219 if (!Blk1) 220 return Blk2; 221 } 222 while (Blk2->BlkNum < Blk1->BlkNum) { 223 Blk2 = Blk2->IDom; 224 if (!Blk2) 225 return Blk1; 226 } 227 } 228 return Blk1; 229 } 230 231 /// FindDominators - Calculate the dominator tree for the subset of the CFG 232 /// corresponding to the basic blocks on the BlockList. This uses the 233 /// algorithm from: "A Simple, Fast Dominance Algorithm" by Cooper, Harvey 234 /// and Kennedy, published in Software--Practice and Experience, 2001, 235 /// 4:1-10. Because the CFG subset does not include any edges leading into 236 /// blocks that define the value, the results are not the usual dominator 237 /// tree. The CFG subset has a single pseudo-entry node with edges to a set 238 /// of root nodes for blocks that define the value. The dominators for this 239 /// subset CFG are not the standard dominators but they are adequate for 240 /// placing PHIs within the subset CFG. 241 void FindDominators(BlockListTy *BlockList, BBInfo *PseudoEntry) { 242 bool Changed; 243 do { 244 Changed = false; 245 // Iterate over the list in reverse order, i.e., forward on CFG edges. 246 for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(), 247 E = BlockList->rend(); I != E; ++I) { 248 BBInfo *Info = *I; 249 BBInfo *NewIDom = nullptr; 250 251 // Iterate through the block's predecessors. 252 for (unsigned p = 0; p != Info->NumPreds; ++p) { 253 BBInfo *Pred = Info->Preds[p]; 254 255 // Treat an unreachable predecessor as a definition with 'undef'. 256 if (Pred->BlkNum == 0) { 257 Pred->AvailableVal = Traits::GetUndefVal(Pred->BB, Updater); 258 (*AvailableVals)[Pred->BB] = Pred->AvailableVal; 259 Pred->DefBB = Pred; 260 Pred->BlkNum = PseudoEntry->BlkNum; 261 PseudoEntry->BlkNum++; 262 } 263 264 if (!NewIDom) 265 NewIDom = Pred; 266 else 267 NewIDom = IntersectDominators(NewIDom, Pred); 268 } 269 270 // Check if the IDom value has changed. 271 if (NewIDom && NewIDom != Info->IDom) { 272 Info->IDom = NewIDom; 273 Changed = true; 274 } 275 } 276 } while (Changed); 277 } 278 279 /// IsDefInDomFrontier - Search up the dominator tree from Pred to IDom for 280 /// any blocks containing definitions of the value. If one is found, then 281 /// the successor of Pred is in the dominance frontier for the definition, 282 /// and this function returns true. 283 bool IsDefInDomFrontier(const BBInfo *Pred, const BBInfo *IDom) { 284 for (; Pred != IDom; Pred = Pred->IDom) { 285 if (Pred->DefBB == Pred) 286 return true; 287 } 288 return false; 289 } 290 291 /// FindPHIPlacement - PHIs are needed in the iterated dominance frontiers 292 /// of the known definitions. Iteratively add PHIs in the dom frontiers 293 /// until nothing changes. Along the way, keep track of the nearest 294 /// dominating definitions for non-PHI blocks. 295 void FindPHIPlacement(BlockListTy *BlockList) { 296 bool Changed; 297 do { 298 Changed = false; 299 // Iterate over the list in reverse order, i.e., forward on CFG edges. 300 for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(), 301 E = BlockList->rend(); I != E; ++I) { 302 BBInfo *Info = *I; 303 304 // If this block already needs a PHI, there is nothing to do here. 305 if (Info->DefBB == Info) 306 continue; 307 308 // Default to use the same def as the immediate dominator. 309 BBInfo *NewDefBB = Info->IDom->DefBB; 310 for (unsigned p = 0; p != Info->NumPreds; ++p) { 311 if (IsDefInDomFrontier(Info->Preds[p], Info->IDom)) { 312 // Need a PHI here. 313 NewDefBB = Info; 314 break; 315 } 316 } 317 318 // Check if anything changed. 319 if (NewDefBB != Info->DefBB) { 320 Info->DefBB = NewDefBB; 321 Changed = true; 322 } 323 } 324 } while (Changed); 325 } 326 327 /// FindAvailableVal - If this block requires a PHI, first check if an 328 /// existing PHI matches the PHI placement and reaching definitions computed 329 /// earlier, and if not, create a new PHI. Visit all the block's 330 /// predecessors to calculate the available value for each one and fill in 331 /// the incoming values for a new PHI. 332 void FindAvailableVals(BlockListTy *BlockList) { 333 // Go through the worklist in forward order (i.e., backward through the CFG) 334 // and check if existing PHIs can be used. If not, create empty PHIs where 335 // they are needed. 336 for (typename BlockListTy::iterator I = BlockList->begin(), 337 E = BlockList->end(); I != E; ++I) { 338 BBInfo *Info = *I; 339 // Check if there needs to be a PHI in BB. 340 if (Info->DefBB != Info) 341 continue; 342 343 // Look for an existing PHI. 344 FindExistingPHI(Info->BB, BlockList); 345 if (Info->AvailableVal) 346 continue; 347 348 ValT PHI = Traits::CreateEmptyPHI(Info->BB, Info->NumPreds, Updater); 349 Info->AvailableVal = PHI; 350 (*AvailableVals)[Info->BB] = PHI; 351 } 352 353 // Now go back through the worklist in reverse order to fill in the 354 // arguments for any new PHIs added in the forward traversal. 355 for (typename BlockListTy::reverse_iterator I = BlockList->rbegin(), 356 E = BlockList->rend(); I != E; ++I) { 357 BBInfo *Info = *I; 358 359 if (Info->DefBB != Info) { 360 // Record the available value at join nodes to speed up subsequent 361 // uses of this SSAUpdater for the same value. 362 if (Info->NumPreds > 1) 363 (*AvailableVals)[Info->BB] = Info->DefBB->AvailableVal; 364 continue; 365 } 366 367 // Check if this block contains a newly added PHI. 368 PhiT *PHI = Traits::ValueIsNewPHI(Info->AvailableVal, Updater); 369 if (!PHI) 370 continue; 371 372 // Iterate through the block's predecessors. 373 for (unsigned p = 0; p != Info->NumPreds; ++p) { 374 BBInfo *PredInfo = Info->Preds[p]; 375 BlkT *Pred = PredInfo->BB; 376 // Skip to the nearest preceding definition. 377 if (PredInfo->DefBB != PredInfo) 378 PredInfo = PredInfo->DefBB; 379 Traits::AddPHIOperand(PHI, PredInfo->AvailableVal, Pred); 380 } 381 382 DEBUG(dbgs() << " Inserted PHI: " << *PHI << "\n"); 383 384 // If the client wants to know about all new instructions, tell it. 385 if (InsertedPHIs) InsertedPHIs->push_back(PHI); 386 } 387 } 388 389 /// FindExistingPHI - Look through the PHI nodes in a block to see if any of 390 /// them match what is needed. 391 void FindExistingPHI(BlkT *BB, BlockListTy *BlockList) { 392 for (typename BlkT::iterator BBI = BB->begin(), BBE = BB->end(); 393 BBI != BBE; ++BBI) { 394 PhiT *SomePHI = Traits::InstrIsPHI(&*BBI); 395 if (!SomePHI) 396 break; 397 if (CheckIfPHIMatches(SomePHI)) { 398 RecordMatchingPHIs(BlockList); 399 break; 400 } 401 // Match failed: clear all the PHITag values. 402 for (typename BlockListTy::iterator I = BlockList->begin(), 403 E = BlockList->end(); I != E; ++I) 404 (*I)->PHITag = nullptr; 405 } 406 } 407 408 /// CheckIfPHIMatches - Check if a PHI node matches the placement and values 409 /// in the BBMap. 410 bool CheckIfPHIMatches(PhiT *PHI) { 411 SmallVector<PhiT *, 20> WorkList; 412 WorkList.push_back(PHI); 413 414 // Mark that the block containing this PHI has been visited. 415 BBMap[PHI->getParent()]->PHITag = PHI; 416 417 while (!WorkList.empty()) { 418 PHI = WorkList.pop_back_val(); 419 420 // Iterate through the PHI's incoming values. 421 for (typename Traits::PHI_iterator I = Traits::PHI_begin(PHI), 422 E = Traits::PHI_end(PHI); I != E; ++I) { 423 ValT IncomingVal = I.getIncomingValue(); 424 BBInfo *PredInfo = BBMap[I.getIncomingBlock()]; 425 // Skip to the nearest preceding definition. 426 if (PredInfo->DefBB != PredInfo) 427 PredInfo = PredInfo->DefBB; 428 429 // Check if it matches the expected value. 430 if (PredInfo->AvailableVal) { 431 if (IncomingVal == PredInfo->AvailableVal) 432 continue; 433 return false; 434 } 435 436 // Check if the value is a PHI in the correct block. 437 PhiT *IncomingPHIVal = Traits::ValueIsPHI(IncomingVal, Updater); 438 if (!IncomingPHIVal || IncomingPHIVal->getParent() != PredInfo->BB) 439 return false; 440 441 // If this block has already been visited, check if this PHI matches. 442 if (PredInfo->PHITag) { 443 if (IncomingPHIVal == PredInfo->PHITag) 444 continue; 445 return false; 446 } 447 PredInfo->PHITag = IncomingPHIVal; 448 449 WorkList.push_back(IncomingPHIVal); 450 } 451 } 452 return true; 453 } 454 455 /// RecordMatchingPHIs - For each PHI node that matches, record it in both 456 /// the BBMap and the AvailableVals mapping. 457 void RecordMatchingPHIs(BlockListTy *BlockList) { 458 for (typename BlockListTy::iterator I = BlockList->begin(), 459 E = BlockList->end(); I != E; ++I) 460 if (PhiT *PHI = (*I)->PHITag) { 461 BlkT *BB = PHI->getParent(); 462 ValT PHIVal = Traits::GetPHIValue(PHI); 463 (*AvailableVals)[BB] = PHIVal; 464 BBMap[BB]->AvailableVal = PHIVal; 465 } 466 } 467 }; 468 469 } // end namespace llvm 470 471 #undef DEBUG_TYPE // "ssaupdater" 472 473 #endif // LLVM_TRANSFORMS_UTILS_SSAUPDATERIMPL_H 474