BasicBlockUtils.cpp revision afc36a9520971832dfbebc0333593bf5d3098296
1//===-- BasicBlockUtils.cpp - BasicBlock Utilities -------------------------==// 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 family of functions perform manipulations on basic blocks, and 11// instructions contained within basic blocks. 12// 13//===----------------------------------------------------------------------===// 14 15#include "llvm/Transforms/Utils/BasicBlockUtils.h" 16#include "llvm/Function.h" 17#include "llvm/Instructions.h" 18#include "llvm/IntrinsicInst.h" 19#include "llvm/Constant.h" 20#include "llvm/Type.h" 21#include "llvm/Analysis/AliasAnalysis.h" 22#include "llvm/Analysis/LoopInfo.h" 23#include "llvm/Analysis/Dominators.h" 24#include "llvm/Target/TargetData.h" 25#include "llvm/Transforms/Utils/Local.h" 26#include "llvm/Support/ValueHandle.h" 27#include <algorithm> 28using namespace llvm; 29 30/// DeleteDeadBlock - Delete the specified block, which must have no 31/// predecessors. 32void llvm::DeleteDeadBlock(BasicBlock *BB) { 33 assert((pred_begin(BB) == pred_end(BB) || 34 // Can delete self loop. 35 BB->getSinglePredecessor() == BB) && "Block is not dead!"); 36 TerminatorInst *BBTerm = BB->getTerminator(); 37 38 // Loop through all of our successors and make sure they know that one 39 // of their predecessors is going away. 40 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) 41 BBTerm->getSuccessor(i)->removePredecessor(BB); 42 43 // Zap all the instructions in the block. 44 while (!BB->empty()) { 45 Instruction &I = BB->back(); 46 // If this instruction is used, replace uses with an arbitrary value. 47 // Because control flow can't get here, we don't care what we replace the 48 // value with. Note that since this block is unreachable, and all values 49 // contained within it must dominate their uses, that all uses will 50 // eventually be removed (they are themselves dead). 51 if (!I.use_empty()) 52 I.replaceAllUsesWith(UndefValue::get(I.getType())); 53 BB->getInstList().pop_back(); 54 } 55 56 // Zap the block! 57 BB->eraseFromParent(); 58} 59 60/// FoldSingleEntryPHINodes - We know that BB has one predecessor. If there are 61/// any single-entry PHI nodes in it, fold them away. This handles the case 62/// when all entries to the PHI nodes in a block are guaranteed equal, such as 63/// when the block has exactly one predecessor. 64void llvm::FoldSingleEntryPHINodes(BasicBlock *BB) { 65 if (!isa<PHINode>(BB->begin())) 66 return; 67 68 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 69 if (PN->getIncomingValue(0) != PN) 70 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 71 else 72 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 73 PN->eraseFromParent(); 74 } 75} 76 77 78/// DeleteDeadPHIs - Examine each PHI in the given block and delete it if it 79/// is dead. Also recursively delete any operands that become dead as 80/// a result. This includes tracing the def-use list from the PHI to see if 81/// it is ultimately unused or if it reaches an unused cycle. If a 82/// ValueDeletionListener is specified, it is notified of the deletions. 83void llvm::DeleteDeadPHIs(BasicBlock *BB, ValueDeletionListener *VDL) { 84 // Recursively deleting a PHI may cause multiple PHIs to be deleted 85 // or RAUW'd undef, so use an array of WeakVH for the PHIs to delete. 86 SmallVector<WeakVH, 8> PHIs; 87 for (BasicBlock::iterator I = BB->begin(); 88 PHINode *PN = dyn_cast<PHINode>(I); ++I) 89 PHIs.push_back(PN); 90 91 for (unsigned i = 0, e = PHIs.size(); i != e; ++i) 92 if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*())) 93 RecursivelyDeleteDeadPHINode(PN, VDL); 94} 95 96/// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor, 97/// if possible. The return value indicates success or failure. 98bool llvm::MergeBlockIntoPredecessor(BasicBlock* BB, Pass* P) { 99 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 100 // Can't merge the entry block. 101 if (pred_begin(BB) == pred_end(BB)) return false; 102 103 BasicBlock *PredBB = *PI++; 104 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 105 if (*PI != PredBB) { 106 PredBB = 0; // There are multiple different predecessors... 107 break; 108 } 109 110 // Can't merge if there are multiple predecessors. 111 if (!PredBB) return false; 112 // Don't break self-loops. 113 if (PredBB == BB) return false; 114 // Don't break invokes. 115 if (isa<InvokeInst>(PredBB->getTerminator())) return false; 116 117 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB)); 118 BasicBlock* OnlySucc = BB; 119 for (; SI != SE; ++SI) 120 if (*SI != OnlySucc) { 121 OnlySucc = 0; // There are multiple distinct successors! 122 break; 123 } 124 125 // Can't merge if there are multiple successors. 126 if (!OnlySucc) return false; 127 128 // Can't merge if there is PHI loop. 129 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) { 130 if (PHINode *PN = dyn_cast<PHINode>(BI)) { 131 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 132 if (PN->getIncomingValue(i) == PN) 133 return false; 134 } else 135 break; 136 } 137 138 // Begin by getting rid of unneeded PHIs. 139 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 140 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 141 BB->getInstList().pop_front(); // Delete the phi node... 142 } 143 144 // Delete the unconditional branch from the predecessor... 145 PredBB->getInstList().pop_back(); 146 147 // Move all definitions in the successor to the predecessor... 148 PredBB->getInstList().splice(PredBB->end(), BB->getInstList()); 149 150 // Make all PHI nodes that referred to BB now refer to Pred as their 151 // source... 152 BB->replaceAllUsesWith(PredBB); 153 154 // Inherit predecessors name if it exists. 155 if (!PredBB->hasName()) 156 PredBB->takeName(BB); 157 158 // Finally, erase the old block and update dominator info. 159 if (P) { 160 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) { 161 DomTreeNode* DTN = DT->getNode(BB); 162 DomTreeNode* PredDTN = DT->getNode(PredBB); 163 164 if (DTN) { 165 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end()); 166 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(), 167 DE = Children.end(); DI != DE; ++DI) 168 DT->changeImmediateDominator(*DI, PredDTN); 169 170 DT->eraseNode(BB); 171 } 172 } 173 } 174 175 BB->eraseFromParent(); 176 177 178 return true; 179} 180 181/// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI) 182/// with a value, then remove and delete the original instruction. 183/// 184void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL, 185 BasicBlock::iterator &BI, Value *V) { 186 Instruction &I = *BI; 187 // Replaces all of the uses of the instruction with uses of the value 188 I.replaceAllUsesWith(V); 189 190 // Make sure to propagate a name if there is one already. 191 if (I.hasName() && !V->hasName()) 192 V->takeName(&I); 193 194 // Delete the unnecessary instruction now... 195 BI = BIL.erase(BI); 196} 197 198 199/// ReplaceInstWithInst - Replace the instruction specified by BI with the 200/// instruction specified by I. The original instruction is deleted and BI is 201/// updated to point to the new instruction. 202/// 203void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL, 204 BasicBlock::iterator &BI, Instruction *I) { 205 assert(I->getParent() == 0 && 206 "ReplaceInstWithInst: Instruction already inserted into basic block!"); 207 208 // Insert the new instruction into the basic block... 209 BasicBlock::iterator New = BIL.insert(BI, I); 210 211 // Replace all uses of the old instruction, and delete it. 212 ReplaceInstWithValue(BIL, BI, I); 213 214 // Move BI back to point to the newly inserted instruction 215 BI = New; 216} 217 218/// ReplaceInstWithInst - Replace the instruction specified by From with the 219/// instruction specified by To. 220/// 221void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) { 222 BasicBlock::iterator BI(From); 223 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To); 224} 225 226/// RemoveSuccessor - Change the specified terminator instruction such that its 227/// successor SuccNum no longer exists. Because this reduces the outgoing 228/// degree of the current basic block, the actual terminator instruction itself 229/// may have to be changed. In the case where the last successor of the block 230/// is deleted, a return instruction is inserted in its place which can cause a 231/// surprising change in program behavior if it is not expected. 232/// 233void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) { 234 assert(SuccNum < TI->getNumSuccessors() && 235 "Trying to remove a nonexistant successor!"); 236 237 // If our old successor block contains any PHI nodes, remove the entry in the 238 // PHI nodes that comes from this branch... 239 // 240 BasicBlock *BB = TI->getParent(); 241 TI->getSuccessor(SuccNum)->removePredecessor(BB); 242 243 TerminatorInst *NewTI = 0; 244 switch (TI->getOpcode()) { 245 case Instruction::Br: 246 // If this is a conditional branch... convert to unconditional branch. 247 if (TI->getNumSuccessors() == 2) { 248 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum)); 249 } else { // Otherwise convert to a return instruction... 250 Value *RetVal = 0; 251 252 // Create a value to return... if the function doesn't return null... 253 if (BB->getParent()->getReturnType() != Type::VoidTy) 254 RetVal = Constant::getNullValue(BB->getParent()->getReturnType()); 255 256 // Create the return... 257 NewTI = ReturnInst::Create(RetVal); 258 } 259 break; 260 261 case Instruction::Invoke: // Should convert to call 262 case Instruction::Switch: // Should remove entry 263 default: 264 case Instruction::Ret: // Cannot happen, has no successors! 265 assert(0 && "Unhandled terminator instruction type in RemoveSuccessor!"); 266 abort(); 267 } 268 269 if (NewTI) // If it's a different instruction, replace. 270 ReplaceInstWithInst(TI, NewTI); 271} 272 273/// SplitEdge - Split the edge connecting specified block. Pass P must 274/// not be NULL. 275BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) { 276 TerminatorInst *LatchTerm = BB->getTerminator(); 277 unsigned SuccNum = 0; 278#ifndef NDEBUG 279 unsigned e = LatchTerm->getNumSuccessors(); 280#endif 281 for (unsigned i = 0; ; ++i) { 282 assert(i != e && "Didn't find edge?"); 283 if (LatchTerm->getSuccessor(i) == Succ) { 284 SuccNum = i; 285 break; 286 } 287 } 288 289 // If this is a critical edge, let SplitCriticalEdge do it. 290 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P)) 291 return LatchTerm->getSuccessor(SuccNum); 292 293 // If the edge isn't critical, then BB has a single successor or Succ has a 294 // single pred. Split the block. 295 BasicBlock::iterator SplitPoint; 296 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 297 // If the successor only has a single pred, split the top of the successor 298 // block. 299 assert(SP == BB && "CFG broken"); 300 SP = NULL; 301 return SplitBlock(Succ, Succ->begin(), P); 302 } else { 303 // Otherwise, if BB has a single successor, split it at the bottom of the 304 // block. 305 assert(BB->getTerminator()->getNumSuccessors() == 1 && 306 "Should have a single succ!"); 307 return SplitBlock(BB, BB->getTerminator(), P); 308 } 309} 310 311/// SplitBlock - Split the specified block at the specified instruction - every 312/// thing before SplitPt stays in Old and everything starting with SplitPt moves 313/// to a new block. The two blocks are joined by an unconditional branch and 314/// the loop info is updated. 315/// 316BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) { 317 BasicBlock::iterator SplitIt = SplitPt; 318 while (isa<PHINode>(SplitIt)) 319 ++SplitIt; 320 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 321 322 // The new block lives in whichever loop the old one did. 323 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>()) 324 if (Loop *L = LI->getLoopFor(Old)) 325 L->addBasicBlockToLoop(New, LI->getBase()); 326 327 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) 328 { 329 // Old dominates New. New node domiantes all other nodes dominated by Old. 330 DomTreeNode *OldNode = DT->getNode(Old); 331 std::vector<DomTreeNode *> Children; 332 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end(); 333 I != E; ++I) 334 Children.push_back(*I); 335 336 DomTreeNode *NewNode = DT->addNewBlock(New,Old); 337 338 for (std::vector<DomTreeNode *>::iterator I = Children.begin(), 339 E = Children.end(); I != E; ++I) 340 DT->changeImmediateDominator(*I, NewNode); 341 } 342 343 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) 344 DF->splitBlock(Old); 345 346 return New; 347} 348 349 350/// SplitBlockPredecessors - This method transforms BB by introducing a new 351/// basic block into the function, and moving some of the predecessors of BB to 352/// be predecessors of the new block. The new predecessors are indicated by the 353/// Preds array, which has NumPreds elements in it. The new block is given a 354/// suffix of 'Suffix'. 355/// 356/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree and 357/// DominanceFrontier, but no other analyses. 358BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB, 359 BasicBlock *const *Preds, 360 unsigned NumPreds, const char *Suffix, 361 Pass *P) { 362 // Create new basic block, insert right before the original block. 363 BasicBlock *NewBB = 364 BasicBlock::Create(BB->getName()+Suffix, BB->getParent(), BB); 365 366 // The new block unconditionally branches to the old block. 367 BranchInst *BI = BranchInst::Create(BB, NewBB); 368 369 // Move the edges from Preds to point to NewBB instead of BB. 370 for (unsigned i = 0; i != NumPreds; ++i) 371 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB); 372 373 // Update dominator tree and dominator frontier if available. 374 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0; 375 if (DT) 376 DT->splitBlock(NewBB); 377 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0) 378 DF->splitBlock(NewBB); 379 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0; 380 381 382 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI 383 // node becomes an incoming value for BB's phi node. However, if the Preds 384 // list is empty, we need to insert dummy entries into the PHI nodes in BB to 385 // account for the newly created predecessor. 386 if (NumPreds == 0) { 387 // Insert dummy values as the incoming value. 388 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) 389 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB); 390 return NewBB; 391 } 392 393 // Otherwise, create a new PHI node in NewBB for each PHI node in BB. 394 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) { 395 PHINode *PN = cast<PHINode>(I++); 396 397 // Check to see if all of the values coming in are the same. If so, we 398 // don't need to create a new PHI node. 399 Value *InVal = PN->getIncomingValueForBlock(Preds[0]); 400 for (unsigned i = 1; i != NumPreds; ++i) 401 if (InVal != PN->getIncomingValueForBlock(Preds[i])) { 402 InVal = 0; 403 break; 404 } 405 406 if (InVal) { 407 // If all incoming values for the new PHI would be the same, just don't 408 // make a new PHI. Instead, just remove the incoming values from the old 409 // PHI. 410 for (unsigned i = 0; i != NumPreds; ++i) 411 PN->removeIncomingValue(Preds[i], false); 412 } else { 413 // If the values coming into the block are not the same, we need a PHI. 414 // Create the new PHI node, insert it into NewBB at the end of the block 415 PHINode *NewPHI = 416 PHINode::Create(PN->getType(), PN->getName()+".ph", BI); 417 if (AA) AA->copyValue(PN, NewPHI); 418 419 // Move all of the PHI values for 'Preds' to the new PHI. 420 for (unsigned i = 0; i != NumPreds; ++i) { 421 Value *V = PN->removeIncomingValue(Preds[i], false); 422 NewPHI->addIncoming(V, Preds[i]); 423 } 424 InVal = NewPHI; 425 } 426 427 // Add an incoming value to the PHI node in the loop for the preheader 428 // edge. 429 PN->addIncoming(InVal, NewBB); 430 431 // Check to see if we can eliminate this phi node. 432 if (Value *V = PN->hasConstantValue(DT != 0)) { 433 Instruction *I = dyn_cast<Instruction>(V); 434 if (!I || DT == 0 || DT->dominates(I, PN)) { 435 PN->replaceAllUsesWith(V); 436 if (AA) AA->deleteValue(PN); 437 PN->eraseFromParent(); 438 } 439 } 440 } 441 442 return NewBB; 443} 444 445/// AreEquivalentAddressValues - Test if A and B will obviously have the same 446/// value. This includes recognizing that %t0 and %t1 will have the same 447/// value in code like this: 448/// %t0 = getelementptr \@a, 0, 3 449/// store i32 0, i32* %t0 450/// %t1 = getelementptr \@a, 0, 3 451/// %t2 = load i32* %t1 452/// 453static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 454 // Test if the values are trivially equivalent. 455 if (A == B) return true; 456 457 // Test if the values come form identical arithmetic instructions. 458 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 459 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 460 if (const Instruction *BI = dyn_cast<Instruction>(B)) 461 if (cast<Instruction>(A)->isIdenticalTo(BI)) 462 return true; 463 464 // Otherwise they may not be equivalent. 465 return false; 466} 467 468/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 469/// instruction before ScanFrom) checking to see if we have the value at the 470/// memory address *Ptr locally available within a small number of instructions. 471/// If the value is available, return it. 472/// 473/// If not, return the iterator for the last validated instruction that the 474/// value would be live through. If we scanned the entire block and didn't find 475/// something that invalidates *Ptr or provides it, ScanFrom would be left at 476/// begin() and this returns null. ScanFrom could also be left 477/// 478/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 479/// it is set to 0, it will scan the whole block. You can also optionally 480/// specify an alias analysis implementation, which makes this more precise. 481Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 482 BasicBlock::iterator &ScanFrom, 483 unsigned MaxInstsToScan, 484 AliasAnalysis *AA) { 485 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 486 487 // If we're using alias analysis to disambiguate get the size of *Ptr. 488 unsigned AccessSize = 0; 489 if (AA) { 490 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 491 AccessSize = AA->getTargetData().getTypeStoreSizeInBits(AccessTy); 492 } 493 494 while (ScanFrom != ScanBB->begin()) { 495 // We must ignore debug info directives when counting (otherwise they 496 // would affect codegen). 497 Instruction *Inst = --ScanFrom; 498 if (isa<DbgInfoIntrinsic>(Inst)) 499 continue; 500 // We skip pointer-to-pointer bitcasts, which are NOPs. 501 // It is necessary for correctness to skip those that feed into a 502 // llvm.dbg.declare, as these are not present when debugging is off. 503 if (isa<BitCastInst>(Inst) && isa<PointerType>(Inst->getType())) 504 continue; 505 506 // Restore ScanFrom to expected value in case next test succeeds 507 ScanFrom++; 508 509 // Don't scan huge blocks. 510 if (MaxInstsToScan-- == 0) return 0; 511 512 --ScanFrom; 513 // If this is a load of Ptr, the loaded value is available. 514 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 515 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 516 return LI; 517 518 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 519 // If this is a store through Ptr, the value is available! 520 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 521 return SI->getOperand(0); 522 523 // If Ptr is an alloca and this is a store to a different alloca, ignore 524 // the store. This is a trivial form of alias analysis that is important 525 // for reg2mem'd code. 526 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 527 (isa<AllocaInst>(SI->getOperand(1)) || 528 isa<GlobalVariable>(SI->getOperand(1)))) 529 continue; 530 531 // If we have alias analysis and it says the store won't modify the loaded 532 // value, ignore the store. 533 if (AA && 534 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 535 continue; 536 537 // Otherwise the store that may or may not alias the pointer, bail out. 538 ++ScanFrom; 539 return 0; 540 } 541 542 // If this is some other instruction that may clobber Ptr, bail out. 543 if (Inst->mayWriteToMemory()) { 544 // If alias analysis claims that it really won't modify the load, 545 // ignore it. 546 if (AA && 547 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 548 continue; 549 550 // May modify the pointer, bail out. 551 ++ScanFrom; 552 return 0; 553 } 554 } 555 556 // Got to the start of the block, we didn't find it, but are done for this 557 // block. 558 return 0; 559} 560 561/// CopyPrecedingStopPoint - If I is immediately preceded by a StopPoint, 562/// make a copy of the stoppoint before InsertPos (presumably before copying 563/// or moving I). 564void llvm::CopyPrecedingStopPoint(Instruction *I, 565 BasicBlock::iterator InsertPos) { 566 if (I != I->getParent()->begin()) { 567 BasicBlock::iterator BBI = I; --BBI; 568 if (DbgStopPointInst *DSPI = dyn_cast<DbgStopPointInst>(BBI)) { 569 CallInst *newDSPI = DSPI->clone(); 570 newDSPI->insertBefore(InsertPos); 571 } 572 } 573} 574