BasicBlockUtils.cpp revision db6fa2964176c34f0e878e101427c28782c93419
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. 82void llvm::DeleteDeadPHIs(BasicBlock *BB) { 83 // Recursively deleting a PHI may cause multiple PHIs to be deleted 84 // or RAUW'd undef, so use an array of WeakVH for the PHIs to delete. 85 SmallVector<WeakVH, 8> PHIs; 86 for (BasicBlock::iterator I = BB->begin(); 87 PHINode *PN = dyn_cast<PHINode>(I); ++I) 88 PHIs.push_back(PN); 89 90 for (unsigned i = 0, e = PHIs.size(); i != e; ++i) 91 if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*())) 92 RecursivelyDeleteDeadPHINode(PN); 93} 94 95/// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor, 96/// if possible. The return value indicates success or failure. 97bool llvm::MergeBlockIntoPredecessor(BasicBlock* BB, Pass* P) { 98 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 99 // Can't merge the entry block. 100 if (pred_begin(BB) == pred_end(BB)) return false; 101 102 BasicBlock *PredBB = *PI++; 103 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 104 if (*PI != PredBB) { 105 PredBB = 0; // There are multiple different predecessors... 106 break; 107 } 108 109 // Can't merge if there are multiple predecessors. 110 if (!PredBB) return false; 111 // Don't break self-loops. 112 if (PredBB == BB) return false; 113 // Don't break invokes. 114 if (isa<InvokeInst>(PredBB->getTerminator())) return false; 115 116 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB)); 117 BasicBlock* OnlySucc = BB; 118 for (; SI != SE; ++SI) 119 if (*SI != OnlySucc) { 120 OnlySucc = 0; // There are multiple distinct successors! 121 break; 122 } 123 124 // Can't merge if there are multiple successors. 125 if (!OnlySucc) return false; 126 127 // Can't merge if there is PHI loop. 128 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) { 129 if (PHINode *PN = dyn_cast<PHINode>(BI)) { 130 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 131 if (PN->getIncomingValue(i) == PN) 132 return false; 133 } else 134 break; 135 } 136 137 // Begin by getting rid of unneeded PHIs. 138 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 139 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 140 BB->getInstList().pop_front(); // Delete the phi node... 141 } 142 143 // Delete the unconditional branch from the predecessor... 144 PredBB->getInstList().pop_back(); 145 146 // Move all definitions in the successor to the predecessor... 147 PredBB->getInstList().splice(PredBB->end(), BB->getInstList()); 148 149 // Make all PHI nodes that referred to BB now refer to Pred as their 150 // source... 151 BB->replaceAllUsesWith(PredBB); 152 153 // Inherit predecessors name if it exists. 154 if (!PredBB->hasName()) 155 PredBB->takeName(BB); 156 157 // Finally, erase the old block and update dominator info. 158 if (P) { 159 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) { 160 DomTreeNode* DTN = DT->getNode(BB); 161 DomTreeNode* PredDTN = DT->getNode(PredBB); 162 163 if (DTN) { 164 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end()); 165 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(), 166 DE = Children.end(); DI != DE; ++DI) 167 DT->changeImmediateDominator(*DI, PredDTN); 168 169 DT->eraseNode(BB); 170 } 171 } 172 } 173 174 BB->eraseFromParent(); 175 176 177 return true; 178} 179 180/// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI) 181/// with a value, then remove and delete the original instruction. 182/// 183void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL, 184 BasicBlock::iterator &BI, Value *V) { 185 Instruction &I = *BI; 186 // Replaces all of the uses of the instruction with uses of the value 187 I.replaceAllUsesWith(V); 188 189 // Make sure to propagate a name if there is one already. 190 if (I.hasName() && !V->hasName()) 191 V->takeName(&I); 192 193 // Delete the unnecessary instruction now... 194 BI = BIL.erase(BI); 195} 196 197 198/// ReplaceInstWithInst - Replace the instruction specified by BI with the 199/// instruction specified by I. The original instruction is deleted and BI is 200/// updated to point to the new instruction. 201/// 202void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL, 203 BasicBlock::iterator &BI, Instruction *I) { 204 assert(I->getParent() == 0 && 205 "ReplaceInstWithInst: Instruction already inserted into basic block!"); 206 207 // Insert the new instruction into the basic block... 208 BasicBlock::iterator New = BIL.insert(BI, I); 209 210 // Replace all uses of the old instruction, and delete it. 211 ReplaceInstWithValue(BIL, BI, I); 212 213 // Move BI back to point to the newly inserted instruction 214 BI = New; 215} 216 217/// ReplaceInstWithInst - Replace the instruction specified by From with the 218/// instruction specified by To. 219/// 220void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) { 221 BasicBlock::iterator BI(From); 222 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To); 223} 224 225/// RemoveSuccessor - Change the specified terminator instruction such that its 226/// successor SuccNum no longer exists. Because this reduces the outgoing 227/// degree of the current basic block, the actual terminator instruction itself 228/// may have to be changed. In the case where the last successor of the block 229/// is deleted, a return instruction is inserted in its place which can cause a 230/// surprising change in program behavior if it is not expected. 231/// 232void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) { 233 assert(SuccNum < TI->getNumSuccessors() && 234 "Trying to remove a nonexistant successor!"); 235 236 // If our old successor block contains any PHI nodes, remove the entry in the 237 // PHI nodes that comes from this branch... 238 // 239 BasicBlock *BB = TI->getParent(); 240 TI->getSuccessor(SuccNum)->removePredecessor(BB); 241 242 TerminatorInst *NewTI = 0; 243 switch (TI->getOpcode()) { 244 case Instruction::Br: 245 // If this is a conditional branch... convert to unconditional branch. 246 if (TI->getNumSuccessors() == 2) { 247 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum)); 248 } else { // Otherwise convert to a return instruction... 249 Value *RetVal = 0; 250 251 // Create a value to return... if the function doesn't return null... 252 if (BB->getParent()->getReturnType() != Type::VoidTy) 253 RetVal = Constant::getNullValue(BB->getParent()->getReturnType()); 254 255 // Create the return... 256 NewTI = ReturnInst::Create(RetVal); 257 } 258 break; 259 260 case Instruction::Invoke: // Should convert to call 261 case Instruction::Switch: // Should remove entry 262 default: 263 case Instruction::Ret: // Cannot happen, has no successors! 264 assert(0 && "Unhandled terminator instruction type in RemoveSuccessor!"); 265 abort(); 266 } 267 268 if (NewTI) // If it's a different instruction, replace. 269 ReplaceInstWithInst(TI, NewTI); 270} 271 272/// SplitEdge - Split the edge connecting specified block. Pass P must 273/// not be NULL. 274BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) { 275 TerminatorInst *LatchTerm = BB->getTerminator(); 276 unsigned SuccNum = 0; 277#ifndef NDEBUG 278 unsigned e = LatchTerm->getNumSuccessors(); 279#endif 280 for (unsigned i = 0; ; ++i) { 281 assert(i != e && "Didn't find edge?"); 282 if (LatchTerm->getSuccessor(i) == Succ) { 283 SuccNum = i; 284 break; 285 } 286 } 287 288 // If this is a critical edge, let SplitCriticalEdge do it. 289 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P)) 290 return LatchTerm->getSuccessor(SuccNum); 291 292 // If the edge isn't critical, then BB has a single successor or Succ has a 293 // single pred. Split the block. 294 BasicBlock::iterator SplitPoint; 295 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 296 // If the successor only has a single pred, split the top of the successor 297 // block. 298 assert(SP == BB && "CFG broken"); 299 SP = NULL; 300 return SplitBlock(Succ, Succ->begin(), P); 301 } else { 302 // Otherwise, if BB has a single successor, split it at the bottom of the 303 // block. 304 assert(BB->getTerminator()->getNumSuccessors() == 1 && 305 "Should have a single succ!"); 306 return SplitBlock(BB, BB->getTerminator(), P); 307 } 308} 309 310/// SplitBlock - Split the specified block at the specified instruction - every 311/// thing before SplitPt stays in Old and everything starting with SplitPt moves 312/// to a new block. The two blocks are joined by an unconditional branch and 313/// the loop info is updated. 314/// 315BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) { 316 BasicBlock::iterator SplitIt = SplitPt; 317 while (isa<PHINode>(SplitIt)) 318 ++SplitIt; 319 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 320 321 // The new block lives in whichever loop the old one did. 322 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>()) 323 if (Loop *L = LI->getLoopFor(Old)) 324 L->addBasicBlockToLoop(New, LI->getBase()); 325 326 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) 327 { 328 // Old dominates New. New node domiantes all other nodes dominated by Old. 329 DomTreeNode *OldNode = DT->getNode(Old); 330 std::vector<DomTreeNode *> Children; 331 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end(); 332 I != E; ++I) 333 Children.push_back(*I); 334 335 DomTreeNode *NewNode = DT->addNewBlock(New,Old); 336 337 for (std::vector<DomTreeNode *>::iterator I = Children.begin(), 338 E = Children.end(); I != E; ++I) 339 DT->changeImmediateDominator(*I, NewNode); 340 } 341 342 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) 343 DF->splitBlock(Old); 344 345 return New; 346} 347 348 349/// SplitBlockPredecessors - This method transforms BB by introducing a new 350/// basic block into the function, and moving some of the predecessors of BB to 351/// be predecessors of the new block. The new predecessors are indicated by the 352/// Preds array, which has NumPreds elements in it. The new block is given a 353/// suffix of 'Suffix'. 354/// 355/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree and 356/// DominanceFrontier, but no other analyses. 357BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB, 358 BasicBlock *const *Preds, 359 unsigned NumPreds, const char *Suffix, 360 Pass *P) { 361 // Create new basic block, insert right before the original block. 362 BasicBlock *NewBB = 363 BasicBlock::Create(BB->getName()+Suffix, BB->getParent(), BB); 364 365 // The new block unconditionally branches to the old block. 366 BranchInst *BI = BranchInst::Create(BB, NewBB); 367 368 // Move the edges from Preds to point to NewBB instead of BB. 369 for (unsigned i = 0; i != NumPreds; ++i) 370 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB); 371 372 // Update dominator tree and dominator frontier if available. 373 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0; 374 if (DT) 375 DT->splitBlock(NewBB); 376 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0) 377 DF->splitBlock(NewBB); 378 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0; 379 380 381 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI 382 // node becomes an incoming value for BB's phi node. However, if the Preds 383 // list is empty, we need to insert dummy entries into the PHI nodes in BB to 384 // account for the newly created predecessor. 385 if (NumPreds == 0) { 386 // Insert dummy values as the incoming value. 387 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) 388 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB); 389 return NewBB; 390 } 391 392 // Otherwise, create a new PHI node in NewBB for each PHI node in BB. 393 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) { 394 PHINode *PN = cast<PHINode>(I++); 395 396 // Check to see if all of the values coming in are the same. If so, we 397 // don't need to create a new PHI node. 398 Value *InVal = PN->getIncomingValueForBlock(Preds[0]); 399 for (unsigned i = 1; i != NumPreds; ++i) 400 if (InVal != PN->getIncomingValueForBlock(Preds[i])) { 401 InVal = 0; 402 break; 403 } 404 405 if (InVal) { 406 // If all incoming values for the new PHI would be the same, just don't 407 // make a new PHI. Instead, just remove the incoming values from the old 408 // PHI. 409 for (unsigned i = 0; i != NumPreds; ++i) 410 PN->removeIncomingValue(Preds[i], false); 411 } else { 412 // If the values coming into the block are not the same, we need a PHI. 413 // Create the new PHI node, insert it into NewBB at the end of the block 414 PHINode *NewPHI = 415 PHINode::Create(PN->getType(), PN->getName()+".ph", BI); 416 if (AA) AA->copyValue(PN, NewPHI); 417 418 // Move all of the PHI values for 'Preds' to the new PHI. 419 for (unsigned i = 0; i != NumPreds; ++i) { 420 Value *V = PN->removeIncomingValue(Preds[i], false); 421 NewPHI->addIncoming(V, Preds[i]); 422 } 423 InVal = NewPHI; 424 } 425 426 // Add an incoming value to the PHI node in the loop for the preheader 427 // edge. 428 PN->addIncoming(InVal, NewBB); 429 430 // Check to see if we can eliminate this phi node. 431 if (Value *V = PN->hasConstantValue(DT != 0)) { 432 Instruction *I = dyn_cast<Instruction>(V); 433 if (!I || DT == 0 || DT->dominates(I, PN)) { 434 PN->replaceAllUsesWith(V); 435 if (AA) AA->deleteValue(PN); 436 PN->eraseFromParent(); 437 } 438 } 439 } 440 441 return NewBB; 442} 443 444/// AreEquivalentAddressValues - Test if A and B will obviously have the same 445/// value. This includes recognizing that %t0 and %t1 will have the same 446/// value in code like this: 447/// %t0 = getelementptr \@a, 0, 3 448/// store i32 0, i32* %t0 449/// %t1 = getelementptr \@a, 0, 3 450/// %t2 = load i32* %t1 451/// 452static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 453 // Test if the values are trivially equivalent. 454 if (A == B) return true; 455 456 // Test if the values come form identical arithmetic instructions. 457 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 458 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 459 if (const Instruction *BI = dyn_cast<Instruction>(B)) 460 if (cast<Instruction>(A)->isIdenticalTo(BI)) 461 return true; 462 463 // Otherwise they may not be equivalent. 464 return false; 465} 466 467/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 468/// instruction before ScanFrom) checking to see if we have the value at the 469/// memory address *Ptr locally available within a small number of instructions. 470/// If the value is available, return it. 471/// 472/// If not, return the iterator for the last validated instruction that the 473/// value would be live through. If we scanned the entire block and didn't find 474/// something that invalidates *Ptr or provides it, ScanFrom would be left at 475/// begin() and this returns null. ScanFrom could also be left 476/// 477/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 478/// it is set to 0, it will scan the whole block. You can also optionally 479/// specify an alias analysis implementation, which makes this more precise. 480Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 481 BasicBlock::iterator &ScanFrom, 482 unsigned MaxInstsToScan, 483 AliasAnalysis *AA) { 484 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 485 486 // If we're using alias analysis to disambiguate get the size of *Ptr. 487 unsigned AccessSize = 0; 488 if (AA) { 489 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 490 AccessSize = AA->getTargetData().getTypeStoreSizeInBits(AccessTy); 491 } 492 493 while (ScanFrom != ScanBB->begin()) { 494 // We must ignore debug info directives when counting (otherwise they 495 // would affect codegen). 496 Instruction *Inst = --ScanFrom; 497 if (isa<DbgInfoIntrinsic>(Inst)) 498 continue; 499 // We skip pointer-to-pointer bitcasts, which are NOPs. 500 // It is necessary for correctness to skip those that feed into a 501 // llvm.dbg.declare, as these are not present when debugging is off. 502 if (isa<BitCastInst>(Inst) && isa<PointerType>(Inst->getType())) 503 continue; 504 505 // Restore ScanFrom to expected value in case next test succeeds 506 ScanFrom++; 507 508 // Don't scan huge blocks. 509 if (MaxInstsToScan-- == 0) return 0; 510 511 --ScanFrom; 512 // If this is a load of Ptr, the loaded value is available. 513 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 514 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 515 return LI; 516 517 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 518 // If this is a store through Ptr, the value is available! 519 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 520 return SI->getOperand(0); 521 522 // If Ptr is an alloca and this is a store to a different alloca, ignore 523 // the store. This is a trivial form of alias analysis that is important 524 // for reg2mem'd code. 525 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 526 (isa<AllocaInst>(SI->getOperand(1)) || 527 isa<GlobalVariable>(SI->getOperand(1)))) 528 continue; 529 530 // If we have alias analysis and it says the store won't modify the loaded 531 // value, ignore the store. 532 if (AA && 533 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 534 continue; 535 536 // Otherwise the store that may or may not alias the pointer, bail out. 537 ++ScanFrom; 538 return 0; 539 } 540 541 // If this is some other instruction that may clobber Ptr, bail out. 542 if (Inst->mayWriteToMemory()) { 543 // If alias analysis claims that it really won't modify the load, 544 // ignore it. 545 if (AA && 546 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 547 continue; 548 549 // May modify the pointer, bail out. 550 ++ScanFrom; 551 return 0; 552 } 553 } 554 555 // Got to the start of the block, we didn't find it, but are done for this 556 // block. 557 return 0; 558} 559 560/// CopyPrecedingStopPoint - If I is immediately preceded by a StopPoint, 561/// make a copy of the stoppoint before InsertPos (presumably before copying 562/// or moving I). 563void llvm::CopyPrecedingStopPoint(Instruction *I, 564 BasicBlock::iterator InsertPos) { 565 if (I != I->getParent()->begin()) { 566 BasicBlock::iterator BBI = I; --BBI; 567 if (DbgStopPointInst *DSPI = dyn_cast<DbgStopPointInst>(BBI)) { 568 CallInst *newDSPI = DSPI->clone(); 569 newDSPI->insertBefore(InsertPos); 570 } 571 } 572} 573