BasicBlockUtils.cpp revision 5622f07a21b799964dc172925b9ebc38191859f6
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/Constant.h" 19#include "llvm/Type.h" 20#include "llvm/Analysis/AliasAnalysis.h" 21#include "llvm/Analysis/LoopInfo.h" 22#include "llvm/Analysis/Dominators.h" 23#include "llvm/Target/TargetData.h" 24#include <algorithm> 25using namespace llvm; 26 27/// DeleteDeadBlock - Delete the specified block, which must have no 28/// predecessors. 29void llvm::DeleteDeadBlock(BasicBlock *BB) { 30 assert((pred_begin(BB) == pred_end(BB) || 31 // Can delete self loop. 32 BB->getSinglePredecessor() == BB) && "Block is not dead!"); 33 TerminatorInst *BBTerm = BB->getTerminator(); 34 35 // Loop through all of our successors and make sure they know that one 36 // of their predecessors is going away. 37 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) 38 BBTerm->getSuccessor(i)->removePredecessor(BB); 39 40 // Zap all the instructions in the block. 41 while (!BB->empty()) { 42 Instruction &I = BB->back(); 43 // If this instruction is used, replace uses with an arbitrary value. 44 // Because control flow can't get here, we don't care what we replace the 45 // value with. Note that since this block is unreachable, and all values 46 // contained within it must dominate their uses, that all uses will 47 // eventually be removed (they are themselves dead). 48 if (!I.use_empty()) 49 I.replaceAllUsesWith(UndefValue::get(I.getType())); 50 BB->getInstList().pop_back(); 51 } 52 53 // Zap the block! 54 BB->eraseFromParent(); 55} 56 57/// FoldSingleEntryPHINodes - We know that BB has one predecessor. If there are 58/// any single-entry PHI nodes in it, fold them away. This handles the case 59/// when all entries to the PHI nodes in a block are guaranteed equal, such as 60/// when the block has exactly one predecessor. 61void llvm::FoldSingleEntryPHINodes(BasicBlock *BB) { 62 if (!isa<PHINode>(BB->begin())) 63 return; 64 65 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 66 if (PN->getIncomingValue(0) != PN) 67 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 68 else 69 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 70 PN->eraseFromParent(); 71 } 72} 73 74 75/// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor, 76/// if possible. The return value indicates success or failure. 77bool llvm::MergeBlockIntoPredecessor(BasicBlock* BB, Pass* P) { 78 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 79 // Can't merge the entry block. 80 if (pred_begin(BB) == pred_end(BB)) return false; 81 82 BasicBlock *PredBB = *PI++; 83 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 84 if (*PI != PredBB) { 85 PredBB = 0; // There are multiple different predecessors... 86 break; 87 } 88 89 // Can't merge if there are multiple predecessors. 90 if (!PredBB) return false; 91 // Don't break self-loops. 92 if (PredBB == BB) return false; 93 // Don't break invokes. 94 if (isa<InvokeInst>(PredBB->getTerminator())) return false; 95 96 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB)); 97 BasicBlock* OnlySucc = BB; 98 for (; SI != SE; ++SI) 99 if (*SI != OnlySucc) { 100 OnlySucc = 0; // There are multiple distinct successors! 101 break; 102 } 103 104 // Can't merge if there are multiple successors. 105 if (!OnlySucc) return false; 106 107 // Can't merge if there is PHI loop. 108 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) { 109 if (PHINode *PN = dyn_cast<PHINode>(BI)) { 110 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 111 if (PN->getIncomingValue(i) == PN) 112 return false; 113 } else 114 break; 115 } 116 117 // Begin by getting rid of unneeded PHIs. 118 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 119 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 120 BB->getInstList().pop_front(); // Delete the phi node... 121 } 122 123 // Delete the unconditional branch from the predecessor... 124 PredBB->getInstList().pop_back(); 125 126 // Move all definitions in the successor to the predecessor... 127 PredBB->getInstList().splice(PredBB->end(), BB->getInstList()); 128 129 // Make all PHI nodes that referred to BB now refer to Pred as their 130 // source... 131 BB->replaceAllUsesWith(PredBB); 132 133 // Inherit predecessors name if it exists. 134 if (!PredBB->hasName()) 135 PredBB->takeName(BB); 136 137 // Finally, erase the old block and update dominator info. 138 if (P) { 139 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) { 140 DomTreeNode* DTN = DT->getNode(BB); 141 DomTreeNode* PredDTN = DT->getNode(PredBB); 142 143 if (DTN) { 144 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end()); 145 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(), 146 DE = Children.end(); DI != DE; ++DI) 147 DT->changeImmediateDominator(*DI, PredDTN); 148 149 DT->eraseNode(BB); 150 } 151 } 152 } 153 154 BB->eraseFromParent(); 155 156 157 return true; 158} 159 160/// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI) 161/// with a value, then remove and delete the original instruction. 162/// 163void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL, 164 BasicBlock::iterator &BI, Value *V) { 165 Instruction &I = *BI; 166 // Replaces all of the uses of the instruction with uses of the value 167 I.replaceAllUsesWith(V); 168 169 // Make sure to propagate a name if there is one already. 170 if (I.hasName() && !V->hasName()) 171 V->takeName(&I); 172 173 // Delete the unnecessary instruction now... 174 BI = BIL.erase(BI); 175} 176 177 178/// ReplaceInstWithInst - Replace the instruction specified by BI with the 179/// instruction specified by I. The original instruction is deleted and BI is 180/// updated to point to the new instruction. 181/// 182void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL, 183 BasicBlock::iterator &BI, Instruction *I) { 184 assert(I->getParent() == 0 && 185 "ReplaceInstWithInst: Instruction already inserted into basic block!"); 186 187 // Insert the new instruction into the basic block... 188 BasicBlock::iterator New = BIL.insert(BI, I); 189 190 // Replace all uses of the old instruction, and delete it. 191 ReplaceInstWithValue(BIL, BI, I); 192 193 // Move BI back to point to the newly inserted instruction 194 BI = New; 195} 196 197/// ReplaceInstWithInst - Replace the instruction specified by From with the 198/// instruction specified by To. 199/// 200void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) { 201 BasicBlock::iterator BI(From); 202 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To); 203} 204 205/// RemoveSuccessor - Change the specified terminator instruction such that its 206/// successor SuccNum no longer exists. Because this reduces the outgoing 207/// degree of the current basic block, the actual terminator instruction itself 208/// may have to be changed. In the case where the last successor of the block 209/// is deleted, a return instruction is inserted in its place which can cause a 210/// surprising change in program behavior if it is not expected. 211/// 212void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) { 213 assert(SuccNum < TI->getNumSuccessors() && 214 "Trying to remove a nonexistant successor!"); 215 216 // If our old successor block contains any PHI nodes, remove the entry in the 217 // PHI nodes that comes from this branch... 218 // 219 BasicBlock *BB = TI->getParent(); 220 TI->getSuccessor(SuccNum)->removePredecessor(BB); 221 222 TerminatorInst *NewTI = 0; 223 switch (TI->getOpcode()) { 224 case Instruction::Br: 225 // If this is a conditional branch... convert to unconditional branch. 226 if (TI->getNumSuccessors() == 2) { 227 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum)); 228 } else { // Otherwise convert to a return instruction... 229 Value *RetVal = 0; 230 231 // Create a value to return... if the function doesn't return null... 232 if (BB->getParent()->getReturnType() != Type::VoidTy) 233 RetVal = Constant::getNullValue(BB->getParent()->getReturnType()); 234 235 // Create the return... 236 NewTI = ReturnInst::Create(RetVal); 237 } 238 break; 239 240 case Instruction::Invoke: // Should convert to call 241 case Instruction::Switch: // Should remove entry 242 default: 243 case Instruction::Ret: // Cannot happen, has no successors! 244 assert(0 && "Unhandled terminator instruction type in RemoveSuccessor!"); 245 abort(); 246 } 247 248 if (NewTI) // If it's a different instruction, replace. 249 ReplaceInstWithInst(TI, NewTI); 250} 251 252/// SplitEdge - Split the edge connecting specified block. Pass P must 253/// not be NULL. 254BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) { 255 TerminatorInst *LatchTerm = BB->getTerminator(); 256 unsigned SuccNum = 0; 257#ifndef NDEBUG 258 unsigned e = LatchTerm->getNumSuccessors(); 259#endif 260 for (unsigned i = 0; ; ++i) { 261 assert(i != e && "Didn't find edge?"); 262 if (LatchTerm->getSuccessor(i) == Succ) { 263 SuccNum = i; 264 break; 265 } 266 } 267 268 // If this is a critical edge, let SplitCriticalEdge do it. 269 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P)) 270 return LatchTerm->getSuccessor(SuccNum); 271 272 // If the edge isn't critical, then BB has a single successor or Succ has a 273 // single pred. Split the block. 274 BasicBlock::iterator SplitPoint; 275 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 276 // If the successor only has a single pred, split the top of the successor 277 // block. 278 assert(SP == BB && "CFG broken"); 279 SP = NULL; 280 return SplitBlock(Succ, Succ->begin(), P); 281 } else { 282 // Otherwise, if BB has a single successor, split it at the bottom of the 283 // block. 284 assert(BB->getTerminator()->getNumSuccessors() == 1 && 285 "Should have a single succ!"); 286 return SplitBlock(BB, BB->getTerminator(), P); 287 } 288} 289 290/// SplitBlock - Split the specified block at the specified instruction - every 291/// thing before SplitPt stays in Old and everything starting with SplitPt moves 292/// to a new block. The two blocks are joined by an unconditional branch and 293/// the loop info is updated. 294/// 295BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) { 296 BasicBlock::iterator SplitIt = SplitPt; 297 while (isa<PHINode>(SplitIt)) 298 ++SplitIt; 299 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 300 301 // The new block lives in whichever loop the old one did. 302 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>()) 303 if (Loop *L = LI->getLoopFor(Old)) 304 L->addBasicBlockToLoop(New, LI->getBase()); 305 306 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) 307 { 308 // Old dominates New. New node domiantes all other nodes dominated by Old. 309 DomTreeNode *OldNode = DT->getNode(Old); 310 std::vector<DomTreeNode *> Children; 311 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end(); 312 I != E; ++I) 313 Children.push_back(*I); 314 315 DomTreeNode *NewNode = DT->addNewBlock(New,Old); 316 317 for (std::vector<DomTreeNode *>::iterator I = Children.begin(), 318 E = Children.end(); I != E; ++I) 319 DT->changeImmediateDominator(*I, NewNode); 320 } 321 322 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) 323 DF->splitBlock(Old); 324 325 return New; 326} 327 328 329/// SplitBlockPredecessors - This method transforms BB by introducing a new 330/// basic block into the function, and moving some of the predecessors of BB to 331/// be predecessors of the new block. The new predecessors are indicated by the 332/// Preds array, which has NumPreds elements in it. The new block is given a 333/// suffix of 'Suffix'. 334/// 335/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree and 336/// DominanceFrontier, but no other analyses. 337BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB, 338 BasicBlock *const *Preds, 339 unsigned NumPreds, const char *Suffix, 340 Pass *P) { 341 // Create new basic block, insert right before the original block. 342 BasicBlock *NewBB = 343 BasicBlock::Create(BB->getName()+Suffix, BB->getParent(), BB); 344 345 // The new block unconditionally branches to the old block. 346 BranchInst *BI = BranchInst::Create(BB, NewBB); 347 348 // Move the edges from Preds to point to NewBB instead of BB. 349 for (unsigned i = 0; i != NumPreds; ++i) 350 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB); 351 352 // Update dominator tree and dominator frontier if available. 353 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0; 354 if (DT) 355 DT->splitBlock(NewBB); 356 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0) 357 DF->splitBlock(NewBB); 358 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0; 359 360 361 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI 362 // node becomes an incoming value for BB's phi node. However, if the Preds 363 // list is empty, we need to insert dummy entries into the PHI nodes in BB to 364 // account for the newly created predecessor. 365 if (NumPreds == 0) { 366 // Insert dummy values as the incoming value. 367 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) 368 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB); 369 return NewBB; 370 } 371 372 // Otherwise, create a new PHI node in NewBB for each PHI node in BB. 373 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) { 374 PHINode *PN = cast<PHINode>(I++); 375 376 // Check to see if all of the values coming in are the same. If so, we 377 // don't need to create a new PHI node. 378 Value *InVal = PN->getIncomingValueForBlock(Preds[0]); 379 for (unsigned i = 1; i != NumPreds; ++i) 380 if (InVal != PN->getIncomingValueForBlock(Preds[i])) { 381 InVal = 0; 382 break; 383 } 384 385 if (InVal) { 386 // If all incoming values for the new PHI would be the same, just don't 387 // make a new PHI. Instead, just remove the incoming values from the old 388 // PHI. 389 for (unsigned i = 0; i != NumPreds; ++i) 390 PN->removeIncomingValue(Preds[i], false); 391 } else { 392 // If the values coming into the block are not the same, we need a PHI. 393 // Create the new PHI node, insert it into NewBB at the end of the block 394 PHINode *NewPHI = 395 PHINode::Create(PN->getType(), PN->getName()+".ph", BI); 396 if (AA) AA->copyValue(PN, NewPHI); 397 398 // Move all of the PHI values for 'Preds' to the new PHI. 399 for (unsigned i = 0; i != NumPreds; ++i) { 400 Value *V = PN->removeIncomingValue(Preds[i], false); 401 NewPHI->addIncoming(V, Preds[i]); 402 } 403 InVal = NewPHI; 404 } 405 406 // Add an incoming value to the PHI node in the loop for the preheader 407 // edge. 408 PN->addIncoming(InVal, NewBB); 409 410 // Check to see if we can eliminate this phi node. 411 if (Value *V = PN->hasConstantValue(DT != 0)) { 412 Instruction *I = dyn_cast<Instruction>(V); 413 if (!I || DT == 0 || DT->dominates(I, PN)) { 414 PN->replaceAllUsesWith(V); 415 if (AA) AA->deleteValue(PN); 416 PN->eraseFromParent(); 417 } 418 } 419 } 420 421 return NewBB; 422} 423 424/// AreEquivalentAddressValues - Test if A and B will obviously have the same 425/// value. This includes recognizing that %t0 and %t1 will have the same 426/// value in code like this: 427/// %t0 = getelementptr @a, 0, 3 428/// store i32 0, i32* %t0 429/// %t1 = getelementptr @a, 0, 3 430/// %t2 = load i32* %t1 431/// 432static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 433 // Test if the values are trivially equivalent. 434 if (A == B) return true; 435 436 // Test if the values come form identical arithmetic instructions. 437 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 438 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 439 if (const Instruction *BI = dyn_cast<Instruction>(B)) 440 if (cast<Instruction>(A)->isIdenticalTo(BI)) 441 return true; 442 443 // Otherwise they may not be equivalent. 444 return false; 445} 446 447/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 448/// instruction before ScanFrom) checking to see if we have the value at the 449/// memory address *Ptr locally available within a small number of instructions. 450/// If the value is available, return it. 451/// 452/// If not, return the iterator for the last validated instruction that the 453/// value would be live through. If we scanned the entire block and didn't find 454/// something that invalidates *Ptr or provides it, ScanFrom would be left at 455/// begin() and this returns null. ScanFrom could also be left 456/// 457/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 458/// it is set to 0, it will scan the whole block. You can also optionally 459/// specify an alias analysis implementation, which makes this more precise. 460Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 461 BasicBlock::iterator &ScanFrom, 462 unsigned MaxInstsToScan, 463 AliasAnalysis *AA) { 464 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 465 466 // If we're using alias analysis to disambiguate get the size of *Ptr. 467 unsigned AccessSize = 0; 468 if (AA) { 469 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 470 AccessSize = AA->getTargetData().getTypeStoreSizeInBits(AccessTy); 471 } 472 473 while (ScanFrom != ScanBB->begin()) { 474 // Don't scan huge blocks. 475 if (MaxInstsToScan-- == 0) return 0; 476 477 Instruction *Inst = --ScanFrom; 478 479 // If this is a load of Ptr, the loaded value is available. 480 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 481 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 482 return LI; 483 484 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 485 // If this is a store through Ptr, the value is available! 486 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 487 return SI->getOperand(0); 488 489 // If Ptr is an alloca and this is a store to a different alloca, ignore 490 // the store. This is a trivial form of alias analysis that is important 491 // for reg2mem'd code. 492 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 493 (isa<AllocaInst>(SI->getOperand(1)) || 494 isa<GlobalVariable>(SI->getOperand(1)))) 495 continue; 496 497 // If we have alias analysis and it says the store won't modify the loaded 498 // value, ignore the store. 499 if (AA && 500 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 501 continue; 502 503 // Otherwise the store that may or may not alias the pointer, bail out. 504 ++ScanFrom; 505 return 0; 506 } 507 508 // If this is some other instruction that may clobber Ptr, bail out. 509 if (Inst->mayWriteToMemory()) { 510 // If alias analysis claims that it really won't modify the load, 511 // ignore it. 512 if (AA && 513 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 514 continue; 515 516 // May modify the pointer, bail out. 517 ++ScanFrom; 518 return 0; 519 } 520 } 521 522 // Got to the start of the block, we didn't find it, but are done for this 523 // block. 524 return 0; 525} 526