BasicBlockUtils.cpp revision 0f8b53f19d29013ab18f3d444cea1e6305405611
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 <algorithm> 26using namespace llvm; 27 28/// DeleteDeadBlock - Delete the specified block, which must have no 29/// predecessors. 30void llvm::DeleteDeadBlock(BasicBlock *BB) { 31 assert((pred_begin(BB) == pred_end(BB) || 32 // Can delete self loop. 33 BB->getSinglePredecessor() == BB) && "Block is not dead!"); 34 TerminatorInst *BBTerm = BB->getTerminator(); 35 36 // Loop through all of our successors and make sure they know that one 37 // of their predecessors is going away. 38 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) 39 BBTerm->getSuccessor(i)->removePredecessor(BB); 40 41 // Zap all the instructions in the block. 42 while (!BB->empty()) { 43 Instruction &I = BB->back(); 44 // If this instruction is used, replace uses with an arbitrary value. 45 // Because control flow can't get here, we don't care what we replace the 46 // value with. Note that since this block is unreachable, and all values 47 // contained within it must dominate their uses, that all uses will 48 // eventually be removed (they are themselves dead). 49 if (!I.use_empty()) 50 I.replaceAllUsesWith(UndefValue::get(I.getType())); 51 BB->getInstList().pop_back(); 52 } 53 54 // Zap the block! 55 BB->eraseFromParent(); 56} 57 58/// FoldSingleEntryPHINodes - We know that BB has one predecessor. If there are 59/// any single-entry PHI nodes in it, fold them away. This handles the case 60/// when all entries to the PHI nodes in a block are guaranteed equal, such as 61/// when the block has exactly one predecessor. 62void llvm::FoldSingleEntryPHINodes(BasicBlock *BB) { 63 if (!isa<PHINode>(BB->begin())) 64 return; 65 66 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 67 if (PN->getIncomingValue(0) != PN) 68 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 69 else 70 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 71 PN->eraseFromParent(); 72 } 73} 74 75 76/// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor, 77/// if possible. The return value indicates success or failure. 78bool llvm::MergeBlockIntoPredecessor(BasicBlock* BB, Pass* P) { 79 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 80 // Can't merge the entry block. 81 if (pred_begin(BB) == pred_end(BB)) return false; 82 83 BasicBlock *PredBB = *PI++; 84 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 85 if (*PI != PredBB) { 86 PredBB = 0; // There are multiple different predecessors... 87 break; 88 } 89 90 // Can't merge if there are multiple predecessors. 91 if (!PredBB) return false; 92 // Don't break self-loops. 93 if (PredBB == BB) return false; 94 // Don't break invokes. 95 if (isa<InvokeInst>(PredBB->getTerminator())) return false; 96 97 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB)); 98 BasicBlock* OnlySucc = BB; 99 for (; SI != SE; ++SI) 100 if (*SI != OnlySucc) { 101 OnlySucc = 0; // There are multiple distinct successors! 102 break; 103 } 104 105 // Can't merge if there are multiple successors. 106 if (!OnlySucc) return false; 107 108 // Can't merge if there is PHI loop. 109 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) { 110 if (PHINode *PN = dyn_cast<PHINode>(BI)) { 111 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 112 if (PN->getIncomingValue(i) == PN) 113 return false; 114 } else 115 break; 116 } 117 118 // Begin by getting rid of unneeded PHIs. 119 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 120 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 121 BB->getInstList().pop_front(); // Delete the phi node... 122 } 123 124 // Delete the unconditional branch from the predecessor... 125 PredBB->getInstList().pop_back(); 126 127 // Move all definitions in the successor to the predecessor... 128 PredBB->getInstList().splice(PredBB->end(), BB->getInstList()); 129 130 // Make all PHI nodes that referred to BB now refer to Pred as their 131 // source... 132 BB->replaceAllUsesWith(PredBB); 133 134 // Inherit predecessors name if it exists. 135 if (!PredBB->hasName()) 136 PredBB->takeName(BB); 137 138 // Finally, erase the old block and update dominator info. 139 if (P) { 140 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) { 141 DomTreeNode* DTN = DT->getNode(BB); 142 DomTreeNode* PredDTN = DT->getNode(PredBB); 143 144 if (DTN) { 145 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end()); 146 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(), 147 DE = Children.end(); DI != DE; ++DI) 148 DT->changeImmediateDominator(*DI, PredDTN); 149 150 DT->eraseNode(BB); 151 } 152 } 153 } 154 155 BB->eraseFromParent(); 156 157 158 return true; 159} 160 161/// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI) 162/// with a value, then remove and delete the original instruction. 163/// 164void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL, 165 BasicBlock::iterator &BI, Value *V) { 166 Instruction &I = *BI; 167 // Replaces all of the uses of the instruction with uses of the value 168 I.replaceAllUsesWith(V); 169 170 // Make sure to propagate a name if there is one already. 171 if (I.hasName() && !V->hasName()) 172 V->takeName(&I); 173 174 // Delete the unnecessary instruction now... 175 BI = BIL.erase(BI); 176} 177 178 179/// ReplaceInstWithInst - Replace the instruction specified by BI with the 180/// instruction specified by I. The original instruction is deleted and BI is 181/// updated to point to the new instruction. 182/// 183void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL, 184 BasicBlock::iterator &BI, Instruction *I) { 185 assert(I->getParent() == 0 && 186 "ReplaceInstWithInst: Instruction already inserted into basic block!"); 187 188 // Insert the new instruction into the basic block... 189 BasicBlock::iterator New = BIL.insert(BI, I); 190 191 // Replace all uses of the old instruction, and delete it. 192 ReplaceInstWithValue(BIL, BI, I); 193 194 // Move BI back to point to the newly inserted instruction 195 BI = New; 196} 197 198/// ReplaceInstWithInst - Replace the instruction specified by From with the 199/// instruction specified by To. 200/// 201void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) { 202 BasicBlock::iterator BI(From); 203 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To); 204} 205 206/// RemoveSuccessor - Change the specified terminator instruction such that its 207/// successor SuccNum no longer exists. Because this reduces the outgoing 208/// degree of the current basic block, the actual terminator instruction itself 209/// may have to be changed. In the case where the last successor of the block 210/// is deleted, a return instruction is inserted in its place which can cause a 211/// surprising change in program behavior if it is not expected. 212/// 213void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) { 214 assert(SuccNum < TI->getNumSuccessors() && 215 "Trying to remove a nonexistant successor!"); 216 217 // If our old successor block contains any PHI nodes, remove the entry in the 218 // PHI nodes that comes from this branch... 219 // 220 BasicBlock *BB = TI->getParent(); 221 TI->getSuccessor(SuccNum)->removePredecessor(BB); 222 223 TerminatorInst *NewTI = 0; 224 switch (TI->getOpcode()) { 225 case Instruction::Br: 226 // If this is a conditional branch... convert to unconditional branch. 227 if (TI->getNumSuccessors() == 2) { 228 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum)); 229 } else { // Otherwise convert to a return instruction... 230 Value *RetVal = 0; 231 232 // Create a value to return... if the function doesn't return null... 233 if (BB->getParent()->getReturnType() != Type::VoidTy) 234 RetVal = Constant::getNullValue(BB->getParent()->getReturnType()); 235 236 // Create the return... 237 NewTI = ReturnInst::Create(RetVal); 238 } 239 break; 240 241 case Instruction::Invoke: // Should convert to call 242 case Instruction::Switch: // Should remove entry 243 default: 244 case Instruction::Ret: // Cannot happen, has no successors! 245 assert(0 && "Unhandled terminator instruction type in RemoveSuccessor!"); 246 abort(); 247 } 248 249 if (NewTI) // If it's a different instruction, replace. 250 ReplaceInstWithInst(TI, NewTI); 251} 252 253/// SplitEdge - Split the edge connecting specified block. Pass P must 254/// not be NULL. 255BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) { 256 TerminatorInst *LatchTerm = BB->getTerminator(); 257 unsigned SuccNum = 0; 258#ifndef NDEBUG 259 unsigned e = LatchTerm->getNumSuccessors(); 260#endif 261 for (unsigned i = 0; ; ++i) { 262 assert(i != e && "Didn't find edge?"); 263 if (LatchTerm->getSuccessor(i) == Succ) { 264 SuccNum = i; 265 break; 266 } 267 } 268 269 // If this is a critical edge, let SplitCriticalEdge do it. 270 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P)) 271 return LatchTerm->getSuccessor(SuccNum); 272 273 // If the edge isn't critical, then BB has a single successor or Succ has a 274 // single pred. Split the block. 275 BasicBlock::iterator SplitPoint; 276 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 277 // If the successor only has a single pred, split the top of the successor 278 // block. 279 assert(SP == BB && "CFG broken"); 280 SP = NULL; 281 return SplitBlock(Succ, Succ->begin(), P); 282 } else { 283 // Otherwise, if BB has a single successor, split it at the bottom of the 284 // block. 285 assert(BB->getTerminator()->getNumSuccessors() == 1 && 286 "Should have a single succ!"); 287 return SplitBlock(BB, BB->getTerminator(), P); 288 } 289} 290 291/// SplitBlock - Split the specified block at the specified instruction - every 292/// thing before SplitPt stays in Old and everything starting with SplitPt moves 293/// to a new block. The two blocks are joined by an unconditional branch and 294/// the loop info is updated. 295/// 296BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) { 297 BasicBlock::iterator SplitIt = SplitPt; 298 while (isa<PHINode>(SplitIt)) 299 ++SplitIt; 300 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 301 302 // The new block lives in whichever loop the old one did. 303 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>()) 304 if (Loop *L = LI->getLoopFor(Old)) 305 L->addBasicBlockToLoop(New, LI->getBase()); 306 307 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) 308 { 309 // Old dominates New. New node domiantes all other nodes dominated by Old. 310 DomTreeNode *OldNode = DT->getNode(Old); 311 std::vector<DomTreeNode *> Children; 312 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end(); 313 I != E; ++I) 314 Children.push_back(*I); 315 316 DomTreeNode *NewNode = DT->addNewBlock(New,Old); 317 318 for (std::vector<DomTreeNode *>::iterator I = Children.begin(), 319 E = Children.end(); I != E; ++I) 320 DT->changeImmediateDominator(*I, NewNode); 321 } 322 323 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) 324 DF->splitBlock(Old); 325 326 return New; 327} 328 329 330/// SplitBlockPredecessors - This method transforms BB by introducing a new 331/// basic block into the function, and moving some of the predecessors of BB to 332/// be predecessors of the new block. The new predecessors are indicated by the 333/// Preds array, which has NumPreds elements in it. The new block is given a 334/// suffix of 'Suffix'. 335/// 336/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree and 337/// DominanceFrontier, but no other analyses. 338BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB, 339 BasicBlock *const *Preds, 340 unsigned NumPreds, const char *Suffix, 341 Pass *P) { 342 // Create new basic block, insert right before the original block. 343 BasicBlock *NewBB = 344 BasicBlock::Create(BB->getName()+Suffix, BB->getParent(), BB); 345 346 // The new block unconditionally branches to the old block. 347 BranchInst *BI = BranchInst::Create(BB, NewBB); 348 349 // Move the edges from Preds to point to NewBB instead of BB. 350 for (unsigned i = 0; i != NumPreds; ++i) 351 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB); 352 353 // Update dominator tree and dominator frontier if available. 354 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0; 355 if (DT) 356 DT->splitBlock(NewBB); 357 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0) 358 DF->splitBlock(NewBB); 359 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0; 360 361 362 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI 363 // node becomes an incoming value for BB's phi node. However, if the Preds 364 // list is empty, we need to insert dummy entries into the PHI nodes in BB to 365 // account for the newly created predecessor. 366 if (NumPreds == 0) { 367 // Insert dummy values as the incoming value. 368 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) 369 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB); 370 return NewBB; 371 } 372 373 // Otherwise, create a new PHI node in NewBB for each PHI node in BB. 374 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) { 375 PHINode *PN = cast<PHINode>(I++); 376 377 // Check to see if all of the values coming in are the same. If so, we 378 // don't need to create a new PHI node. 379 Value *InVal = PN->getIncomingValueForBlock(Preds[0]); 380 for (unsigned i = 1; i != NumPreds; ++i) 381 if (InVal != PN->getIncomingValueForBlock(Preds[i])) { 382 InVal = 0; 383 break; 384 } 385 386 if (InVal) { 387 // If all incoming values for the new PHI would be the same, just don't 388 // make a new PHI. Instead, just remove the incoming values from the old 389 // PHI. 390 for (unsigned i = 0; i != NumPreds; ++i) 391 PN->removeIncomingValue(Preds[i], false); 392 } else { 393 // If the values coming into the block are not the same, we need a PHI. 394 // Create the new PHI node, insert it into NewBB at the end of the block 395 PHINode *NewPHI = 396 PHINode::Create(PN->getType(), PN->getName()+".ph", BI); 397 if (AA) AA->copyValue(PN, NewPHI); 398 399 // Move all of the PHI values for 'Preds' to the new PHI. 400 for (unsigned i = 0; i != NumPreds; ++i) { 401 Value *V = PN->removeIncomingValue(Preds[i], false); 402 NewPHI->addIncoming(V, Preds[i]); 403 } 404 InVal = NewPHI; 405 } 406 407 // Add an incoming value to the PHI node in the loop for the preheader 408 // edge. 409 PN->addIncoming(InVal, NewBB); 410 411 // Check to see if we can eliminate this phi node. 412 if (Value *V = PN->hasConstantValue(DT != 0)) { 413 Instruction *I = dyn_cast<Instruction>(V); 414 if (!I || DT == 0 || DT->dominates(I, PN)) { 415 PN->replaceAllUsesWith(V); 416 if (AA) AA->deleteValue(PN); 417 PN->eraseFromParent(); 418 } 419 } 420 } 421 422 return NewBB; 423} 424 425/// AreEquivalentAddressValues - Test if A and B will obviously have the same 426/// value. This includes recognizing that %t0 and %t1 will have the same 427/// value in code like this: 428/// %t0 = getelementptr \@a, 0, 3 429/// store i32 0, i32* %t0 430/// %t1 = getelementptr \@a, 0, 3 431/// %t2 = load i32* %t1 432/// 433static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 434 // Test if the values are trivially equivalent. 435 if (A == B) return true; 436 437 // Test if the values come form identical arithmetic instructions. 438 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 439 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 440 if (const Instruction *BI = dyn_cast<Instruction>(B)) 441 if (cast<Instruction>(A)->isIdenticalTo(BI)) 442 return true; 443 444 // Otherwise they may not be equivalent. 445 return false; 446} 447 448/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 449/// instruction before ScanFrom) checking to see if we have the value at the 450/// memory address *Ptr locally available within a small number of instructions. 451/// If the value is available, return it. 452/// 453/// If not, return the iterator for the last validated instruction that the 454/// value would be live through. If we scanned the entire block and didn't find 455/// something that invalidates *Ptr or provides it, ScanFrom would be left at 456/// begin() and this returns null. ScanFrom could also be left 457/// 458/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 459/// it is set to 0, it will scan the whole block. You can also optionally 460/// specify an alias analysis implementation, which makes this more precise. 461Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 462 BasicBlock::iterator &ScanFrom, 463 unsigned MaxInstsToScan, 464 AliasAnalysis *AA) { 465 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 466 467 // If we're using alias analysis to disambiguate get the size of *Ptr. 468 unsigned AccessSize = 0; 469 if (AA) { 470 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 471 AccessSize = AA->getTargetData().getTypeStoreSizeInBits(AccessTy); 472 } 473 474 while (ScanFrom != ScanBB->begin()) { 475 // We must ignore debug info directives when counting (otherwise they 476 // would affect codegen). 477 Instruction *Inst = --ScanFrom; 478 if (isa<DbgInfoIntrinsic>(Inst)) 479 continue; 480 // Restore ScanFrom to expected value in case next test succeeds 481 ScanFrom++; 482 483 // Don't scan huge blocks. 484 if (MaxInstsToScan-- == 0) return 0; 485 486 --ScanFrom; 487 // If this is a load of Ptr, the loaded value is available. 488 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 489 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 490 return LI; 491 492 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 493 // If this is a store through Ptr, the value is available! 494 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 495 return SI->getOperand(0); 496 497 // If Ptr is an alloca and this is a store to a different alloca, ignore 498 // the store. This is a trivial form of alias analysis that is important 499 // for reg2mem'd code. 500 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 501 (isa<AllocaInst>(SI->getOperand(1)) || 502 isa<GlobalVariable>(SI->getOperand(1)))) 503 continue; 504 505 // If we have alias analysis and it says the store won't modify the loaded 506 // value, ignore the store. 507 if (AA && 508 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 509 continue; 510 511 // Otherwise the store that may or may not alias the pointer, bail out. 512 ++ScanFrom; 513 return 0; 514 } 515 516 // If this is some other instruction that may clobber Ptr, bail out. 517 if (Inst->mayWriteToMemory()) { 518 // If alias analysis claims that it really won't modify the load, 519 // ignore it. 520 if (AA && 521 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 522 continue; 523 524 // May modify the pointer, bail out. 525 ++ScanFrom; 526 return 0; 527 } 528 } 529 530 // Got to the start of the block, we didn't find it, but are done for this 531 // block. 532 return 0; 533} 534 535/// CopyPrecedingStopPoint - If I is immediately preceded by a StopPoint, 536/// make a copy of the stoppoint before InsertPos (presumably before copying 537/// or moving I). 538void llvm::CopyPrecedingStopPoint(Instruction *I, 539 BasicBlock::iterator InsertPos) { 540 if (I != I->getParent()->begin()) { 541 BasicBlock::iterator BBI = I; --BBI; 542 if (DbgStopPointInst *DSPI = dyn_cast<DbgStopPointInst>(BBI)) { 543 DbgStopPointInst *newDSPI = 544 reinterpret_cast<DbgStopPointInst*>(DSPI->clone()); 545 newDSPI->insertBefore(InsertPos); 546 } 547 } 548} 549