BasicBlockUtils.cpp revision 44a29e066a24e88bdf127e88be4380a5f259c4b4
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/Transforms/Scalar.h" 27#include "llvm/Support/ErrorHandling.h" 28#include "llvm/Support/ValueHandle.h" 29#include <algorithm> 30using namespace llvm; 31 32/// DeleteDeadBlock - Delete the specified block, which must have no 33/// predecessors. 34void llvm::DeleteDeadBlock(BasicBlock *BB) { 35 assert((pred_begin(BB) == pred_end(BB) || 36 // Can delete self loop. 37 BB->getSinglePredecessor() == BB) && "Block is not dead!"); 38 TerminatorInst *BBTerm = BB->getTerminator(); 39 40 // Loop through all of our successors and make sure they know that one 41 // of their predecessors is going away. 42 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) 43 BBTerm->getSuccessor(i)->removePredecessor(BB); 44 45 // Zap all the instructions in the block. 46 while (!BB->empty()) { 47 Instruction &I = BB->back(); 48 // If this instruction is used, replace uses with an arbitrary value. 49 // Because control flow can't get here, we don't care what we replace the 50 // value with. Note that since this block is unreachable, and all values 51 // contained within it must dominate their uses, that all uses will 52 // eventually be removed (they are themselves dead). 53 if (!I.use_empty()) 54 I.replaceAllUsesWith(UndefValue::get(I.getType())); 55 BB->getInstList().pop_back(); 56 } 57 58 // Zap the block! 59 BB->eraseFromParent(); 60} 61 62/// FoldSingleEntryPHINodes - We know that BB has one predecessor. If there are 63/// any single-entry PHI nodes in it, fold them away. This handles the case 64/// when all entries to the PHI nodes in a block are guaranteed equal, such as 65/// when the block has exactly one predecessor. 66void llvm::FoldSingleEntryPHINodes(BasicBlock *BB) { 67 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 68 if (PN->getIncomingValue(0) != PN) 69 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 70 else 71 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 72 PN->eraseFromParent(); 73 } 74} 75 76 77/// DeleteDeadPHIs - Examine each PHI in the given block and delete it if it 78/// is dead. Also recursively delete any operands that become dead as 79/// a result. This includes tracing the def-use list from the PHI to see if 80/// it is ultimately unused or if it reaches an unused cycle. 81void llvm::DeleteDeadPHIs(BasicBlock *BB) { 82 // Recursively deleting a PHI may cause multiple PHIs to be deleted 83 // or RAUW'd undef, so use an array of WeakVH for the PHIs to delete. 84 SmallVector<WeakVH, 8> PHIs; 85 for (BasicBlock::iterator I = BB->begin(); 86 PHINode *PN = dyn_cast<PHINode>(I); ++I) 87 PHIs.push_back(PN); 88 89 for (unsigned i = 0, e = PHIs.size(); i != e; ++i) 90 if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*())) 91 RecursivelyDeleteDeadPHINode(PN); 92} 93 94/// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor, 95/// if possible. The return value indicates success or failure. 96bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, Pass *P) { 97 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 98 // Can't merge the entry block. Don't merge away blocks who have their 99 // address taken: this is a bug if the predecessor block is the entry node 100 // (because we'd end up taking the address of the entry) and undesirable in 101 // any case. 102 if (pred_begin(BB) == pred_end(BB) || 103 BB->hasAddressTaken()) return false; 104 105 BasicBlock *PredBB = *PI++; 106 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 107 if (*PI != PredBB) { 108 PredBB = 0; // There are multiple different predecessors... 109 break; 110 } 111 112 // Can't merge if there are multiple predecessors. 113 if (!PredBB) return false; 114 // Don't break self-loops. 115 if (PredBB == BB) return false; 116 // Don't break invokes. 117 if (isa<InvokeInst>(PredBB->getTerminator())) return false; 118 119 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB)); 120 BasicBlock* OnlySucc = BB; 121 for (; SI != SE; ++SI) 122 if (*SI != OnlySucc) { 123 OnlySucc = 0; // There are multiple distinct successors! 124 break; 125 } 126 127 // Can't merge if there are multiple successors. 128 if (!OnlySucc) return false; 129 130 // Can't merge if there is PHI loop. 131 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) { 132 if (PHINode *PN = dyn_cast<PHINode>(BI)) { 133 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 134 if (PN->getIncomingValue(i) == PN) 135 return false; 136 } else 137 break; 138 } 139 140 // Begin by getting rid of unneeded PHIs. 141 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 142 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 143 BB->getInstList().pop_front(); // Delete the phi node... 144 } 145 146 // Delete the unconditional branch from the predecessor... 147 PredBB->getInstList().pop_back(); 148 149 // Move all definitions in the successor to the predecessor... 150 PredBB->getInstList().splice(PredBB->end(), BB->getInstList()); 151 152 // Make all PHI nodes that referred to BB now refer to Pred as their 153 // source... 154 BB->replaceAllUsesWith(PredBB); 155 156 // Inherit predecessors name if it exists. 157 if (!PredBB->hasName()) 158 PredBB->takeName(BB); 159 160 // Finally, erase the old block and update dominator info. 161 if (P) { 162 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) { 163 DomTreeNode* DTN = DT->getNode(BB); 164 DomTreeNode* PredDTN = DT->getNode(PredBB); 165 166 if (DTN) { 167 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end()); 168 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(), 169 DE = Children.end(); DI != DE; ++DI) 170 DT->changeImmediateDominator(*DI, PredDTN); 171 172 DT->eraseNode(BB); 173 } 174 } 175 } 176 177 BB->eraseFromParent(); 178 179 180 return true; 181} 182 183/// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI) 184/// with a value, then remove and delete the original instruction. 185/// 186void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL, 187 BasicBlock::iterator &BI, Value *V) { 188 Instruction &I = *BI; 189 // Replaces all of the uses of the instruction with uses of the value 190 I.replaceAllUsesWith(V); 191 192 // Make sure to propagate a name if there is one already. 193 if (I.hasName() && !V->hasName()) 194 V->takeName(&I); 195 196 // Delete the unnecessary instruction now... 197 BI = BIL.erase(BI); 198} 199 200 201/// ReplaceInstWithInst - Replace the instruction specified by BI with the 202/// instruction specified by I. The original instruction is deleted and BI is 203/// updated to point to the new instruction. 204/// 205void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL, 206 BasicBlock::iterator &BI, Instruction *I) { 207 assert(I->getParent() == 0 && 208 "ReplaceInstWithInst: Instruction already inserted into basic block!"); 209 210 // Insert the new instruction into the basic block... 211 BasicBlock::iterator New = BIL.insert(BI, I); 212 213 // Replace all uses of the old instruction, and delete it. 214 ReplaceInstWithValue(BIL, BI, I); 215 216 // Move BI back to point to the newly inserted instruction 217 BI = New; 218} 219 220/// ReplaceInstWithInst - Replace the instruction specified by From with the 221/// instruction specified by To. 222/// 223void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) { 224 BasicBlock::iterator BI(From); 225 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To); 226} 227 228/// RemoveSuccessor - Change the specified terminator instruction such that its 229/// successor SuccNum no longer exists. Because this reduces the outgoing 230/// degree of the current basic block, the actual terminator instruction itself 231/// may have to be changed. In the case where the last successor of the block 232/// is deleted, a return instruction is inserted in its place which can cause a 233/// surprising change in program behavior if it is not expected. 234/// 235void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) { 236 assert(SuccNum < TI->getNumSuccessors() && 237 "Trying to remove a nonexistant successor!"); 238 239 // If our old successor block contains any PHI nodes, remove the entry in the 240 // PHI nodes that comes from this branch... 241 // 242 BasicBlock *BB = TI->getParent(); 243 TI->getSuccessor(SuccNum)->removePredecessor(BB); 244 245 TerminatorInst *NewTI = 0; 246 switch (TI->getOpcode()) { 247 case Instruction::Br: 248 // If this is a conditional branch... convert to unconditional branch. 249 if (TI->getNumSuccessors() == 2) { 250 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum)); 251 } else { // Otherwise convert to a return instruction... 252 Value *RetVal = 0; 253 254 // Create a value to return... if the function doesn't return null... 255 if (BB->getParent()->getReturnType() != Type::getVoidTy(TI->getContext())) 256 RetVal = Constant::getNullValue(BB->getParent()->getReturnType()); 257 258 // Create the return... 259 NewTI = ReturnInst::Create(TI->getContext(), RetVal); 260 } 261 break; 262 263 case Instruction::Invoke: // Should convert to call 264 case Instruction::Switch: // Should remove entry 265 default: 266 case Instruction::Ret: // Cannot happen, has no successors! 267 llvm_unreachable("Unhandled terminator instruction type in RemoveSuccessor!"); 268 } 269 270 if (NewTI) // If it's a different instruction, replace. 271 ReplaceInstWithInst(TI, NewTI); 272} 273 274/// SplitEdge - Split the edge connecting specified block. Pass P must 275/// not be NULL. 276BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) { 277 TerminatorInst *LatchTerm = BB->getTerminator(); 278 unsigned SuccNum = 0; 279#ifndef NDEBUG 280 unsigned e = LatchTerm->getNumSuccessors(); 281#endif 282 for (unsigned i = 0; ; ++i) { 283 assert(i != e && "Didn't find edge?"); 284 if (LatchTerm->getSuccessor(i) == Succ) { 285 SuccNum = i; 286 break; 287 } 288 } 289 290 // If this is a critical edge, let SplitCriticalEdge do it. 291 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P)) 292 return LatchTerm->getSuccessor(SuccNum); 293 294 // If the edge isn't critical, then BB has a single successor or Succ has a 295 // single pred. Split the block. 296 BasicBlock::iterator SplitPoint; 297 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 298 // If the successor only has a single pred, split the top of the successor 299 // block. 300 assert(SP == BB && "CFG broken"); 301 SP = NULL; 302 return SplitBlock(Succ, Succ->begin(), P); 303 } else { 304 // Otherwise, if BB has a single successor, split it at the bottom of the 305 // block. 306 assert(BB->getTerminator()->getNumSuccessors() == 1 && 307 "Should have a single succ!"); 308 return SplitBlock(BB, BB->getTerminator(), P); 309 } 310} 311 312/// SplitBlock - Split the specified block at the specified instruction - every 313/// thing before SplitPt stays in Old and everything starting with SplitPt moves 314/// to a new block. The two blocks are joined by an unconditional branch and 315/// the loop info is updated. 316/// 317BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) { 318 BasicBlock::iterator SplitIt = SplitPt; 319 while (isa<PHINode>(SplitIt)) 320 ++SplitIt; 321 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 322 323 // The new block lives in whichever loop the old one did. This preserves 324 // LCSSA as well, because we force the split point to be after any PHI nodes. 325 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>()) 326 if (Loop *L = LI->getLoopFor(Old)) 327 L->addBasicBlockToLoop(New, LI->getBase()); 328 329 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) 330 { 331 // Old dominates New. New node domiantes all other nodes dominated by Old. 332 DomTreeNode *OldNode = DT->getNode(Old); 333 std::vector<DomTreeNode *> Children; 334 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end(); 335 I != E; ++I) 336 Children.push_back(*I); 337 338 DomTreeNode *NewNode = DT->addNewBlock(New,Old); 339 340 for (std::vector<DomTreeNode *>::iterator I = Children.begin(), 341 E = Children.end(); I != E; ++I) 342 DT->changeImmediateDominator(*I, NewNode); 343 } 344 345 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) 346 DF->splitBlock(Old); 347 348 return New; 349} 350 351 352/// SplitBlockPredecessors - This method transforms BB by introducing a new 353/// basic block into the function, and moving some of the predecessors of BB to 354/// be predecessors of the new block. The new predecessors are indicated by the 355/// Preds array, which has NumPreds elements in it. The new block is given a 356/// suffix of 'Suffix'. 357/// 358/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree, 359/// DominanceFrontier, LoopInfo, and LCCSA but no other analyses. 360/// In particular, it does not preserve LoopSimplify (because it's 361/// complicated to handle the case where one of the edges being split 362/// is an exit of a loop with other exits). 363/// 364BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB, 365 BasicBlock *const *Preds, 366 unsigned NumPreds, const char *Suffix, 367 Pass *P) { 368 // Create new basic block, insert right before the original block. 369 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), BB->getName()+Suffix, 370 BB->getParent(), BB); 371 372 // The new block unconditionally branches to the old block. 373 BranchInst *BI = BranchInst::Create(BB, NewBB); 374 375 LoopInfo *LI = P ? P->getAnalysisIfAvailable<LoopInfo>() : 0; 376 Loop *L = LI ? LI->getLoopFor(BB) : 0; 377 bool PreserveLCSSA = P->mustPreserveAnalysisID(LCSSAID); 378 379 // Move the edges from Preds to point to NewBB instead of BB. 380 // While here, if we need to preserve loop analyses, collect 381 // some information about how this split will affect loops. 382 bool HasLoopExit = false; 383 bool IsLoopEntry = !!L; 384 bool SplitMakesNewLoopHeader = false; 385 for (unsigned i = 0; i != NumPreds; ++i) { 386 // This is slightly more strict than necessary; the minimum requirement 387 // is that there be no more than one indirectbr branching to BB. And 388 // all BlockAddress uses would need to be updated. 389 assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) && 390 "Cannot split an edge from an IndirectBrInst"); 391 392 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB); 393 394 if (LI) { 395 // If we need to preserve LCSSA, determine if any of 396 // the preds is a loop exit. 397 if (PreserveLCSSA) 398 if (Loop *PL = LI->getLoopFor(Preds[i])) 399 if (!PL->contains(BB)) 400 HasLoopExit = true; 401 // If we need to preserve LoopInfo, note whether any of the 402 // preds crosses an interesting loop boundary. 403 if (L) { 404 if (L->contains(Preds[i])) 405 IsLoopEntry = false; 406 else 407 SplitMakesNewLoopHeader = true; 408 } 409 } 410 } 411 412 // Update dominator tree and dominator frontier if available. 413 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0; 414 if (DT) 415 DT->splitBlock(NewBB); 416 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0) 417 DF->splitBlock(NewBB); 418 419 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI 420 // node becomes an incoming value for BB's phi node. However, if the Preds 421 // list is empty, we need to insert dummy entries into the PHI nodes in BB to 422 // account for the newly created predecessor. 423 if (NumPreds == 0) { 424 // Insert dummy values as the incoming value. 425 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) 426 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB); 427 return NewBB; 428 } 429 430 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0; 431 432 if (L) { 433 if (IsLoopEntry) { 434 // Add the new block to the nearest enclosing loop (and not an 435 // adjacent loop). To find this, examine each of the predecessors and 436 // determine which loops enclose them, and select the most-nested loop 437 // which contains the loop containing the block being split. 438 Loop *InnermostPredLoop = 0; 439 for (unsigned i = 0; i != NumPreds; ++i) 440 if (Loop *PredLoop = LI->getLoopFor(Preds[i])) { 441 // Seek a loop which actually contains the block being split (to 442 // avoid adjacent loops). 443 while (PredLoop && !PredLoop->contains(BB)) 444 PredLoop = PredLoop->getParentLoop(); 445 // Select the most-nested of these loops which contains the block. 446 if (PredLoop && 447 PredLoop->contains(BB) && 448 (!InnermostPredLoop || 449 InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth())) 450 InnermostPredLoop = PredLoop; 451 } 452 if (InnermostPredLoop) 453 InnermostPredLoop->addBasicBlockToLoop(NewBB, LI->getBase()); 454 } else { 455 L->addBasicBlockToLoop(NewBB, LI->getBase()); 456 if (SplitMakesNewLoopHeader) 457 L->moveToHeader(NewBB); 458 } 459 } 460 461 // Otherwise, create a new PHI node in NewBB for each PHI node in BB. 462 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) { 463 PHINode *PN = cast<PHINode>(I++); 464 465 // Check to see if all of the values coming in are the same. If so, we 466 // don't need to create a new PHI node, unless it's needed for LCSSA. 467 Value *InVal = 0; 468 if (!HasLoopExit) { 469 InVal = PN->getIncomingValueForBlock(Preds[0]); 470 for (unsigned i = 1; i != NumPreds; ++i) 471 if (InVal != PN->getIncomingValueForBlock(Preds[i])) { 472 InVal = 0; 473 break; 474 } 475 } 476 477 if (InVal) { 478 // If all incoming values for the new PHI would be the same, just don't 479 // make a new PHI. Instead, just remove the incoming values from the old 480 // PHI. 481 for (unsigned i = 0; i != NumPreds; ++i) 482 PN->removeIncomingValue(Preds[i], false); 483 } else { 484 // If the values coming into the block are not the same, we need a PHI. 485 // Create the new PHI node, insert it into NewBB at the end of the block 486 PHINode *NewPHI = 487 PHINode::Create(PN->getType(), PN->getName()+".ph", BI); 488 if (AA) AA->copyValue(PN, NewPHI); 489 490 // Move all of the PHI values for 'Preds' to the new PHI. 491 for (unsigned i = 0; i != NumPreds; ++i) { 492 Value *V = PN->removeIncomingValue(Preds[i], false); 493 NewPHI->addIncoming(V, Preds[i]); 494 } 495 InVal = NewPHI; 496 } 497 498 // Add an incoming value to the PHI node in the loop for the preheader 499 // edge. 500 PN->addIncoming(InVal, NewBB); 501 } 502 503 return NewBB; 504} 505 506/// FindFunctionBackedges - Analyze the specified function to find all of the 507/// loop backedges in the function and return them. This is a relatively cheap 508/// (compared to computing dominators and loop info) analysis. 509/// 510/// The output is added to Result, as pairs of <from,to> edge info. 511void llvm::FindFunctionBackedges(const Function &F, 512 SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) { 513 const BasicBlock *BB = &F.getEntryBlock(); 514 if (succ_begin(BB) == succ_end(BB)) 515 return; 516 517 SmallPtrSet<const BasicBlock*, 8> Visited; 518 SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack; 519 SmallPtrSet<const BasicBlock*, 8> InStack; 520 521 Visited.insert(BB); 522 VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 523 InStack.insert(BB); 524 do { 525 std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back(); 526 const BasicBlock *ParentBB = Top.first; 527 succ_const_iterator &I = Top.second; 528 529 bool FoundNew = false; 530 while (I != succ_end(ParentBB)) { 531 BB = *I++; 532 if (Visited.insert(BB)) { 533 FoundNew = true; 534 break; 535 } 536 // Successor is in VisitStack, it's a back edge. 537 if (InStack.count(BB)) 538 Result.push_back(std::make_pair(ParentBB, BB)); 539 } 540 541 if (FoundNew) { 542 // Go down one level if there is a unvisited successor. 543 InStack.insert(BB); 544 VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 545 } else { 546 // Go up one level. 547 InStack.erase(VisitStack.pop_back_val().first); 548 } 549 } while (!VisitStack.empty()); 550 551 552} 553 554 555 556/// AreEquivalentAddressValues - Test if A and B will obviously have the same 557/// value. This includes recognizing that %t0 and %t1 will have the same 558/// value in code like this: 559/// %t0 = getelementptr \@a, 0, 3 560/// store i32 0, i32* %t0 561/// %t1 = getelementptr \@a, 0, 3 562/// %t2 = load i32* %t1 563/// 564static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 565 // Test if the values are trivially equivalent. 566 if (A == B) return true; 567 568 // Test if the values come from identical arithmetic instructions. 569 // Use isIdenticalToWhenDefined instead of isIdenticalTo because 570 // this function is only used when one address use dominates the 571 // other, which means that they'll always either have the same 572 // value or one of them will have an undefined value. 573 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 574 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 575 if (const Instruction *BI = dyn_cast<Instruction>(B)) 576 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 577 return true; 578 579 // Otherwise they may not be equivalent. 580 return false; 581} 582 583/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 584/// instruction before ScanFrom) checking to see if we have the value at the 585/// memory address *Ptr locally available within a small number of instructions. 586/// If the value is available, return it. 587/// 588/// If not, return the iterator for the last validated instruction that the 589/// value would be live through. If we scanned the entire block and didn't find 590/// something that invalidates *Ptr or provides it, ScanFrom would be left at 591/// begin() and this returns null. ScanFrom could also be left 592/// 593/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 594/// it is set to 0, it will scan the whole block. You can also optionally 595/// specify an alias analysis implementation, which makes this more precise. 596Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 597 BasicBlock::iterator &ScanFrom, 598 unsigned MaxInstsToScan, 599 AliasAnalysis *AA) { 600 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 601 602 // If we're using alias analysis to disambiguate get the size of *Ptr. 603 unsigned AccessSize = 0; 604 if (AA) { 605 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 606 AccessSize = AA->getTypeStoreSize(AccessTy); 607 } 608 609 while (ScanFrom != ScanBB->begin()) { 610 // We must ignore debug info directives when counting (otherwise they 611 // would affect codegen). 612 Instruction *Inst = --ScanFrom; 613 if (isa<DbgInfoIntrinsic>(Inst)) 614 continue; 615 // We skip pointer-to-pointer bitcasts, which are NOPs. 616 // It is necessary for correctness to skip those that feed into a 617 // llvm.dbg.declare, as these are not present when debugging is off. 618 if (isa<BitCastInst>(Inst) && isa<PointerType>(Inst->getType())) 619 continue; 620 621 // Restore ScanFrom to expected value in case next test succeeds 622 ScanFrom++; 623 624 // Don't scan huge blocks. 625 if (MaxInstsToScan-- == 0) return 0; 626 627 --ScanFrom; 628 // If this is a load of Ptr, the loaded value is available. 629 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 630 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 631 return LI; 632 633 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 634 // If this is a store through Ptr, the value is available! 635 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 636 return SI->getOperand(0); 637 638 // If Ptr is an alloca and this is a store to a different alloca, ignore 639 // the store. This is a trivial form of alias analysis that is important 640 // for reg2mem'd code. 641 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 642 (isa<AllocaInst>(SI->getOperand(1)) || 643 isa<GlobalVariable>(SI->getOperand(1)))) 644 continue; 645 646 // If we have alias analysis and it says the store won't modify the loaded 647 // value, ignore the store. 648 if (AA && 649 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 650 continue; 651 652 // Otherwise the store that may or may not alias the pointer, bail out. 653 ++ScanFrom; 654 return 0; 655 } 656 657 // If this is some other instruction that may clobber Ptr, bail out. 658 if (Inst->mayWriteToMemory()) { 659 // If alias analysis claims that it really won't modify the load, 660 // ignore it. 661 if (AA && 662 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 663 continue; 664 665 // May modify the pointer, bail out. 666 ++ScanFrom; 667 return 0; 668 } 669 } 670 671 // Got to the start of the block, we didn't find it, but are done for this 672 // block. 673 return 0; 674} 675 676