BasicBlockUtils.cpp revision ae23daf63afccd68be965ff4f7acafa818d76aaa
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. 81bool 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 bool Changed = false; 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 Changed |= RecursivelyDeleteDeadPHINode(PN); 93 94 return Changed; 95} 96 97/// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor, 98/// if possible. The return value indicates success or failure. 99bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, Pass *P) { 100 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 101 // Can't merge the entry block. Don't merge away blocks who have their 102 // address taken: this is a bug if the predecessor block is the entry node 103 // (because we'd end up taking the address of the entry) and undesirable in 104 // any case. 105 if (pred_begin(BB) == pred_end(BB) || 106 BB->hasAddressTaken()) return false; 107 108 BasicBlock *PredBB = *PI++; 109 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 110 if (*PI != PredBB) { 111 PredBB = 0; // There are multiple different predecessors... 112 break; 113 } 114 115 // Can't merge if there are multiple predecessors. 116 if (!PredBB) return false; 117 // Don't break self-loops. 118 if (PredBB == BB) return false; 119 // Don't break invokes. 120 if (isa<InvokeInst>(PredBB->getTerminator())) return false; 121 122 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB)); 123 BasicBlock* OnlySucc = BB; 124 for (; SI != SE; ++SI) 125 if (*SI != OnlySucc) { 126 OnlySucc = 0; // There are multiple distinct successors! 127 break; 128 } 129 130 // Can't merge if there are multiple successors. 131 if (!OnlySucc) return false; 132 133 // Can't merge if there is PHI loop. 134 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) { 135 if (PHINode *PN = dyn_cast<PHINode>(BI)) { 136 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 137 if (PN->getIncomingValue(i) == PN) 138 return false; 139 } else 140 break; 141 } 142 143 // Begin by getting rid of unneeded PHIs. 144 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 145 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 146 BB->getInstList().pop_front(); // Delete the phi node... 147 } 148 149 // Delete the unconditional branch from the predecessor... 150 PredBB->getInstList().pop_back(); 151 152 // Move all definitions in the successor to the predecessor... 153 PredBB->getInstList().splice(PredBB->end(), BB->getInstList()); 154 155 // Make all PHI nodes that referred to BB now refer to Pred as their 156 // source... 157 BB->replaceAllUsesWith(PredBB); 158 159 // Inherit predecessors name if it exists. 160 if (!PredBB->hasName()) 161 PredBB->takeName(BB); 162 163 // Finally, erase the old block and update dominator info. 164 if (P) { 165 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) { 166 DomTreeNode* DTN = DT->getNode(BB); 167 DomTreeNode* PredDTN = DT->getNode(PredBB); 168 169 if (DTN) { 170 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end()); 171 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(), 172 DE = Children.end(); DI != DE; ++DI) 173 DT->changeImmediateDominator(*DI, PredDTN); 174 175 DT->eraseNode(BB); 176 } 177 } 178 } 179 180 BB->eraseFromParent(); 181 182 183 return true; 184} 185 186/// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI) 187/// with a value, then remove and delete the original instruction. 188/// 189void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL, 190 BasicBlock::iterator &BI, Value *V) { 191 Instruction &I = *BI; 192 // Replaces all of the uses of the instruction with uses of the value 193 I.replaceAllUsesWith(V); 194 195 // Make sure to propagate a name if there is one already. 196 if (I.hasName() && !V->hasName()) 197 V->takeName(&I); 198 199 // Delete the unnecessary instruction now... 200 BI = BIL.erase(BI); 201} 202 203 204/// ReplaceInstWithInst - Replace the instruction specified by BI with the 205/// instruction specified by I. The original instruction is deleted and BI is 206/// updated to point to the new instruction. 207/// 208void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL, 209 BasicBlock::iterator &BI, Instruction *I) { 210 assert(I->getParent() == 0 && 211 "ReplaceInstWithInst: Instruction already inserted into basic block!"); 212 213 // Insert the new instruction into the basic block... 214 BasicBlock::iterator New = BIL.insert(BI, I); 215 216 // Replace all uses of the old instruction, and delete it. 217 ReplaceInstWithValue(BIL, BI, I); 218 219 // Move BI back to point to the newly inserted instruction 220 BI = New; 221} 222 223/// ReplaceInstWithInst - Replace the instruction specified by From with the 224/// instruction specified by To. 225/// 226void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) { 227 BasicBlock::iterator BI(From); 228 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To); 229} 230 231/// RemoveSuccessor - Change the specified terminator instruction such that its 232/// successor SuccNum no longer exists. Because this reduces the outgoing 233/// degree of the current basic block, the actual terminator instruction itself 234/// may have to be changed. In the case where the last successor of the block 235/// is deleted, a return instruction is inserted in its place which can cause a 236/// surprising change in program behavior if it is not expected. 237/// 238void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) { 239 assert(SuccNum < TI->getNumSuccessors() && 240 "Trying to remove a nonexistant successor!"); 241 242 // If our old successor block contains any PHI nodes, remove the entry in the 243 // PHI nodes that comes from this branch... 244 // 245 BasicBlock *BB = TI->getParent(); 246 TI->getSuccessor(SuccNum)->removePredecessor(BB); 247 248 TerminatorInst *NewTI = 0; 249 switch (TI->getOpcode()) { 250 case Instruction::Br: 251 // If this is a conditional branch... convert to unconditional branch. 252 if (TI->getNumSuccessors() == 2) { 253 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum)); 254 } else { // Otherwise convert to a return instruction... 255 Value *RetVal = 0; 256 257 // Create a value to return... if the function doesn't return null... 258 if (!BB->getParent()->getReturnType()->isVoidTy()) 259 RetVal = Constant::getNullValue(BB->getParent()->getReturnType()); 260 261 // Create the return... 262 NewTI = ReturnInst::Create(TI->getContext(), RetVal); 263 } 264 break; 265 266 case Instruction::Invoke: // Should convert to call 267 case Instruction::Switch: // Should remove entry 268 default: 269 case Instruction::Ret: // Cannot happen, has no successors! 270 llvm_unreachable("Unhandled terminator instruction type in RemoveSuccessor!"); 271 } 272 273 if (NewTI) // If it's a different instruction, replace. 274 ReplaceInstWithInst(TI, NewTI); 275} 276 277/// GetSuccessorNumber - Search for the specified successor of basic block BB 278/// and return its position in the terminator instruction's list of 279/// successors. It is an error to call this with a block that is not a 280/// successor. 281unsigned llvm::GetSuccessorNumber(BasicBlock *BB, BasicBlock *Succ) { 282 TerminatorInst *Term = BB->getTerminator(); 283#ifndef NDEBUG 284 unsigned e = Term->getNumSuccessors(); 285#endif 286 for (unsigned i = 0; ; ++i) { 287 assert(i != e && "Didn't find edge?"); 288 if (Term->getSuccessor(i) == Succ) 289 return i; 290 } 291 return 0; 292} 293 294/// SplitEdge - Split the edge connecting specified block. Pass P must 295/// not be NULL. 296BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) { 297 unsigned SuccNum = GetSuccessorNumber(BB, Succ); 298 299 // If this is a critical edge, let SplitCriticalEdge do it. 300 TerminatorInst *LatchTerm = BB->getTerminator(); 301 if (SplitCriticalEdge(LatchTerm, SuccNum, P)) 302 return LatchTerm->getSuccessor(SuccNum); 303 304 // If the edge isn't critical, then BB has a single successor or Succ has a 305 // single pred. Split the block. 306 BasicBlock::iterator SplitPoint; 307 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 308 // If the successor only has a single pred, split the top of the successor 309 // block. 310 assert(SP == BB && "CFG broken"); 311 SP = NULL; 312 return SplitBlock(Succ, Succ->begin(), P); 313 } else { 314 // Otherwise, if BB has a single successor, split it at the bottom of the 315 // block. 316 assert(BB->getTerminator()->getNumSuccessors() == 1 && 317 "Should have a single succ!"); 318 return SplitBlock(BB, BB->getTerminator(), P); 319 } 320} 321 322/// SplitBlock - Split the specified block at the specified instruction - every 323/// thing before SplitPt stays in Old and everything starting with SplitPt moves 324/// to a new block. The two blocks are joined by an unconditional branch and 325/// the loop info is updated. 326/// 327BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) { 328 BasicBlock::iterator SplitIt = SplitPt; 329 while (isa<PHINode>(SplitIt)) 330 ++SplitIt; 331 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 332 333 // The new block lives in whichever loop the old one did. This preserves 334 // LCSSA as well, because we force the split point to be after any PHI nodes. 335 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>()) 336 if (Loop *L = LI->getLoopFor(Old)) 337 L->addBasicBlockToLoop(New, LI->getBase()); 338 339 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) 340 { 341 // Old dominates New. New node domiantes all other nodes dominated by Old. 342 DomTreeNode *OldNode = DT->getNode(Old); 343 std::vector<DomTreeNode *> Children; 344 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end(); 345 I != E; ++I) 346 Children.push_back(*I); 347 348 DomTreeNode *NewNode = DT->addNewBlock(New,Old); 349 350 for (std::vector<DomTreeNode *>::iterator I = Children.begin(), 351 E = Children.end(); I != E; ++I) 352 DT->changeImmediateDominator(*I, NewNode); 353 } 354 355 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) 356 DF->splitBlock(Old); 357 358 return New; 359} 360 361 362/// SplitBlockPredecessors - This method transforms BB by introducing a new 363/// basic block into the function, and moving some of the predecessors of BB to 364/// be predecessors of the new block. The new predecessors are indicated by the 365/// Preds array, which has NumPreds elements in it. The new block is given a 366/// suffix of 'Suffix'. 367/// 368/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree, 369/// DominanceFrontier, LoopInfo, and LCCSA but no other analyses. 370/// In particular, it does not preserve LoopSimplify (because it's 371/// complicated to handle the case where one of the edges being split 372/// is an exit of a loop with other exits). 373/// 374BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB, 375 BasicBlock *const *Preds, 376 unsigned NumPreds, const char *Suffix, 377 Pass *P) { 378 // Create new basic block, insert right before the original block. 379 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), BB->getName()+Suffix, 380 BB->getParent(), BB); 381 382 // The new block unconditionally branches to the old block. 383 BranchInst *BI = BranchInst::Create(BB, NewBB); 384 385 LoopInfo *LI = P ? P->getAnalysisIfAvailable<LoopInfo>() : 0; 386 Loop *L = LI ? LI->getLoopFor(BB) : 0; 387 bool PreserveLCSSA = P->mustPreserveAnalysisID(LCSSAID); 388 389 // Move the edges from Preds to point to NewBB instead of BB. 390 // While here, if we need to preserve loop analyses, collect 391 // some information about how this split will affect loops. 392 bool HasLoopExit = false; 393 bool IsLoopEntry = !!L; 394 bool SplitMakesNewLoopHeader = false; 395 for (unsigned i = 0; i != NumPreds; ++i) { 396 // This is slightly more strict than necessary; the minimum requirement 397 // is that there be no more than one indirectbr branching to BB. And 398 // all BlockAddress uses would need to be updated. 399 assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) && 400 "Cannot split an edge from an IndirectBrInst"); 401 402 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB); 403 404 if (LI) { 405 // If we need to preserve LCSSA, determine if any of 406 // the preds is a loop exit. 407 if (PreserveLCSSA) 408 if (Loop *PL = LI->getLoopFor(Preds[i])) 409 if (!PL->contains(BB)) 410 HasLoopExit = true; 411 // If we need to preserve LoopInfo, note whether any of the 412 // preds crosses an interesting loop boundary. 413 if (L) { 414 if (L->contains(Preds[i])) 415 IsLoopEntry = false; 416 else 417 SplitMakesNewLoopHeader = true; 418 } 419 } 420 } 421 422 // Update dominator tree and dominator frontier if available. 423 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0; 424 if (DT) 425 DT->splitBlock(NewBB); 426 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0) 427 DF->splitBlock(NewBB); 428 429 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI 430 // node becomes an incoming value for BB's phi node. However, if the Preds 431 // list is empty, we need to insert dummy entries into the PHI nodes in BB to 432 // account for the newly created predecessor. 433 if (NumPreds == 0) { 434 // Insert dummy values as the incoming value. 435 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) 436 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB); 437 return NewBB; 438 } 439 440 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0; 441 442 if (L) { 443 if (IsLoopEntry) { 444 // Add the new block to the nearest enclosing loop (and not an 445 // adjacent loop). To find this, examine each of the predecessors and 446 // determine which loops enclose them, and select the most-nested loop 447 // which contains the loop containing the block being split. 448 Loop *InnermostPredLoop = 0; 449 for (unsigned i = 0; i != NumPreds; ++i) 450 if (Loop *PredLoop = LI->getLoopFor(Preds[i])) { 451 // Seek a loop which actually contains the block being split (to 452 // avoid adjacent loops). 453 while (PredLoop && !PredLoop->contains(BB)) 454 PredLoop = PredLoop->getParentLoop(); 455 // Select the most-nested of these loops which contains the block. 456 if (PredLoop && 457 PredLoop->contains(BB) && 458 (!InnermostPredLoop || 459 InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth())) 460 InnermostPredLoop = PredLoop; 461 } 462 if (InnermostPredLoop) 463 InnermostPredLoop->addBasicBlockToLoop(NewBB, LI->getBase()); 464 } else { 465 L->addBasicBlockToLoop(NewBB, LI->getBase()); 466 if (SplitMakesNewLoopHeader) 467 L->moveToHeader(NewBB); 468 } 469 } 470 471 // Otherwise, create a new PHI node in NewBB for each PHI node in BB. 472 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) { 473 PHINode *PN = cast<PHINode>(I++); 474 475 // Check to see if all of the values coming in are the same. If so, we 476 // don't need to create a new PHI node, unless it's needed for LCSSA. 477 Value *InVal = 0; 478 if (!HasLoopExit) { 479 InVal = PN->getIncomingValueForBlock(Preds[0]); 480 for (unsigned i = 1; i != NumPreds; ++i) 481 if (InVal != PN->getIncomingValueForBlock(Preds[i])) { 482 InVal = 0; 483 break; 484 } 485 } 486 487 if (InVal) { 488 // If all incoming values for the new PHI would be the same, just don't 489 // make a new PHI. Instead, just remove the incoming values from the old 490 // PHI. 491 for (unsigned i = 0; i != NumPreds; ++i) 492 PN->removeIncomingValue(Preds[i], false); 493 } else { 494 // If the values coming into the block are not the same, we need a PHI. 495 // Create the new PHI node, insert it into NewBB at the end of the block 496 PHINode *NewPHI = 497 PHINode::Create(PN->getType(), PN->getName()+".ph", BI); 498 if (AA) AA->copyValue(PN, NewPHI); 499 500 // Move all of the PHI values for 'Preds' to the new PHI. 501 for (unsigned i = 0; i != NumPreds; ++i) { 502 Value *V = PN->removeIncomingValue(Preds[i], false); 503 NewPHI->addIncoming(V, Preds[i]); 504 } 505 InVal = NewPHI; 506 } 507 508 // Add an incoming value to the PHI node in the loop for the preheader 509 // edge. 510 PN->addIncoming(InVal, NewBB); 511 } 512 513 return NewBB; 514} 515 516/// FindFunctionBackedges - Analyze the specified function to find all of the 517/// loop backedges in the function and return them. This is a relatively cheap 518/// (compared to computing dominators and loop info) analysis. 519/// 520/// The output is added to Result, as pairs of <from,to> edge info. 521void llvm::FindFunctionBackedges(const Function &F, 522 SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) { 523 const BasicBlock *BB = &F.getEntryBlock(); 524 if (succ_begin(BB) == succ_end(BB)) 525 return; 526 527 SmallPtrSet<const BasicBlock*, 8> Visited; 528 SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack; 529 SmallPtrSet<const BasicBlock*, 8> InStack; 530 531 Visited.insert(BB); 532 VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 533 InStack.insert(BB); 534 do { 535 std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back(); 536 const BasicBlock *ParentBB = Top.first; 537 succ_const_iterator &I = Top.second; 538 539 bool FoundNew = false; 540 while (I != succ_end(ParentBB)) { 541 BB = *I++; 542 if (Visited.insert(BB)) { 543 FoundNew = true; 544 break; 545 } 546 // Successor is in VisitStack, it's a back edge. 547 if (InStack.count(BB)) 548 Result.push_back(std::make_pair(ParentBB, BB)); 549 } 550 551 if (FoundNew) { 552 // Go down one level if there is a unvisited successor. 553 InStack.insert(BB); 554 VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 555 } else { 556 // Go up one level. 557 InStack.erase(VisitStack.pop_back_val().first); 558 } 559 } while (!VisitStack.empty()); 560 561 562} 563 564 565 566/// AreEquivalentAddressValues - Test if A and B will obviously have the same 567/// value. This includes recognizing that %t0 and %t1 will have the same 568/// value in code like this: 569/// %t0 = getelementptr \@a, 0, 3 570/// store i32 0, i32* %t0 571/// %t1 = getelementptr \@a, 0, 3 572/// %t2 = load i32* %t1 573/// 574static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 575 // Test if the values are trivially equivalent. 576 if (A == B) return true; 577 578 // Test if the values come from identical arithmetic instructions. 579 // Use isIdenticalToWhenDefined instead of isIdenticalTo because 580 // this function is only used when one address use dominates the 581 // other, which means that they'll always either have the same 582 // value or one of them will have an undefined value. 583 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 584 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 585 if (const Instruction *BI = dyn_cast<Instruction>(B)) 586 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 587 return true; 588 589 // Otherwise they may not be equivalent. 590 return false; 591} 592 593/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 594/// instruction before ScanFrom) checking to see if we have the value at the 595/// memory address *Ptr locally available within a small number of instructions. 596/// If the value is available, return it. 597/// 598/// If not, return the iterator for the last validated instruction that the 599/// value would be live through. If we scanned the entire block and didn't find 600/// something that invalidates *Ptr or provides it, ScanFrom would be left at 601/// begin() and this returns null. ScanFrom could also be left 602/// 603/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 604/// it is set to 0, it will scan the whole block. You can also optionally 605/// specify an alias analysis implementation, which makes this more precise. 606Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 607 BasicBlock::iterator &ScanFrom, 608 unsigned MaxInstsToScan, 609 AliasAnalysis *AA) { 610 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 611 612 // If we're using alias analysis to disambiguate get the size of *Ptr. 613 unsigned AccessSize = 0; 614 if (AA) { 615 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 616 AccessSize = AA->getTypeStoreSize(AccessTy); 617 } 618 619 while (ScanFrom != ScanBB->begin()) { 620 // We must ignore debug info directives when counting (otherwise they 621 // would affect codegen). 622 Instruction *Inst = --ScanFrom; 623 if (isa<DbgInfoIntrinsic>(Inst)) 624 continue; 625 626 // Restore ScanFrom to expected value in case next test succeeds 627 ScanFrom++; 628 629 // Don't scan huge blocks. 630 if (MaxInstsToScan-- == 0) return 0; 631 632 --ScanFrom; 633 // If this is a load of Ptr, the loaded value is available. 634 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 635 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 636 return LI; 637 638 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 639 // If this is a store through Ptr, the value is available! 640 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 641 return SI->getOperand(0); 642 643 // If Ptr is an alloca and this is a store to a different alloca, ignore 644 // the store. This is a trivial form of alias analysis that is important 645 // for reg2mem'd code. 646 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 647 (isa<AllocaInst>(SI->getOperand(1)) || 648 isa<GlobalVariable>(SI->getOperand(1)))) 649 continue; 650 651 // If we have alias analysis and it says the store won't modify the loaded 652 // value, ignore the store. 653 if (AA && 654 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 655 continue; 656 657 // Otherwise the store that may or may not alias the pointer, bail out. 658 ++ScanFrom; 659 return 0; 660 } 661 662 // If this is some other instruction that may clobber Ptr, bail out. 663 if (Inst->mayWriteToMemory()) { 664 // If alias analysis claims that it really won't modify the load, 665 // ignore it. 666 if (AA && 667 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 668 continue; 669 670 // May modify the pointer, bail out. 671 ++ScanFrom; 672 return 0; 673 } 674 } 675 676 // Got to the start of the block, we didn't find it, but are done for this 677 // block. 678 return 0; 679} 680 681