BasicBlockUtils.cpp revision 882029269e0cf4b497993b8e9a754429ef035fac
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/LLVMContext.h" 20#include "llvm/Constant.h" 21#include "llvm/Type.h" 22#include "llvm/Analysis/AliasAnalysis.h" 23#include "llvm/Analysis/LoopInfo.h" 24#include "llvm/Analysis/Dominators.h" 25#include "llvm/Target/TargetData.h" 26#include "llvm/Transforms/Utils/Local.h" 27#include "llvm/Transforms/Scalar.h" 28#include "llvm/Support/ErrorHandling.h" 29#include "llvm/Support/ValueHandle.h" 30#include <algorithm> 31using namespace llvm; 32 33/// DeleteDeadBlock - Delete the specified block, which must have no 34/// predecessors. 35void llvm::DeleteDeadBlock(BasicBlock *BB) { 36 assert((pred_begin(BB) == pred_end(BB) || 37 // Can delete self loop. 38 BB->getSinglePredecessor() == BB) && "Block is not dead!"); 39 TerminatorInst *BBTerm = BB->getTerminator(); 40 41 // Loop through all of our successors and make sure they know that one 42 // of their predecessors is going away. 43 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) 44 BBTerm->getSuccessor(i)->removePredecessor(BB); 45 46 // Zap all the instructions in the block. 47 while (!BB->empty()) { 48 Instruction &I = BB->back(); 49 // If this instruction is used, replace uses with an arbitrary value. 50 // Because control flow can't get here, we don't care what we replace the 51 // value with. Note that since this block is unreachable, and all values 52 // contained within it must dominate their uses, that all uses will 53 // eventually be removed (they are themselves dead). 54 if (!I.use_empty()) 55 I.replaceAllUsesWith(UndefValue::get(I.getType())); 56 BB->getInstList().pop_back(); 57 } 58 59 // Zap the block! 60 BB->eraseFromParent(); 61} 62 63/// FoldSingleEntryPHINodes - We know that BB has one predecessor. If there are 64/// any single-entry PHI nodes in it, fold them away. This handles the case 65/// when all entries to the PHI nodes in a block are guaranteed equal, such as 66/// when the block has exactly one predecessor. 67void llvm::FoldSingleEntryPHINodes(BasicBlock *BB) { 68 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 69 if (PN->getIncomingValue(0) != PN) 70 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 71 else 72 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 73 PN->eraseFromParent(); 74 } 75} 76 77 78/// DeleteDeadPHIs - Examine each PHI in the given block and delete it if it 79/// is dead. Also recursively delete any operands that become dead as 80/// a result. This includes tracing the def-use list from the PHI to see if 81/// it is ultimately unused or if it reaches an unused cycle. 82void llvm::DeleteDeadPHIs(BasicBlock *BB) { 83 // Recursively deleting a PHI may cause multiple PHIs to be deleted 84 // or RAUW'd undef, so use an array of WeakVH for the PHIs to delete. 85 SmallVector<WeakVH, 8> PHIs; 86 for (BasicBlock::iterator I = BB->begin(); 87 PHINode *PN = dyn_cast<PHINode>(I); ++I) 88 PHIs.push_back(PN); 89 90 for (unsigned i = 0, e = PHIs.size(); i != e; ++i) 91 if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*())) 92 RecursivelyDeleteDeadPHINode(PN); 93} 94 95/// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor, 96/// if possible. The return value indicates success or failure. 97bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, Pass *P) { 98 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 99 // Can't merge the entry block. Don't merge away blocks who have their 100 // address taken: this is a bug if the predecessor block is the entry node 101 // (because we'd end up taking the address of the entry) and undesirable in 102 // any case. 103 if (pred_begin(BB) == pred_end(BB) || 104 BB->hasAddressTaken()) return false; 105 106 BasicBlock *PredBB = *PI++; 107 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 108 if (*PI != PredBB) { 109 PredBB = 0; // There are multiple different predecessors... 110 break; 111 } 112 113 // Can't merge if there are multiple predecessors. 114 if (!PredBB) return false; 115 // Don't break self-loops. 116 if (PredBB == BB) return false; 117 // Don't break invokes. 118 if (isa<InvokeInst>(PredBB->getTerminator())) return false; 119 120 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB)); 121 BasicBlock* OnlySucc = BB; 122 for (; SI != SE; ++SI) 123 if (*SI != OnlySucc) { 124 OnlySucc = 0; // There are multiple distinct successors! 125 break; 126 } 127 128 // Can't merge if there are multiple successors. 129 if (!OnlySucc) return false; 130 131 // Can't merge if there is PHI loop. 132 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) { 133 if (PHINode *PN = dyn_cast<PHINode>(BI)) { 134 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 135 if (PN->getIncomingValue(i) == PN) 136 return false; 137 } else 138 break; 139 } 140 141 // Begin by getting rid of unneeded PHIs. 142 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 143 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 144 BB->getInstList().pop_front(); // Delete the phi node... 145 } 146 147 // Delete the unconditional branch from the predecessor... 148 PredBB->getInstList().pop_back(); 149 150 // Move all definitions in the successor to the predecessor... 151 PredBB->getInstList().splice(PredBB->end(), BB->getInstList()); 152 153 // Make all PHI nodes that referred to BB now refer to Pred as their 154 // source... 155 BB->replaceAllUsesWith(PredBB); 156 157 // Inherit predecessors name if it exists. 158 if (!PredBB->hasName()) 159 PredBB->takeName(BB); 160 161 // Finally, erase the old block and update dominator info. 162 if (P) { 163 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) { 164 DomTreeNode* DTN = DT->getNode(BB); 165 DomTreeNode* PredDTN = DT->getNode(PredBB); 166 167 if (DTN) { 168 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end()); 169 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(), 170 DE = Children.end(); DI != DE; ++DI) 171 DT->changeImmediateDominator(*DI, PredDTN); 172 173 DT->eraseNode(BB); 174 } 175 } 176 } 177 178 BB->eraseFromParent(); 179 180 181 return true; 182} 183 184/// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI) 185/// with a value, then remove and delete the original instruction. 186/// 187void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL, 188 BasicBlock::iterator &BI, Value *V) { 189 Instruction &I = *BI; 190 // Replaces all of the uses of the instruction with uses of the value 191 I.replaceAllUsesWith(V); 192 193 // Make sure to propagate a name if there is one already. 194 if (I.hasName() && !V->hasName()) 195 V->takeName(&I); 196 197 // Delete the unnecessary instruction now... 198 BI = BIL.erase(BI); 199} 200 201 202/// ReplaceInstWithInst - Replace the instruction specified by BI with the 203/// instruction specified by I. The original instruction is deleted and BI is 204/// updated to point to the new instruction. 205/// 206void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL, 207 BasicBlock::iterator &BI, Instruction *I) { 208 assert(I->getParent() == 0 && 209 "ReplaceInstWithInst: Instruction already inserted into basic block!"); 210 211 // Insert the new instruction into the basic block... 212 BasicBlock::iterator New = BIL.insert(BI, I); 213 214 // Replace all uses of the old instruction, and delete it. 215 ReplaceInstWithValue(BIL, BI, I); 216 217 // Move BI back to point to the newly inserted instruction 218 BI = New; 219} 220 221/// ReplaceInstWithInst - Replace the instruction specified by From with the 222/// instruction specified by To. 223/// 224void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) { 225 BasicBlock::iterator BI(From); 226 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To); 227} 228 229/// RemoveSuccessor - Change the specified terminator instruction such that its 230/// successor SuccNum no longer exists. Because this reduces the outgoing 231/// degree of the current basic block, the actual terminator instruction itself 232/// may have to be changed. In the case where the last successor of the block 233/// is deleted, a return instruction is inserted in its place which can cause a 234/// surprising change in program behavior if it is not expected. 235/// 236void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) { 237 assert(SuccNum < TI->getNumSuccessors() && 238 "Trying to remove a nonexistant successor!"); 239 240 // If our old successor block contains any PHI nodes, remove the entry in the 241 // PHI nodes that comes from this branch... 242 // 243 BasicBlock *BB = TI->getParent(); 244 TI->getSuccessor(SuccNum)->removePredecessor(BB); 245 246 TerminatorInst *NewTI = 0; 247 switch (TI->getOpcode()) { 248 case Instruction::Br: 249 // If this is a conditional branch... convert to unconditional branch. 250 if (TI->getNumSuccessors() == 2) { 251 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum)); 252 } else { // Otherwise convert to a return instruction... 253 Value *RetVal = 0; 254 255 // Create a value to return... if the function doesn't return null... 256 if (BB->getParent()->getReturnType() != Type::getVoidTy(TI->getContext())) 257 RetVal = Constant::getNullValue(BB->getParent()->getReturnType()); 258 259 // Create the return... 260 NewTI = ReturnInst::Create(TI->getContext(), RetVal); 261 } 262 break; 263 264 case Instruction::Invoke: // Should convert to call 265 case Instruction::Switch: // Should remove entry 266 default: 267 case Instruction::Ret: // Cannot happen, has no successors! 268 llvm_unreachable("Unhandled terminator instruction type in RemoveSuccessor!"); 269 } 270 271 if (NewTI) // If it's a different instruction, replace. 272 ReplaceInstWithInst(TI, NewTI); 273} 274 275/// SplitEdge - Split the edge connecting specified block. Pass P must 276/// not be NULL. 277BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) { 278 assert(!isa<IndirectBrInst>(BB->getTerminator()) && 279 "Cannot split an edge from an IndirectBrInst"); 280 TerminatorInst *LatchTerm = BB->getTerminator(); 281 unsigned SuccNum = 0; 282#ifndef NDEBUG 283 unsigned e = LatchTerm->getNumSuccessors(); 284#endif 285 for (unsigned i = 0; ; ++i) { 286 assert(i != e && "Didn't find edge?"); 287 if (LatchTerm->getSuccessor(i) == Succ) { 288 SuccNum = i; 289 break; 290 } 291 } 292 293 // If this is a critical edge, let SplitCriticalEdge do it. 294 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P)) 295 return LatchTerm->getSuccessor(SuccNum); 296 297 // If the edge isn't critical, then BB has a single successor or Succ has a 298 // single pred. Split the block. 299 BasicBlock::iterator SplitPoint; 300 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 301 // If the successor only has a single pred, split the top of the successor 302 // block. 303 assert(SP == BB && "CFG broken"); 304 SP = NULL; 305 return SplitBlock(Succ, Succ->begin(), P); 306 } else { 307 // Otherwise, if BB has a single successor, split it at the bottom of the 308 // block. 309 assert(BB->getTerminator()->getNumSuccessors() == 1 && 310 "Should have a single succ!"); 311 return SplitBlock(BB, BB->getTerminator(), P); 312 } 313} 314 315/// SplitBlock - Split the specified block at the specified instruction - every 316/// thing before SplitPt stays in Old and everything starting with SplitPt moves 317/// to a new block. The two blocks are joined by an unconditional branch and 318/// the loop info is updated. 319/// 320BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) { 321 BasicBlock::iterator SplitIt = SplitPt; 322 while (isa<PHINode>(SplitIt)) 323 ++SplitIt; 324 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 325 326 // The new block lives in whichever loop the old one did. This preserves 327 // LCSSA as well, because we force the split point to be after any PHI nodes. 328 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>()) 329 if (Loop *L = LI->getLoopFor(Old)) 330 L->addBasicBlockToLoop(New, LI->getBase()); 331 332 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>()) 333 { 334 // Old dominates New. New node domiantes all other nodes dominated by Old. 335 DomTreeNode *OldNode = DT->getNode(Old); 336 std::vector<DomTreeNode *> Children; 337 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end(); 338 I != E; ++I) 339 Children.push_back(*I); 340 341 DomTreeNode *NewNode = DT->addNewBlock(New,Old); 342 343 for (std::vector<DomTreeNode *>::iterator I = Children.begin(), 344 E = Children.end(); I != E; ++I) 345 DT->changeImmediateDominator(*I, NewNode); 346 } 347 348 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>()) 349 DF->splitBlock(Old); 350 351 return New; 352} 353 354 355/// SplitBlockPredecessors - This method transforms BB by introducing a new 356/// basic block into the function, and moving some of the predecessors of BB to 357/// be predecessors of the new block. The new predecessors are indicated by the 358/// Preds array, which has NumPreds elements in it. The new block is given a 359/// suffix of 'Suffix'. 360/// 361/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree, 362/// DominanceFrontier, LoopInfo, and LCCSA but no other analyses. 363/// In particular, it does not preserve LoopSimplify (because it's 364/// complicated to handle the case where one of the edges being split 365/// is an exit of a loop with other exits). 366/// 367BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB, 368 BasicBlock *const *Preds, 369 unsigned NumPreds, const char *Suffix, 370 Pass *P) { 371 // Create new basic block, insert right before the original block. 372 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), BB->getName()+Suffix, 373 BB->getParent(), BB); 374 375 // The new block unconditionally branches to the old block. 376 BranchInst *BI = BranchInst::Create(BB, NewBB); 377 378 LoopInfo *LI = P ? P->getAnalysisIfAvailable<LoopInfo>() : 0; 379 Loop *L = LI ? LI->getLoopFor(BB) : 0; 380 bool PreserveLCSSA = P->mustPreserveAnalysisID(LCSSAID); 381 382 // Move the edges from Preds to point to NewBB instead of BB. 383 // While here, if we need to preserve loop analyses, collect 384 // some information about how this split will affect loops. 385 bool HasLoopExit = false; 386 bool IsLoopEntry = !!L; 387 bool SplitMakesNewLoopHeader = false; 388 for (unsigned i = 0; i != NumPreds; ++i) { 389 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB); 390 391 if (LI) { 392 // If we need to preserve LCSSA, determine if any of 393 // the preds is a loop exit. 394 if (PreserveLCSSA) 395 if (Loop *PL = LI->getLoopFor(Preds[i])) 396 if (!PL->contains(BB)) 397 HasLoopExit = true; 398 // If we need to preserve LoopInfo, note whether any of the 399 // preds crosses an interesting loop boundary. 400 if (L) { 401 if (L->contains(Preds[i])) 402 IsLoopEntry = false; 403 else 404 SplitMakesNewLoopHeader = true; 405 } 406 } 407 } 408 409 // Update dominator tree and dominator frontier if available. 410 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0; 411 if (DT) 412 DT->splitBlock(NewBB); 413 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0) 414 DF->splitBlock(NewBB); 415 416 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI 417 // node becomes an incoming value for BB's phi node. However, if the Preds 418 // list is empty, we need to insert dummy entries into the PHI nodes in BB to 419 // account for the newly created predecessor. 420 if (NumPreds == 0) { 421 // Insert dummy values as the incoming value. 422 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) 423 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB); 424 return NewBB; 425 } 426 427 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0; 428 429 if (L) { 430 if (IsLoopEntry) { 431 // Add the new block to the nearest enclosing loop (and not an 432 // adjacent loop). To find this, examine each of the predecessors and 433 // determine which loops enclose them, and select the most-nested loop 434 // which contains the loop containing the block being split. 435 Loop *InnermostPredLoop = 0; 436 for (unsigned i = 0; i != NumPreds; ++i) 437 if (Loop *PredLoop = LI->getLoopFor(Preds[i])) { 438 // Seek a loop which actually contains the block being split (to 439 // avoid adjacent loops). 440 while (PredLoop && !PredLoop->contains(BB)) 441 PredLoop = PredLoop->getParentLoop(); 442 // Select the most-nested of these loops which contains the block. 443 if (PredLoop && 444 PredLoop->contains(BB) && 445 (!InnermostPredLoop || 446 InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth())) 447 InnermostPredLoop = PredLoop; 448 } 449 if (InnermostPredLoop) 450 InnermostPredLoop->addBasicBlockToLoop(NewBB, LI->getBase()); 451 } else { 452 L->addBasicBlockToLoop(NewBB, LI->getBase()); 453 if (SplitMakesNewLoopHeader) 454 L->moveToHeader(NewBB); 455 } 456 } 457 458 // Otherwise, create a new PHI node in NewBB for each PHI node in BB. 459 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) { 460 PHINode *PN = cast<PHINode>(I++); 461 462 // Check to see if all of the values coming in are the same. If so, we 463 // don't need to create a new PHI node, unless it's needed for LCSSA. 464 Value *InVal = 0; 465 if (!HasLoopExit) { 466 InVal = PN->getIncomingValueForBlock(Preds[0]); 467 for (unsigned i = 1; i != NumPreds; ++i) 468 if (InVal != PN->getIncomingValueForBlock(Preds[i])) { 469 InVal = 0; 470 break; 471 } 472 } 473 474 if (InVal) { 475 // If all incoming values for the new PHI would be the same, just don't 476 // make a new PHI. Instead, just remove the incoming values from the old 477 // PHI. 478 for (unsigned i = 0; i != NumPreds; ++i) 479 PN->removeIncomingValue(Preds[i], false); 480 } else { 481 // If the values coming into the block are not the same, we need a PHI. 482 // Create the new PHI node, insert it into NewBB at the end of the block 483 PHINode *NewPHI = 484 PHINode::Create(PN->getType(), PN->getName()+".ph", BI); 485 if (AA) AA->copyValue(PN, NewPHI); 486 487 // Move all of the PHI values for 'Preds' to the new PHI. 488 for (unsigned i = 0; i != NumPreds; ++i) { 489 Value *V = PN->removeIncomingValue(Preds[i], false); 490 NewPHI->addIncoming(V, Preds[i]); 491 } 492 InVal = NewPHI; 493 } 494 495 // Add an incoming value to the PHI node in the loop for the preheader 496 // edge. 497 PN->addIncoming(InVal, NewBB); 498 } 499 500 return NewBB; 501} 502 503/// FindFunctionBackedges - Analyze the specified function to find all of the 504/// loop backedges in the function and return them. This is a relatively cheap 505/// (compared to computing dominators and loop info) analysis. 506/// 507/// The output is added to Result, as pairs of <from,to> edge info. 508void llvm::FindFunctionBackedges(const Function &F, 509 SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) { 510 const BasicBlock *BB = &F.getEntryBlock(); 511 if (succ_begin(BB) == succ_end(BB)) 512 return; 513 514 SmallPtrSet<const BasicBlock*, 8> Visited; 515 SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack; 516 SmallPtrSet<const BasicBlock*, 8> InStack; 517 518 Visited.insert(BB); 519 VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 520 InStack.insert(BB); 521 do { 522 std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back(); 523 const BasicBlock *ParentBB = Top.first; 524 succ_const_iterator &I = Top.second; 525 526 bool FoundNew = false; 527 while (I != succ_end(ParentBB)) { 528 BB = *I++; 529 if (Visited.insert(BB)) { 530 FoundNew = true; 531 break; 532 } 533 // Successor is in VisitStack, it's a back edge. 534 if (InStack.count(BB)) 535 Result.push_back(std::make_pair(ParentBB, BB)); 536 } 537 538 if (FoundNew) { 539 // Go down one level if there is a unvisited successor. 540 InStack.insert(BB); 541 VisitStack.push_back(std::make_pair(BB, succ_begin(BB))); 542 } else { 543 // Go up one level. 544 InStack.erase(VisitStack.pop_back_val().first); 545 } 546 } while (!VisitStack.empty()); 547 548 549} 550 551 552 553/// AreEquivalentAddressValues - Test if A and B will obviously have the same 554/// value. This includes recognizing that %t0 and %t1 will have the same 555/// value in code like this: 556/// %t0 = getelementptr \@a, 0, 3 557/// store i32 0, i32* %t0 558/// %t1 = getelementptr \@a, 0, 3 559/// %t2 = load i32* %t1 560/// 561static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 562 // Test if the values are trivially equivalent. 563 if (A == B) return true; 564 565 // Test if the values come from identical arithmetic instructions. 566 // Use isIdenticalToWhenDefined instead of isIdenticalTo because 567 // this function is only used when one address use dominates the 568 // other, which means that they'll always either have the same 569 // value or one of them will have an undefined value. 570 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 571 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 572 if (const Instruction *BI = dyn_cast<Instruction>(B)) 573 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 574 return true; 575 576 // Otherwise they may not be equivalent. 577 return false; 578} 579 580/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 581/// instruction before ScanFrom) checking to see if we have the value at the 582/// memory address *Ptr locally available within a small number of instructions. 583/// If the value is available, return it. 584/// 585/// If not, return the iterator for the last validated instruction that the 586/// value would be live through. If we scanned the entire block and didn't find 587/// something that invalidates *Ptr or provides it, ScanFrom would be left at 588/// begin() and this returns null. ScanFrom could also be left 589/// 590/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 591/// it is set to 0, it will scan the whole block. You can also optionally 592/// specify an alias analysis implementation, which makes this more precise. 593Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 594 BasicBlock::iterator &ScanFrom, 595 unsigned MaxInstsToScan, 596 AliasAnalysis *AA) { 597 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 598 599 // If we're using alias analysis to disambiguate get the size of *Ptr. 600 unsigned AccessSize = 0; 601 if (AA) { 602 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 603 AccessSize = AA->getTypeStoreSize(AccessTy); 604 } 605 606 while (ScanFrom != ScanBB->begin()) { 607 // We must ignore debug info directives when counting (otherwise they 608 // would affect codegen). 609 Instruction *Inst = --ScanFrom; 610 if (isa<DbgInfoIntrinsic>(Inst)) 611 continue; 612 // We skip pointer-to-pointer bitcasts, which are NOPs. 613 // It is necessary for correctness to skip those that feed into a 614 // llvm.dbg.declare, as these are not present when debugging is off. 615 if (isa<BitCastInst>(Inst) && isa<PointerType>(Inst->getType())) 616 continue; 617 618 // Restore ScanFrom to expected value in case next test succeeds 619 ScanFrom++; 620 621 // Don't scan huge blocks. 622 if (MaxInstsToScan-- == 0) return 0; 623 624 --ScanFrom; 625 // If this is a load of Ptr, the loaded value is available. 626 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 627 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 628 return LI; 629 630 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 631 // If this is a store through Ptr, the value is available! 632 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 633 return SI->getOperand(0); 634 635 // If Ptr is an alloca and this is a store to a different alloca, ignore 636 // the store. This is a trivial form of alias analysis that is important 637 // for reg2mem'd code. 638 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 639 (isa<AllocaInst>(SI->getOperand(1)) || 640 isa<GlobalVariable>(SI->getOperand(1)))) 641 continue; 642 643 // If we have alias analysis and it says the store won't modify the loaded 644 // value, ignore the store. 645 if (AA && 646 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 647 continue; 648 649 // Otherwise the store that may or may not alias the pointer, bail out. 650 ++ScanFrom; 651 return 0; 652 } 653 654 // If this is some other instruction that may clobber Ptr, bail out. 655 if (Inst->mayWriteToMemory()) { 656 // If alias analysis claims that it really won't modify the load, 657 // ignore it. 658 if (AA && 659 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 660 continue; 661 662 // May modify the pointer, bail out. 663 ++ScanFrom; 664 return 0; 665 } 666 } 667 668 // Got to the start of the block, we didn't find it, but are done for this 669 // block. 670 return 0; 671} 672 673/// CopyPrecedingStopPoint - If I is immediately preceded by a StopPoint, 674/// make a copy of the stoppoint before InsertPos (presumably before copying 675/// or moving I). 676void llvm::CopyPrecedingStopPoint(Instruction *I, 677 BasicBlock::iterator InsertPos) { 678 if (I != I->getParent()->begin()) { 679 BasicBlock::iterator BBI = I; --BBI; 680 if (DbgStopPointInst *DSPI = dyn_cast<DbgStopPointInst>(BBI)) { 681 CallInst *newDSPI = cast<CallInst>(DSPI->clone()); 682 newDSPI->insertBefore(InsertPos); 683 } 684 } 685} 686