JumpThreading.cpp revision 40dd12e7080c5df253fd70b468368e3144a43c0c
1//===- JumpThreading.cpp - Thread control through conditional blocks ------===// 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 file implements the Jump Threading pass. 11// 12//===----------------------------------------------------------------------===// 13 14#define DEBUG_TYPE "jump-threading" 15#include "llvm/Transforms/Scalar.h" 16#include "llvm/IntrinsicInst.h" 17#include "llvm/LLVMContext.h" 18#include "llvm/Pass.h" 19#include "llvm/Analysis/ConstantFolding.h" 20#include "llvm/Transforms/Utils/BasicBlockUtils.h" 21#include "llvm/Transforms/Utils/Local.h" 22#include "llvm/Transforms/Utils/SSAUpdater.h" 23#include "llvm/Target/TargetData.h" 24#include "llvm/ADT/DenseMap.h" 25#include "llvm/ADT/Statistic.h" 26#include "llvm/ADT/STLExtras.h" 27#include "llvm/ADT/SmallPtrSet.h" 28#include "llvm/ADT/SmallSet.h" 29#include "llvm/Support/CommandLine.h" 30#include "llvm/Support/Debug.h" 31#include "llvm/Support/raw_ostream.h" 32using namespace llvm; 33 34STATISTIC(NumThreads, "Number of jumps threaded"); 35STATISTIC(NumFolds, "Number of terminators folded"); 36STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi"); 37 38static cl::opt<unsigned> 39Threshold("jump-threading-threshold", 40 cl::desc("Max block size to duplicate for jump threading"), 41 cl::init(6), cl::Hidden); 42 43namespace { 44 /// This pass performs 'jump threading', which looks at blocks that have 45 /// multiple predecessors and multiple successors. If one or more of the 46 /// predecessors of the block can be proven to always jump to one of the 47 /// successors, we forward the edge from the predecessor to the successor by 48 /// duplicating the contents of this block. 49 /// 50 /// An example of when this can occur is code like this: 51 /// 52 /// if () { ... 53 /// X = 4; 54 /// } 55 /// if (X < 3) { 56 /// 57 /// In this case, the unconditional branch at the end of the first if can be 58 /// revectored to the false side of the second if. 59 /// 60 class JumpThreading : public FunctionPass { 61 TargetData *TD; 62#ifdef NDEBUG 63 SmallPtrSet<BasicBlock*, 16> LoopHeaders; 64#else 65 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; 66#endif 67 public: 68 static char ID; // Pass identification 69 JumpThreading() : FunctionPass(&ID) {} 70 71 bool runOnFunction(Function &F); 72 void FindLoopHeaders(Function &F); 73 74 bool ProcessBlock(BasicBlock *BB); 75 bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB); 76 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 77 BasicBlock *PredBB); 78 79 BasicBlock *FactorCommonPHIPreds(PHINode *PN, Value *Val); 80 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 81 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 82 83 bool ProcessJumpOnPHI(PHINode *PN); 84 bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); 85 bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); 86 87 bool SimplifyPartiallyRedundantLoad(LoadInst *LI); 88 }; 89} 90 91char JumpThreading::ID = 0; 92static RegisterPass<JumpThreading> 93X("jump-threading", "Jump Threading"); 94 95// Public interface to the Jump Threading pass 96FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 97 98/// runOnFunction - Top level algorithm. 99/// 100bool JumpThreading::runOnFunction(Function &F) { 101 DEBUG(errs() << "Jump threading on function '" << F.getName() << "'\n"); 102 TD = getAnalysisIfAvailable<TargetData>(); 103 104 FindLoopHeaders(F); 105 106 bool AnotherIteration = true, EverChanged = false; 107 while (AnotherIteration) { 108 AnotherIteration = false; 109 bool Changed = false; 110 for (Function::iterator I = F.begin(), E = F.end(); I != E;) { 111 BasicBlock *BB = I; 112 while (ProcessBlock(BB)) 113 Changed = true; 114 115 ++I; 116 117 // If the block is trivially dead, zap it. This eliminates the successor 118 // edges which simplifies the CFG. 119 if (pred_begin(BB) == pred_end(BB) && 120 BB != &BB->getParent()->getEntryBlock()) { 121 DEBUG(errs() << " JT: Deleting dead block '" << BB->getName() 122 << "' with terminator: " << *BB->getTerminator() << '\n'); 123 LoopHeaders.erase(BB); 124 DeleteDeadBlock(BB); 125 Changed = true; 126 } 127 } 128 AnotherIteration = Changed; 129 EverChanged |= Changed; 130 } 131 132 LoopHeaders.clear(); 133 return EverChanged; 134} 135 136/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 137/// thread across it. 138static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { 139 /// Ignore PHI nodes, these will be flattened when duplication happens. 140 BasicBlock::const_iterator I = BB->getFirstNonPHI(); 141 142 // Sum up the cost of each instruction until we get to the terminator. Don't 143 // include the terminator because the copy won't include it. 144 unsigned Size = 0; 145 for (; !isa<TerminatorInst>(I); ++I) { 146 // Debugger intrinsics don't incur code size. 147 if (isa<DbgInfoIntrinsic>(I)) continue; 148 149 // If this is a pointer->pointer bitcast, it is free. 150 if (isa<BitCastInst>(I) && isa<PointerType>(I->getType())) 151 continue; 152 153 // All other instructions count for at least one unit. 154 ++Size; 155 156 // Calls are more expensive. If they are non-intrinsic calls, we model them 157 // as having cost of 4. If they are a non-vector intrinsic, we model them 158 // as having cost of 2 total, and if they are a vector intrinsic, we model 159 // them as having cost 1. 160 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 161 if (!isa<IntrinsicInst>(CI)) 162 Size += 3; 163 else if (!isa<VectorType>(CI->getType())) 164 Size += 1; 165 } 166 } 167 168 // Threading through a switch statement is particularly profitable. If this 169 // block ends in a switch, decrease its cost to make it more likely to happen. 170 if (isa<SwitchInst>(I)) 171 Size = Size > 6 ? Size-6 : 0; 172 173 return Size; 174} 175 176 177 178/// FindLoopHeaders - We do not want jump threading to turn proper loop 179/// structures into irreducible loops. Doing this breaks up the loop nesting 180/// hierarchy and pessimizes later transformations. To prevent this from 181/// happening, we first have to find the loop headers. Here we approximate this 182/// by finding targets of backedges in the CFG. 183/// 184/// Note that there definitely are cases when we want to allow threading of 185/// edges across a loop header. For example, threading a jump from outside the 186/// loop (the preheader) to an exit block of the loop is definitely profitable. 187/// It is also almost always profitable to thread backedges from within the loop 188/// to exit blocks, and is often profitable to thread backedges to other blocks 189/// within the loop (forming a nested loop). This simple analysis is not rich 190/// enough to track all of these properties and keep it up-to-date as the CFG 191/// mutates, so we don't allow any of these transformations. 192/// 193void JumpThreading::FindLoopHeaders(Function &F) { 194 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; 195 FindFunctionBackedges(F, Edges); 196 197 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 198 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); 199} 200 201 202/// FactorCommonPHIPreds - If there are multiple preds with the same incoming 203/// value for the PHI, factor them together so we get one block to thread for 204/// the whole group. 205/// This is important for things like "phi i1 [true, true, false, true, x]" 206/// where we only need to clone the block for the true blocks once. 207/// 208BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Value *Val) { 209 SmallVector<BasicBlock*, 16> CommonPreds; 210 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 211 if (PN->getIncomingValue(i) == Val) 212 CommonPreds.push_back(PN->getIncomingBlock(i)); 213 214 if (CommonPreds.size() == 1) 215 return CommonPreds[0]; 216 217 DEBUG(errs() << " Factoring out " << CommonPreds.size() 218 << " common predecessors.\n"); 219 return SplitBlockPredecessors(PN->getParent(), 220 &CommonPreds[0], CommonPreds.size(), 221 ".thr_comm", this); 222} 223 224 225/// GetBestDestForBranchOnUndef - If we determine that the specified block ends 226/// in an undefined jump, decide which block is best to revector to. 227/// 228/// Since we can pick an arbitrary destination, we pick the successor with the 229/// fewest predecessors. This should reduce the in-degree of the others. 230/// 231static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) { 232 TerminatorInst *BBTerm = BB->getTerminator(); 233 unsigned MinSucc = 0; 234 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); 235 // Compute the successor with the minimum number of predecessors. 236 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 237 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { 238 TestBB = BBTerm->getSuccessor(i); 239 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 240 if (NumPreds < MinNumPreds) 241 MinSucc = i; 242 } 243 244 return MinSucc; 245} 246 247/// ProcessBlock - If there are any predecessors whose control can be threaded 248/// through to a successor, transform them now. 249bool JumpThreading::ProcessBlock(BasicBlock *BB) { 250 // If this block has a single predecessor, and if that pred has a single 251 // successor, merge the blocks. This encourages recursive jump threading 252 // because now the condition in this block can be threaded through 253 // predecessors of our predecessor block. 254 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) 255 if (SinglePred->getTerminator()->getNumSuccessors() == 1 && 256 SinglePred != BB) { 257 // If SinglePred was a loop header, BB becomes one. 258 if (LoopHeaders.erase(SinglePred)) 259 LoopHeaders.insert(BB); 260 261 // Remember if SinglePred was the entry block of the function. If so, we 262 // will need to move BB back to the entry position. 263 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); 264 MergeBasicBlockIntoOnlyPred(BB); 265 266 if (isEntry && BB != &BB->getParent()->getEntryBlock()) 267 BB->moveBefore(&BB->getParent()->getEntryBlock()); 268 return true; 269 } 270 271 // See if this block ends with a branch or switch. If so, see if the 272 // condition is a phi node. If so, and if an entry of the phi node is a 273 // constant, we can thread the block. 274 Value *Condition; 275 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 276 // Can't thread an unconditional jump. 277 if (BI->isUnconditional()) return false; 278 Condition = BI->getCondition(); 279 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) 280 Condition = SI->getCondition(); 281 else 282 return false; // Must be an invoke. 283 284 // If the terminator of this block is branching on a constant, simplify the 285 // terminator to an unconditional branch. This can occur due to threading in 286 // other blocks. 287 if (isa<ConstantInt>(Condition)) { 288 DEBUG(errs() << " In block '" << BB->getName() 289 << "' folding terminator: " << *BB->getTerminator() << '\n'); 290 ++NumFolds; 291 ConstantFoldTerminator(BB); 292 return true; 293 } 294 295 // If the terminator is branching on an undef, we can pick any of the 296 // successors to branch to. Let GetBestDestForJumpOnUndef decide. 297 if (isa<UndefValue>(Condition)) { 298 unsigned BestSucc = GetBestDestForJumpOnUndef(BB); 299 300 // Fold the branch/switch. 301 TerminatorInst *BBTerm = BB->getTerminator(); 302 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { 303 if (i == BestSucc) continue; 304 BBTerm->getSuccessor(i)->removePredecessor(BB); 305 } 306 307 DEBUG(errs() << " In block '" << BB->getName() 308 << "' folding undef terminator: " << *BBTerm << '\n'); 309 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm); 310 BBTerm->eraseFromParent(); 311 return true; 312 } 313 314 Instruction *CondInst = dyn_cast<Instruction>(Condition); 315 316 // If the condition is an instruction defined in another block, see if a 317 // predecessor has the same condition: 318 // br COND, BBX, BBY 319 // BBX: 320 // br COND, BBZ, BBW 321 if (!Condition->hasOneUse() && // Multiple uses. 322 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition. 323 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 324 if (isa<BranchInst>(BB->getTerminator())) { 325 for (; PI != E; ++PI) 326 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 327 if (PBI->isConditional() && PBI->getCondition() == Condition && 328 ProcessBranchOnDuplicateCond(*PI, BB)) 329 return true; 330 } else { 331 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator"); 332 for (; PI != E; ++PI) 333 if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator())) 334 if (PSI->getCondition() == Condition && 335 ProcessSwitchOnDuplicateCond(*PI, BB)) 336 return true; 337 } 338 } 339 340 // All the rest of our checks depend on the condition being an instruction. 341 if (CondInst == 0) 342 return false; 343 344 // See if this is a phi node in the current block. 345 if (PHINode *PN = dyn_cast<PHINode>(CondInst)) 346 if (PN->getParent() == BB) 347 return ProcessJumpOnPHI(PN); 348 349 // If this is a conditional branch whose condition is and/or of a phi, try to 350 // simplify it. 351 if ((CondInst->getOpcode() == Instruction::And || 352 CondInst->getOpcode() == Instruction::Or) && 353 isa<BranchInst>(BB->getTerminator()) && 354 ProcessBranchOnLogical(CondInst, BB, 355 CondInst->getOpcode() == Instruction::And)) 356 return true; 357 358 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { 359 if (isa<PHINode>(CondCmp->getOperand(0))) { 360 // If we have "br (phi != 42)" and the phi node has any constant values 361 // as operands, we can thread through this block. 362 // 363 // If we have "br (cmp phi, x)" and the phi node contains x such that the 364 // comparison uniquely identifies the branch target, we can thread 365 // through this block. 366 367 if (ProcessBranchOnCompare(CondCmp, BB)) 368 return true; 369 } 370 371 // If we have a comparison, loop over the predecessors to see if there is 372 // a condition with the same value. 373 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 374 for (; PI != E; ++PI) 375 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 376 if (PBI->isConditional() && *PI != BB) { 377 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) { 378 if (CI->getOperand(0) == CondCmp->getOperand(0) && 379 CI->getOperand(1) == CondCmp->getOperand(1) && 380 CI->getPredicate() == CondCmp->getPredicate()) { 381 // TODO: Could handle things like (x != 4) --> (x == 17) 382 if (ProcessBranchOnDuplicateCond(*PI, BB)) 383 return true; 384 } 385 } 386 } 387 } 388 389 // Check for some cases that are worth simplifying. Right now we want to look 390 // for loads that are used by a switch or by the condition for the branch. If 391 // we see one, check to see if it's partially redundant. If so, insert a PHI 392 // which can then be used to thread the values. 393 // 394 // This is particularly important because reg2mem inserts loads and stores all 395 // over the place, and this blocks jump threading if we don't zap them. 396 Value *SimplifyValue = CondInst; 397 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 398 if (isa<Constant>(CondCmp->getOperand(1))) 399 SimplifyValue = CondCmp->getOperand(0); 400 401 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 402 if (SimplifyPartiallyRedundantLoad(LI)) 403 return true; 404 405 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 406 // "(X == 4)" thread through this block. 407 408 return false; 409} 410 411/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that 412/// block that jump on exactly the same condition. This means that we almost 413/// always know the direction of the edge in the DESTBB: 414/// PREDBB: 415/// br COND, DESTBB, BBY 416/// DESTBB: 417/// br COND, BBZ, BBW 418/// 419/// If DESTBB has multiple predecessors, we can't just constant fold the branch 420/// in DESTBB, we have to thread over it. 421bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, 422 BasicBlock *BB) { 423 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); 424 425 // If both successors of PredBB go to DESTBB, we don't know anything. We can 426 // fold the branch to an unconditional one, which allows other recursive 427 // simplifications. 428 bool BranchDir; 429 if (PredBI->getSuccessor(1) != BB) 430 BranchDir = true; 431 else if (PredBI->getSuccessor(0) != BB) 432 BranchDir = false; 433 else { 434 DEBUG(errs() << " In block '" << PredBB->getName() 435 << "' folding terminator: " << *PredBB->getTerminator() << '\n'); 436 ++NumFolds; 437 ConstantFoldTerminator(PredBB); 438 return true; 439 } 440 441 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); 442 443 // If the dest block has one predecessor, just fix the branch condition to a 444 // constant and fold it. 445 if (BB->getSinglePredecessor()) { 446 DEBUG(errs() << " In block '" << BB->getName() 447 << "' folding condition to '" << BranchDir << "': " 448 << *BB->getTerminator() << '\n'); 449 ++NumFolds; 450 Value *OldCond = DestBI->getCondition(); 451 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()), 452 BranchDir)); 453 ConstantFoldTerminator(BB); 454 RecursivelyDeleteTriviallyDeadInstructions(OldCond); 455 return true; 456 } 457 458 459 // Next, figure out which successor we are threading to. 460 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); 461 462 // Ok, try to thread it! 463 return ThreadEdge(BB, PredBB, SuccBB); 464} 465 466/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that 467/// block that switch on exactly the same condition. This means that we almost 468/// always know the direction of the edge in the DESTBB: 469/// PREDBB: 470/// switch COND [... DESTBB, BBY ... ] 471/// DESTBB: 472/// switch COND [... BBZ, BBW ] 473/// 474/// Optimizing switches like this is very important, because simplifycfg builds 475/// switches out of repeated 'if' conditions. 476bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, 477 BasicBlock *DestBB) { 478 // Can't thread edge to self. 479 if (PredBB == DestBB) 480 return false; 481 482 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); 483 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); 484 485 // There are a variety of optimizations that we can potentially do on these 486 // blocks: we order them from most to least preferable. 487 488 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB 489 // directly to their destination. This does not introduce *any* code size 490 // growth. Skip debug info first. 491 BasicBlock::iterator BBI = DestBB->begin(); 492 while (isa<DbgInfoIntrinsic>(BBI)) 493 BBI++; 494 495 // FIXME: Thread if it just contains a PHI. 496 if (isa<SwitchInst>(BBI)) { 497 bool MadeChange = false; 498 // Ignore the default edge for now. 499 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { 500 ConstantInt *DestVal = DestSI->getCaseValue(i); 501 BasicBlock *DestSucc = DestSI->getSuccessor(i); 502 503 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if 504 // PredSI has an explicit case for it. If so, forward. If it is covered 505 // by the default case, we can't update PredSI. 506 unsigned PredCase = PredSI->findCaseValue(DestVal); 507 if (PredCase == 0) continue; 508 509 // If PredSI doesn't go to DestBB on this value, then it won't reach the 510 // case on this condition. 511 if (PredSI->getSuccessor(PredCase) != DestBB && 512 DestSI->getSuccessor(i) != DestBB) 513 continue; 514 515 // Otherwise, we're safe to make the change. Make sure that the edge from 516 // DestSI to DestSucc is not critical and has no PHI nodes. 517 DEBUG(errs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI); 518 DEBUG(errs() << "THROUGH: " << *DestSI); 519 520 // If the destination has PHI nodes, just split the edge for updating 521 // simplicity. 522 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ 523 SplitCriticalEdge(DestSI, i, this); 524 DestSucc = DestSI->getSuccessor(i); 525 } 526 FoldSingleEntryPHINodes(DestSucc); 527 PredSI->setSuccessor(PredCase, DestSucc); 528 MadeChange = true; 529 } 530 531 if (MadeChange) 532 return true; 533 } 534 535 return false; 536} 537 538 539/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 540/// load instruction, eliminate it by replacing it with a PHI node. This is an 541/// important optimization that encourages jump threading, and needs to be run 542/// interlaced with other jump threading tasks. 543bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 544 // Don't hack volatile loads. 545 if (LI->isVolatile()) return false; 546 547 // If the load is defined in a block with exactly one predecessor, it can't be 548 // partially redundant. 549 BasicBlock *LoadBB = LI->getParent(); 550 if (LoadBB->getSinglePredecessor()) 551 return false; 552 553 Value *LoadedPtr = LI->getOperand(0); 554 555 // If the loaded operand is defined in the LoadBB, it can't be available. 556 // FIXME: Could do PHI translation, that would be fun :) 557 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 558 if (PtrOp->getParent() == LoadBB) 559 return false; 560 561 // Scan a few instructions up from the load, to see if it is obviously live at 562 // the entry to its block. 563 BasicBlock::iterator BBIt = LI; 564 565 if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB, 566 BBIt, 6)) { 567 // If the value if the load is locally available within the block, just use 568 // it. This frequently occurs for reg2mem'd allocas. 569 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 570 571 // If the returned value is the load itself, replace with an undef. This can 572 // only happen in dead loops. 573 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); 574 LI->replaceAllUsesWith(AvailableVal); 575 LI->eraseFromParent(); 576 return true; 577 } 578 579 // Otherwise, if we scanned the whole block and got to the top of the block, 580 // we know the block is locally transparent to the load. If not, something 581 // might clobber its value. 582 if (BBIt != LoadBB->begin()) 583 return false; 584 585 586 SmallPtrSet<BasicBlock*, 8> PredsScanned; 587 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 588 AvailablePredsTy AvailablePreds; 589 BasicBlock *OneUnavailablePred = 0; 590 591 // If we got here, the loaded value is transparent through to the start of the 592 // block. Check to see if it is available in any of the predecessor blocks. 593 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 594 PI != PE; ++PI) { 595 BasicBlock *PredBB = *PI; 596 597 // If we already scanned this predecessor, skip it. 598 if (!PredsScanned.insert(PredBB)) 599 continue; 600 601 // Scan the predecessor to see if the value is available in the pred. 602 BBIt = PredBB->end(); 603 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); 604 if (!PredAvailable) { 605 OneUnavailablePred = PredBB; 606 continue; 607 } 608 609 // If so, this load is partially redundant. Remember this info so that we 610 // can create a PHI node. 611 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 612 } 613 614 // If the loaded value isn't available in any predecessor, it isn't partially 615 // redundant. 616 if (AvailablePreds.empty()) return false; 617 618 // Okay, the loaded value is available in at least one (and maybe all!) 619 // predecessors. If the value is unavailable in more than one unique 620 // predecessor, we want to insert a merge block for those common predecessors. 621 // This ensures that we only have to insert one reload, thus not increasing 622 // code size. 623 BasicBlock *UnavailablePred = 0; 624 625 // If there is exactly one predecessor where the value is unavailable, the 626 // already computed 'OneUnavailablePred' block is it. If it ends in an 627 // unconditional branch, we know that it isn't a critical edge. 628 if (PredsScanned.size() == AvailablePreds.size()+1 && 629 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 630 UnavailablePred = OneUnavailablePred; 631 } else if (PredsScanned.size() != AvailablePreds.size()) { 632 // Otherwise, we had multiple unavailable predecessors or we had a critical 633 // edge from the one. 634 SmallVector<BasicBlock*, 8> PredsToSplit; 635 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 636 637 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 638 AvailablePredSet.insert(AvailablePreds[i].first); 639 640 // Add all the unavailable predecessors to the PredsToSplit list. 641 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 642 PI != PE; ++PI) 643 if (!AvailablePredSet.count(*PI)) 644 PredsToSplit.push_back(*PI); 645 646 // Split them out to their own block. 647 UnavailablePred = 648 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), 649 "thread-split", this); 650 } 651 652 // If the value isn't available in all predecessors, then there will be 653 // exactly one where it isn't available. Insert a load on that edge and add 654 // it to the AvailablePreds list. 655 if (UnavailablePred) { 656 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 657 "Can't handle critical edge here!"); 658 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", 659 UnavailablePred->getTerminator()); 660 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 661 } 662 663 // Now we know that each predecessor of this block has a value in 664 // AvailablePreds, sort them for efficient access as we're walking the preds. 665 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 666 667 // Create a PHI node at the start of the block for the PRE'd load value. 668 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); 669 PN->takeName(LI); 670 671 // Insert new entries into the PHI for each predecessor. A single block may 672 // have multiple entries here. 673 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; 674 ++PI) { 675 AvailablePredsTy::iterator I = 676 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 677 std::make_pair(*PI, (Value*)0)); 678 679 assert(I != AvailablePreds.end() && I->first == *PI && 680 "Didn't find entry for predecessor!"); 681 682 PN->addIncoming(I->second, I->first); 683 } 684 685 //cerr << "PRE: " << *LI << *PN << "\n"; 686 687 LI->replaceAllUsesWith(PN); 688 LI->eraseFromParent(); 689 690 return true; 691} 692 693 694/// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in 695/// the current block. See if there are any simplifications we can do based on 696/// inputs to the phi node. 697/// 698bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { 699 BasicBlock *BB = PN->getParent(); 700 701 // See if the phi node has any constant integer or undef values. If so, we 702 // can determine where the corresponding predecessor will branch. 703 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 704 Value *PredVal = PN->getIncomingValue(i); 705 706 // Check to see if this input is a constant integer. If so, the direction 707 // of the branch is predictable. 708 if (ConstantInt *CI = dyn_cast<ConstantInt>(PredVal)) { 709 // Merge any common predecessors that will act the same. 710 BasicBlock *PredBB = FactorCommonPHIPreds(PN, CI); 711 712 BasicBlock *SuccBB; 713 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 714 SuccBB = BI->getSuccessor(CI->isZero()); 715 else { 716 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); 717 SuccBB = SI->getSuccessor(SI->findCaseValue(CI)); 718 } 719 720 // Ok, try to thread it! 721 return ThreadEdge(BB, PredBB, SuccBB); 722 } 723 724 // If the input is an undef, then it doesn't matter which way it will go. 725 // Pick an arbitrary dest and thread the edge. 726 if (UndefValue *UV = dyn_cast<UndefValue>(PredVal)) { 727 // Merge any common predecessors that will act the same. 728 BasicBlock *PredBB = FactorCommonPHIPreds(PN, UV); 729 BasicBlock *SuccBB = 730 BB->getTerminator()->getSuccessor(GetBestDestForJumpOnUndef(BB)); 731 732 // Ok, try to thread it! 733 return ThreadEdge(BB, PredBB, SuccBB); 734 } 735 } 736 737 // If the incoming values are all variables, we don't know the destination of 738 // any predecessors. However, if any of the predecessor blocks end in an 739 // unconditional branch, we can *duplicate* the jump into that block in order 740 // to further encourage jump threading and to eliminate cases where we have 741 // branch on a phi of an icmp (branch on icmp is much better). 742 743 // We don't want to do this tranformation for switches, because we don't 744 // really want to duplicate a switch. 745 if (isa<SwitchInst>(BB->getTerminator())) 746 return false; 747 748 // Look for unconditional branch predecessors. 749 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 750 BasicBlock *PredBB = PN->getIncomingBlock(i); 751 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator())) 752 if (PredBr->isUnconditional() && 753 // Try to duplicate BB into PredBB. 754 DuplicateCondBranchOnPHIIntoPred(BB, PredBB)) 755 return true; 756 } 757 758 return false; 759} 760 761 762/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch 763/// whose condition is an AND/OR where one side is PN. If PN has constant 764/// operands that permit us to evaluate the condition for some operand, thread 765/// through the block. For example with: 766/// br (and X, phi(Y, Z, false)) 767/// the predecessor corresponding to the 'false' will always jump to the false 768/// destination of the branch. 769/// 770bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB, 771 bool isAnd) { 772 // If this is a binary operator tree of the same AND/OR opcode, check the 773 // LHS/RHS. 774 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) 775 if ((isAnd && BO->getOpcode() == Instruction::And) || 776 (!isAnd && BO->getOpcode() == Instruction::Or)) { 777 if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd)) 778 return true; 779 if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd)) 780 return true; 781 } 782 783 // If this isn't a PHI node, we can't handle it. 784 PHINode *PN = dyn_cast<PHINode>(V); 785 if (!PN || PN->getParent() != BB) return false; 786 787 // We can only do the simplification for phi nodes of 'false' with AND or 788 // 'true' with OR. See if we have any entries in the phi for this. 789 unsigned PredNo = ~0U; 790 ConstantInt *PredCst = ConstantInt::get(Type::getInt1Ty(BB->getContext()), 791 !isAnd); 792 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 793 if (PN->getIncomingValue(i) == PredCst) { 794 PredNo = i; 795 break; 796 } 797 } 798 799 // If no match, bail out. 800 if (PredNo == ~0U) 801 return false; 802 803 // If so, we can actually do this threading. Merge any common predecessors 804 // that will act the same. 805 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 806 807 // Next, figure out which successor we are threading to. If this was an AND, 808 // the constant must be FALSE, and we must be targeting the 'false' block. 809 // If this is an OR, the constant must be TRUE, and we must be targeting the 810 // 'true' block. 811 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd); 812 813 // Ok, try to thread it! 814 return ThreadEdge(BB, PredBB, SuccBB); 815} 816 817/// GetResultOfComparison - Given an icmp/fcmp predicate and the left and right 818/// hand sides of the compare instruction, try to determine the result. If the 819/// result can not be determined, a null pointer is returned. 820static Constant *GetResultOfComparison(CmpInst::Predicate pred, 821 Value *LHS, Value *RHS, 822 LLVMContext &Context) { 823 if (Constant *CLHS = dyn_cast<Constant>(LHS)) 824 if (Constant *CRHS = dyn_cast<Constant>(RHS)) 825 return ConstantExpr::getCompare(pred, CLHS, CRHS); 826 827 if (LHS == RHS) 828 if (isa<IntegerType>(LHS->getType()) || isa<PointerType>(LHS->getType())) 829 return ICmpInst::isTrueWhenEqual(pred) ? 830 ConstantInt::getTrue(Context) : ConstantInt::getFalse(Context); 831 832 return 0; 833} 834 835/// ProcessBranchOnCompare - We found a branch on a comparison between a phi 836/// node and a value. If we can identify when the comparison is true between 837/// the phi inputs and the value, we can fold the compare for that edge and 838/// thread through it. 839bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) { 840 PHINode *PN = cast<PHINode>(Cmp->getOperand(0)); 841 Value *RHS = Cmp->getOperand(1); 842 843 // If the phi isn't in the current block, an incoming edge to this block 844 // doesn't control the destination. 845 if (PN->getParent() != BB) 846 return false; 847 848 // We can do this simplification if any comparisons fold to true or false. 849 // See if any do. 850 Value *PredVal = 0; 851 bool TrueDirection = false; 852 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 853 PredVal = PN->getIncomingValue(i); 854 855 Constant *Res = GetResultOfComparison(Cmp->getPredicate(), PredVal, 856 RHS, Cmp->getContext()); 857 if (!Res) { 858 PredVal = 0; 859 continue; 860 } 861 862 // If this folded to a constant expr, we can't do anything. 863 if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) { 864 TrueDirection = ResC->getZExtValue(); 865 break; 866 } 867 // If this folded to undef, just go the false way. 868 if (isa<UndefValue>(Res)) { 869 TrueDirection = false; 870 break; 871 } 872 873 // Otherwise, we can't fold this input. 874 PredVal = 0; 875 } 876 877 // If no match, bail out. 878 if (PredVal == 0) 879 return false; 880 881 // If so, we can actually do this threading. Merge any common predecessors 882 // that will act the same. 883 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredVal); 884 885 // Next, get our successor. 886 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection); 887 888 // Ok, try to thread it! 889 return ThreadEdge(BB, PredBB, SuccBB); 890} 891 892 893/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new 894/// predecessor to the PHIBB block. If it has PHI nodes, add entries for 895/// NewPred using the entries from OldPred (suitably mapped). 896static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, 897 BasicBlock *OldPred, 898 BasicBlock *NewPred, 899 DenseMap<Instruction*, Value*> &ValueMap) { 900 for (BasicBlock::iterator PNI = PHIBB->begin(); 901 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) { 902 // Ok, we have a PHI node. Figure out what the incoming value was for the 903 // DestBlock. 904 Value *IV = PN->getIncomingValueForBlock(OldPred); 905 906 // Remap the value if necessary. 907 if (Instruction *Inst = dyn_cast<Instruction>(IV)) { 908 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst); 909 if (I != ValueMap.end()) 910 IV = I->second; 911 } 912 913 PN->addIncoming(IV, NewPred); 914 } 915} 916 917/// ThreadEdge - We have decided that it is safe and profitable to thread an 918/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this 919/// change. 920bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, 921 BasicBlock *SuccBB) { 922 // If threading to the same block as we come from, we would infinite loop. 923 if (SuccBB == BB) { 924 DEBUG(errs() << " Not threading across BB '" << BB->getName() 925 << "' - would thread to self!\n"); 926 return false; 927 } 928 929 // If threading this would thread across a loop header, don't thread the edge. 930 // See the comments above FindLoopHeaders for justifications and caveats. 931 if (LoopHeaders.count(BB)) { 932 DEBUG(errs() << " Not threading from '" << PredBB->getName() 933 << "' across loop header BB '" << BB->getName() 934 << "' to dest BB '" << SuccBB->getName() 935 << "' - it might create an irreducible loop!\n"); 936 return false; 937 } 938 939 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 940 if (JumpThreadCost > Threshold) { 941 DEBUG(errs() << " Not threading BB '" << BB->getName() 942 << "' - Cost is too high: " << JumpThreadCost << "\n"); 943 return false; 944 } 945 946 // And finally, do it! 947 DEBUG(errs() << " Threading edge from '" << PredBB->getName() << "' to '" 948 << SuccBB->getName() << "' with cost: " << JumpThreadCost 949 << ", across block:\n " 950 << *BB << "\n"); 951 952 // We are going to have to map operands from the original BB block to the new 953 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 954 // account for entry from PredBB. 955 DenseMap<Instruction*, Value*> ValueMapping; 956 957 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), 958 BB->getName()+".thread", 959 BB->getParent(), BB); 960 NewBB->moveAfter(PredBB); 961 962 BasicBlock::iterator BI = BB->begin(); 963 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 964 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 965 966 // Clone the non-phi instructions of BB into NewBB, keeping track of the 967 // mapping and using it to remap operands in the cloned instructions. 968 for (; !isa<TerminatorInst>(BI); ++BI) { 969 Instruction *New = BI->clone(); 970 New->setName(BI->getName()); 971 NewBB->getInstList().push_back(New); 972 ValueMapping[BI] = New; 973 974 // Remap operands to patch up intra-block references. 975 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 976 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 977 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 978 if (I != ValueMapping.end()) 979 New->setOperand(i, I->second); 980 } 981 } 982 983 // We didn't copy the terminator from BB over to NewBB, because there is now 984 // an unconditional jump to SuccBB. Insert the unconditional jump. 985 BranchInst::Create(SuccBB, NewBB); 986 987 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 988 // PHI nodes for NewBB now. 989 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping); 990 991 // If there were values defined in BB that are used outside the block, then we 992 // now have to update all uses of the value to use either the original value, 993 // the cloned value, or some PHI derived value. This can require arbitrary 994 // PHI insertion, of which we are prepared to do, clean these up now. 995 SSAUpdater SSAUpdate; 996 SmallVector<Use*, 16> UsesToRename; 997 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 998 // Scan all uses of this instruction to see if it is used outside of its 999 // block, and if so, record them in UsesToRename. 1000 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1001 ++UI) { 1002 Instruction *User = cast<Instruction>(*UI); 1003 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1004 if (UserPN->getIncomingBlock(UI) == BB) 1005 continue; 1006 } else if (User->getParent() == BB) 1007 continue; 1008 1009 UsesToRename.push_back(&UI.getUse()); 1010 } 1011 1012 // If there are no uses outside the block, we're done with this instruction. 1013 if (UsesToRename.empty()) 1014 continue; 1015 1016 DEBUG(errs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1017 1018 // We found a use of I outside of BB. Rename all uses of I that are outside 1019 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1020 // with the two values we know. 1021 SSAUpdate.Initialize(I); 1022 SSAUpdate.AddAvailableValue(BB, I); 1023 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]); 1024 1025 while (!UsesToRename.empty()) 1026 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1027 DEBUG(errs() << "\n"); 1028 } 1029 1030 1031 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 1032 // NewBB instead of BB. This eliminates predecessors from BB, which requires 1033 // us to simplify any PHI nodes in BB. 1034 TerminatorInst *PredTerm = PredBB->getTerminator(); 1035 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 1036 if (PredTerm->getSuccessor(i) == BB) { 1037 BB->removePredecessor(PredBB); 1038 PredTerm->setSuccessor(i, NewBB); 1039 } 1040 1041 // At this point, the IR is fully up to date and consistent. Do a quick scan 1042 // over the new instructions and zap any that are constants or dead. This 1043 // frequently happens because of phi translation. 1044 BI = NewBB->begin(); 1045 for (BasicBlock::iterator E = NewBB->end(); BI != E; ) { 1046 Instruction *Inst = BI++; 1047 if (Constant *C = ConstantFoldInstruction(Inst, BB->getContext(), TD)) { 1048 Inst->replaceAllUsesWith(C); 1049 Inst->eraseFromParent(); 1050 continue; 1051 } 1052 1053 RecursivelyDeleteTriviallyDeadInstructions(Inst); 1054 } 1055 1056 // Threaded an edge! 1057 ++NumThreads; 1058 return true; 1059} 1060 1061/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch 1062/// to BB which contains an i1 PHI node and a conditional branch on that PHI. 1063/// If we can duplicate the contents of BB up into PredBB do so now, this 1064/// improves the odds that the branch will be on an analyzable instruction like 1065/// a compare. 1066bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 1067 BasicBlock *PredBB) { 1068 // If BB is a loop header, then duplicating this block outside the loop would 1069 // cause us to transform this into an irreducible loop, don't do this. 1070 // See the comments above FindLoopHeaders for justifications and caveats. 1071 if (LoopHeaders.count(BB)) { 1072 DEBUG(errs() << " Not duplicating loop header '" << BB->getName() 1073 << "' into predecessor block '" << PredBB->getName() 1074 << "' - it might create an irreducible loop!\n"); 1075 return false; 1076 } 1077 1078 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB); 1079 if (DuplicationCost > Threshold) { 1080 DEBUG(errs() << " Not duplicating BB '" << BB->getName() 1081 << "' - Cost is too high: " << DuplicationCost << "\n"); 1082 return false; 1083 } 1084 1085 // Okay, we decided to do this! Clone all the instructions in BB onto the end 1086 // of PredBB. 1087 DEBUG(errs() << " Duplicating block '" << BB->getName() << "' into end of '" 1088 << PredBB->getName() << "' to eliminate branch on phi. Cost: " 1089 << DuplicationCost << " block is:" << *BB << "\n"); 1090 1091 // We are going to have to map operands from the original BB block into the 1092 // PredBB block. Evaluate PHI nodes in BB. 1093 DenseMap<Instruction*, Value*> ValueMapping; 1094 1095 BasicBlock::iterator BI = BB->begin(); 1096 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1097 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1098 1099 BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator()); 1100 1101 // Clone the non-phi instructions of BB into PredBB, keeping track of the 1102 // mapping and using it to remap operands in the cloned instructions. 1103 for (; BI != BB->end(); ++BI) { 1104 Instruction *New = BI->clone(); 1105 New->setName(BI->getName()); 1106 PredBB->getInstList().insert(OldPredBranch, New); 1107 ValueMapping[BI] = New; 1108 1109 // Remap operands to patch up intra-block references. 1110 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1111 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1112 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1113 if (I != ValueMapping.end()) 1114 New->setOperand(i, I->second); 1115 } 1116 } 1117 1118 // Check to see if the targets of the branch had PHI nodes. If so, we need to 1119 // add entries to the PHI nodes for branch from PredBB now. 1120 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator()); 1121 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB, 1122 ValueMapping); 1123 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB, 1124 ValueMapping); 1125 1126 // If there were values defined in BB that are used outside the block, then we 1127 // now have to update all uses of the value to use either the original value, 1128 // the cloned value, or some PHI derived value. This can require arbitrary 1129 // PHI insertion, of which we are prepared to do, clean these up now. 1130 SSAUpdater SSAUpdate; 1131 SmallVector<Use*, 16> UsesToRename; 1132 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1133 // Scan all uses of this instruction to see if it is used outside of its 1134 // block, and if so, record them in UsesToRename. 1135 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1136 ++UI) { 1137 Instruction *User = cast<Instruction>(*UI); 1138 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1139 if (UserPN->getIncomingBlock(UI) == BB) 1140 continue; 1141 } else if (User->getParent() == BB) 1142 continue; 1143 1144 UsesToRename.push_back(&UI.getUse()); 1145 } 1146 1147 // If there are no uses outside the block, we're done with this instruction. 1148 if (UsesToRename.empty()) 1149 continue; 1150 1151 DEBUG(errs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1152 1153 // We found a use of I outside of BB. Rename all uses of I that are outside 1154 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1155 // with the two values we know. 1156 SSAUpdate.Initialize(I); 1157 SSAUpdate.AddAvailableValue(BB, I); 1158 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]); 1159 1160 while (!UsesToRename.empty()) 1161 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1162 DEBUG(errs() << "\n"); 1163 } 1164 1165 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge 1166 // that we nuked. 1167 BB->removePredecessor(PredBB); 1168 1169 // Remove the unconditional branch at the end of the PredBB block. 1170 OldPredBranch->eraseFromParent(); 1171 1172 ++NumDupes; 1173 return true; 1174} 1175 1176 1177