JumpThreading.cpp revision fc6e29d4ab52b7d3efd83846ed495a9ca7e51e49
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/InstructionSimplify.h" 20#include "llvm/Analysis/LazyValueInfo.h" 21#include "llvm/Analysis/Loads.h" 22#include "llvm/Transforms/Utils/BasicBlockUtils.h" 23#include "llvm/Transforms/Utils/Local.h" 24#include "llvm/Transforms/Utils/SSAUpdater.h" 25#include "llvm/Target/TargetData.h" 26#include "llvm/ADT/DenseMap.h" 27#include "llvm/ADT/DenseSet.h" 28#include "llvm/ADT/Statistic.h" 29#include "llvm/ADT/STLExtras.h" 30#include "llvm/ADT/SmallPtrSet.h" 31#include "llvm/ADT/SmallSet.h" 32#include "llvm/Support/CommandLine.h" 33#include "llvm/Support/Debug.h" 34#include "llvm/Support/ValueHandle.h" 35#include "llvm/Support/raw_ostream.h" 36using namespace llvm; 37 38STATISTIC(NumThreads, "Number of jumps threaded"); 39STATISTIC(NumFolds, "Number of terminators folded"); 40STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi"); 41 42static cl::opt<unsigned> 43Threshold("jump-threading-threshold", 44 cl::desc("Max block size to duplicate for jump threading"), 45 cl::init(6), cl::Hidden); 46 47// Turn on use of LazyValueInfo. 48static cl::opt<bool> 49EnableLVI("enable-jump-threading-lvi", 50 cl::desc("Use LVI for jump threading"), 51 cl::init(true), 52 cl::ReallyHidden); 53 54 55 56namespace { 57 /// This pass performs 'jump threading', which looks at blocks that have 58 /// multiple predecessors and multiple successors. If one or more of the 59 /// predecessors of the block can be proven to always jump to one of the 60 /// successors, we forward the edge from the predecessor to the successor by 61 /// duplicating the contents of this block. 62 /// 63 /// An example of when this can occur is code like this: 64 /// 65 /// if () { ... 66 /// X = 4; 67 /// } 68 /// if (X < 3) { 69 /// 70 /// In this case, the unconditional branch at the end of the first if can be 71 /// revectored to the false side of the second if. 72 /// 73 class JumpThreading : public FunctionPass { 74 TargetData *TD; 75 LazyValueInfo *LVI; 76#ifdef NDEBUG 77 SmallPtrSet<BasicBlock*, 16> LoopHeaders; 78#else 79 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; 80#endif 81 DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet; 82 83 // RAII helper for updating the recursion stack. 84 struct RecursionSetRemover { 85 DenseSet<std::pair<Value*, BasicBlock*> > &TheSet; 86 std::pair<Value*, BasicBlock*> ThePair; 87 88 RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S, 89 std::pair<Value*, BasicBlock*> P) 90 : TheSet(S), ThePair(P) { } 91 92 ~RecursionSetRemover() { 93 TheSet.erase(ThePair); 94 } 95 }; 96 public: 97 static char ID; // Pass identification 98 JumpThreading() : FunctionPass(ID) {} 99 100 bool runOnFunction(Function &F); 101 102 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 103 if (EnableLVI) { 104 AU.addRequired<LazyValueInfo>(); 105 AU.addPreserved<LazyValueInfo>(); 106 } 107 } 108 109 void FindLoopHeaders(Function &F); 110 bool ProcessBlock(BasicBlock *BB); 111 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs, 112 BasicBlock *SuccBB); 113 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 114 const SmallVectorImpl<BasicBlock *> &PredBBs); 115 116 typedef SmallVectorImpl<std::pair<ConstantInt*, 117 BasicBlock*> > PredValueInfo; 118 119 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, 120 PredValueInfo &Result); 121 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB); 122 123 124 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 125 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 126 127 bool ProcessBranchOnPHI(PHINode *PN); 128 bool ProcessBranchOnXOR(BinaryOperator *BO); 129 130 bool SimplifyPartiallyRedundantLoad(LoadInst *LI); 131 }; 132} 133 134char JumpThreading::ID = 0; 135INITIALIZE_PASS(JumpThreading, "jump-threading", 136 "Jump Threading", false, false); 137 138// Public interface to the Jump Threading pass 139FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 140 141/// runOnFunction - Top level algorithm. 142/// 143bool JumpThreading::runOnFunction(Function &F) { 144 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n"); 145 TD = getAnalysisIfAvailable<TargetData>(); 146 LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0; 147 148 FindLoopHeaders(F); 149 150 bool Changed, EverChanged = false; 151 do { 152 Changed = false; 153 for (Function::iterator I = F.begin(), E = F.end(); I != E;) { 154 BasicBlock *BB = I; 155 // Thread all of the branches we can over this block. 156 while (ProcessBlock(BB)) 157 Changed = true; 158 159 ++I; 160 161 // If the block is trivially dead, zap it. This eliminates the successor 162 // edges which simplifies the CFG. 163 if (pred_begin(BB) == pred_end(BB) && 164 BB != &BB->getParent()->getEntryBlock()) { 165 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName() 166 << "' with terminator: " << *BB->getTerminator() << '\n'); 167 LoopHeaders.erase(BB); 168 if (LVI) LVI->eraseBlock(BB); 169 DeleteDeadBlock(BB); 170 Changed = true; 171 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 172 // Can't thread an unconditional jump, but if the block is "almost 173 // empty", we can replace uses of it with uses of the successor and make 174 // this dead. 175 if (BI->isUnconditional() && 176 BB != &BB->getParent()->getEntryBlock()) { 177 BasicBlock::iterator BBI = BB->getFirstNonPHI(); 178 // Ignore dbg intrinsics. 179 while (isa<DbgInfoIntrinsic>(BBI)) 180 ++BBI; 181 // If the terminator is the only non-phi instruction, try to nuke it. 182 if (BBI->isTerminator()) { 183 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the 184 // block, we have to make sure it isn't in the LoopHeaders set. We 185 // reinsert afterward if needed. 186 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB); 187 BasicBlock *Succ = BI->getSuccessor(0); 188 189 // FIXME: It is always conservatively correct to drop the info 190 // for a block even if it doesn't get erased. This isn't totally 191 // awesome, but it allows us to use AssertingVH to prevent nasty 192 // dangling pointer issues within LazyValueInfo. 193 if (LVI) LVI->eraseBlock(BB); 194 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) { 195 Changed = true; 196 // If we deleted BB and BB was the header of a loop, then the 197 // successor is now the header of the loop. 198 BB = Succ; 199 } 200 201 if (ErasedFromLoopHeaders) 202 LoopHeaders.insert(BB); 203 } 204 } 205 } 206 } 207 EverChanged |= Changed; 208 } while (Changed); 209 210 LoopHeaders.clear(); 211 return EverChanged; 212} 213 214/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 215/// thread across it. 216static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { 217 /// Ignore PHI nodes, these will be flattened when duplication happens. 218 BasicBlock::const_iterator I = BB->getFirstNonPHI(); 219 220 // FIXME: THREADING will delete values that are just used to compute the 221 // branch, so they shouldn't count against the duplication cost. 222 223 224 // Sum up the cost of each instruction until we get to the terminator. Don't 225 // include the terminator because the copy won't include it. 226 unsigned Size = 0; 227 for (; !isa<TerminatorInst>(I); ++I) { 228 // Debugger intrinsics don't incur code size. 229 if (isa<DbgInfoIntrinsic>(I)) continue; 230 231 // If this is a pointer->pointer bitcast, it is free. 232 if (isa<BitCastInst>(I) && I->getType()->isPointerTy()) 233 continue; 234 235 // All other instructions count for at least one unit. 236 ++Size; 237 238 // Calls are more expensive. If they are non-intrinsic calls, we model them 239 // as having cost of 4. If they are a non-vector intrinsic, we model them 240 // as having cost of 2 total, and if they are a vector intrinsic, we model 241 // them as having cost 1. 242 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 243 if (!isa<IntrinsicInst>(CI)) 244 Size += 3; 245 else if (!CI->getType()->isVectorTy()) 246 Size += 1; 247 } 248 } 249 250 // Threading through a switch statement is particularly profitable. If this 251 // block ends in a switch, decrease its cost to make it more likely to happen. 252 if (isa<SwitchInst>(I)) 253 Size = Size > 6 ? Size-6 : 0; 254 255 return Size; 256} 257 258/// FindLoopHeaders - We do not want jump threading to turn proper loop 259/// structures into irreducible loops. Doing this breaks up the loop nesting 260/// hierarchy and pessimizes later transformations. To prevent this from 261/// happening, we first have to find the loop headers. Here we approximate this 262/// by finding targets of backedges in the CFG. 263/// 264/// Note that there definitely are cases when we want to allow threading of 265/// edges across a loop header. For example, threading a jump from outside the 266/// loop (the preheader) to an exit block of the loop is definitely profitable. 267/// It is also almost always profitable to thread backedges from within the loop 268/// to exit blocks, and is often profitable to thread backedges to other blocks 269/// within the loop (forming a nested loop). This simple analysis is not rich 270/// enough to track all of these properties and keep it up-to-date as the CFG 271/// mutates, so we don't allow any of these transformations. 272/// 273void JumpThreading::FindLoopHeaders(Function &F) { 274 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; 275 FindFunctionBackedges(F, Edges); 276 277 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 278 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); 279} 280 281// Helper method for ComputeValueKnownInPredecessors. If Value is a 282// ConstantInt, push it. If it's an undef, push 0. Otherwise, do nothing. 283static void PushConstantIntOrUndef(SmallVectorImpl<std::pair<ConstantInt*, 284 BasicBlock*> > &Result, 285 Constant *Value, BasicBlock* BB){ 286 if (ConstantInt *FoldedCInt = dyn_cast<ConstantInt>(Value)) 287 Result.push_back(std::make_pair(FoldedCInt, BB)); 288 else if (isa<UndefValue>(Value)) 289 Result.push_back(std::make_pair((ConstantInt*)0, BB)); 290} 291 292/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see 293/// if we can infer that the value is a known ConstantInt in any of our 294/// predecessors. If so, return the known list of value and pred BB in the 295/// result vector. If a value is known to be undef, it is returned as null. 296/// 297/// This returns true if there were any known values. 298/// 299bool JumpThreading:: 300ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){ 301 // This method walks up use-def chains recursively. Because of this, we could 302 // get into an infinite loop going around loops in the use-def chain. To 303 // prevent this, keep track of what (value, block) pairs we've already visited 304 // and terminate the search if we loop back to them 305 if (!RecursionSet.insert(std::make_pair(V, BB)).second) 306 return false; 307 308 // An RAII help to remove this pair from the recursion set once the recursion 309 // stack pops back out again. 310 RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB)); 311 312 // If V is a constantint, then it is known in all predecessors. 313 if (isa<ConstantInt>(V) || isa<UndefValue>(V)) { 314 ConstantInt *CI = dyn_cast<ConstantInt>(V); 315 316 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 317 Result.push_back(std::make_pair(CI, *PI)); 318 319 return true; 320 } 321 322 // If V is a non-instruction value, or an instruction in a different block, 323 // then it can't be derived from a PHI. 324 Instruction *I = dyn_cast<Instruction>(V); 325 if (I == 0 || I->getParent() != BB) { 326 327 // Okay, if this is a live-in value, see if it has a known value at the end 328 // of any of our predecessors. 329 // 330 // FIXME: This should be an edge property, not a block end property. 331 /// TODO: Per PR2563, we could infer value range information about a 332 /// predecessor based on its terminator. 333 // 334 if (LVI) { 335 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if 336 // "I" is a non-local compare-with-a-constant instruction. This would be 337 // able to handle value inequalities better, for example if the compare is 338 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available. 339 // Perhaps getConstantOnEdge should be smart enough to do this? 340 341 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 342 BasicBlock *P = *PI; 343 // If the value is known by LazyValueInfo to be a constant in a 344 // predecessor, use that information to try to thread this block. 345 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB); 346 if (PredCst == 0 || 347 (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst))) 348 continue; 349 350 Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), P)); 351 } 352 353 return !Result.empty(); 354 } 355 356 return false; 357 } 358 359 /// If I is a PHI node, then we know the incoming values for any constants. 360 if (PHINode *PN = dyn_cast<PHINode>(I)) { 361 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 362 Value *InVal = PN->getIncomingValue(i); 363 if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) { 364 ConstantInt *CI = dyn_cast<ConstantInt>(InVal); 365 Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i))); 366 } else if (LVI) { 367 Constant *CI = LVI->getConstantOnEdge(InVal, 368 PN->getIncomingBlock(i), BB); 369 // LVI returns null is no value could be determined. 370 if (!CI) continue; 371 PushConstantIntOrUndef(Result, CI, PN->getIncomingBlock(i)); 372 } 373 } 374 375 return !Result.empty(); 376 } 377 378 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals; 379 380 // Handle some boolean conditions. 381 if (I->getType()->getPrimitiveSizeInBits() == 1) { 382 // X | true -> true 383 // X & false -> false 384 if (I->getOpcode() == Instruction::Or || 385 I->getOpcode() == Instruction::And) { 386 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals); 387 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals); 388 389 if (LHSVals.empty() && RHSVals.empty()) 390 return false; 391 392 ConstantInt *InterestingVal; 393 if (I->getOpcode() == Instruction::Or) 394 InterestingVal = ConstantInt::getTrue(I->getContext()); 395 else 396 InterestingVal = ConstantInt::getFalse(I->getContext()); 397 398 SmallPtrSet<BasicBlock*, 4> LHSKnownBBs; 399 400 // Scan for the sentinel. If we find an undef, force it to the 401 // interesting value: x|undef -> true and x&undef -> false. 402 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) 403 if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) { 404 Result.push_back(LHSVals[i]); 405 Result.back().first = InterestingVal; 406 LHSKnownBBs.insert(LHSVals[i].second); 407 } 408 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i) 409 if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) { 410 // If we already inferred a value for this block on the LHS, don't 411 // re-add it. 412 if (!LHSKnownBBs.count(RHSVals[i].second)) { 413 Result.push_back(RHSVals[i]); 414 Result.back().first = InterestingVal; 415 } 416 } 417 418 return !Result.empty(); 419 } 420 421 // Handle the NOT form of XOR. 422 if (I->getOpcode() == Instruction::Xor && 423 isa<ConstantInt>(I->getOperand(1)) && 424 cast<ConstantInt>(I->getOperand(1))->isOne()) { 425 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result); 426 if (Result.empty()) 427 return false; 428 429 // Invert the known values. 430 for (unsigned i = 0, e = Result.size(); i != e; ++i) 431 if (Result[i].first) 432 Result[i].first = 433 cast<ConstantInt>(ConstantExpr::getNot(Result[i].first)); 434 435 return true; 436 } 437 438 // Try to simplify some other binary operator values. 439 } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) { 440 if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) { 441 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals; 442 ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals); 443 444 // Try to use constant folding to simplify the binary operator. 445 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { 446 Constant *V = LHSVals[i].first ? LHSVals[i].first : 447 cast<Constant>(UndefValue::get(BO->getType())); 448 Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI); 449 450 PushConstantIntOrUndef(Result, Folded, LHSVals[i].second); 451 } 452 } 453 454 return !Result.empty(); 455 } 456 457 // Handle compare with phi operand, where the PHI is defined in this block. 458 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) { 459 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0)); 460 if (PN && PN->getParent() == BB) { 461 // We can do this simplification if any comparisons fold to true or false. 462 // See if any do. 463 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 464 BasicBlock *PredBB = PN->getIncomingBlock(i); 465 Value *LHS = PN->getIncomingValue(i); 466 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB); 467 468 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD); 469 if (Res == 0) { 470 if (!LVI || !isa<Constant>(RHS)) 471 continue; 472 473 LazyValueInfo::Tristate 474 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS, 475 cast<Constant>(RHS), PredBB, BB); 476 if (ResT == LazyValueInfo::Unknown) 477 continue; 478 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT); 479 } 480 481 if (Constant *ConstRes = dyn_cast<Constant>(Res)) 482 PushConstantIntOrUndef(Result, ConstRes, PredBB); 483 } 484 485 return !Result.empty(); 486 } 487 488 489 // If comparing a live-in value against a constant, see if we know the 490 // live-in value on any predecessors. 491 if (LVI && isa<Constant>(Cmp->getOperand(1)) && 492 Cmp->getType()->isIntegerTy()) { 493 if (!isa<Instruction>(Cmp->getOperand(0)) || 494 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) { 495 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1)); 496 497 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){ 498 BasicBlock *P = *PI; 499 // If the value is known by LazyValueInfo to be a constant in a 500 // predecessor, use that information to try to thread this block. 501 LazyValueInfo::Tristate Res = 502 LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0), 503 RHSCst, P, BB); 504 if (Res == LazyValueInfo::Unknown) 505 continue; 506 507 Constant *ResC = ConstantInt::get(Cmp->getType(), Res); 508 Result.push_back(std::make_pair(cast<ConstantInt>(ResC), P)); 509 } 510 511 return !Result.empty(); 512 } 513 514 // Try to find a constant value for the LHS of a comparison, 515 // and evaluate it statically if we can. 516 if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) { 517 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals; 518 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals); 519 520 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { 521 Constant *V = LHSVals[i].first ? LHSVals[i].first : 522 cast<Constant>(UndefValue::get(CmpConst->getType())); 523 Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(), 524 V, CmpConst); 525 PushConstantIntOrUndef(Result, Folded, LHSVals[i].second); 526 } 527 528 return !Result.empty(); 529 } 530 } 531 } 532 533 if (LVI) { 534 // If all else fails, see if LVI can figure out a constant value for us. 535 Constant *CI = LVI->getConstant(V, BB); 536 ConstantInt *CInt = dyn_cast_or_null<ConstantInt>(CI); 537 if (CInt) { 538 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 539 Result.push_back(std::make_pair(CInt, *PI)); 540 } 541 542 return !Result.empty(); 543 } 544 545 return false; 546} 547 548 549 550/// GetBestDestForBranchOnUndef - If we determine that the specified block ends 551/// in an undefined jump, decide which block is best to revector to. 552/// 553/// Since we can pick an arbitrary destination, we pick the successor with the 554/// fewest predecessors. This should reduce the in-degree of the others. 555/// 556static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) { 557 TerminatorInst *BBTerm = BB->getTerminator(); 558 unsigned MinSucc = 0; 559 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); 560 // Compute the successor with the minimum number of predecessors. 561 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 562 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { 563 TestBB = BBTerm->getSuccessor(i); 564 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 565 if (NumPreds < MinNumPreds) 566 MinSucc = i; 567 } 568 569 return MinSucc; 570} 571 572/// ProcessBlock - If there are any predecessors whose control can be threaded 573/// through to a successor, transform them now. 574bool JumpThreading::ProcessBlock(BasicBlock *BB) { 575 // If the block is trivially dead, just return and let the caller nuke it. 576 // This simplifies other transformations. 577 if (pred_begin(BB) == pred_end(BB) && 578 BB != &BB->getParent()->getEntryBlock()) 579 return false; 580 581 // If this block has a single predecessor, and if that pred has a single 582 // successor, merge the blocks. This encourages recursive jump threading 583 // because now the condition in this block can be threaded through 584 // predecessors of our predecessor block. 585 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) { 586 if (SinglePred->getTerminator()->getNumSuccessors() == 1 && 587 SinglePred != BB) { 588 // If SinglePred was a loop header, BB becomes one. 589 if (LoopHeaders.erase(SinglePred)) 590 LoopHeaders.insert(BB); 591 592 // Remember if SinglePred was the entry block of the function. If so, we 593 // will need to move BB back to the entry position. 594 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); 595 if (LVI) LVI->eraseBlock(SinglePred); 596 MergeBasicBlockIntoOnlyPred(BB); 597 598 if (isEntry && BB != &BB->getParent()->getEntryBlock()) 599 BB->moveBefore(&BB->getParent()->getEntryBlock()); 600 return true; 601 } 602 } 603 604 // Look to see if the terminator is a branch of switch, if not we can't thread 605 // it. 606 Value *Condition; 607 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 608 // Can't thread an unconditional jump. 609 if (BI->isUnconditional()) return false; 610 Condition = BI->getCondition(); 611 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) 612 Condition = SI->getCondition(); 613 else 614 return false; // Must be an invoke. 615 616 // If the terminator of this block is branching on a constant, simplify the 617 // terminator to an unconditional branch. This can occur due to threading in 618 // other blocks. 619 if (isa<ConstantInt>(Condition)) { 620 DEBUG(dbgs() << " In block '" << BB->getName() 621 << "' folding terminator: " << *BB->getTerminator() << '\n'); 622 ++NumFolds; 623 ConstantFoldTerminator(BB); 624 return true; 625 } 626 627 // If the terminator is branching on an undef, we can pick any of the 628 // successors to branch to. Let GetBestDestForJumpOnUndef decide. 629 if (isa<UndefValue>(Condition)) { 630 unsigned BestSucc = GetBestDestForJumpOnUndef(BB); 631 632 // Fold the branch/switch. 633 TerminatorInst *BBTerm = BB->getTerminator(); 634 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { 635 if (i == BestSucc) continue; 636 RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD); 637 } 638 639 DEBUG(dbgs() << " In block '" << BB->getName() 640 << "' folding undef terminator: " << *BBTerm << '\n'); 641 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm); 642 BBTerm->eraseFromParent(); 643 return true; 644 } 645 646 Instruction *CondInst = dyn_cast<Instruction>(Condition); 647 648 // If the condition is an instruction defined in another block, see if a 649 // predecessor has the same condition: 650 // br COND, BBX, BBY 651 // BBX: 652 // br COND, BBZ, BBW 653 if (!LVI && 654 !Condition->hasOneUse() && // Multiple uses. 655 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition. 656 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 657 if (isa<BranchInst>(BB->getTerminator())) { 658 for (; PI != E; ++PI) { 659 BasicBlock *P = *PI; 660 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator())) 661 if (PBI->isConditional() && PBI->getCondition() == Condition && 662 ProcessBranchOnDuplicateCond(P, BB)) 663 return true; 664 } 665 } else { 666 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator"); 667 for (; PI != E; ++PI) { 668 BasicBlock *P = *PI; 669 if (SwitchInst *PSI = dyn_cast<SwitchInst>(P->getTerminator())) 670 if (PSI->getCondition() == Condition && 671 ProcessSwitchOnDuplicateCond(P, BB)) 672 return true; 673 } 674 } 675 } 676 677 // All the rest of our checks depend on the condition being an instruction. 678 if (CondInst == 0) { 679 // FIXME: Unify this with code below. 680 if (LVI && ProcessThreadableEdges(Condition, BB)) 681 return true; 682 return false; 683 } 684 685 686 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { 687 if (!LVI && 688 (!isa<PHINode>(CondCmp->getOperand(0)) || 689 cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) { 690 // If we have a comparison, loop over the predecessors to see if there is 691 // a condition with a lexically identical value. 692 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 693 for (; PI != E; ++PI) { 694 BasicBlock *P = *PI; 695 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator())) 696 if (PBI->isConditional() && P != BB) { 697 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) { 698 if (CI->getOperand(0) == CondCmp->getOperand(0) && 699 CI->getOperand(1) == CondCmp->getOperand(1) && 700 CI->getPredicate() == CondCmp->getPredicate()) { 701 // TODO: Could handle things like (x != 4) --> (x == 17) 702 if (ProcessBranchOnDuplicateCond(P, BB)) 703 return true; 704 } 705 } 706 } 707 } 708 } 709 710 // For a comparison where the LHS is outside this block, it's possible 711 // that we've branched on it before. Used LVI to see if we can simplify 712 // the branch based on that. 713 BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); 714 Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1)); 715 pred_iterator PI = pred_begin(BB), PE = pred_end(BB); 716 if (LVI && CondBr && CondConst && CondBr->isConditional() && PI != PE && 717 (!isa<Instruction>(CondCmp->getOperand(0)) || 718 cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) { 719 // For predecessor edge, determine if the comparison is true or false 720 // on that edge. If they're all true or all false, we can simplify the 721 // branch. 722 // FIXME: We could handle mixed true/false by duplicating code. 723 LazyValueInfo::Tristate Baseline = 724 LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0), 725 CondConst, *PI, BB); 726 if (Baseline != LazyValueInfo::Unknown) { 727 // Check that all remaining incoming values match the first one. 728 while (++PI != PE) { 729 LazyValueInfo::Tristate Ret = LVI->getPredicateOnEdge( 730 CondCmp->getPredicate(), 731 CondCmp->getOperand(0), 732 CondConst, *PI, BB); 733 if (Ret != Baseline) break; 734 } 735 736 // If we terminated early, then one of the values didn't match. 737 if (PI == PE) { 738 unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0; 739 unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1; 740 RemovePredecessorAndSimplify(CondBr->getSuccessor(ToRemove), BB, TD); 741 BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr); 742 CondBr->eraseFromParent(); 743 return true; 744 } 745 } 746 } 747 } 748 749 // Check for some cases that are worth simplifying. Right now we want to look 750 // for loads that are used by a switch or by the condition for the branch. If 751 // we see one, check to see if it's partially redundant. If so, insert a PHI 752 // which can then be used to thread the values. 753 // 754 Value *SimplifyValue = CondInst; 755 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 756 if (isa<Constant>(CondCmp->getOperand(1))) 757 SimplifyValue = CondCmp->getOperand(0); 758 759 // TODO: There are other places where load PRE would be profitable, such as 760 // more complex comparisons. 761 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 762 if (SimplifyPartiallyRedundantLoad(LI)) 763 return true; 764 765 766 // Handle a variety of cases where we are branching on something derived from 767 // a PHI node in the current block. If we can prove that any predecessors 768 // compute a predictable value based on a PHI node, thread those predecessors. 769 // 770 if (ProcessThreadableEdges(CondInst, BB)) 771 return true; 772 773 // If this is an otherwise-unfoldable branch on a phi node in the current 774 // block, see if we can simplify. 775 if (PHINode *PN = dyn_cast<PHINode>(CondInst)) 776 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 777 return ProcessBranchOnPHI(PN); 778 779 780 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify. 781 if (CondInst->getOpcode() == Instruction::Xor && 782 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 783 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst)); 784 785 786 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 787 // "(X == 4)", thread through this block. 788 789 return false; 790} 791 792/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that 793/// block that jump on exactly the same condition. This means that we almost 794/// always know the direction of the edge in the DESTBB: 795/// PREDBB: 796/// br COND, DESTBB, BBY 797/// DESTBB: 798/// br COND, BBZ, BBW 799/// 800/// If DESTBB has multiple predecessors, we can't just constant fold the branch 801/// in DESTBB, we have to thread over it. 802bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, 803 BasicBlock *BB) { 804 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); 805 806 // If both successors of PredBB go to DESTBB, we don't know anything. We can 807 // fold the branch to an unconditional one, which allows other recursive 808 // simplifications. 809 bool BranchDir; 810 if (PredBI->getSuccessor(1) != BB) 811 BranchDir = true; 812 else if (PredBI->getSuccessor(0) != BB) 813 BranchDir = false; 814 else { 815 DEBUG(dbgs() << " In block '" << PredBB->getName() 816 << "' folding terminator: " << *PredBB->getTerminator() << '\n'); 817 ++NumFolds; 818 ConstantFoldTerminator(PredBB); 819 return true; 820 } 821 822 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); 823 824 // If the dest block has one predecessor, just fix the branch condition to a 825 // constant and fold it. 826 if (BB->getSinglePredecessor()) { 827 DEBUG(dbgs() << " In block '" << BB->getName() 828 << "' folding condition to '" << BranchDir << "': " 829 << *BB->getTerminator() << '\n'); 830 ++NumFolds; 831 Value *OldCond = DestBI->getCondition(); 832 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()), 833 BranchDir)); 834 // Delete dead instructions before we fold the branch. Folding the branch 835 // can eliminate edges from the CFG which can end up deleting OldCond. 836 RecursivelyDeleteTriviallyDeadInstructions(OldCond); 837 ConstantFoldTerminator(BB); 838 return true; 839 } 840 841 842 // Next, figure out which successor we are threading to. 843 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); 844 845 SmallVector<BasicBlock*, 2> Preds; 846 Preds.push_back(PredBB); 847 848 // Ok, try to thread it! 849 return ThreadEdge(BB, Preds, SuccBB); 850} 851 852/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that 853/// block that switch on exactly the same condition. This means that we almost 854/// always know the direction of the edge in the DESTBB: 855/// PREDBB: 856/// switch COND [... DESTBB, BBY ... ] 857/// DESTBB: 858/// switch COND [... BBZ, BBW ] 859/// 860/// Optimizing switches like this is very important, because simplifycfg builds 861/// switches out of repeated 'if' conditions. 862bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, 863 BasicBlock *DestBB) { 864 // Can't thread edge to self. 865 if (PredBB == DestBB) 866 return false; 867 868 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); 869 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); 870 871 // There are a variety of optimizations that we can potentially do on these 872 // blocks: we order them from most to least preferable. 873 874 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB 875 // directly to their destination. This does not introduce *any* code size 876 // growth. Skip debug info first. 877 BasicBlock::iterator BBI = DestBB->begin(); 878 while (isa<DbgInfoIntrinsic>(BBI)) 879 BBI++; 880 881 // FIXME: Thread if it just contains a PHI. 882 if (isa<SwitchInst>(BBI)) { 883 bool MadeChange = false; 884 // Ignore the default edge for now. 885 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { 886 ConstantInt *DestVal = DestSI->getCaseValue(i); 887 BasicBlock *DestSucc = DestSI->getSuccessor(i); 888 889 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if 890 // PredSI has an explicit case for it. If so, forward. If it is covered 891 // by the default case, we can't update PredSI. 892 unsigned PredCase = PredSI->findCaseValue(DestVal); 893 if (PredCase == 0) continue; 894 895 // If PredSI doesn't go to DestBB on this value, then it won't reach the 896 // case on this condition. 897 if (PredSI->getSuccessor(PredCase) != DestBB && 898 DestSI->getSuccessor(i) != DestBB) 899 continue; 900 901 // Do not forward this if it already goes to this destination, this would 902 // be an infinite loop. 903 if (PredSI->getSuccessor(PredCase) == DestSucc) 904 continue; 905 906 // Otherwise, we're safe to make the change. Make sure that the edge from 907 // DestSI to DestSucc is not critical and has no PHI nodes. 908 DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI); 909 DEBUG(dbgs() << "THROUGH: " << *DestSI); 910 911 // If the destination has PHI nodes, just split the edge for updating 912 // simplicity. 913 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ 914 SplitCriticalEdge(DestSI, i, this); 915 DestSucc = DestSI->getSuccessor(i); 916 } 917 FoldSingleEntryPHINodes(DestSucc); 918 PredSI->setSuccessor(PredCase, DestSucc); 919 MadeChange = true; 920 } 921 922 if (MadeChange) 923 return true; 924 } 925 926 return false; 927} 928 929 930/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 931/// load instruction, eliminate it by replacing it with a PHI node. This is an 932/// important optimization that encourages jump threading, and needs to be run 933/// interlaced with other jump threading tasks. 934bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 935 // Don't hack volatile loads. 936 if (LI->isVolatile()) return false; 937 938 // If the load is defined in a block with exactly one predecessor, it can't be 939 // partially redundant. 940 BasicBlock *LoadBB = LI->getParent(); 941 if (LoadBB->getSinglePredecessor()) 942 return false; 943 944 Value *LoadedPtr = LI->getOperand(0); 945 946 // If the loaded operand is defined in the LoadBB, it can't be available. 947 // TODO: Could do simple PHI translation, that would be fun :) 948 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 949 if (PtrOp->getParent() == LoadBB) 950 return false; 951 952 // Scan a few instructions up from the load, to see if it is obviously live at 953 // the entry to its block. 954 BasicBlock::iterator BBIt = LI; 955 956 if (Value *AvailableVal = 957 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) { 958 // If the value if the load is locally available within the block, just use 959 // it. This frequently occurs for reg2mem'd allocas. 960 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 961 962 // If the returned value is the load itself, replace with an undef. This can 963 // only happen in dead loops. 964 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); 965 LI->replaceAllUsesWith(AvailableVal); 966 LI->eraseFromParent(); 967 return true; 968 } 969 970 // Otherwise, if we scanned the whole block and got to the top of the block, 971 // we know the block is locally transparent to the load. If not, something 972 // might clobber its value. 973 if (BBIt != LoadBB->begin()) 974 return false; 975 976 977 SmallPtrSet<BasicBlock*, 8> PredsScanned; 978 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 979 AvailablePredsTy AvailablePreds; 980 BasicBlock *OneUnavailablePred = 0; 981 982 // If we got here, the loaded value is transparent through to the start of the 983 // block. Check to see if it is available in any of the predecessor blocks. 984 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 985 PI != PE; ++PI) { 986 BasicBlock *PredBB = *PI; 987 988 // If we already scanned this predecessor, skip it. 989 if (!PredsScanned.insert(PredBB)) 990 continue; 991 992 // Scan the predecessor to see if the value is available in the pred. 993 BBIt = PredBB->end(); 994 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); 995 if (!PredAvailable) { 996 OneUnavailablePred = PredBB; 997 continue; 998 } 999 1000 // If so, this load is partially redundant. Remember this info so that we 1001 // can create a PHI node. 1002 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 1003 } 1004 1005 // If the loaded value isn't available in any predecessor, it isn't partially 1006 // redundant. 1007 if (AvailablePreds.empty()) return false; 1008 1009 // Okay, the loaded value is available in at least one (and maybe all!) 1010 // predecessors. If the value is unavailable in more than one unique 1011 // predecessor, we want to insert a merge block for those common predecessors. 1012 // This ensures that we only have to insert one reload, thus not increasing 1013 // code size. 1014 BasicBlock *UnavailablePred = 0; 1015 1016 // If there is exactly one predecessor where the value is unavailable, the 1017 // already computed 'OneUnavailablePred' block is it. If it ends in an 1018 // unconditional branch, we know that it isn't a critical edge. 1019 if (PredsScanned.size() == AvailablePreds.size()+1 && 1020 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 1021 UnavailablePred = OneUnavailablePred; 1022 } else if (PredsScanned.size() != AvailablePreds.size()) { 1023 // Otherwise, we had multiple unavailable predecessors or we had a critical 1024 // edge from the one. 1025 SmallVector<BasicBlock*, 8> PredsToSplit; 1026 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 1027 1028 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 1029 AvailablePredSet.insert(AvailablePreds[i].first); 1030 1031 // Add all the unavailable predecessors to the PredsToSplit list. 1032 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 1033 PI != PE; ++PI) { 1034 BasicBlock *P = *PI; 1035 // If the predecessor is an indirect goto, we can't split the edge. 1036 if (isa<IndirectBrInst>(P->getTerminator())) 1037 return false; 1038 1039 if (!AvailablePredSet.count(P)) 1040 PredsToSplit.push_back(P); 1041 } 1042 1043 // Split them out to their own block. 1044 UnavailablePred = 1045 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), 1046 "thread-pre-split", this); 1047 } 1048 1049 // If the value isn't available in all predecessors, then there will be 1050 // exactly one where it isn't available. Insert a load on that edge and add 1051 // it to the AvailablePreds list. 1052 if (UnavailablePred) { 1053 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 1054 "Can't handle critical edge here!"); 1055 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false, 1056 LI->getAlignment(), 1057 UnavailablePred->getTerminator()); 1058 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 1059 } 1060 1061 // Now we know that each predecessor of this block has a value in 1062 // AvailablePreds, sort them for efficient access as we're walking the preds. 1063 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 1064 1065 // Create a PHI node at the start of the block for the PRE'd load value. 1066 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); 1067 PN->takeName(LI); 1068 1069 // Insert new entries into the PHI for each predecessor. A single block may 1070 // have multiple entries here. 1071 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; 1072 ++PI) { 1073 BasicBlock *P = *PI; 1074 AvailablePredsTy::iterator I = 1075 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 1076 std::make_pair(P, (Value*)0)); 1077 1078 assert(I != AvailablePreds.end() && I->first == P && 1079 "Didn't find entry for predecessor!"); 1080 1081 PN->addIncoming(I->second, I->first); 1082 } 1083 1084 //cerr << "PRE: " << *LI << *PN << "\n"; 1085 1086 LI->replaceAllUsesWith(PN); 1087 LI->eraseFromParent(); 1088 1089 return true; 1090} 1091 1092/// FindMostPopularDest - The specified list contains multiple possible 1093/// threadable destinations. Pick the one that occurs the most frequently in 1094/// the list. 1095static BasicBlock * 1096FindMostPopularDest(BasicBlock *BB, 1097 const SmallVectorImpl<std::pair<BasicBlock*, 1098 BasicBlock*> > &PredToDestList) { 1099 assert(!PredToDestList.empty()); 1100 1101 // Determine popularity. If there are multiple possible destinations, we 1102 // explicitly choose to ignore 'undef' destinations. We prefer to thread 1103 // blocks with known and real destinations to threading undef. We'll handle 1104 // them later if interesting. 1105 DenseMap<BasicBlock*, unsigned> DestPopularity; 1106 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1107 if (PredToDestList[i].second) 1108 DestPopularity[PredToDestList[i].second]++; 1109 1110 // Find the most popular dest. 1111 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin(); 1112 BasicBlock *MostPopularDest = DPI->first; 1113 unsigned Popularity = DPI->second; 1114 SmallVector<BasicBlock*, 4> SamePopularity; 1115 1116 for (++DPI; DPI != DestPopularity.end(); ++DPI) { 1117 // If the popularity of this entry isn't higher than the popularity we've 1118 // seen so far, ignore it. 1119 if (DPI->second < Popularity) 1120 ; // ignore. 1121 else if (DPI->second == Popularity) { 1122 // If it is the same as what we've seen so far, keep track of it. 1123 SamePopularity.push_back(DPI->first); 1124 } else { 1125 // If it is more popular, remember it. 1126 SamePopularity.clear(); 1127 MostPopularDest = DPI->first; 1128 Popularity = DPI->second; 1129 } 1130 } 1131 1132 // Okay, now we know the most popular destination. If there is more than 1133 // destination, we need to determine one. This is arbitrary, but we need 1134 // to make a deterministic decision. Pick the first one that appears in the 1135 // successor list. 1136 if (!SamePopularity.empty()) { 1137 SamePopularity.push_back(MostPopularDest); 1138 TerminatorInst *TI = BB->getTerminator(); 1139 for (unsigned i = 0; ; ++i) { 1140 assert(i != TI->getNumSuccessors() && "Didn't find any successor!"); 1141 1142 if (std::find(SamePopularity.begin(), SamePopularity.end(), 1143 TI->getSuccessor(i)) == SamePopularity.end()) 1144 continue; 1145 1146 MostPopularDest = TI->getSuccessor(i); 1147 break; 1148 } 1149 } 1150 1151 // Okay, we have finally picked the most popular destination. 1152 return MostPopularDest; 1153} 1154 1155bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) { 1156 // If threading this would thread across a loop header, don't even try to 1157 // thread the edge. 1158 if (LoopHeaders.count(BB)) 1159 return false; 1160 1161 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues; 1162 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues)) 1163 return false; 1164 1165 assert(!PredValues.empty() && 1166 "ComputeValueKnownInPredecessors returned true with no values"); 1167 1168 DEBUG(dbgs() << "IN BB: " << *BB; 1169 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1170 dbgs() << " BB '" << BB->getName() << "': FOUND condition = "; 1171 if (PredValues[i].first) 1172 dbgs() << *PredValues[i].first; 1173 else 1174 dbgs() << "UNDEF"; 1175 dbgs() << " for pred '" << PredValues[i].second->getName() 1176 << "'.\n"; 1177 }); 1178 1179 // Decide what we want to thread through. Convert our list of known values to 1180 // a list of known destinations for each pred. This also discards duplicate 1181 // predecessors and keeps track of the undefined inputs (which are represented 1182 // as a null dest in the PredToDestList). 1183 SmallPtrSet<BasicBlock*, 16> SeenPreds; 1184 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList; 1185 1186 BasicBlock *OnlyDest = 0; 1187 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL; 1188 1189 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1190 BasicBlock *Pred = PredValues[i].second; 1191 if (!SeenPreds.insert(Pred)) 1192 continue; // Duplicate predecessor entry. 1193 1194 // If the predecessor ends with an indirect goto, we can't change its 1195 // destination. 1196 if (isa<IndirectBrInst>(Pred->getTerminator())) 1197 continue; 1198 1199 ConstantInt *Val = PredValues[i].first; 1200 1201 BasicBlock *DestBB; 1202 if (Val == 0) // Undef. 1203 DestBB = 0; 1204 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 1205 DestBB = BI->getSuccessor(Val->isZero()); 1206 else { 1207 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); 1208 DestBB = SI->getSuccessor(SI->findCaseValue(Val)); 1209 } 1210 1211 // If we have exactly one destination, remember it for efficiency below. 1212 if (i == 0) 1213 OnlyDest = DestBB; 1214 else if (OnlyDest != DestBB) 1215 OnlyDest = MultipleDestSentinel; 1216 1217 PredToDestList.push_back(std::make_pair(Pred, DestBB)); 1218 } 1219 1220 // If all edges were unthreadable, we fail. 1221 if (PredToDestList.empty()) 1222 return false; 1223 1224 // Determine which is the most common successor. If we have many inputs and 1225 // this block is a switch, we want to start by threading the batch that goes 1226 // to the most popular destination first. If we only know about one 1227 // threadable destination (the common case) we can avoid this. 1228 BasicBlock *MostPopularDest = OnlyDest; 1229 1230 if (MostPopularDest == MultipleDestSentinel) 1231 MostPopularDest = FindMostPopularDest(BB, PredToDestList); 1232 1233 // Now that we know what the most popular destination is, factor all 1234 // predecessors that will jump to it into a single predecessor. 1235 SmallVector<BasicBlock*, 16> PredsToFactor; 1236 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1237 if (PredToDestList[i].second == MostPopularDest) { 1238 BasicBlock *Pred = PredToDestList[i].first; 1239 1240 // This predecessor may be a switch or something else that has multiple 1241 // edges to the block. Factor each of these edges by listing them 1242 // according to # occurrences in PredsToFactor. 1243 TerminatorInst *PredTI = Pred->getTerminator(); 1244 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i) 1245 if (PredTI->getSuccessor(i) == BB) 1246 PredsToFactor.push_back(Pred); 1247 } 1248 1249 // If the threadable edges are branching on an undefined value, we get to pick 1250 // the destination that these predecessors should get to. 1251 if (MostPopularDest == 0) 1252 MostPopularDest = BB->getTerminator()-> 1253 getSuccessor(GetBestDestForJumpOnUndef(BB)); 1254 1255 // Ok, try to thread it! 1256 return ThreadEdge(BB, PredsToFactor, MostPopularDest); 1257} 1258 1259/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on 1260/// a PHI node in the current block. See if there are any simplifications we 1261/// can do based on inputs to the phi node. 1262/// 1263bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) { 1264 BasicBlock *BB = PN->getParent(); 1265 1266 // TODO: We could make use of this to do it once for blocks with common PHI 1267 // values. 1268 SmallVector<BasicBlock*, 1> PredBBs; 1269 PredBBs.resize(1); 1270 1271 // If any of the predecessor blocks end in an unconditional branch, we can 1272 // *duplicate* the conditional branch into that block in order to further 1273 // encourage jump threading and to eliminate cases where we have branch on a 1274 // phi of an icmp (branch on icmp is much better). 1275 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1276 BasicBlock *PredBB = PN->getIncomingBlock(i); 1277 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator())) 1278 if (PredBr->isUnconditional()) { 1279 PredBBs[0] = PredBB; 1280 // Try to duplicate BB into PredBB. 1281 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs)) 1282 return true; 1283 } 1284 } 1285 1286 return false; 1287} 1288 1289/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on 1290/// a xor instruction in the current block. See if there are any 1291/// simplifications we can do based on inputs to the xor. 1292/// 1293bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) { 1294 BasicBlock *BB = BO->getParent(); 1295 1296 // If either the LHS or RHS of the xor is a constant, don't do this 1297 // optimization. 1298 if (isa<ConstantInt>(BO->getOperand(0)) || 1299 isa<ConstantInt>(BO->getOperand(1))) 1300 return false; 1301 1302 // If the first instruction in BB isn't a phi, we won't be able to infer 1303 // anything special about any particular predecessor. 1304 if (!isa<PHINode>(BB->front())) 1305 return false; 1306 1307 // If we have a xor as the branch input to this block, and we know that the 1308 // LHS or RHS of the xor in any predecessor is true/false, then we can clone 1309 // the condition into the predecessor and fix that value to true, saving some 1310 // logical ops on that path and encouraging other paths to simplify. 1311 // 1312 // This copies something like this: 1313 // 1314 // BB: 1315 // %X = phi i1 [1], [%X'] 1316 // %Y = icmp eq i32 %A, %B 1317 // %Z = xor i1 %X, %Y 1318 // br i1 %Z, ... 1319 // 1320 // Into: 1321 // BB': 1322 // %Y = icmp ne i32 %A, %B 1323 // br i1 %Z, ... 1324 1325 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues; 1326 bool isLHS = true; 1327 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) { 1328 assert(XorOpValues.empty()); 1329 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues)) 1330 return false; 1331 isLHS = false; 1332 } 1333 1334 assert(!XorOpValues.empty() && 1335 "ComputeValueKnownInPredecessors returned true with no values"); 1336 1337 // Scan the information to see which is most popular: true or false. The 1338 // predecessors can be of the set true, false, or undef. 1339 unsigned NumTrue = 0, NumFalse = 0; 1340 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1341 if (!XorOpValues[i].first) continue; // Ignore undefs for the count. 1342 if (XorOpValues[i].first->isZero()) 1343 ++NumFalse; 1344 else 1345 ++NumTrue; 1346 } 1347 1348 // Determine which value to split on, true, false, or undef if neither. 1349 ConstantInt *SplitVal = 0; 1350 if (NumTrue > NumFalse) 1351 SplitVal = ConstantInt::getTrue(BB->getContext()); 1352 else if (NumTrue != 0 || NumFalse != 0) 1353 SplitVal = ConstantInt::getFalse(BB->getContext()); 1354 1355 // Collect all of the blocks that this can be folded into so that we can 1356 // factor this once and clone it once. 1357 SmallVector<BasicBlock*, 8> BlocksToFoldInto; 1358 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1359 if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue; 1360 1361 BlocksToFoldInto.push_back(XorOpValues[i].second); 1362 } 1363 1364 // If we inferred a value for all of the predecessors, then duplication won't 1365 // help us. However, we can just replace the LHS or RHS with the constant. 1366 if (BlocksToFoldInto.size() == 1367 cast<PHINode>(BB->front()).getNumIncomingValues()) { 1368 if (SplitVal == 0) { 1369 // If all preds provide undef, just nuke the xor, because it is undef too. 1370 BO->replaceAllUsesWith(UndefValue::get(BO->getType())); 1371 BO->eraseFromParent(); 1372 } else if (SplitVal->isZero()) { 1373 // If all preds provide 0, replace the xor with the other input. 1374 BO->replaceAllUsesWith(BO->getOperand(isLHS)); 1375 BO->eraseFromParent(); 1376 } else { 1377 // If all preds provide 1, set the computed value to 1. 1378 BO->setOperand(!isLHS, SplitVal); 1379 } 1380 1381 return true; 1382 } 1383 1384 // Try to duplicate BB into PredBB. 1385 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto); 1386} 1387 1388 1389/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new 1390/// predecessor to the PHIBB block. If it has PHI nodes, add entries for 1391/// NewPred using the entries from OldPred (suitably mapped). 1392static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, 1393 BasicBlock *OldPred, 1394 BasicBlock *NewPred, 1395 DenseMap<Instruction*, Value*> &ValueMap) { 1396 for (BasicBlock::iterator PNI = PHIBB->begin(); 1397 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) { 1398 // Ok, we have a PHI node. Figure out what the incoming value was for the 1399 // DestBlock. 1400 Value *IV = PN->getIncomingValueForBlock(OldPred); 1401 1402 // Remap the value if necessary. 1403 if (Instruction *Inst = dyn_cast<Instruction>(IV)) { 1404 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst); 1405 if (I != ValueMap.end()) 1406 IV = I->second; 1407 } 1408 1409 PN->addIncoming(IV, NewPred); 1410 } 1411} 1412 1413/// ThreadEdge - We have decided that it is safe and profitable to factor the 1414/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB 1415/// across BB. Transform the IR to reflect this change. 1416bool JumpThreading::ThreadEdge(BasicBlock *BB, 1417 const SmallVectorImpl<BasicBlock*> &PredBBs, 1418 BasicBlock *SuccBB) { 1419 // If threading to the same block as we come from, we would infinite loop. 1420 if (SuccBB == BB) { 1421 DEBUG(dbgs() << " Not threading across BB '" << BB->getName() 1422 << "' - would thread to self!\n"); 1423 return false; 1424 } 1425 1426 // If threading this would thread across a loop header, don't thread the edge. 1427 // See the comments above FindLoopHeaders for justifications and caveats. 1428 if (LoopHeaders.count(BB)) { 1429 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName() 1430 << "' to dest BB '" << SuccBB->getName() 1431 << "' - it might create an irreducible loop!\n"); 1432 return false; 1433 } 1434 1435 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 1436 if (JumpThreadCost > Threshold) { 1437 DEBUG(dbgs() << " Not threading BB '" << BB->getName() 1438 << "' - Cost is too high: " << JumpThreadCost << "\n"); 1439 return false; 1440 } 1441 1442 // And finally, do it! Start by factoring the predecessors is needed. 1443 BasicBlock *PredBB; 1444 if (PredBBs.size() == 1) 1445 PredBB = PredBBs[0]; 1446 else { 1447 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1448 << " common predecessors.\n"); 1449 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), 1450 ".thr_comm", this); 1451 } 1452 1453 // And finally, do it! 1454 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '" 1455 << SuccBB->getName() << "' with cost: " << JumpThreadCost 1456 << ", across block:\n " 1457 << *BB << "\n"); 1458 1459 if (LVI) 1460 LVI->threadEdge(PredBB, BB, SuccBB); 1461 1462 // We are going to have to map operands from the original BB block to the new 1463 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 1464 // account for entry from PredBB. 1465 DenseMap<Instruction*, Value*> ValueMapping; 1466 1467 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), 1468 BB->getName()+".thread", 1469 BB->getParent(), BB); 1470 NewBB->moveAfter(PredBB); 1471 1472 BasicBlock::iterator BI = BB->begin(); 1473 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1474 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1475 1476 // Clone the non-phi instructions of BB into NewBB, keeping track of the 1477 // mapping and using it to remap operands in the cloned instructions. 1478 for (; !isa<TerminatorInst>(BI); ++BI) { 1479 Instruction *New = BI->clone(); 1480 New->setName(BI->getName()); 1481 NewBB->getInstList().push_back(New); 1482 ValueMapping[BI] = New; 1483 1484 // Remap operands to patch up intra-block references. 1485 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1486 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1487 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1488 if (I != ValueMapping.end()) 1489 New->setOperand(i, I->second); 1490 } 1491 } 1492 1493 // We didn't copy the terminator from BB over to NewBB, because there is now 1494 // an unconditional jump to SuccBB. Insert the unconditional jump. 1495 BranchInst::Create(SuccBB, NewBB); 1496 1497 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 1498 // PHI nodes for NewBB now. 1499 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping); 1500 1501 // If there were values defined in BB that are used outside the block, then we 1502 // now have to update all uses of the value to use either the original value, 1503 // the cloned value, or some PHI derived value. This can require arbitrary 1504 // PHI insertion, of which we are prepared to do, clean these up now. 1505 SSAUpdater SSAUpdate; 1506 SmallVector<Use*, 16> UsesToRename; 1507 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1508 // Scan all uses of this instruction to see if it is used outside of its 1509 // block, and if so, record them in UsesToRename. 1510 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1511 ++UI) { 1512 Instruction *User = cast<Instruction>(*UI); 1513 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1514 if (UserPN->getIncomingBlock(UI) == BB) 1515 continue; 1516 } else if (User->getParent() == BB) 1517 continue; 1518 1519 UsesToRename.push_back(&UI.getUse()); 1520 } 1521 1522 // If there are no uses outside the block, we're done with this instruction. 1523 if (UsesToRename.empty()) 1524 continue; 1525 1526 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1527 1528 // We found a use of I outside of BB. Rename all uses of I that are outside 1529 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1530 // with the two values we know. 1531 SSAUpdate.Initialize(I->getType(), I->getName()); 1532 SSAUpdate.AddAvailableValue(BB, I); 1533 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]); 1534 1535 while (!UsesToRename.empty()) 1536 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1537 DEBUG(dbgs() << "\n"); 1538 } 1539 1540 1541 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 1542 // NewBB instead of BB. This eliminates predecessors from BB, which requires 1543 // us to simplify any PHI nodes in BB. 1544 TerminatorInst *PredTerm = PredBB->getTerminator(); 1545 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 1546 if (PredTerm->getSuccessor(i) == BB) { 1547 RemovePredecessorAndSimplify(BB, PredBB, TD); 1548 PredTerm->setSuccessor(i, NewBB); 1549 } 1550 1551 // At this point, the IR is fully up to date and consistent. Do a quick scan 1552 // over the new instructions and zap any that are constants or dead. This 1553 // frequently happens because of phi translation. 1554 SimplifyInstructionsInBlock(NewBB, TD); 1555 1556 // Threaded an edge! 1557 ++NumThreads; 1558 return true; 1559} 1560 1561/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch 1562/// to BB which contains an i1 PHI node and a conditional branch on that PHI. 1563/// If we can duplicate the contents of BB up into PredBB do so now, this 1564/// improves the odds that the branch will be on an analyzable instruction like 1565/// a compare. 1566bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 1567 const SmallVectorImpl<BasicBlock *> &PredBBs) { 1568 assert(!PredBBs.empty() && "Can't handle an empty set"); 1569 1570 // If BB is a loop header, then duplicating this block outside the loop would 1571 // cause us to transform this into an irreducible loop, don't do this. 1572 // See the comments above FindLoopHeaders for justifications and caveats. 1573 if (LoopHeaders.count(BB)) { 1574 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName() 1575 << "' into predecessor block '" << PredBBs[0]->getName() 1576 << "' - it might create an irreducible loop!\n"); 1577 return false; 1578 } 1579 1580 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB); 1581 if (DuplicationCost > Threshold) { 1582 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName() 1583 << "' - Cost is too high: " << DuplicationCost << "\n"); 1584 return false; 1585 } 1586 1587 // And finally, do it! Start by factoring the predecessors is needed. 1588 BasicBlock *PredBB; 1589 if (PredBBs.size() == 1) 1590 PredBB = PredBBs[0]; 1591 else { 1592 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1593 << " common predecessors.\n"); 1594 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), 1595 ".thr_comm", this); 1596 } 1597 1598 // Okay, we decided to do this! Clone all the instructions in BB onto the end 1599 // of PredBB. 1600 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '" 1601 << PredBB->getName() << "' to eliminate branch on phi. Cost: " 1602 << DuplicationCost << " block is:" << *BB << "\n"); 1603 1604 // Unless PredBB ends with an unconditional branch, split the edge so that we 1605 // can just clone the bits from BB into the end of the new PredBB. 1606 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator()); 1607 1608 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) { 1609 PredBB = SplitEdge(PredBB, BB, this); 1610 OldPredBranch = cast<BranchInst>(PredBB->getTerminator()); 1611 } 1612 1613 // We are going to have to map operands from the original BB block into the 1614 // PredBB block. Evaluate PHI nodes in BB. 1615 DenseMap<Instruction*, Value*> ValueMapping; 1616 1617 BasicBlock::iterator BI = BB->begin(); 1618 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1619 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1620 1621 // Clone the non-phi instructions of BB into PredBB, keeping track of the 1622 // mapping and using it to remap operands in the cloned instructions. 1623 for (; BI != BB->end(); ++BI) { 1624 Instruction *New = BI->clone(); 1625 1626 // Remap operands to patch up intra-block references. 1627 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1628 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1629 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1630 if (I != ValueMapping.end()) 1631 New->setOperand(i, I->second); 1632 } 1633 1634 // If this instruction can be simplified after the operands are updated, 1635 // just use the simplified value instead. This frequently happens due to 1636 // phi translation. 1637 if (Value *IV = SimplifyInstruction(New, TD)) { 1638 delete New; 1639 ValueMapping[BI] = IV; 1640 } else { 1641 // Otherwise, insert the new instruction into the block. 1642 New->setName(BI->getName()); 1643 PredBB->getInstList().insert(OldPredBranch, New); 1644 ValueMapping[BI] = New; 1645 } 1646 } 1647 1648 // Check to see if the targets of the branch had PHI nodes. If so, we need to 1649 // add entries to the PHI nodes for branch from PredBB now. 1650 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator()); 1651 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB, 1652 ValueMapping); 1653 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB, 1654 ValueMapping); 1655 1656 // If there were values defined in BB that are used outside the block, then we 1657 // now have to update all uses of the value to use either the original value, 1658 // the cloned value, or some PHI derived value. This can require arbitrary 1659 // PHI insertion, of which we are prepared to do, clean these up now. 1660 SSAUpdater SSAUpdate; 1661 SmallVector<Use*, 16> UsesToRename; 1662 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1663 // Scan all uses of this instruction to see if it is used outside of its 1664 // block, and if so, record them in UsesToRename. 1665 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1666 ++UI) { 1667 Instruction *User = cast<Instruction>(*UI); 1668 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1669 if (UserPN->getIncomingBlock(UI) == BB) 1670 continue; 1671 } else if (User->getParent() == BB) 1672 continue; 1673 1674 UsesToRename.push_back(&UI.getUse()); 1675 } 1676 1677 // If there are no uses outside the block, we're done with this instruction. 1678 if (UsesToRename.empty()) 1679 continue; 1680 1681 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1682 1683 // We found a use of I outside of BB. Rename all uses of I that are outside 1684 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1685 // with the two values we know. 1686 SSAUpdate.Initialize(I->getType(), I->getName()); 1687 SSAUpdate.AddAvailableValue(BB, I); 1688 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]); 1689 1690 while (!UsesToRename.empty()) 1691 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1692 DEBUG(dbgs() << "\n"); 1693 } 1694 1695 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge 1696 // that we nuked. 1697 RemovePredecessorAndSimplify(BB, PredBB, TD); 1698 1699 // Remove the unconditional branch at the end of the PredBB block. 1700 OldPredBranch->eraseFromParent(); 1701 1702 ++NumDupes; 1703 return true; 1704} 1705 1706 1707