JumpThreading.cpp revision 660cab32fe5105bcaa17daa4704c24065ac0a7e6
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/Statistic.h" 28#include "llvm/ADT/STLExtras.h" 29#include "llvm/ADT/SmallPtrSet.h" 30#include "llvm/ADT/SmallSet.h" 31#include "llvm/Support/CommandLine.h" 32#include "llvm/Support/Debug.h" 33#include "llvm/Support/ValueHandle.h" 34#include "llvm/Support/raw_ostream.h" 35using namespace llvm; 36 37STATISTIC(NumThreads, "Number of jumps threaded"); 38STATISTIC(NumFolds, "Number of terminators folded"); 39STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi"); 40 41static cl::opt<unsigned> 42Threshold("jump-threading-threshold", 43 cl::desc("Max block size to duplicate for jump threading"), 44 cl::init(6), cl::Hidden); 45 46// Turn on use of LazyValueInfo. 47static cl::opt<bool> 48EnableLVI("enable-jump-threading-lvi", 49 cl::desc("Use LVI for jump threading"), 50 cl::init(true), 51 cl::ReallyHidden); 52 53 54 55namespace { 56 /// This pass performs 'jump threading', which looks at blocks that have 57 /// multiple predecessors and multiple successors. If one or more of the 58 /// predecessors of the block can be proven to always jump to one of the 59 /// successors, we forward the edge from the predecessor to the successor by 60 /// duplicating the contents of this block. 61 /// 62 /// An example of when this can occur is code like this: 63 /// 64 /// if () { ... 65 /// X = 4; 66 /// } 67 /// if (X < 3) { 68 /// 69 /// In this case, the unconditional branch at the end of the first if can be 70 /// revectored to the false side of the second if. 71 /// 72 class JumpThreading : public FunctionPass { 73 TargetData *TD; 74 LazyValueInfo *LVI; 75#ifdef NDEBUG 76 SmallPtrSet<BasicBlock*, 16> LoopHeaders; 77#else 78 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; 79#endif 80 public: 81 static char ID; // Pass identification 82 JumpThreading() : FunctionPass(ID) {} 83 84 bool runOnFunction(Function &F); 85 86 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 87 if (EnableLVI) 88 AU.addRequired<LazyValueInfo>(); 89 } 90 91 void FindLoopHeaders(Function &F); 92 bool ProcessBlock(BasicBlock *BB); 93 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs, 94 BasicBlock *SuccBB); 95 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 96 const SmallVectorImpl<BasicBlock *> &PredBBs); 97 98 typedef SmallVectorImpl<std::pair<ConstantInt*, 99 BasicBlock*> > PredValueInfo; 100 101 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, 102 PredValueInfo &Result); 103 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB); 104 105 106 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 107 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 108 109 bool ProcessBranchOnPHI(PHINode *PN); 110 bool ProcessBranchOnXOR(BinaryOperator *BO); 111 112 bool SimplifyPartiallyRedundantLoad(LoadInst *LI); 113 }; 114} 115 116char JumpThreading::ID = 0; 117INITIALIZE_PASS(JumpThreading, "jump-threading", 118 "Jump Threading", false, false); 119 120// Public interface to the Jump Threading pass 121FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 122 123/// runOnFunction - Top level algorithm. 124/// 125bool JumpThreading::runOnFunction(Function &F) { 126 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n"); 127 TD = getAnalysisIfAvailable<TargetData>(); 128 LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0; 129 130 FindLoopHeaders(F); 131 132 bool Changed, EverChanged = false; 133 do { 134 Changed = false; 135 for (Function::iterator I = F.begin(), E = F.end(); I != E;) { 136 BasicBlock *BB = I; 137 // Thread all of the branches we can over this block. 138 while (ProcessBlock(BB)) 139 Changed = true; 140 141 ++I; 142 143 // If the block is trivially dead, zap it. This eliminates the successor 144 // edges which simplifies the CFG. 145 if (pred_begin(BB) == pred_end(BB) && 146 BB != &BB->getParent()->getEntryBlock()) { 147 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName() 148 << "' with terminator: " << *BB->getTerminator() << '\n'); 149 LoopHeaders.erase(BB); 150 if (LVI) LVI->eraseBlock(BB); 151 DeleteDeadBlock(BB); 152 Changed = true; 153 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 154 // Can't thread an unconditional jump, but if the block is "almost 155 // empty", we can replace uses of it with uses of the successor and make 156 // this dead. 157 if (BI->isUnconditional() && 158 BB != &BB->getParent()->getEntryBlock()) { 159 BasicBlock::iterator BBI = BB->getFirstNonPHI(); 160 // Ignore dbg intrinsics. 161 while (isa<DbgInfoIntrinsic>(BBI)) 162 ++BBI; 163 // If the terminator is the only non-phi instruction, try to nuke it. 164 if (BBI->isTerminator()) { 165 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the 166 // block, we have to make sure it isn't in the LoopHeaders set. We 167 // reinsert afterward if needed. 168 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB); 169 BasicBlock *Succ = BI->getSuccessor(0); 170 171 // FIXME: It is always conservatively correct to drop the info 172 // for a block even if it doesn't get erased. This isn't totally 173 // awesome, but it allows us to use AssertingVH to prevent nasty 174 // dangling pointer issues within LazyValueInfo. 175 if (LVI) LVI->eraseBlock(BB); 176 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) { 177 Changed = true; 178 // If we deleted BB and BB was the header of a loop, then the 179 // successor is now the header of the loop. 180 BB = Succ; 181 } 182 183 if (ErasedFromLoopHeaders) 184 LoopHeaders.insert(BB); 185 } 186 } 187 } 188 } 189 EverChanged |= Changed; 190 } while (Changed); 191 192 LoopHeaders.clear(); 193 return EverChanged; 194} 195 196/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 197/// thread across it. 198static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { 199 /// Ignore PHI nodes, these will be flattened when duplication happens. 200 BasicBlock::const_iterator I = BB->getFirstNonPHI(); 201 202 // FIXME: THREADING will delete values that are just used to compute the 203 // branch, so they shouldn't count against the duplication cost. 204 205 206 // Sum up the cost of each instruction until we get to the terminator. Don't 207 // include the terminator because the copy won't include it. 208 unsigned Size = 0; 209 for (; !isa<TerminatorInst>(I); ++I) { 210 // Debugger intrinsics don't incur code size. 211 if (isa<DbgInfoIntrinsic>(I)) continue; 212 213 // If this is a pointer->pointer bitcast, it is free. 214 if (isa<BitCastInst>(I) && I->getType()->isPointerTy()) 215 continue; 216 217 // All other instructions count for at least one unit. 218 ++Size; 219 220 // Calls are more expensive. If they are non-intrinsic calls, we model them 221 // as having cost of 4. If they are a non-vector intrinsic, we model them 222 // as having cost of 2 total, and if they are a vector intrinsic, we model 223 // them as having cost 1. 224 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 225 if (!isa<IntrinsicInst>(CI)) 226 Size += 3; 227 else if (!CI->getType()->isVectorTy()) 228 Size += 1; 229 } 230 } 231 232 // Threading through a switch statement is particularly profitable. If this 233 // block ends in a switch, decrease its cost to make it more likely to happen. 234 if (isa<SwitchInst>(I)) 235 Size = Size > 6 ? Size-6 : 0; 236 237 return Size; 238} 239 240/// FindLoopHeaders - We do not want jump threading to turn proper loop 241/// structures into irreducible loops. Doing this breaks up the loop nesting 242/// hierarchy and pessimizes later transformations. To prevent this from 243/// happening, we first have to find the loop headers. Here we approximate this 244/// by finding targets of backedges in the CFG. 245/// 246/// Note that there definitely are cases when we want to allow threading of 247/// edges across a loop header. For example, threading a jump from outside the 248/// loop (the preheader) to an exit block of the loop is definitely profitable. 249/// It is also almost always profitable to thread backedges from within the loop 250/// to exit blocks, and is often profitable to thread backedges to other blocks 251/// within the loop (forming a nested loop). This simple analysis is not rich 252/// enough to track all of these properties and keep it up-to-date as the CFG 253/// mutates, so we don't allow any of these transformations. 254/// 255void JumpThreading::FindLoopHeaders(Function &F) { 256 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; 257 FindFunctionBackedges(F, Edges); 258 259 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 260 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); 261} 262 263/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see 264/// if we can infer that the value is a known ConstantInt in any of our 265/// predecessors. If so, return the known list of value and pred BB in the 266/// result vector. If a value is known to be undef, it is returned as null. 267/// 268/// This returns true if there were any known values. 269/// 270bool JumpThreading:: 271ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){ 272 // If V is a constantint, then it is known in all predecessors. 273 if (isa<ConstantInt>(V) || isa<UndefValue>(V)) { 274 ConstantInt *CI = dyn_cast<ConstantInt>(V); 275 276 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 277 Result.push_back(std::make_pair(CI, *PI)); 278 return true; 279 } 280 281 // If V is a non-instruction value, or an instruction in a different block, 282 // then it can't be derived from a PHI. 283 Instruction *I = dyn_cast<Instruction>(V); 284 if (I == 0 || I->getParent() != BB) { 285 286 // Okay, if this is a live-in value, see if it has a known value at the end 287 // of any of our predecessors. 288 // 289 // FIXME: This should be an edge property, not a block end property. 290 /// TODO: Per PR2563, we could infer value range information about a 291 /// predecessor based on its terminator. 292 // 293 if (LVI) { 294 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if 295 // "I" is a non-local compare-with-a-constant instruction. This would be 296 // able to handle value inequalities better, for example if the compare is 297 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available. 298 // Perhaps getConstantOnEdge should be smart enough to do this? 299 300 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 301 BasicBlock *P = *PI; 302 // If the value is known by LazyValueInfo to be a constant in a 303 // predecessor, use that information to try to thread this block. 304 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB); 305 if (PredCst == 0 || 306 (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst))) 307 continue; 308 309 Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), P)); 310 } 311 312 return !Result.empty(); 313 } 314 315 return false; 316 } 317 318 /// If I is a PHI node, then we know the incoming values for any constants. 319 if (PHINode *PN = dyn_cast<PHINode>(I)) { 320 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 321 Value *InVal = PN->getIncomingValue(i); 322 if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) { 323 ConstantInt *CI = dyn_cast<ConstantInt>(InVal); 324 Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i))); 325 } else if (LVI) { 326 Constant *CI = LVI->getConstantOnEdge(InVal, 327 PN->getIncomingBlock(i), BB); 328 ConstantInt *CInt = dyn_cast_or_null<ConstantInt>(CI); 329 if (CInt) 330 Result.push_back(std::make_pair(CInt, PN->getIncomingBlock(i))); 331 } 332 } 333 return !Result.empty(); 334 } 335 336 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals; 337 338 // Handle some boolean conditions. 339 if (I->getType()->getPrimitiveSizeInBits() == 1) { 340 // X | true -> true 341 // X & false -> false 342 if (I->getOpcode() == Instruction::Or || 343 I->getOpcode() == Instruction::And) { 344 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals); 345 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals); 346 347 if (LHSVals.empty() && RHSVals.empty()) 348 return false; 349 350 ConstantInt *InterestingVal; 351 if (I->getOpcode() == Instruction::Or) 352 InterestingVal = ConstantInt::getTrue(I->getContext()); 353 else 354 InterestingVal = ConstantInt::getFalse(I->getContext()); 355 356 SmallPtrSet<BasicBlock*, 4> LHSKnownBBs; 357 358 // Scan for the sentinel. If we find an undef, force it to the 359 // interesting value: x|undef -> true and x&undef -> false. 360 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) 361 if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) { 362 Result.push_back(LHSVals[i]); 363 Result.back().first = InterestingVal; 364 LHSKnownBBs.insert(LHSVals[i].second); 365 } 366 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i) 367 if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) { 368 // If we already inferred a value for this block on the LHS, don't 369 // re-add it. 370 if (!LHSKnownBBs.count(RHSVals[i].second)) { 371 Result.push_back(RHSVals[i]); 372 Result.back().first = InterestingVal; 373 } 374 } 375 return !Result.empty(); 376 377 // Try to process a few other binary operator patterns. 378 } else if (isa<BinaryOperator>(I)) { 379 380 } 381 382 // Handle the NOT form of XOR. 383 if (I->getOpcode() == Instruction::Xor && 384 isa<ConstantInt>(I->getOperand(1)) && 385 cast<ConstantInt>(I->getOperand(1))->isOne()) { 386 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result); 387 if (Result.empty()) 388 return false; 389 390 // Invert the known values. 391 for (unsigned i = 0, e = Result.size(); i != e; ++i) 392 if (Result[i].first) 393 Result[i].first = 394 cast<ConstantInt>(ConstantExpr::getNot(Result[i].first)); 395 return true; 396 } 397 398 // Try to simplify some other binary operator values. 399 } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) { 400 // AND or OR of a value with itself is that value. 401 ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)); 402 if (CI && (BO->getOpcode() == Instruction::And || 403 BO->getOpcode() == Instruction::Or)) { 404 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals; 405 ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals); 406 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) 407 if (LHSVals[i].first == CI) 408 Result.push_back(std::make_pair(CI, LHSVals[i].second)); 409 410 return !Result.empty(); 411 } 412 } 413 414 // Handle compare with phi operand, where the PHI is defined in this block. 415 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) { 416 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0)); 417 if (PN && PN->getParent() == BB) { 418 // We can do this simplification if any comparisons fold to true or false. 419 // See if any do. 420 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 421 BasicBlock *PredBB = PN->getIncomingBlock(i); 422 Value *LHS = PN->getIncomingValue(i); 423 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB); 424 425 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD); 426 if (Res == 0) { 427 if (!LVI || !isa<Constant>(RHS)) 428 continue; 429 430 LazyValueInfo::Tristate 431 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS, 432 cast<Constant>(RHS), PredBB, BB); 433 if (ResT == LazyValueInfo::Unknown) 434 continue; 435 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT); 436 } 437 438 if (isa<UndefValue>(Res)) 439 Result.push_back(std::make_pair((ConstantInt*)0, PredBB)); 440 else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res)) 441 Result.push_back(std::make_pair(CI, PredBB)); 442 } 443 444 return !Result.empty(); 445 } 446 447 448 // If comparing a live-in value against a constant, see if we know the 449 // live-in value on any predecessors. 450 if (LVI && isa<Constant>(Cmp->getOperand(1)) && 451 Cmp->getType()->isIntegerTy()) { 452 if (!isa<Instruction>(Cmp->getOperand(0)) || 453 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) { 454 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1)); 455 456 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){ 457 BasicBlock *P = *PI; 458 // If the value is known by LazyValueInfo to be a constant in a 459 // predecessor, use that information to try to thread this block. 460 LazyValueInfo::Tristate Res = 461 LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0), 462 RHSCst, P, BB); 463 if (Res == LazyValueInfo::Unknown) 464 continue; 465 466 Constant *ResC = ConstantInt::get(Cmp->getType(), Res); 467 Result.push_back(std::make_pair(cast<ConstantInt>(ResC), P)); 468 } 469 470 return !Result.empty(); 471 } 472 473 // Try to find a constant value for the LHS of an equality comparison, 474 // and evaluate it statically if we can. 475 if (Cmp->getPredicate() == CmpInst::ICMP_EQ || 476 Cmp->getPredicate() == CmpInst::ICMP_NE) { 477 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals; 478 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals); 479 480 ConstantInt *True = ConstantInt::getTrue(I->getContext()); 481 ConstantInt *False = ConstantInt::getFalse(I->getContext()); 482 if (Cmp->getPredicate() == CmpInst::ICMP_NE) std::swap(True, False); 483 484 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { 485 if (LHSVals[i].first == Cmp->getOperand(1)) 486 Result.push_back(std::make_pair(True, LHSVals[i].second)); 487 else 488 Result.push_back(std::make_pair(False, LHSVals[i].second)); 489 } 490 491 return !Result.empty(); 492 } 493 } 494 } 495 496 if (LVI) { 497 // If all else fails, see if LVI can figure out a constant value for us. 498 Constant *CI = LVI->getConstant(V, BB); 499 ConstantInt *CInt = dyn_cast_or_null<ConstantInt>(CI); 500 if (CInt) { 501 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 502 Result.push_back(std::make_pair(CInt, *PI)); 503 } 504 505 return !Result.empty(); 506 } 507 508 return false; 509} 510 511 512 513/// GetBestDestForBranchOnUndef - If we determine that the specified block ends 514/// in an undefined jump, decide which block is best to revector to. 515/// 516/// Since we can pick an arbitrary destination, we pick the successor with the 517/// fewest predecessors. This should reduce the in-degree of the others. 518/// 519static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) { 520 TerminatorInst *BBTerm = BB->getTerminator(); 521 unsigned MinSucc = 0; 522 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); 523 // Compute the successor with the minimum number of predecessors. 524 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 525 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { 526 TestBB = BBTerm->getSuccessor(i); 527 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 528 if (NumPreds < MinNumPreds) 529 MinSucc = i; 530 } 531 532 return MinSucc; 533} 534 535/// ProcessBlock - If there are any predecessors whose control can be threaded 536/// through to a successor, transform them now. 537bool JumpThreading::ProcessBlock(BasicBlock *BB) { 538 // If the block is trivially dead, just return and let the caller nuke it. 539 // This simplifies other transformations. 540 if (pred_begin(BB) == pred_end(BB) && 541 BB != &BB->getParent()->getEntryBlock()) 542 return false; 543 544 // If this block has a single predecessor, and if that pred has a single 545 // successor, merge the blocks. This encourages recursive jump threading 546 // because now the condition in this block can be threaded through 547 // predecessors of our predecessor block. 548 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) { 549 if (SinglePred->getTerminator()->getNumSuccessors() == 1 && 550 SinglePred != BB) { 551 // If SinglePred was a loop header, BB becomes one. 552 if (LoopHeaders.erase(SinglePred)) 553 LoopHeaders.insert(BB); 554 555 // Remember if SinglePred was the entry block of the function. If so, we 556 // will need to move BB back to the entry position. 557 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); 558 if (LVI) LVI->eraseBlock(SinglePred); 559 MergeBasicBlockIntoOnlyPred(BB); 560 561 if (isEntry && BB != &BB->getParent()->getEntryBlock()) 562 BB->moveBefore(&BB->getParent()->getEntryBlock()); 563 return true; 564 } 565 } 566 567 // Look to see if the terminator is a branch of switch, if not we can't thread 568 // it. 569 Value *Condition; 570 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 571 // Can't thread an unconditional jump. 572 if (BI->isUnconditional()) return false; 573 Condition = BI->getCondition(); 574 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) 575 Condition = SI->getCondition(); 576 else 577 return false; // Must be an invoke. 578 579 // If the terminator of this block is branching on a constant, simplify the 580 // terminator to an unconditional branch. This can occur due to threading in 581 // other blocks. 582 if (isa<ConstantInt>(Condition)) { 583 DEBUG(dbgs() << " In block '" << BB->getName() 584 << "' folding terminator: " << *BB->getTerminator() << '\n'); 585 ++NumFolds; 586 ConstantFoldTerminator(BB); 587 return true; 588 } 589 590 // If the terminator is branching on an undef, we can pick any of the 591 // successors to branch to. Let GetBestDestForJumpOnUndef decide. 592 if (isa<UndefValue>(Condition)) { 593 unsigned BestSucc = GetBestDestForJumpOnUndef(BB); 594 595 // Fold the branch/switch. 596 TerminatorInst *BBTerm = BB->getTerminator(); 597 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { 598 if (i == BestSucc) continue; 599 RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD); 600 } 601 602 DEBUG(dbgs() << " In block '" << BB->getName() 603 << "' folding undef terminator: " << *BBTerm << '\n'); 604 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm); 605 BBTerm->eraseFromParent(); 606 return true; 607 } 608 609 Instruction *CondInst = dyn_cast<Instruction>(Condition); 610 611 // If the condition is an instruction defined in another block, see if a 612 // predecessor has the same condition: 613 // br COND, BBX, BBY 614 // BBX: 615 // br COND, BBZ, BBW 616 if (!LVI && 617 !Condition->hasOneUse() && // Multiple uses. 618 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition. 619 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 620 if (isa<BranchInst>(BB->getTerminator())) { 621 for (; PI != E; ++PI) { 622 BasicBlock *P = *PI; 623 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator())) 624 if (PBI->isConditional() && PBI->getCondition() == Condition && 625 ProcessBranchOnDuplicateCond(P, BB)) 626 return true; 627 } 628 } else { 629 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator"); 630 for (; PI != E; ++PI) { 631 BasicBlock *P = *PI; 632 if (SwitchInst *PSI = dyn_cast<SwitchInst>(P->getTerminator())) 633 if (PSI->getCondition() == Condition && 634 ProcessSwitchOnDuplicateCond(P, BB)) 635 return true; 636 } 637 } 638 } 639 640 // All the rest of our checks depend on the condition being an instruction. 641 if (CondInst == 0) { 642 // FIXME: Unify this with code below. 643 if (LVI && ProcessThreadableEdges(Condition, BB)) 644 return true; 645 return false; 646 } 647 648 649 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { 650 if (!LVI && 651 (!isa<PHINode>(CondCmp->getOperand(0)) || 652 cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) { 653 // If we have a comparison, loop over the predecessors to see if there is 654 // a condition with a lexically identical value. 655 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 656 for (; PI != E; ++PI) { 657 BasicBlock *P = *PI; 658 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator())) 659 if (PBI->isConditional() && P != BB) { 660 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) { 661 if (CI->getOperand(0) == CondCmp->getOperand(0) && 662 CI->getOperand(1) == CondCmp->getOperand(1) && 663 CI->getPredicate() == CondCmp->getPredicate()) { 664 // TODO: Could handle things like (x != 4) --> (x == 17) 665 if (ProcessBranchOnDuplicateCond(P, BB)) 666 return true; 667 } 668 } 669 } 670 } 671 } 672 673 // For a comparison where the LHS is outside this block, it's possible 674 // that we've branch on it before. Used LVI to see if we can simplify 675 // the branch based on that. 676 BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); 677 Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1)); 678 if (LVI && CondBr && CondConst && CondBr->isConditional() && 679 (!isa<Instruction>(CondCmp->getOperand(0)) || 680 cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) { 681 // For predecessor edge, determine if the comparison is true or false 682 // on that edge. If they're all true or all false, we can simplify the 683 // branch. 684 // FIXME: We could handle mixed true/false by duplicating code. 685 unsigned Trues = 0, Falses = 0, predcount = 0; 686 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB);PI != PE; ++PI){ 687 ++predcount; 688 LazyValueInfo::Tristate Ret = 689 LVI->getPredicateOnEdge(CondCmp->getPredicate(), 690 CondCmp->getOperand(0), CondConst, *PI, BB); 691 if (Ret == LazyValueInfo::True) 692 ++Trues; 693 else if (Ret == LazyValueInfo::False) 694 ++Falses; 695 } 696 697 // If we can determine the branch direction statically, converted 698 // the conditional branch to an unconditional one. 699 if (Trues && Trues == predcount) { 700 RemovePredecessorAndSimplify(CondBr->getSuccessor(1), BB, TD); 701 BranchInst::Create(CondBr->getSuccessor(0), CondBr); 702 CondBr->eraseFromParent(); 703 return true; 704 } else if (Falses && Falses == predcount) { 705 RemovePredecessorAndSimplify(CondBr->getSuccessor(0), BB, TD); 706 BranchInst::Create(CondBr->getSuccessor(1), CondBr); 707 CondBr->eraseFromParent(); 708 return true; 709 } 710 } 711 } 712 713 // Check for some cases that are worth simplifying. Right now we want to look 714 // for loads that are used by a switch or by the condition for the branch. If 715 // we see one, check to see if it's partially redundant. If so, insert a PHI 716 // which can then be used to thread the values. 717 // 718 Value *SimplifyValue = CondInst; 719 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 720 if (isa<Constant>(CondCmp->getOperand(1))) 721 SimplifyValue = CondCmp->getOperand(0); 722 723 // TODO: There are other places where load PRE would be profitable, such as 724 // more complex comparisons. 725 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 726 if (SimplifyPartiallyRedundantLoad(LI)) 727 return true; 728 729 730 // Handle a variety of cases where we are branching on something derived from 731 // a PHI node in the current block. If we can prove that any predecessors 732 // compute a predictable value based on a PHI node, thread those predecessors. 733 // 734 if (ProcessThreadableEdges(CondInst, BB)) 735 return true; 736 737 // If this is an otherwise-unfoldable branch on a phi node in the current 738 // block, see if we can simplify. 739 if (PHINode *PN = dyn_cast<PHINode>(CondInst)) 740 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 741 return ProcessBranchOnPHI(PN); 742 743 744 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify. 745 if (CondInst->getOpcode() == Instruction::Xor && 746 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 747 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst)); 748 749 750 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 751 // "(X == 4)", thread through this block. 752 753 return false; 754} 755 756/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that 757/// block that jump on exactly the same condition. This means that we almost 758/// always know the direction of the edge in the DESTBB: 759/// PREDBB: 760/// br COND, DESTBB, BBY 761/// DESTBB: 762/// br COND, BBZ, BBW 763/// 764/// If DESTBB has multiple predecessors, we can't just constant fold the branch 765/// in DESTBB, we have to thread over it. 766bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, 767 BasicBlock *BB) { 768 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); 769 770 // If both successors of PredBB go to DESTBB, we don't know anything. We can 771 // fold the branch to an unconditional one, which allows other recursive 772 // simplifications. 773 bool BranchDir; 774 if (PredBI->getSuccessor(1) != BB) 775 BranchDir = true; 776 else if (PredBI->getSuccessor(0) != BB) 777 BranchDir = false; 778 else { 779 DEBUG(dbgs() << " In block '" << PredBB->getName() 780 << "' folding terminator: " << *PredBB->getTerminator() << '\n'); 781 ++NumFolds; 782 ConstantFoldTerminator(PredBB); 783 return true; 784 } 785 786 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); 787 788 // If the dest block has one predecessor, just fix the branch condition to a 789 // constant and fold it. 790 if (BB->getSinglePredecessor()) { 791 DEBUG(dbgs() << " In block '" << BB->getName() 792 << "' folding condition to '" << BranchDir << "': " 793 << *BB->getTerminator() << '\n'); 794 ++NumFolds; 795 Value *OldCond = DestBI->getCondition(); 796 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()), 797 BranchDir)); 798 // Delete dead instructions before we fold the branch. Folding the branch 799 // can eliminate edges from the CFG which can end up deleting OldCond. 800 RecursivelyDeleteTriviallyDeadInstructions(OldCond); 801 ConstantFoldTerminator(BB); 802 return true; 803 } 804 805 806 // Next, figure out which successor we are threading to. 807 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); 808 809 SmallVector<BasicBlock*, 2> Preds; 810 Preds.push_back(PredBB); 811 812 // Ok, try to thread it! 813 return ThreadEdge(BB, Preds, SuccBB); 814} 815 816/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that 817/// block that switch on exactly the same condition. This means that we almost 818/// always know the direction of the edge in the DESTBB: 819/// PREDBB: 820/// switch COND [... DESTBB, BBY ... ] 821/// DESTBB: 822/// switch COND [... BBZ, BBW ] 823/// 824/// Optimizing switches like this is very important, because simplifycfg builds 825/// switches out of repeated 'if' conditions. 826bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, 827 BasicBlock *DestBB) { 828 // Can't thread edge to self. 829 if (PredBB == DestBB) 830 return false; 831 832 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); 833 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); 834 835 // There are a variety of optimizations that we can potentially do on these 836 // blocks: we order them from most to least preferable. 837 838 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB 839 // directly to their destination. This does not introduce *any* code size 840 // growth. Skip debug info first. 841 BasicBlock::iterator BBI = DestBB->begin(); 842 while (isa<DbgInfoIntrinsic>(BBI)) 843 BBI++; 844 845 // FIXME: Thread if it just contains a PHI. 846 if (isa<SwitchInst>(BBI)) { 847 bool MadeChange = false; 848 // Ignore the default edge for now. 849 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { 850 ConstantInt *DestVal = DestSI->getCaseValue(i); 851 BasicBlock *DestSucc = DestSI->getSuccessor(i); 852 853 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if 854 // PredSI has an explicit case for it. If so, forward. If it is covered 855 // by the default case, we can't update PredSI. 856 unsigned PredCase = PredSI->findCaseValue(DestVal); 857 if (PredCase == 0) continue; 858 859 // If PredSI doesn't go to DestBB on this value, then it won't reach the 860 // case on this condition. 861 if (PredSI->getSuccessor(PredCase) != DestBB && 862 DestSI->getSuccessor(i) != DestBB) 863 continue; 864 865 // Do not forward this if it already goes to this destination, this would 866 // be an infinite loop. 867 if (PredSI->getSuccessor(PredCase) == DestSucc) 868 continue; 869 870 // Otherwise, we're safe to make the change. Make sure that the edge from 871 // DestSI to DestSucc is not critical and has no PHI nodes. 872 DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI); 873 DEBUG(dbgs() << "THROUGH: " << *DestSI); 874 875 // If the destination has PHI nodes, just split the edge for updating 876 // simplicity. 877 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ 878 SplitCriticalEdge(DestSI, i, this); 879 DestSucc = DestSI->getSuccessor(i); 880 } 881 FoldSingleEntryPHINodes(DestSucc); 882 PredSI->setSuccessor(PredCase, DestSucc); 883 MadeChange = true; 884 } 885 886 if (MadeChange) 887 return true; 888 } 889 890 return false; 891} 892 893 894/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 895/// load instruction, eliminate it by replacing it with a PHI node. This is an 896/// important optimization that encourages jump threading, and needs to be run 897/// interlaced with other jump threading tasks. 898bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 899 // Don't hack volatile loads. 900 if (LI->isVolatile()) return false; 901 902 // If the load is defined in a block with exactly one predecessor, it can't be 903 // partially redundant. 904 BasicBlock *LoadBB = LI->getParent(); 905 if (LoadBB->getSinglePredecessor()) 906 return false; 907 908 Value *LoadedPtr = LI->getOperand(0); 909 910 // If the loaded operand is defined in the LoadBB, it can't be available. 911 // TODO: Could do simple PHI translation, that would be fun :) 912 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 913 if (PtrOp->getParent() == LoadBB) 914 return false; 915 916 // Scan a few instructions up from the load, to see if it is obviously live at 917 // the entry to its block. 918 BasicBlock::iterator BBIt = LI; 919 920 if (Value *AvailableVal = 921 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) { 922 // If the value if the load is locally available within the block, just use 923 // it. This frequently occurs for reg2mem'd allocas. 924 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 925 926 // If the returned value is the load itself, replace with an undef. This can 927 // only happen in dead loops. 928 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); 929 LI->replaceAllUsesWith(AvailableVal); 930 LI->eraseFromParent(); 931 return true; 932 } 933 934 // Otherwise, if we scanned the whole block and got to the top of the block, 935 // we know the block is locally transparent to the load. If not, something 936 // might clobber its value. 937 if (BBIt != LoadBB->begin()) 938 return false; 939 940 941 SmallPtrSet<BasicBlock*, 8> PredsScanned; 942 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 943 AvailablePredsTy AvailablePreds; 944 BasicBlock *OneUnavailablePred = 0; 945 946 // If we got here, the loaded value is transparent through to the start of the 947 // block. Check to see if it is available in any of the predecessor blocks. 948 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 949 PI != PE; ++PI) { 950 BasicBlock *PredBB = *PI; 951 952 // If we already scanned this predecessor, skip it. 953 if (!PredsScanned.insert(PredBB)) 954 continue; 955 956 // Scan the predecessor to see if the value is available in the pred. 957 BBIt = PredBB->end(); 958 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); 959 if (!PredAvailable) { 960 OneUnavailablePred = PredBB; 961 continue; 962 } 963 964 // If so, this load is partially redundant. Remember this info so that we 965 // can create a PHI node. 966 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 967 } 968 969 // If the loaded value isn't available in any predecessor, it isn't partially 970 // redundant. 971 if (AvailablePreds.empty()) return false; 972 973 // Okay, the loaded value is available in at least one (and maybe all!) 974 // predecessors. If the value is unavailable in more than one unique 975 // predecessor, we want to insert a merge block for those common predecessors. 976 // This ensures that we only have to insert one reload, thus not increasing 977 // code size. 978 BasicBlock *UnavailablePred = 0; 979 980 // If there is exactly one predecessor where the value is unavailable, the 981 // already computed 'OneUnavailablePred' block is it. If it ends in an 982 // unconditional branch, we know that it isn't a critical edge. 983 if (PredsScanned.size() == AvailablePreds.size()+1 && 984 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 985 UnavailablePred = OneUnavailablePred; 986 } else if (PredsScanned.size() != AvailablePreds.size()) { 987 // Otherwise, we had multiple unavailable predecessors or we had a critical 988 // edge from the one. 989 SmallVector<BasicBlock*, 8> PredsToSplit; 990 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 991 992 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 993 AvailablePredSet.insert(AvailablePreds[i].first); 994 995 // Add all the unavailable predecessors to the PredsToSplit list. 996 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 997 PI != PE; ++PI) { 998 BasicBlock *P = *PI; 999 // If the predecessor is an indirect goto, we can't split the edge. 1000 if (isa<IndirectBrInst>(P->getTerminator())) 1001 return false; 1002 1003 if (!AvailablePredSet.count(P)) 1004 PredsToSplit.push_back(P); 1005 } 1006 1007 // Split them out to their own block. 1008 UnavailablePred = 1009 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), 1010 "thread-pre-split", this); 1011 } 1012 1013 // If the value isn't available in all predecessors, then there will be 1014 // exactly one where it isn't available. Insert a load on that edge and add 1015 // it to the AvailablePreds list. 1016 if (UnavailablePred) { 1017 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 1018 "Can't handle critical edge here!"); 1019 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false, 1020 LI->getAlignment(), 1021 UnavailablePred->getTerminator()); 1022 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 1023 } 1024 1025 // Now we know that each predecessor of this block has a value in 1026 // AvailablePreds, sort them for efficient access as we're walking the preds. 1027 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 1028 1029 // Create a PHI node at the start of the block for the PRE'd load value. 1030 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); 1031 PN->takeName(LI); 1032 1033 // Insert new entries into the PHI for each predecessor. A single block may 1034 // have multiple entries here. 1035 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; 1036 ++PI) { 1037 BasicBlock *P = *PI; 1038 AvailablePredsTy::iterator I = 1039 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 1040 std::make_pair(P, (Value*)0)); 1041 1042 assert(I != AvailablePreds.end() && I->first == P && 1043 "Didn't find entry for predecessor!"); 1044 1045 PN->addIncoming(I->second, I->first); 1046 } 1047 1048 //cerr << "PRE: " << *LI << *PN << "\n"; 1049 1050 LI->replaceAllUsesWith(PN); 1051 LI->eraseFromParent(); 1052 1053 return true; 1054} 1055 1056/// FindMostPopularDest - The specified list contains multiple possible 1057/// threadable destinations. Pick the one that occurs the most frequently in 1058/// the list. 1059static BasicBlock * 1060FindMostPopularDest(BasicBlock *BB, 1061 const SmallVectorImpl<std::pair<BasicBlock*, 1062 BasicBlock*> > &PredToDestList) { 1063 assert(!PredToDestList.empty()); 1064 1065 // Determine popularity. If there are multiple possible destinations, we 1066 // explicitly choose to ignore 'undef' destinations. We prefer to thread 1067 // blocks with known and real destinations to threading undef. We'll handle 1068 // them later if interesting. 1069 DenseMap<BasicBlock*, unsigned> DestPopularity; 1070 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1071 if (PredToDestList[i].second) 1072 DestPopularity[PredToDestList[i].second]++; 1073 1074 // Find the most popular dest. 1075 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin(); 1076 BasicBlock *MostPopularDest = DPI->first; 1077 unsigned Popularity = DPI->second; 1078 SmallVector<BasicBlock*, 4> SamePopularity; 1079 1080 for (++DPI; DPI != DestPopularity.end(); ++DPI) { 1081 // If the popularity of this entry isn't higher than the popularity we've 1082 // seen so far, ignore it. 1083 if (DPI->second < Popularity) 1084 ; // ignore. 1085 else if (DPI->second == Popularity) { 1086 // If it is the same as what we've seen so far, keep track of it. 1087 SamePopularity.push_back(DPI->first); 1088 } else { 1089 // If it is more popular, remember it. 1090 SamePopularity.clear(); 1091 MostPopularDest = DPI->first; 1092 Popularity = DPI->second; 1093 } 1094 } 1095 1096 // Okay, now we know the most popular destination. If there is more than 1097 // destination, we need to determine one. This is arbitrary, but we need 1098 // to make a deterministic decision. Pick the first one that appears in the 1099 // successor list. 1100 if (!SamePopularity.empty()) { 1101 SamePopularity.push_back(MostPopularDest); 1102 TerminatorInst *TI = BB->getTerminator(); 1103 for (unsigned i = 0; ; ++i) { 1104 assert(i != TI->getNumSuccessors() && "Didn't find any successor!"); 1105 1106 if (std::find(SamePopularity.begin(), SamePopularity.end(), 1107 TI->getSuccessor(i)) == SamePopularity.end()) 1108 continue; 1109 1110 MostPopularDest = TI->getSuccessor(i); 1111 break; 1112 } 1113 } 1114 1115 // Okay, we have finally picked the most popular destination. 1116 return MostPopularDest; 1117} 1118 1119bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) { 1120 // If threading this would thread across a loop header, don't even try to 1121 // thread the edge. 1122 if (LoopHeaders.count(BB)) 1123 return false; 1124 1125 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues; 1126 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues)) 1127 return false; 1128 assert(!PredValues.empty() && 1129 "ComputeValueKnownInPredecessors returned true with no values"); 1130 1131 DEBUG(dbgs() << "IN BB: " << *BB; 1132 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1133 dbgs() << " BB '" << BB->getName() << "': FOUND condition = "; 1134 if (PredValues[i].first) 1135 dbgs() << *PredValues[i].first; 1136 else 1137 dbgs() << "UNDEF"; 1138 dbgs() << " for pred '" << PredValues[i].second->getName() 1139 << "'.\n"; 1140 }); 1141 1142 // Decide what we want to thread through. Convert our list of known values to 1143 // a list of known destinations for each pred. This also discards duplicate 1144 // predecessors and keeps track of the undefined inputs (which are represented 1145 // as a null dest in the PredToDestList). 1146 SmallPtrSet<BasicBlock*, 16> SeenPreds; 1147 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList; 1148 1149 BasicBlock *OnlyDest = 0; 1150 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL; 1151 1152 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1153 BasicBlock *Pred = PredValues[i].second; 1154 if (!SeenPreds.insert(Pred)) 1155 continue; // Duplicate predecessor entry. 1156 1157 // If the predecessor ends with an indirect goto, we can't change its 1158 // destination. 1159 if (isa<IndirectBrInst>(Pred->getTerminator())) 1160 continue; 1161 1162 ConstantInt *Val = PredValues[i].first; 1163 1164 BasicBlock *DestBB; 1165 if (Val == 0) // Undef. 1166 DestBB = 0; 1167 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 1168 DestBB = BI->getSuccessor(Val->isZero()); 1169 else { 1170 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); 1171 DestBB = SI->getSuccessor(SI->findCaseValue(Val)); 1172 } 1173 1174 // If we have exactly one destination, remember it for efficiency below. 1175 if (i == 0) 1176 OnlyDest = DestBB; 1177 else if (OnlyDest != DestBB) 1178 OnlyDest = MultipleDestSentinel; 1179 1180 PredToDestList.push_back(std::make_pair(Pred, DestBB)); 1181 } 1182 1183 // If all edges were unthreadable, we fail. 1184 if (PredToDestList.empty()) 1185 return false; 1186 1187 // Determine which is the most common successor. If we have many inputs and 1188 // this block is a switch, we want to start by threading the batch that goes 1189 // to the most popular destination first. If we only know about one 1190 // threadable destination (the common case) we can avoid this. 1191 BasicBlock *MostPopularDest = OnlyDest; 1192 1193 if (MostPopularDest == MultipleDestSentinel) 1194 MostPopularDest = FindMostPopularDest(BB, PredToDestList); 1195 1196 // Now that we know what the most popular destination is, factor all 1197 // predecessors that will jump to it into a single predecessor. 1198 SmallVector<BasicBlock*, 16> PredsToFactor; 1199 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1200 if (PredToDestList[i].second == MostPopularDest) { 1201 BasicBlock *Pred = PredToDestList[i].first; 1202 1203 // This predecessor may be a switch or something else that has multiple 1204 // edges to the block. Factor each of these edges by listing them 1205 // according to # occurrences in PredsToFactor. 1206 TerminatorInst *PredTI = Pred->getTerminator(); 1207 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i) 1208 if (PredTI->getSuccessor(i) == BB) 1209 PredsToFactor.push_back(Pred); 1210 } 1211 1212 // If the threadable edges are branching on an undefined value, we get to pick 1213 // the destination that these predecessors should get to. 1214 if (MostPopularDest == 0) 1215 MostPopularDest = BB->getTerminator()-> 1216 getSuccessor(GetBestDestForJumpOnUndef(BB)); 1217 1218 // Ok, try to thread it! 1219 return ThreadEdge(BB, PredsToFactor, MostPopularDest); 1220} 1221 1222/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on 1223/// a PHI node in the current block. See if there are any simplifications we 1224/// can do based on inputs to the phi node. 1225/// 1226bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) { 1227 BasicBlock *BB = PN->getParent(); 1228 1229 // TODO: We could make use of this to do it once for blocks with common PHI 1230 // values. 1231 SmallVector<BasicBlock*, 1> PredBBs; 1232 PredBBs.resize(1); 1233 1234 // If any of the predecessor blocks end in an unconditional branch, we can 1235 // *duplicate* the conditional branch into that block in order to further 1236 // encourage jump threading and to eliminate cases where we have branch on a 1237 // phi of an icmp (branch on icmp is much better). 1238 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1239 BasicBlock *PredBB = PN->getIncomingBlock(i); 1240 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator())) 1241 if (PredBr->isUnconditional()) { 1242 PredBBs[0] = PredBB; 1243 // Try to duplicate BB into PredBB. 1244 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs)) 1245 return true; 1246 } 1247 } 1248 1249 return false; 1250} 1251 1252/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on 1253/// a xor instruction in the current block. See if there are any 1254/// simplifications we can do based on inputs to the xor. 1255/// 1256bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) { 1257 BasicBlock *BB = BO->getParent(); 1258 1259 // If either the LHS or RHS of the xor is a constant, don't do this 1260 // optimization. 1261 if (isa<ConstantInt>(BO->getOperand(0)) || 1262 isa<ConstantInt>(BO->getOperand(1))) 1263 return false; 1264 1265 // If the first instruction in BB isn't a phi, we won't be able to infer 1266 // anything special about any particular predecessor. 1267 if (!isa<PHINode>(BB->front())) 1268 return false; 1269 1270 // If we have a xor as the branch input to this block, and we know that the 1271 // LHS or RHS of the xor in any predecessor is true/false, then we can clone 1272 // the condition into the predecessor and fix that value to true, saving some 1273 // logical ops on that path and encouraging other paths to simplify. 1274 // 1275 // This copies something like this: 1276 // 1277 // BB: 1278 // %X = phi i1 [1], [%X'] 1279 // %Y = icmp eq i32 %A, %B 1280 // %Z = xor i1 %X, %Y 1281 // br i1 %Z, ... 1282 // 1283 // Into: 1284 // BB': 1285 // %Y = icmp ne i32 %A, %B 1286 // br i1 %Z, ... 1287 1288 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues; 1289 bool isLHS = true; 1290 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) { 1291 assert(XorOpValues.empty()); 1292 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues)) 1293 return false; 1294 isLHS = false; 1295 } 1296 1297 assert(!XorOpValues.empty() && 1298 "ComputeValueKnownInPredecessors returned true with no values"); 1299 1300 // Scan the information to see which is most popular: true or false. The 1301 // predecessors can be of the set true, false, or undef. 1302 unsigned NumTrue = 0, NumFalse = 0; 1303 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1304 if (!XorOpValues[i].first) continue; // Ignore undefs for the count. 1305 if (XorOpValues[i].first->isZero()) 1306 ++NumFalse; 1307 else 1308 ++NumTrue; 1309 } 1310 1311 // Determine which value to split on, true, false, or undef if neither. 1312 ConstantInt *SplitVal = 0; 1313 if (NumTrue > NumFalse) 1314 SplitVal = ConstantInt::getTrue(BB->getContext()); 1315 else if (NumTrue != 0 || NumFalse != 0) 1316 SplitVal = ConstantInt::getFalse(BB->getContext()); 1317 1318 // Collect all of the blocks that this can be folded into so that we can 1319 // factor this once and clone it once. 1320 SmallVector<BasicBlock*, 8> BlocksToFoldInto; 1321 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1322 if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue; 1323 1324 BlocksToFoldInto.push_back(XorOpValues[i].second); 1325 } 1326 1327 // If we inferred a value for all of the predecessors, then duplication won't 1328 // help us. However, we can just replace the LHS or RHS with the constant. 1329 if (BlocksToFoldInto.size() == 1330 cast<PHINode>(BB->front()).getNumIncomingValues()) { 1331 if (SplitVal == 0) { 1332 // If all preds provide undef, just nuke the xor, because it is undef too. 1333 BO->replaceAllUsesWith(UndefValue::get(BO->getType())); 1334 BO->eraseFromParent(); 1335 } else if (SplitVal->isZero()) { 1336 // If all preds provide 0, replace the xor with the other input. 1337 BO->replaceAllUsesWith(BO->getOperand(isLHS)); 1338 BO->eraseFromParent(); 1339 } else { 1340 // If all preds provide 1, set the computed value to 1. 1341 BO->setOperand(!isLHS, SplitVal); 1342 } 1343 1344 return true; 1345 } 1346 1347 // Try to duplicate BB into PredBB. 1348 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto); 1349} 1350 1351 1352/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new 1353/// predecessor to the PHIBB block. If it has PHI nodes, add entries for 1354/// NewPred using the entries from OldPred (suitably mapped). 1355static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, 1356 BasicBlock *OldPred, 1357 BasicBlock *NewPred, 1358 DenseMap<Instruction*, Value*> &ValueMap) { 1359 for (BasicBlock::iterator PNI = PHIBB->begin(); 1360 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) { 1361 // Ok, we have a PHI node. Figure out what the incoming value was for the 1362 // DestBlock. 1363 Value *IV = PN->getIncomingValueForBlock(OldPred); 1364 1365 // Remap the value if necessary. 1366 if (Instruction *Inst = dyn_cast<Instruction>(IV)) { 1367 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst); 1368 if (I != ValueMap.end()) 1369 IV = I->second; 1370 } 1371 1372 PN->addIncoming(IV, NewPred); 1373 } 1374} 1375 1376/// ThreadEdge - We have decided that it is safe and profitable to factor the 1377/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB 1378/// across BB. Transform the IR to reflect this change. 1379bool JumpThreading::ThreadEdge(BasicBlock *BB, 1380 const SmallVectorImpl<BasicBlock*> &PredBBs, 1381 BasicBlock *SuccBB) { 1382 // If threading to the same block as we come from, we would infinite loop. 1383 if (SuccBB == BB) { 1384 DEBUG(dbgs() << " Not threading across BB '" << BB->getName() 1385 << "' - would thread to self!\n"); 1386 return false; 1387 } 1388 1389 // If threading this would thread across a loop header, don't thread the edge. 1390 // See the comments above FindLoopHeaders for justifications and caveats. 1391 if (LoopHeaders.count(BB)) { 1392 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName() 1393 << "' to dest BB '" << SuccBB->getName() 1394 << "' - it might create an irreducible loop!\n"); 1395 return false; 1396 } 1397 1398 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 1399 if (JumpThreadCost > Threshold) { 1400 DEBUG(dbgs() << " Not threading BB '" << BB->getName() 1401 << "' - Cost is too high: " << JumpThreadCost << "\n"); 1402 return false; 1403 } 1404 1405 // And finally, do it! Start by factoring the predecessors is needed. 1406 BasicBlock *PredBB; 1407 if (PredBBs.size() == 1) 1408 PredBB = PredBBs[0]; 1409 else { 1410 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1411 << " common predecessors.\n"); 1412 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), 1413 ".thr_comm", this); 1414 } 1415 1416 // And finally, do it! 1417 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '" 1418 << SuccBB->getName() << "' with cost: " << JumpThreadCost 1419 << ", across block:\n " 1420 << *BB << "\n"); 1421 1422 if (LVI) 1423 LVI->threadEdge(PredBB, BB, SuccBB); 1424 1425 // We are going to have to map operands from the original BB block to the new 1426 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 1427 // account for entry from PredBB. 1428 DenseMap<Instruction*, Value*> ValueMapping; 1429 1430 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), 1431 BB->getName()+".thread", 1432 BB->getParent(), BB); 1433 NewBB->moveAfter(PredBB); 1434 1435 BasicBlock::iterator BI = BB->begin(); 1436 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1437 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1438 1439 // Clone the non-phi instructions of BB into NewBB, keeping track of the 1440 // mapping and using it to remap operands in the cloned instructions. 1441 for (; !isa<TerminatorInst>(BI); ++BI) { 1442 Instruction *New = BI->clone(); 1443 New->setName(BI->getName()); 1444 NewBB->getInstList().push_back(New); 1445 ValueMapping[BI] = New; 1446 1447 // Remap operands to patch up intra-block references. 1448 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1449 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1450 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1451 if (I != ValueMapping.end()) 1452 New->setOperand(i, I->second); 1453 } 1454 } 1455 1456 // We didn't copy the terminator from BB over to NewBB, because there is now 1457 // an unconditional jump to SuccBB. Insert the unconditional jump. 1458 BranchInst::Create(SuccBB, NewBB); 1459 1460 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 1461 // PHI nodes for NewBB now. 1462 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping); 1463 1464 // If there were values defined in BB that are used outside the block, then we 1465 // now have to update all uses of the value to use either the original value, 1466 // the cloned value, or some PHI derived value. This can require arbitrary 1467 // PHI insertion, of which we are prepared to do, clean these up now. 1468 SSAUpdater SSAUpdate; 1469 SmallVector<Use*, 16> UsesToRename; 1470 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1471 // Scan all uses of this instruction to see if it is used outside of its 1472 // block, and if so, record them in UsesToRename. 1473 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1474 ++UI) { 1475 Instruction *User = cast<Instruction>(*UI); 1476 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1477 if (UserPN->getIncomingBlock(UI) == BB) 1478 continue; 1479 } else if (User->getParent() == BB) 1480 continue; 1481 1482 UsesToRename.push_back(&UI.getUse()); 1483 } 1484 1485 // If there are no uses outside the block, we're done with this instruction. 1486 if (UsesToRename.empty()) 1487 continue; 1488 1489 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1490 1491 // We found a use of I outside of BB. Rename all uses of I that are outside 1492 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1493 // with the two values we know. 1494 SSAUpdate.Initialize(I); 1495 SSAUpdate.AddAvailableValue(BB, I); 1496 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]); 1497 1498 while (!UsesToRename.empty()) 1499 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1500 DEBUG(dbgs() << "\n"); 1501 } 1502 1503 1504 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 1505 // NewBB instead of BB. This eliminates predecessors from BB, which requires 1506 // us to simplify any PHI nodes in BB. 1507 TerminatorInst *PredTerm = PredBB->getTerminator(); 1508 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 1509 if (PredTerm->getSuccessor(i) == BB) { 1510 RemovePredecessorAndSimplify(BB, PredBB, TD); 1511 PredTerm->setSuccessor(i, NewBB); 1512 } 1513 1514 // At this point, the IR is fully up to date and consistent. Do a quick scan 1515 // over the new instructions and zap any that are constants or dead. This 1516 // frequently happens because of phi translation. 1517 SimplifyInstructionsInBlock(NewBB, TD); 1518 1519 // Threaded an edge! 1520 ++NumThreads; 1521 return true; 1522} 1523 1524/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch 1525/// to BB which contains an i1 PHI node and a conditional branch on that PHI. 1526/// If we can duplicate the contents of BB up into PredBB do so now, this 1527/// improves the odds that the branch will be on an analyzable instruction like 1528/// a compare. 1529bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 1530 const SmallVectorImpl<BasicBlock *> &PredBBs) { 1531 assert(!PredBBs.empty() && "Can't handle an empty set"); 1532 1533 // If BB is a loop header, then duplicating this block outside the loop would 1534 // cause us to transform this into an irreducible loop, don't do this. 1535 // See the comments above FindLoopHeaders for justifications and caveats. 1536 if (LoopHeaders.count(BB)) { 1537 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName() 1538 << "' into predecessor block '" << PredBBs[0]->getName() 1539 << "' - it might create an irreducible loop!\n"); 1540 return false; 1541 } 1542 1543 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB); 1544 if (DuplicationCost > Threshold) { 1545 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName() 1546 << "' - Cost is too high: " << DuplicationCost << "\n"); 1547 return false; 1548 } 1549 1550 // And finally, do it! Start by factoring the predecessors is needed. 1551 BasicBlock *PredBB; 1552 if (PredBBs.size() == 1) 1553 PredBB = PredBBs[0]; 1554 else { 1555 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1556 << " common predecessors.\n"); 1557 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), 1558 ".thr_comm", this); 1559 } 1560 1561 // Okay, we decided to do this! Clone all the instructions in BB onto the end 1562 // of PredBB. 1563 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '" 1564 << PredBB->getName() << "' to eliminate branch on phi. Cost: " 1565 << DuplicationCost << " block is:" << *BB << "\n"); 1566 1567 // Unless PredBB ends with an unconditional branch, split the edge so that we 1568 // can just clone the bits from BB into the end of the new PredBB. 1569 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator()); 1570 1571 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) { 1572 PredBB = SplitEdge(PredBB, BB, this); 1573 OldPredBranch = cast<BranchInst>(PredBB->getTerminator()); 1574 } 1575 1576 // We are going to have to map operands from the original BB block into the 1577 // PredBB block. Evaluate PHI nodes in BB. 1578 DenseMap<Instruction*, Value*> ValueMapping; 1579 1580 BasicBlock::iterator BI = BB->begin(); 1581 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1582 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1583 1584 // Clone the non-phi instructions of BB into PredBB, keeping track of the 1585 // mapping and using it to remap operands in the cloned instructions. 1586 for (; BI != BB->end(); ++BI) { 1587 Instruction *New = BI->clone(); 1588 1589 // Remap operands to patch up intra-block references. 1590 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1591 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1592 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1593 if (I != ValueMapping.end()) 1594 New->setOperand(i, I->second); 1595 } 1596 1597 // If this instruction can be simplified after the operands are updated, 1598 // just use the simplified value instead. This frequently happens due to 1599 // phi translation. 1600 if (Value *IV = SimplifyInstruction(New, TD)) { 1601 delete New; 1602 ValueMapping[BI] = IV; 1603 } else { 1604 // Otherwise, insert the new instruction into the block. 1605 New->setName(BI->getName()); 1606 PredBB->getInstList().insert(OldPredBranch, New); 1607 ValueMapping[BI] = New; 1608 } 1609 } 1610 1611 // Check to see if the targets of the branch had PHI nodes. If so, we need to 1612 // add entries to the PHI nodes for branch from PredBB now. 1613 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator()); 1614 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB, 1615 ValueMapping); 1616 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB, 1617 ValueMapping); 1618 1619 // If there were values defined in BB that are used outside the block, then we 1620 // now have to update all uses of the value to use either the original value, 1621 // the cloned value, or some PHI derived value. This can require arbitrary 1622 // PHI insertion, of which we are prepared to do, clean these up now. 1623 SSAUpdater SSAUpdate; 1624 SmallVector<Use*, 16> UsesToRename; 1625 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1626 // Scan all uses of this instruction to see if it is used outside of its 1627 // block, and if so, record them in UsesToRename. 1628 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1629 ++UI) { 1630 Instruction *User = cast<Instruction>(*UI); 1631 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1632 if (UserPN->getIncomingBlock(UI) == BB) 1633 continue; 1634 } else if (User->getParent() == BB) 1635 continue; 1636 1637 UsesToRename.push_back(&UI.getUse()); 1638 } 1639 1640 // If there are no uses outside the block, we're done with this instruction. 1641 if (UsesToRename.empty()) 1642 continue; 1643 1644 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1645 1646 // We found a use of I outside of BB. Rename all uses of I that are outside 1647 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1648 // with the two values we know. 1649 SSAUpdate.Initialize(I); 1650 SSAUpdate.AddAvailableValue(BB, I); 1651 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]); 1652 1653 while (!UsesToRename.empty()) 1654 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1655 DEBUG(dbgs() << "\n"); 1656 } 1657 1658 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge 1659 // that we nuked. 1660 RemovePredecessorAndSimplify(BB, PredBB, TD); 1661 1662 // Remove the unconditional branch at the end of the PredBB block. 1663 OldPredBranch->eraseFromParent(); 1664 1665 ++NumDupes; 1666 return true; 1667} 1668 1669 1670