JumpThreading.cpp revision bdabacdccae980b82b5d92e10697b8aeb3b94cfa
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; 447 if (V == 0) V = 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; 522 if (V == 0) V = 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 = 730 LVI->getPredicateOnEdge(CondCmp->getPredicate(), 731 CondCmp->getOperand(0), CondConst, *PI, BB); 732 if (Ret != Baseline) break; 733 } 734 735 // If we terminated early, then one of the values didn't match. 736 if (PI == PE) { 737 unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0; 738 unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1; 739 RemovePredecessorAndSimplify(CondBr->getSuccessor(ToRemove), BB, TD); 740 BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr); 741 CondBr->eraseFromParent(); 742 return true; 743 } 744 } 745 } 746 } 747 748 // Check for some cases that are worth simplifying. Right now we want to look 749 // for loads that are used by a switch or by the condition for the branch. If 750 // we see one, check to see if it's partially redundant. If so, insert a PHI 751 // which can then be used to thread the values. 752 // 753 Value *SimplifyValue = CondInst; 754 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 755 if (isa<Constant>(CondCmp->getOperand(1))) 756 SimplifyValue = CondCmp->getOperand(0); 757 758 // TODO: There are other places where load PRE would be profitable, such as 759 // more complex comparisons. 760 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 761 if (SimplifyPartiallyRedundantLoad(LI)) 762 return true; 763 764 765 // Handle a variety of cases where we are branching on something derived from 766 // a PHI node in the current block. If we can prove that any predecessors 767 // compute a predictable value based on a PHI node, thread those predecessors. 768 // 769 if (ProcessThreadableEdges(CondInst, BB)) 770 return true; 771 772 // If this is an otherwise-unfoldable branch on a phi node in the current 773 // block, see if we can simplify. 774 if (PHINode *PN = dyn_cast<PHINode>(CondInst)) 775 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 776 return ProcessBranchOnPHI(PN); 777 778 779 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify. 780 if (CondInst->getOpcode() == Instruction::Xor && 781 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 782 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst)); 783 784 785 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 786 // "(X == 4)", thread through this block. 787 788 return false; 789} 790 791/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that 792/// block that jump on exactly the same condition. This means that we almost 793/// always know the direction of the edge in the DESTBB: 794/// PREDBB: 795/// br COND, DESTBB, BBY 796/// DESTBB: 797/// br COND, BBZ, BBW 798/// 799/// If DESTBB has multiple predecessors, we can't just constant fold the branch 800/// in DESTBB, we have to thread over it. 801bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, 802 BasicBlock *BB) { 803 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); 804 805 // If both successors of PredBB go to DESTBB, we don't know anything. We can 806 // fold the branch to an unconditional one, which allows other recursive 807 // simplifications. 808 bool BranchDir; 809 if (PredBI->getSuccessor(1) != BB) 810 BranchDir = true; 811 else if (PredBI->getSuccessor(0) != BB) 812 BranchDir = false; 813 else { 814 DEBUG(dbgs() << " In block '" << PredBB->getName() 815 << "' folding terminator: " << *PredBB->getTerminator() << '\n'); 816 ++NumFolds; 817 ConstantFoldTerminator(PredBB); 818 return true; 819 } 820 821 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); 822 823 // If the dest block has one predecessor, just fix the branch condition to a 824 // constant and fold it. 825 if (BB->getSinglePredecessor()) { 826 DEBUG(dbgs() << " In block '" << BB->getName() 827 << "' folding condition to '" << BranchDir << "': " 828 << *BB->getTerminator() << '\n'); 829 ++NumFolds; 830 Value *OldCond = DestBI->getCondition(); 831 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()), 832 BranchDir)); 833 // Delete dead instructions before we fold the branch. Folding the branch 834 // can eliminate edges from the CFG which can end up deleting OldCond. 835 RecursivelyDeleteTriviallyDeadInstructions(OldCond); 836 ConstantFoldTerminator(BB); 837 return true; 838 } 839 840 841 // Next, figure out which successor we are threading to. 842 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); 843 844 SmallVector<BasicBlock*, 2> Preds; 845 Preds.push_back(PredBB); 846 847 // Ok, try to thread it! 848 return ThreadEdge(BB, Preds, SuccBB); 849} 850 851/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that 852/// block that switch on exactly the same condition. This means that we almost 853/// always know the direction of the edge in the DESTBB: 854/// PREDBB: 855/// switch COND [... DESTBB, BBY ... ] 856/// DESTBB: 857/// switch COND [... BBZ, BBW ] 858/// 859/// Optimizing switches like this is very important, because simplifycfg builds 860/// switches out of repeated 'if' conditions. 861bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, 862 BasicBlock *DestBB) { 863 // Can't thread edge to self. 864 if (PredBB == DestBB) 865 return false; 866 867 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); 868 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); 869 870 // There are a variety of optimizations that we can potentially do on these 871 // blocks: we order them from most to least preferable. 872 873 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB 874 // directly to their destination. This does not introduce *any* code size 875 // growth. Skip debug info first. 876 BasicBlock::iterator BBI = DestBB->begin(); 877 while (isa<DbgInfoIntrinsic>(BBI)) 878 BBI++; 879 880 // FIXME: Thread if it just contains a PHI. 881 if (isa<SwitchInst>(BBI)) { 882 bool MadeChange = false; 883 // Ignore the default edge for now. 884 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { 885 ConstantInt *DestVal = DestSI->getCaseValue(i); 886 BasicBlock *DestSucc = DestSI->getSuccessor(i); 887 888 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if 889 // PredSI has an explicit case for it. If so, forward. If it is covered 890 // by the default case, we can't update PredSI. 891 unsigned PredCase = PredSI->findCaseValue(DestVal); 892 if (PredCase == 0) continue; 893 894 // If PredSI doesn't go to DestBB on this value, then it won't reach the 895 // case on this condition. 896 if (PredSI->getSuccessor(PredCase) != DestBB && 897 DestSI->getSuccessor(i) != DestBB) 898 continue; 899 900 // Do not forward this if it already goes to this destination, this would 901 // be an infinite loop. 902 if (PredSI->getSuccessor(PredCase) == DestSucc) 903 continue; 904 905 // Otherwise, we're safe to make the change. Make sure that the edge from 906 // DestSI to DestSucc is not critical and has no PHI nodes. 907 DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI); 908 DEBUG(dbgs() << "THROUGH: " << *DestSI); 909 910 // If the destination has PHI nodes, just split the edge for updating 911 // simplicity. 912 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ 913 SplitCriticalEdge(DestSI, i, this); 914 DestSucc = DestSI->getSuccessor(i); 915 } 916 FoldSingleEntryPHINodes(DestSucc); 917 PredSI->setSuccessor(PredCase, DestSucc); 918 MadeChange = true; 919 } 920 921 if (MadeChange) 922 return true; 923 } 924 925 return false; 926} 927 928 929/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 930/// load instruction, eliminate it by replacing it with a PHI node. This is an 931/// important optimization that encourages jump threading, and needs to be run 932/// interlaced with other jump threading tasks. 933bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 934 // Don't hack volatile loads. 935 if (LI->isVolatile()) return false; 936 937 // If the load is defined in a block with exactly one predecessor, it can't be 938 // partially redundant. 939 BasicBlock *LoadBB = LI->getParent(); 940 if (LoadBB->getSinglePredecessor()) 941 return false; 942 943 Value *LoadedPtr = LI->getOperand(0); 944 945 // If the loaded operand is defined in the LoadBB, it can't be available. 946 // TODO: Could do simple PHI translation, that would be fun :) 947 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 948 if (PtrOp->getParent() == LoadBB) 949 return false; 950 951 // Scan a few instructions up from the load, to see if it is obviously live at 952 // the entry to its block. 953 BasicBlock::iterator BBIt = LI; 954 955 if (Value *AvailableVal = 956 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) { 957 // If the value if the load is locally available within the block, just use 958 // it. This frequently occurs for reg2mem'd allocas. 959 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 960 961 // If the returned value is the load itself, replace with an undef. This can 962 // only happen in dead loops. 963 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); 964 LI->replaceAllUsesWith(AvailableVal); 965 LI->eraseFromParent(); 966 return true; 967 } 968 969 // Otherwise, if we scanned the whole block and got to the top of the block, 970 // we know the block is locally transparent to the load. If not, something 971 // might clobber its value. 972 if (BBIt != LoadBB->begin()) 973 return false; 974 975 976 SmallPtrSet<BasicBlock*, 8> PredsScanned; 977 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 978 AvailablePredsTy AvailablePreds; 979 BasicBlock *OneUnavailablePred = 0; 980 981 // If we got here, the loaded value is transparent through to the start of the 982 // block. Check to see if it is available in any of the predecessor blocks. 983 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 984 PI != PE; ++PI) { 985 BasicBlock *PredBB = *PI; 986 987 // If we already scanned this predecessor, skip it. 988 if (!PredsScanned.insert(PredBB)) 989 continue; 990 991 // Scan the predecessor to see if the value is available in the pred. 992 BBIt = PredBB->end(); 993 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); 994 if (!PredAvailable) { 995 OneUnavailablePred = PredBB; 996 continue; 997 } 998 999 // If so, this load is partially redundant. Remember this info so that we 1000 // can create a PHI node. 1001 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 1002 } 1003 1004 // If the loaded value isn't available in any predecessor, it isn't partially 1005 // redundant. 1006 if (AvailablePreds.empty()) return false; 1007 1008 // Okay, the loaded value is available in at least one (and maybe all!) 1009 // predecessors. If the value is unavailable in more than one unique 1010 // predecessor, we want to insert a merge block for those common predecessors. 1011 // This ensures that we only have to insert one reload, thus not increasing 1012 // code size. 1013 BasicBlock *UnavailablePred = 0; 1014 1015 // If there is exactly one predecessor where the value is unavailable, the 1016 // already computed 'OneUnavailablePred' block is it. If it ends in an 1017 // unconditional branch, we know that it isn't a critical edge. 1018 if (PredsScanned.size() == AvailablePreds.size()+1 && 1019 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 1020 UnavailablePred = OneUnavailablePred; 1021 } else if (PredsScanned.size() != AvailablePreds.size()) { 1022 // Otherwise, we had multiple unavailable predecessors or we had a critical 1023 // edge from the one. 1024 SmallVector<BasicBlock*, 8> PredsToSplit; 1025 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 1026 1027 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 1028 AvailablePredSet.insert(AvailablePreds[i].first); 1029 1030 // Add all the unavailable predecessors to the PredsToSplit list. 1031 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 1032 PI != PE; ++PI) { 1033 BasicBlock *P = *PI; 1034 // If the predecessor is an indirect goto, we can't split the edge. 1035 if (isa<IndirectBrInst>(P->getTerminator())) 1036 return false; 1037 1038 if (!AvailablePredSet.count(P)) 1039 PredsToSplit.push_back(P); 1040 } 1041 1042 // Split them out to their own block. 1043 UnavailablePred = 1044 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), 1045 "thread-pre-split", this); 1046 } 1047 1048 // If the value isn't available in all predecessors, then there will be 1049 // exactly one where it isn't available. Insert a load on that edge and add 1050 // it to the AvailablePreds list. 1051 if (UnavailablePred) { 1052 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 1053 "Can't handle critical edge here!"); 1054 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false, 1055 LI->getAlignment(), 1056 UnavailablePred->getTerminator()); 1057 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 1058 } 1059 1060 // Now we know that each predecessor of this block has a value in 1061 // AvailablePreds, sort them for efficient access as we're walking the preds. 1062 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 1063 1064 // Create a PHI node at the start of the block for the PRE'd load value. 1065 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); 1066 PN->takeName(LI); 1067 1068 // Insert new entries into the PHI for each predecessor. A single block may 1069 // have multiple entries here. 1070 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; 1071 ++PI) { 1072 BasicBlock *P = *PI; 1073 AvailablePredsTy::iterator I = 1074 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 1075 std::make_pair(P, (Value*)0)); 1076 1077 assert(I != AvailablePreds.end() && I->first == P && 1078 "Didn't find entry for predecessor!"); 1079 1080 PN->addIncoming(I->second, I->first); 1081 } 1082 1083 //cerr << "PRE: " << *LI << *PN << "\n"; 1084 1085 LI->replaceAllUsesWith(PN); 1086 LI->eraseFromParent(); 1087 1088 return true; 1089} 1090 1091/// FindMostPopularDest - The specified list contains multiple possible 1092/// threadable destinations. Pick the one that occurs the most frequently in 1093/// the list. 1094static BasicBlock * 1095FindMostPopularDest(BasicBlock *BB, 1096 const SmallVectorImpl<std::pair<BasicBlock*, 1097 BasicBlock*> > &PredToDestList) { 1098 assert(!PredToDestList.empty()); 1099 1100 // Determine popularity. If there are multiple possible destinations, we 1101 // explicitly choose to ignore 'undef' destinations. We prefer to thread 1102 // blocks with known and real destinations to threading undef. We'll handle 1103 // them later if interesting. 1104 DenseMap<BasicBlock*, unsigned> DestPopularity; 1105 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1106 if (PredToDestList[i].second) 1107 DestPopularity[PredToDestList[i].second]++; 1108 1109 // Find the most popular dest. 1110 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin(); 1111 BasicBlock *MostPopularDest = DPI->first; 1112 unsigned Popularity = DPI->second; 1113 SmallVector<BasicBlock*, 4> SamePopularity; 1114 1115 for (++DPI; DPI != DestPopularity.end(); ++DPI) { 1116 // If the popularity of this entry isn't higher than the popularity we've 1117 // seen so far, ignore it. 1118 if (DPI->second < Popularity) 1119 ; // ignore. 1120 else if (DPI->second == Popularity) { 1121 // If it is the same as what we've seen so far, keep track of it. 1122 SamePopularity.push_back(DPI->first); 1123 } else { 1124 // If it is more popular, remember it. 1125 SamePopularity.clear(); 1126 MostPopularDest = DPI->first; 1127 Popularity = DPI->second; 1128 } 1129 } 1130 1131 // Okay, now we know the most popular destination. If there is more than 1132 // destination, we need to determine one. This is arbitrary, but we need 1133 // to make a deterministic decision. Pick the first one that appears in the 1134 // successor list. 1135 if (!SamePopularity.empty()) { 1136 SamePopularity.push_back(MostPopularDest); 1137 TerminatorInst *TI = BB->getTerminator(); 1138 for (unsigned i = 0; ; ++i) { 1139 assert(i != TI->getNumSuccessors() && "Didn't find any successor!"); 1140 1141 if (std::find(SamePopularity.begin(), SamePopularity.end(), 1142 TI->getSuccessor(i)) == SamePopularity.end()) 1143 continue; 1144 1145 MostPopularDest = TI->getSuccessor(i); 1146 break; 1147 } 1148 } 1149 1150 // Okay, we have finally picked the most popular destination. 1151 return MostPopularDest; 1152} 1153 1154bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) { 1155 // If threading this would thread across a loop header, don't even try to 1156 // thread the edge. 1157 if (LoopHeaders.count(BB)) 1158 return false; 1159 1160 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues; 1161 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues)) 1162 return false; 1163 1164 assert(!PredValues.empty() && 1165 "ComputeValueKnownInPredecessors returned true with no values"); 1166 1167 DEBUG(dbgs() << "IN BB: " << *BB; 1168 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1169 dbgs() << " BB '" << BB->getName() << "': FOUND condition = "; 1170 if (PredValues[i].first) 1171 dbgs() << *PredValues[i].first; 1172 else 1173 dbgs() << "UNDEF"; 1174 dbgs() << " for pred '" << PredValues[i].second->getName() 1175 << "'.\n"; 1176 }); 1177 1178 // Decide what we want to thread through. Convert our list of known values to 1179 // a list of known destinations for each pred. This also discards duplicate 1180 // predecessors and keeps track of the undefined inputs (which are represented 1181 // as a null dest in the PredToDestList). 1182 SmallPtrSet<BasicBlock*, 16> SeenPreds; 1183 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList; 1184 1185 BasicBlock *OnlyDest = 0; 1186 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL; 1187 1188 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1189 BasicBlock *Pred = PredValues[i].second; 1190 if (!SeenPreds.insert(Pred)) 1191 continue; // Duplicate predecessor entry. 1192 1193 // If the predecessor ends with an indirect goto, we can't change its 1194 // destination. 1195 if (isa<IndirectBrInst>(Pred->getTerminator())) 1196 continue; 1197 1198 ConstantInt *Val = PredValues[i].first; 1199 1200 BasicBlock *DestBB; 1201 if (Val == 0) // Undef. 1202 DestBB = 0; 1203 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 1204 DestBB = BI->getSuccessor(Val->isZero()); 1205 else { 1206 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); 1207 DestBB = SI->getSuccessor(SI->findCaseValue(Val)); 1208 } 1209 1210 // If we have exactly one destination, remember it for efficiency below. 1211 if (i == 0) 1212 OnlyDest = DestBB; 1213 else if (OnlyDest != DestBB) 1214 OnlyDest = MultipleDestSentinel; 1215 1216 PredToDestList.push_back(std::make_pair(Pred, DestBB)); 1217 } 1218 1219 // If all edges were unthreadable, we fail. 1220 if (PredToDestList.empty()) 1221 return false; 1222 1223 // Determine which is the most common successor. If we have many inputs and 1224 // this block is a switch, we want to start by threading the batch that goes 1225 // to the most popular destination first. If we only know about one 1226 // threadable destination (the common case) we can avoid this. 1227 BasicBlock *MostPopularDest = OnlyDest; 1228 1229 if (MostPopularDest == MultipleDestSentinel) 1230 MostPopularDest = FindMostPopularDest(BB, PredToDestList); 1231 1232 // Now that we know what the most popular destination is, factor all 1233 // predecessors that will jump to it into a single predecessor. 1234 SmallVector<BasicBlock*, 16> PredsToFactor; 1235 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1236 if (PredToDestList[i].second == MostPopularDest) { 1237 BasicBlock *Pred = PredToDestList[i].first; 1238 1239 // This predecessor may be a switch or something else that has multiple 1240 // edges to the block. Factor each of these edges by listing them 1241 // according to # occurrences in PredsToFactor. 1242 TerminatorInst *PredTI = Pred->getTerminator(); 1243 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i) 1244 if (PredTI->getSuccessor(i) == BB) 1245 PredsToFactor.push_back(Pred); 1246 } 1247 1248 // If the threadable edges are branching on an undefined value, we get to pick 1249 // the destination that these predecessors should get to. 1250 if (MostPopularDest == 0) 1251 MostPopularDest = BB->getTerminator()-> 1252 getSuccessor(GetBestDestForJumpOnUndef(BB)); 1253 1254 // Ok, try to thread it! 1255 return ThreadEdge(BB, PredsToFactor, MostPopularDest); 1256} 1257 1258/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on 1259/// a PHI node in the current block. See if there are any simplifications we 1260/// can do based on inputs to the phi node. 1261/// 1262bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) { 1263 BasicBlock *BB = PN->getParent(); 1264 1265 // TODO: We could make use of this to do it once for blocks with common PHI 1266 // values. 1267 SmallVector<BasicBlock*, 1> PredBBs; 1268 PredBBs.resize(1); 1269 1270 // If any of the predecessor blocks end in an unconditional branch, we can 1271 // *duplicate* the conditional branch into that block in order to further 1272 // encourage jump threading and to eliminate cases where we have branch on a 1273 // phi of an icmp (branch on icmp is much better). 1274 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1275 BasicBlock *PredBB = PN->getIncomingBlock(i); 1276 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator())) 1277 if (PredBr->isUnconditional()) { 1278 PredBBs[0] = PredBB; 1279 // Try to duplicate BB into PredBB. 1280 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs)) 1281 return true; 1282 } 1283 } 1284 1285 return false; 1286} 1287 1288/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on 1289/// a xor instruction in the current block. See if there are any 1290/// simplifications we can do based on inputs to the xor. 1291/// 1292bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) { 1293 BasicBlock *BB = BO->getParent(); 1294 1295 // If either the LHS or RHS of the xor is a constant, don't do this 1296 // optimization. 1297 if (isa<ConstantInt>(BO->getOperand(0)) || 1298 isa<ConstantInt>(BO->getOperand(1))) 1299 return false; 1300 1301 // If the first instruction in BB isn't a phi, we won't be able to infer 1302 // anything special about any particular predecessor. 1303 if (!isa<PHINode>(BB->front())) 1304 return false; 1305 1306 // If we have a xor as the branch input to this block, and we know that the 1307 // LHS or RHS of the xor in any predecessor is true/false, then we can clone 1308 // the condition into the predecessor and fix that value to true, saving some 1309 // logical ops on that path and encouraging other paths to simplify. 1310 // 1311 // This copies something like this: 1312 // 1313 // BB: 1314 // %X = phi i1 [1], [%X'] 1315 // %Y = icmp eq i32 %A, %B 1316 // %Z = xor i1 %X, %Y 1317 // br i1 %Z, ... 1318 // 1319 // Into: 1320 // BB': 1321 // %Y = icmp ne i32 %A, %B 1322 // br i1 %Z, ... 1323 1324 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues; 1325 bool isLHS = true; 1326 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) { 1327 assert(XorOpValues.empty()); 1328 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues)) 1329 return false; 1330 isLHS = false; 1331 } 1332 1333 assert(!XorOpValues.empty() && 1334 "ComputeValueKnownInPredecessors returned true with no values"); 1335 1336 // Scan the information to see which is most popular: true or false. The 1337 // predecessors can be of the set true, false, or undef. 1338 unsigned NumTrue = 0, NumFalse = 0; 1339 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1340 if (!XorOpValues[i].first) continue; // Ignore undefs for the count. 1341 if (XorOpValues[i].first->isZero()) 1342 ++NumFalse; 1343 else 1344 ++NumTrue; 1345 } 1346 1347 // Determine which value to split on, true, false, or undef if neither. 1348 ConstantInt *SplitVal = 0; 1349 if (NumTrue > NumFalse) 1350 SplitVal = ConstantInt::getTrue(BB->getContext()); 1351 else if (NumTrue != 0 || NumFalse != 0) 1352 SplitVal = ConstantInt::getFalse(BB->getContext()); 1353 1354 // Collect all of the blocks that this can be folded into so that we can 1355 // factor this once and clone it once. 1356 SmallVector<BasicBlock*, 8> BlocksToFoldInto; 1357 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1358 if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue; 1359 1360 BlocksToFoldInto.push_back(XorOpValues[i].second); 1361 } 1362 1363 // If we inferred a value for all of the predecessors, then duplication won't 1364 // help us. However, we can just replace the LHS or RHS with the constant. 1365 if (BlocksToFoldInto.size() == 1366 cast<PHINode>(BB->front()).getNumIncomingValues()) { 1367 if (SplitVal == 0) { 1368 // If all preds provide undef, just nuke the xor, because it is undef too. 1369 BO->replaceAllUsesWith(UndefValue::get(BO->getType())); 1370 BO->eraseFromParent(); 1371 } else if (SplitVal->isZero()) { 1372 // If all preds provide 0, replace the xor with the other input. 1373 BO->replaceAllUsesWith(BO->getOperand(isLHS)); 1374 BO->eraseFromParent(); 1375 } else { 1376 // If all preds provide 1, set the computed value to 1. 1377 BO->setOperand(!isLHS, SplitVal); 1378 } 1379 1380 return true; 1381 } 1382 1383 // Try to duplicate BB into PredBB. 1384 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto); 1385} 1386 1387 1388/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new 1389/// predecessor to the PHIBB block. If it has PHI nodes, add entries for 1390/// NewPred using the entries from OldPred (suitably mapped). 1391static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, 1392 BasicBlock *OldPred, 1393 BasicBlock *NewPred, 1394 DenseMap<Instruction*, Value*> &ValueMap) { 1395 for (BasicBlock::iterator PNI = PHIBB->begin(); 1396 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) { 1397 // Ok, we have a PHI node. Figure out what the incoming value was for the 1398 // DestBlock. 1399 Value *IV = PN->getIncomingValueForBlock(OldPred); 1400 1401 // Remap the value if necessary. 1402 if (Instruction *Inst = dyn_cast<Instruction>(IV)) { 1403 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst); 1404 if (I != ValueMap.end()) 1405 IV = I->second; 1406 } 1407 1408 PN->addIncoming(IV, NewPred); 1409 } 1410} 1411 1412/// ThreadEdge - We have decided that it is safe and profitable to factor the 1413/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB 1414/// across BB. Transform the IR to reflect this change. 1415bool JumpThreading::ThreadEdge(BasicBlock *BB, 1416 const SmallVectorImpl<BasicBlock*> &PredBBs, 1417 BasicBlock *SuccBB) { 1418 // If threading to the same block as we come from, we would infinite loop. 1419 if (SuccBB == BB) { 1420 DEBUG(dbgs() << " Not threading across BB '" << BB->getName() 1421 << "' - would thread to self!\n"); 1422 return false; 1423 } 1424 1425 // If threading this would thread across a loop header, don't thread the edge. 1426 // See the comments above FindLoopHeaders for justifications and caveats. 1427 if (LoopHeaders.count(BB)) { 1428 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName() 1429 << "' to dest BB '" << SuccBB->getName() 1430 << "' - it might create an irreducible loop!\n"); 1431 return false; 1432 } 1433 1434 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 1435 if (JumpThreadCost > Threshold) { 1436 DEBUG(dbgs() << " Not threading BB '" << BB->getName() 1437 << "' - Cost is too high: " << JumpThreadCost << "\n"); 1438 return false; 1439 } 1440 1441 // And finally, do it! Start by factoring the predecessors is needed. 1442 BasicBlock *PredBB; 1443 if (PredBBs.size() == 1) 1444 PredBB = PredBBs[0]; 1445 else { 1446 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1447 << " common predecessors.\n"); 1448 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), 1449 ".thr_comm", this); 1450 } 1451 1452 // And finally, do it! 1453 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '" 1454 << SuccBB->getName() << "' with cost: " << JumpThreadCost 1455 << ", across block:\n " 1456 << *BB << "\n"); 1457 1458 if (LVI) 1459 LVI->threadEdge(PredBB, BB, SuccBB); 1460 1461 // We are going to have to map operands from the original BB block to the new 1462 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 1463 // account for entry from PredBB. 1464 DenseMap<Instruction*, Value*> ValueMapping; 1465 1466 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), 1467 BB->getName()+".thread", 1468 BB->getParent(), BB); 1469 NewBB->moveAfter(PredBB); 1470 1471 BasicBlock::iterator BI = BB->begin(); 1472 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1473 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1474 1475 // Clone the non-phi instructions of BB into NewBB, keeping track of the 1476 // mapping and using it to remap operands in the cloned instructions. 1477 for (; !isa<TerminatorInst>(BI); ++BI) { 1478 Instruction *New = BI->clone(); 1479 New->setName(BI->getName()); 1480 NewBB->getInstList().push_back(New); 1481 ValueMapping[BI] = New; 1482 1483 // Remap operands to patch up intra-block references. 1484 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1485 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1486 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1487 if (I != ValueMapping.end()) 1488 New->setOperand(i, I->second); 1489 } 1490 } 1491 1492 // We didn't copy the terminator from BB over to NewBB, because there is now 1493 // an unconditional jump to SuccBB. Insert the unconditional jump. 1494 BranchInst::Create(SuccBB, NewBB); 1495 1496 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 1497 // PHI nodes for NewBB now. 1498 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping); 1499 1500 // If there were values defined in BB that are used outside the block, then we 1501 // now have to update all uses of the value to use either the original value, 1502 // the cloned value, or some PHI derived value. This can require arbitrary 1503 // PHI insertion, of which we are prepared to do, clean these up now. 1504 SSAUpdater SSAUpdate; 1505 SmallVector<Use*, 16> UsesToRename; 1506 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1507 // Scan all uses of this instruction to see if it is used outside of its 1508 // block, and if so, record them in UsesToRename. 1509 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1510 ++UI) { 1511 Instruction *User = cast<Instruction>(*UI); 1512 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1513 if (UserPN->getIncomingBlock(UI) == BB) 1514 continue; 1515 } else if (User->getParent() == BB) 1516 continue; 1517 1518 UsesToRename.push_back(&UI.getUse()); 1519 } 1520 1521 // If there are no uses outside the block, we're done with this instruction. 1522 if (UsesToRename.empty()) 1523 continue; 1524 1525 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1526 1527 // We found a use of I outside of BB. Rename all uses of I that are outside 1528 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1529 // with the two values we know. 1530 SSAUpdate.Initialize(I->getType(), I->getName()); 1531 SSAUpdate.AddAvailableValue(BB, I); 1532 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]); 1533 1534 while (!UsesToRename.empty()) 1535 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1536 DEBUG(dbgs() << "\n"); 1537 } 1538 1539 1540 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 1541 // NewBB instead of BB. This eliminates predecessors from BB, which requires 1542 // us to simplify any PHI nodes in BB. 1543 TerminatorInst *PredTerm = PredBB->getTerminator(); 1544 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 1545 if (PredTerm->getSuccessor(i) == BB) { 1546 RemovePredecessorAndSimplify(BB, PredBB, TD); 1547 PredTerm->setSuccessor(i, NewBB); 1548 } 1549 1550 // At this point, the IR is fully up to date and consistent. Do a quick scan 1551 // over the new instructions and zap any that are constants or dead. This 1552 // frequently happens because of phi translation. 1553 SimplifyInstructionsInBlock(NewBB, TD); 1554 1555 // Threaded an edge! 1556 ++NumThreads; 1557 return true; 1558} 1559 1560/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch 1561/// to BB which contains an i1 PHI node and a conditional branch on that PHI. 1562/// If we can duplicate the contents of BB up into PredBB do so now, this 1563/// improves the odds that the branch will be on an analyzable instruction like 1564/// a compare. 1565bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 1566 const SmallVectorImpl<BasicBlock *> &PredBBs) { 1567 assert(!PredBBs.empty() && "Can't handle an empty set"); 1568 1569 // If BB is a loop header, then duplicating this block outside the loop would 1570 // cause us to transform this into an irreducible loop, don't do this. 1571 // See the comments above FindLoopHeaders for justifications and caveats. 1572 if (LoopHeaders.count(BB)) { 1573 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName() 1574 << "' into predecessor block '" << PredBBs[0]->getName() 1575 << "' - it might create an irreducible loop!\n"); 1576 return false; 1577 } 1578 1579 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB); 1580 if (DuplicationCost > Threshold) { 1581 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName() 1582 << "' - Cost is too high: " << DuplicationCost << "\n"); 1583 return false; 1584 } 1585 1586 // And finally, do it! Start by factoring the predecessors is needed. 1587 BasicBlock *PredBB; 1588 if (PredBBs.size() == 1) 1589 PredBB = PredBBs[0]; 1590 else { 1591 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1592 << " common predecessors.\n"); 1593 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), 1594 ".thr_comm", this); 1595 } 1596 1597 // Okay, we decided to do this! Clone all the instructions in BB onto the end 1598 // of PredBB. 1599 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '" 1600 << PredBB->getName() << "' to eliminate branch on phi. Cost: " 1601 << DuplicationCost << " block is:" << *BB << "\n"); 1602 1603 // Unless PredBB ends with an unconditional branch, split the edge so that we 1604 // can just clone the bits from BB into the end of the new PredBB. 1605 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator()); 1606 1607 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) { 1608 PredBB = SplitEdge(PredBB, BB, this); 1609 OldPredBranch = cast<BranchInst>(PredBB->getTerminator()); 1610 } 1611 1612 // We are going to have to map operands from the original BB block into the 1613 // PredBB block. Evaluate PHI nodes in BB. 1614 DenseMap<Instruction*, Value*> ValueMapping; 1615 1616 BasicBlock::iterator BI = BB->begin(); 1617 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1618 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1619 1620 // Clone the non-phi instructions of BB into PredBB, keeping track of the 1621 // mapping and using it to remap operands in the cloned instructions. 1622 for (; BI != BB->end(); ++BI) { 1623 Instruction *New = BI->clone(); 1624 1625 // Remap operands to patch up intra-block references. 1626 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1627 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1628 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1629 if (I != ValueMapping.end()) 1630 New->setOperand(i, I->second); 1631 } 1632 1633 // If this instruction can be simplified after the operands are updated, 1634 // just use the simplified value instead. This frequently happens due to 1635 // phi translation. 1636 if (Value *IV = SimplifyInstruction(New, TD)) { 1637 delete New; 1638 ValueMapping[BI] = IV; 1639 } else { 1640 // Otherwise, insert the new instruction into the block. 1641 New->setName(BI->getName()); 1642 PredBB->getInstList().insert(OldPredBranch, New); 1643 ValueMapping[BI] = New; 1644 } 1645 } 1646 1647 // Check to see if the targets of the branch had PHI nodes. If so, we need to 1648 // add entries to the PHI nodes for branch from PredBB now. 1649 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator()); 1650 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB, 1651 ValueMapping); 1652 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB, 1653 ValueMapping); 1654 1655 // If there were values defined in BB that are used outside the block, then we 1656 // now have to update all uses of the value to use either the original value, 1657 // the cloned value, or some PHI derived value. This can require arbitrary 1658 // PHI insertion, of which we are prepared to do, clean these up now. 1659 SSAUpdater SSAUpdate; 1660 SmallVector<Use*, 16> UsesToRename; 1661 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1662 // Scan all uses of this instruction to see if it is used outside of its 1663 // block, and if so, record them in UsesToRename. 1664 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1665 ++UI) { 1666 Instruction *User = cast<Instruction>(*UI); 1667 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1668 if (UserPN->getIncomingBlock(UI) == BB) 1669 continue; 1670 } else if (User->getParent() == BB) 1671 continue; 1672 1673 UsesToRename.push_back(&UI.getUse()); 1674 } 1675 1676 // If there are no uses outside the block, we're done with this instruction. 1677 if (UsesToRename.empty()) 1678 continue; 1679 1680 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1681 1682 // We found a use of I outside of BB. Rename all uses of I that are outside 1683 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1684 // with the two values we know. 1685 SSAUpdate.Initialize(I->getType(), I->getName()); 1686 SSAUpdate.AddAvailableValue(BB, I); 1687 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]); 1688 1689 while (!UsesToRename.empty()) 1690 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1691 DEBUG(dbgs() << "\n"); 1692 } 1693 1694 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge 1695 // that we nuked. 1696 RemovePredecessorAndSimplify(BB, PredBB, TD); 1697 1698 // Remove the unconditional branch at the end of the PredBB block. 1699 OldPredBranch->eraseFromParent(); 1700 1701 ++NumDupes; 1702 return true; 1703} 1704 1705 1706