JumpThreading.cpp revision 263508
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/ADT/DenseMap.h" 17#include "llvm/ADT/DenseSet.h" 18#include "llvm/ADT/STLExtras.h" 19#include "llvm/ADT/SmallPtrSet.h" 20#include "llvm/ADT/SmallSet.h" 21#include "llvm/ADT/Statistic.h" 22#include "llvm/Analysis/CFG.h" 23#include "llvm/Analysis/ConstantFolding.h" 24#include "llvm/Analysis/InstructionSimplify.h" 25#include "llvm/Analysis/LazyValueInfo.h" 26#include "llvm/Analysis/Loads.h" 27#include "llvm/IR/DataLayout.h" 28#include "llvm/IR/IntrinsicInst.h" 29#include "llvm/IR/LLVMContext.h" 30#include "llvm/Pass.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" 35#include "llvm/Target/TargetLibraryInfo.h" 36#include "llvm/Transforms/Utils/BasicBlockUtils.h" 37#include "llvm/Transforms/Utils/Local.h" 38#include "llvm/Transforms/Utils/SSAUpdater.h" 39using namespace llvm; 40 41STATISTIC(NumThreads, "Number of jumps threaded"); 42STATISTIC(NumFolds, "Number of terminators folded"); 43STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi"); 44 45static cl::opt<unsigned> 46Threshold("jump-threading-threshold", 47 cl::desc("Max block size to duplicate for jump threading"), 48 cl::init(6), cl::Hidden); 49 50namespace { 51 // These are at global scope so static functions can use them too. 52 typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo; 53 typedef SmallVector<std::pair<Constant*, BasicBlock*>, 8> PredValueInfoTy; 54 55 // This is used to keep track of what kind of constant we're currently hoping 56 // to find. 57 enum ConstantPreference { 58 WantInteger, 59 WantBlockAddress 60 }; 61 62 /// This pass performs 'jump threading', which looks at blocks that have 63 /// multiple predecessors and multiple successors. If one or more of the 64 /// predecessors of the block can be proven to always jump to one of the 65 /// successors, we forward the edge from the predecessor to the successor by 66 /// duplicating the contents of this block. 67 /// 68 /// An example of when this can occur is code like this: 69 /// 70 /// if () { ... 71 /// X = 4; 72 /// } 73 /// if (X < 3) { 74 /// 75 /// In this case, the unconditional branch at the end of the first if can be 76 /// revectored to the false side of the second if. 77 /// 78 class JumpThreading : public FunctionPass { 79 DataLayout *TD; 80 TargetLibraryInfo *TLI; 81 LazyValueInfo *LVI; 82#ifdef NDEBUG 83 SmallPtrSet<BasicBlock*, 16> LoopHeaders; 84#else 85 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; 86#endif 87 DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet; 88 89 // RAII helper for updating the recursion stack. 90 struct RecursionSetRemover { 91 DenseSet<std::pair<Value*, BasicBlock*> > &TheSet; 92 std::pair<Value*, BasicBlock*> ThePair; 93 94 RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S, 95 std::pair<Value*, BasicBlock*> P) 96 : TheSet(S), ThePair(P) { } 97 98 ~RecursionSetRemover() { 99 TheSet.erase(ThePair); 100 } 101 }; 102 public: 103 static char ID; // Pass identification 104 JumpThreading() : FunctionPass(ID) { 105 initializeJumpThreadingPass(*PassRegistry::getPassRegistry()); 106 } 107 108 bool runOnFunction(Function &F); 109 110 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 111 AU.addRequired<LazyValueInfo>(); 112 AU.addPreserved<LazyValueInfo>(); 113 AU.addRequired<TargetLibraryInfo>(); 114 } 115 116 void FindLoopHeaders(Function &F); 117 bool ProcessBlock(BasicBlock *BB); 118 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs, 119 BasicBlock *SuccBB); 120 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 121 const SmallVectorImpl<BasicBlock *> &PredBBs); 122 123 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, 124 PredValueInfo &Result, 125 ConstantPreference Preference); 126 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB, 127 ConstantPreference Preference); 128 129 bool ProcessBranchOnPHI(PHINode *PN); 130 bool ProcessBranchOnXOR(BinaryOperator *BO); 131 132 bool SimplifyPartiallyRedundantLoad(LoadInst *LI); 133 bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB); 134 }; 135} 136 137char JumpThreading::ID = 0; 138INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading", 139 "Jump Threading", false, false) 140INITIALIZE_PASS_DEPENDENCY(LazyValueInfo) 141INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) 142INITIALIZE_PASS_END(JumpThreading, "jump-threading", 143 "Jump Threading", false, false) 144 145// Public interface to the Jump Threading pass 146FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 147 148/// runOnFunction - Top level algorithm. 149/// 150bool JumpThreading::runOnFunction(Function &F) { 151 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n"); 152 TD = getAnalysisIfAvailable<DataLayout>(); 153 TLI = &getAnalysis<TargetLibraryInfo>(); 154 LVI = &getAnalysis<LazyValueInfo>(); 155 156 FindLoopHeaders(F); 157 158 bool Changed, EverChanged = false; 159 do { 160 Changed = false; 161 for (Function::iterator I = F.begin(), E = F.end(); I != E;) { 162 BasicBlock *BB = I; 163 // Thread all of the branches we can over this block. 164 while (ProcessBlock(BB)) 165 Changed = true; 166 167 ++I; 168 169 // If the block is trivially dead, zap it. This eliminates the successor 170 // edges which simplifies the CFG. 171 if (pred_begin(BB) == pred_end(BB) && 172 BB != &BB->getParent()->getEntryBlock()) { 173 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName() 174 << "' with terminator: " << *BB->getTerminator() << '\n'); 175 LoopHeaders.erase(BB); 176 LVI->eraseBlock(BB); 177 DeleteDeadBlock(BB); 178 Changed = true; 179 continue; 180 } 181 182 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()); 183 184 // Can't thread an unconditional jump, but if the block is "almost 185 // empty", we can replace uses of it with uses of the successor and make 186 // this dead. 187 if (BI && BI->isUnconditional() && 188 BB != &BB->getParent()->getEntryBlock() && 189 // If the terminator is the only non-phi instruction, try to nuke it. 190 BB->getFirstNonPHIOrDbg()->isTerminator()) { 191 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the 192 // block, we have to make sure it isn't in the LoopHeaders set. We 193 // reinsert afterward if needed. 194 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB); 195 BasicBlock *Succ = BI->getSuccessor(0); 196 197 // FIXME: It is always conservatively correct to drop the info 198 // for a block even if it doesn't get erased. This isn't totally 199 // awesome, but it allows us to use AssertingVH to prevent nasty 200 // dangling pointer issues within LazyValueInfo. 201 LVI->eraseBlock(BB); 202 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) { 203 Changed = true; 204 // If we deleted BB and BB was the header of a loop, then the 205 // successor is now the header of the loop. 206 BB = Succ; 207 } 208 209 if (ErasedFromLoopHeaders) 210 LoopHeaders.insert(BB); 211 } 212 } 213 EverChanged |= Changed; 214 } while (Changed); 215 216 LoopHeaders.clear(); 217 return EverChanged; 218} 219 220/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 221/// thread across it. Stop scanning the block when passing the threshold. 222static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB, 223 unsigned Threshold) { 224 /// Ignore PHI nodes, these will be flattened when duplication happens. 225 BasicBlock::const_iterator I = BB->getFirstNonPHI(); 226 227 // FIXME: THREADING will delete values that are just used to compute the 228 // branch, so they shouldn't count against the duplication cost. 229 230 // Sum up the cost of each instruction until we get to the terminator. Don't 231 // include the terminator because the copy won't include it. 232 unsigned Size = 0; 233 for (; !isa<TerminatorInst>(I); ++I) { 234 235 // Stop scanning the block if we've reached the threshold. 236 if (Size > Threshold) 237 return Size; 238 239 // Debugger intrinsics don't incur code size. 240 if (isa<DbgInfoIntrinsic>(I)) continue; 241 242 // If this is a pointer->pointer bitcast, it is free. 243 if (isa<BitCastInst>(I) && I->getType()->isPointerTy()) 244 continue; 245 246 // All other instructions count for at least one unit. 247 ++Size; 248 249 // Calls are more expensive. If they are non-intrinsic calls, we model them 250 // as having cost of 4. If they are a non-vector intrinsic, we model them 251 // as having cost of 2 total, and if they are a vector intrinsic, we model 252 // them as having cost 1. 253 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 254 if (CI->hasFnAttr(Attribute::NoDuplicate)) 255 // Blocks with NoDuplicate are modelled as having infinite cost, so they 256 // are never duplicated. 257 return ~0U; 258 else if (!isa<IntrinsicInst>(CI)) 259 Size += 3; 260 else if (!CI->getType()->isVectorTy()) 261 Size += 1; 262 } 263 } 264 265 // Threading through a switch statement is particularly profitable. If this 266 // block ends in a switch, decrease its cost to make it more likely to happen. 267 if (isa<SwitchInst>(I)) 268 Size = Size > 6 ? Size-6 : 0; 269 270 // The same holds for indirect branches, but slightly more so. 271 if (isa<IndirectBrInst>(I)) 272 Size = Size > 8 ? Size-8 : 0; 273 274 return Size; 275} 276 277/// FindLoopHeaders - We do not want jump threading to turn proper loop 278/// structures into irreducible loops. Doing this breaks up the loop nesting 279/// hierarchy and pessimizes later transformations. To prevent this from 280/// happening, we first have to find the loop headers. Here we approximate this 281/// by finding targets of backedges in the CFG. 282/// 283/// Note that there definitely are cases when we want to allow threading of 284/// edges across a loop header. For example, threading a jump from outside the 285/// loop (the preheader) to an exit block of the loop is definitely profitable. 286/// It is also almost always profitable to thread backedges from within the loop 287/// to exit blocks, and is often profitable to thread backedges to other blocks 288/// within the loop (forming a nested loop). This simple analysis is not rich 289/// enough to track all of these properties and keep it up-to-date as the CFG 290/// mutates, so we don't allow any of these transformations. 291/// 292void JumpThreading::FindLoopHeaders(Function &F) { 293 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; 294 FindFunctionBackedges(F, Edges); 295 296 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 297 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); 298} 299 300/// getKnownConstant - Helper method to determine if we can thread over a 301/// terminator with the given value as its condition, and if so what value to 302/// use for that. What kind of value this is depends on whether we want an 303/// integer or a block address, but an undef is always accepted. 304/// Returns null if Val is null or not an appropriate constant. 305static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) { 306 if (!Val) 307 return 0; 308 309 // Undef is "known" enough. 310 if (UndefValue *U = dyn_cast<UndefValue>(Val)) 311 return U; 312 313 if (Preference == WantBlockAddress) 314 return dyn_cast<BlockAddress>(Val->stripPointerCasts()); 315 316 return dyn_cast<ConstantInt>(Val); 317} 318 319/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see 320/// if we can infer that the value is a known ConstantInt/BlockAddress or undef 321/// in any of our predecessors. If so, return the known list of value and pred 322/// BB in the result vector. 323/// 324/// This returns true if there were any known values. 325/// 326bool JumpThreading:: 327ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result, 328 ConstantPreference Preference) { 329 // This method walks up use-def chains recursively. Because of this, we could 330 // get into an infinite loop going around loops in the use-def chain. To 331 // prevent this, keep track of what (value, block) pairs we've already visited 332 // and terminate the search if we loop back to them 333 if (!RecursionSet.insert(std::make_pair(V, BB)).second) 334 return false; 335 336 // An RAII help to remove this pair from the recursion set once the recursion 337 // stack pops back out again. 338 RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB)); 339 340 // If V is a constant, then it is known in all predecessors. 341 if (Constant *KC = getKnownConstant(V, Preference)) { 342 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 343 Result.push_back(std::make_pair(KC, *PI)); 344 345 return true; 346 } 347 348 // If V is a non-instruction value, or an instruction in a different block, 349 // then it can't be derived from a PHI. 350 Instruction *I = dyn_cast<Instruction>(V); 351 if (I == 0 || I->getParent() != BB) { 352 353 // Okay, if this is a live-in value, see if it has a known value at the end 354 // of any of our predecessors. 355 // 356 // FIXME: This should be an edge property, not a block end property. 357 /// TODO: Per PR2563, we could infer value range information about a 358 /// predecessor based on its terminator. 359 // 360 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if 361 // "I" is a non-local compare-with-a-constant instruction. This would be 362 // able to handle value inequalities better, for example if the compare is 363 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available. 364 // Perhaps getConstantOnEdge should be smart enough to do this? 365 366 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 367 BasicBlock *P = *PI; 368 // If the value is known by LazyValueInfo to be a constant in a 369 // predecessor, use that information to try to thread this block. 370 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB); 371 if (Constant *KC = getKnownConstant(PredCst, Preference)) 372 Result.push_back(std::make_pair(KC, P)); 373 } 374 375 return !Result.empty(); 376 } 377 378 /// If I is a PHI node, then we know the incoming values for any constants. 379 if (PHINode *PN = dyn_cast<PHINode>(I)) { 380 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 381 Value *InVal = PN->getIncomingValue(i); 382 if (Constant *KC = getKnownConstant(InVal, Preference)) { 383 Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i))); 384 } else { 385 Constant *CI = LVI->getConstantOnEdge(InVal, 386 PN->getIncomingBlock(i), BB); 387 if (Constant *KC = getKnownConstant(CI, Preference)) 388 Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i))); 389 } 390 } 391 392 return !Result.empty(); 393 } 394 395 PredValueInfoTy LHSVals, RHSVals; 396 397 // Handle some boolean conditions. 398 if (I->getType()->getPrimitiveSizeInBits() == 1) { 399 assert(Preference == WantInteger && "One-bit non-integer type?"); 400 // X | true -> true 401 // X & false -> false 402 if (I->getOpcode() == Instruction::Or || 403 I->getOpcode() == Instruction::And) { 404 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals, 405 WantInteger); 406 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals, 407 WantInteger); 408 409 if (LHSVals.empty() && RHSVals.empty()) 410 return false; 411 412 ConstantInt *InterestingVal; 413 if (I->getOpcode() == Instruction::Or) 414 InterestingVal = ConstantInt::getTrue(I->getContext()); 415 else 416 InterestingVal = ConstantInt::getFalse(I->getContext()); 417 418 SmallPtrSet<BasicBlock*, 4> LHSKnownBBs; 419 420 // Scan for the sentinel. If we find an undef, force it to the 421 // interesting value: x|undef -> true and x&undef -> false. 422 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) 423 if (LHSVals[i].first == InterestingVal || 424 isa<UndefValue>(LHSVals[i].first)) { 425 Result.push_back(LHSVals[i]); 426 Result.back().first = InterestingVal; 427 LHSKnownBBs.insert(LHSVals[i].second); 428 } 429 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i) 430 if (RHSVals[i].first == InterestingVal || 431 isa<UndefValue>(RHSVals[i].first)) { 432 // If we already inferred a value for this block on the LHS, don't 433 // re-add it. 434 if (!LHSKnownBBs.count(RHSVals[i].second)) { 435 Result.push_back(RHSVals[i]); 436 Result.back().first = InterestingVal; 437 } 438 } 439 440 return !Result.empty(); 441 } 442 443 // Handle the NOT form of XOR. 444 if (I->getOpcode() == Instruction::Xor && 445 isa<ConstantInt>(I->getOperand(1)) && 446 cast<ConstantInt>(I->getOperand(1))->isOne()) { 447 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result, 448 WantInteger); 449 if (Result.empty()) 450 return false; 451 452 // Invert the known values. 453 for (unsigned i = 0, e = Result.size(); i != e; ++i) 454 Result[i].first = ConstantExpr::getNot(Result[i].first); 455 456 return true; 457 } 458 459 // Try to simplify some other binary operator values. 460 } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) { 461 assert(Preference != WantBlockAddress 462 && "A binary operator creating a block address?"); 463 if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) { 464 PredValueInfoTy LHSVals; 465 ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals, 466 WantInteger); 467 468 // Try to use constant folding to simplify the binary operator. 469 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { 470 Constant *V = LHSVals[i].first; 471 Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI); 472 473 if (Constant *KC = getKnownConstant(Folded, WantInteger)) 474 Result.push_back(std::make_pair(KC, LHSVals[i].second)); 475 } 476 } 477 478 return !Result.empty(); 479 } 480 481 // Handle compare with phi operand, where the PHI is defined in this block. 482 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) { 483 assert(Preference == WantInteger && "Compares only produce integers"); 484 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0)); 485 if (PN && PN->getParent() == BB) { 486 // We can do this simplification if any comparisons fold to true or false. 487 // See if any do. 488 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 489 BasicBlock *PredBB = PN->getIncomingBlock(i); 490 Value *LHS = PN->getIncomingValue(i); 491 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB); 492 493 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD); 494 if (Res == 0) { 495 if (!isa<Constant>(RHS)) 496 continue; 497 498 LazyValueInfo::Tristate 499 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS, 500 cast<Constant>(RHS), PredBB, BB); 501 if (ResT == LazyValueInfo::Unknown) 502 continue; 503 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT); 504 } 505 506 if (Constant *KC = getKnownConstant(Res, WantInteger)) 507 Result.push_back(std::make_pair(KC, PredBB)); 508 } 509 510 return !Result.empty(); 511 } 512 513 514 // If comparing a live-in value against a constant, see if we know the 515 // live-in value on any predecessors. 516 if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) { 517 if (!isa<Instruction>(Cmp->getOperand(0)) || 518 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) { 519 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1)); 520 521 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){ 522 BasicBlock *P = *PI; 523 // If the value is known by LazyValueInfo to be a constant in a 524 // predecessor, use that information to try to thread this block. 525 LazyValueInfo::Tristate Res = 526 LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0), 527 RHSCst, P, BB); 528 if (Res == LazyValueInfo::Unknown) 529 continue; 530 531 Constant *ResC = ConstantInt::get(Cmp->getType(), Res); 532 Result.push_back(std::make_pair(ResC, P)); 533 } 534 535 return !Result.empty(); 536 } 537 538 // Try to find a constant value for the LHS of a comparison, 539 // and evaluate it statically if we can. 540 if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) { 541 PredValueInfoTy LHSVals; 542 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals, 543 WantInteger); 544 545 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { 546 Constant *V = LHSVals[i].first; 547 Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(), 548 V, CmpConst); 549 if (Constant *KC = getKnownConstant(Folded, WantInteger)) 550 Result.push_back(std::make_pair(KC, LHSVals[i].second)); 551 } 552 553 return !Result.empty(); 554 } 555 } 556 } 557 558 if (SelectInst *SI = dyn_cast<SelectInst>(I)) { 559 // Handle select instructions where at least one operand is a known constant 560 // and we can figure out the condition value for any predecessor block. 561 Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference); 562 Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference); 563 PredValueInfoTy Conds; 564 if ((TrueVal || FalseVal) && 565 ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds, 566 WantInteger)) { 567 for (unsigned i = 0, e = Conds.size(); i != e; ++i) { 568 Constant *Cond = Conds[i].first; 569 570 // Figure out what value to use for the condition. 571 bool KnownCond; 572 if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) { 573 // A known boolean. 574 KnownCond = CI->isOne(); 575 } else { 576 assert(isa<UndefValue>(Cond) && "Unexpected condition value"); 577 // Either operand will do, so be sure to pick the one that's a known 578 // constant. 579 // FIXME: Do this more cleverly if both values are known constants? 580 KnownCond = (TrueVal != 0); 581 } 582 583 // See if the select has a known constant value for this predecessor. 584 if (Constant *Val = KnownCond ? TrueVal : FalseVal) 585 Result.push_back(std::make_pair(Val, Conds[i].second)); 586 } 587 588 return !Result.empty(); 589 } 590 } 591 592 // If all else fails, see if LVI can figure out a constant value for us. 593 Constant *CI = LVI->getConstant(V, BB); 594 if (Constant *KC = getKnownConstant(CI, Preference)) { 595 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 596 Result.push_back(std::make_pair(KC, *PI)); 597 } 598 599 return !Result.empty(); 600} 601 602 603 604/// GetBestDestForBranchOnUndef - If we determine that the specified block ends 605/// in an undefined jump, decide which block is best to revector to. 606/// 607/// Since we can pick an arbitrary destination, we pick the successor with the 608/// fewest predecessors. This should reduce the in-degree of the others. 609/// 610static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) { 611 TerminatorInst *BBTerm = BB->getTerminator(); 612 unsigned MinSucc = 0; 613 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); 614 // Compute the successor with the minimum number of predecessors. 615 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 616 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { 617 TestBB = BBTerm->getSuccessor(i); 618 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 619 if (NumPreds < MinNumPreds) { 620 MinSucc = i; 621 MinNumPreds = NumPreds; 622 } 623 } 624 625 return MinSucc; 626} 627 628static bool hasAddressTakenAndUsed(BasicBlock *BB) { 629 if (!BB->hasAddressTaken()) return false; 630 631 // If the block has its address taken, it may be a tree of dead constants 632 // hanging off of it. These shouldn't keep the block alive. 633 BlockAddress *BA = BlockAddress::get(BB); 634 BA->removeDeadConstantUsers(); 635 return !BA->use_empty(); 636} 637 638/// ProcessBlock - If there are any predecessors whose control can be threaded 639/// through to a successor, transform them now. 640bool JumpThreading::ProcessBlock(BasicBlock *BB) { 641 // If the block is trivially dead, just return and let the caller nuke it. 642 // This simplifies other transformations. 643 if (pred_begin(BB) == pred_end(BB) && 644 BB != &BB->getParent()->getEntryBlock()) 645 return false; 646 647 // If this block has a single predecessor, and if that pred has a single 648 // successor, merge the blocks. This encourages recursive jump threading 649 // because now the condition in this block can be threaded through 650 // predecessors of our predecessor block. 651 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) { 652 if (SinglePred->getTerminator()->getNumSuccessors() == 1 && 653 SinglePred != BB && !hasAddressTakenAndUsed(BB)) { 654 // If SinglePred was a loop header, BB becomes one. 655 if (LoopHeaders.erase(SinglePred)) 656 LoopHeaders.insert(BB); 657 658 // Remember if SinglePred was the entry block of the function. If so, we 659 // will need to move BB back to the entry position. 660 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); 661 LVI->eraseBlock(SinglePred); 662 MergeBasicBlockIntoOnlyPred(BB); 663 664 if (isEntry && BB != &BB->getParent()->getEntryBlock()) 665 BB->moveBefore(&BB->getParent()->getEntryBlock()); 666 return true; 667 } 668 } 669 670 // What kind of constant we're looking for. 671 ConstantPreference Preference = WantInteger; 672 673 // Look to see if the terminator is a conditional branch, switch or indirect 674 // branch, if not we can't thread it. 675 Value *Condition; 676 Instruction *Terminator = BB->getTerminator(); 677 if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) { 678 // Can't thread an unconditional jump. 679 if (BI->isUnconditional()) return false; 680 Condition = BI->getCondition(); 681 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) { 682 Condition = SI->getCondition(); 683 } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) { 684 // Can't thread indirect branch with no successors. 685 if (IB->getNumSuccessors() == 0) return false; 686 Condition = IB->getAddress()->stripPointerCasts(); 687 Preference = WantBlockAddress; 688 } else { 689 return false; // Must be an invoke. 690 } 691 692 // Run constant folding to see if we can reduce the condition to a simple 693 // constant. 694 if (Instruction *I = dyn_cast<Instruction>(Condition)) { 695 Value *SimpleVal = ConstantFoldInstruction(I, TD, TLI); 696 if (SimpleVal) { 697 I->replaceAllUsesWith(SimpleVal); 698 I->eraseFromParent(); 699 Condition = SimpleVal; 700 } 701 } 702 703 // If the terminator is branching on an undef, we can pick any of the 704 // successors to branch to. Let GetBestDestForJumpOnUndef decide. 705 if (isa<UndefValue>(Condition)) { 706 unsigned BestSucc = GetBestDestForJumpOnUndef(BB); 707 708 // Fold the branch/switch. 709 TerminatorInst *BBTerm = BB->getTerminator(); 710 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { 711 if (i == BestSucc) continue; 712 BBTerm->getSuccessor(i)->removePredecessor(BB, true); 713 } 714 715 DEBUG(dbgs() << " In block '" << BB->getName() 716 << "' folding undef terminator: " << *BBTerm << '\n'); 717 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm); 718 BBTerm->eraseFromParent(); 719 return true; 720 } 721 722 // If the terminator of this block is branching on a constant, simplify the 723 // terminator to an unconditional branch. This can occur due to threading in 724 // other blocks. 725 if (getKnownConstant(Condition, Preference)) { 726 DEBUG(dbgs() << " In block '" << BB->getName() 727 << "' folding terminator: " << *BB->getTerminator() << '\n'); 728 ++NumFolds; 729 ConstantFoldTerminator(BB, true); 730 return true; 731 } 732 733 Instruction *CondInst = dyn_cast<Instruction>(Condition); 734 735 // All the rest of our checks depend on the condition being an instruction. 736 if (CondInst == 0) { 737 // FIXME: Unify this with code below. 738 if (ProcessThreadableEdges(Condition, BB, Preference)) 739 return true; 740 return false; 741 } 742 743 744 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { 745 // For a comparison where the LHS is outside this block, it's possible 746 // that we've branched on it before. Used LVI to see if we can simplify 747 // the branch based on that. 748 BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); 749 Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1)); 750 pred_iterator PI = pred_begin(BB), PE = pred_end(BB); 751 if (CondBr && CondConst && CondBr->isConditional() && PI != PE && 752 (!isa<Instruction>(CondCmp->getOperand(0)) || 753 cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) { 754 // For predecessor edge, determine if the comparison is true or false 755 // on that edge. If they're all true or all false, we can simplify the 756 // branch. 757 // FIXME: We could handle mixed true/false by duplicating code. 758 LazyValueInfo::Tristate Baseline = 759 LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0), 760 CondConst, *PI, BB); 761 if (Baseline != LazyValueInfo::Unknown) { 762 // Check that all remaining incoming values match the first one. 763 while (++PI != PE) { 764 LazyValueInfo::Tristate Ret = 765 LVI->getPredicateOnEdge(CondCmp->getPredicate(), 766 CondCmp->getOperand(0), CondConst, *PI, BB); 767 if (Ret != Baseline) break; 768 } 769 770 // If we terminated early, then one of the values didn't match. 771 if (PI == PE) { 772 unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0; 773 unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1; 774 CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true); 775 BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr); 776 CondBr->eraseFromParent(); 777 return true; 778 } 779 } 780 781 } 782 783 if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB)) 784 return true; 785 } 786 787 // Check for some cases that are worth simplifying. Right now we want to look 788 // for loads that are used by a switch or by the condition for the branch. If 789 // we see one, check to see if it's partially redundant. If so, insert a PHI 790 // which can then be used to thread the values. 791 // 792 Value *SimplifyValue = CondInst; 793 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 794 if (isa<Constant>(CondCmp->getOperand(1))) 795 SimplifyValue = CondCmp->getOperand(0); 796 797 // TODO: There are other places where load PRE would be profitable, such as 798 // more complex comparisons. 799 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 800 if (SimplifyPartiallyRedundantLoad(LI)) 801 return true; 802 803 804 // Handle a variety of cases where we are branching on something derived from 805 // a PHI node in the current block. If we can prove that any predecessors 806 // compute a predictable value based on a PHI node, thread those predecessors. 807 // 808 if (ProcessThreadableEdges(CondInst, BB, Preference)) 809 return true; 810 811 // If this is an otherwise-unfoldable branch on a phi node in the current 812 // block, see if we can simplify. 813 if (PHINode *PN = dyn_cast<PHINode>(CondInst)) 814 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 815 return ProcessBranchOnPHI(PN); 816 817 818 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify. 819 if (CondInst->getOpcode() == Instruction::Xor && 820 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 821 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst)); 822 823 824 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 825 // "(X == 4)", thread through this block. 826 827 return false; 828} 829 830/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 831/// load instruction, eliminate it by replacing it with a PHI node. This is an 832/// important optimization that encourages jump threading, and needs to be run 833/// interlaced with other jump threading tasks. 834bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 835 // Don't hack volatile/atomic loads. 836 if (!LI->isSimple()) return false; 837 838 // If the load is defined in a block with exactly one predecessor, it can't be 839 // partially redundant. 840 BasicBlock *LoadBB = LI->getParent(); 841 if (LoadBB->getSinglePredecessor()) 842 return false; 843 844 // If the load is defined in a landing pad, it can't be partially redundant, 845 // because the edges between the invoke and the landing pad cannot have other 846 // instructions between them. 847 if (LoadBB->isLandingPad()) 848 return false; 849 850 Value *LoadedPtr = LI->getOperand(0); 851 852 // If the loaded operand is defined in the LoadBB, it can't be available. 853 // TODO: Could do simple PHI translation, that would be fun :) 854 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 855 if (PtrOp->getParent() == LoadBB) 856 return false; 857 858 // Scan a few instructions up from the load, to see if it is obviously live at 859 // the entry to its block. 860 BasicBlock::iterator BBIt = LI; 861 862 if (Value *AvailableVal = 863 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) { 864 // If the value if the load is locally available within the block, just use 865 // it. This frequently occurs for reg2mem'd allocas. 866 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 867 868 // If the returned value is the load itself, replace with an undef. This can 869 // only happen in dead loops. 870 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); 871 LI->replaceAllUsesWith(AvailableVal); 872 LI->eraseFromParent(); 873 return true; 874 } 875 876 // Otherwise, if we scanned the whole block and got to the top of the block, 877 // we know the block is locally transparent to the load. If not, something 878 // might clobber its value. 879 if (BBIt != LoadBB->begin()) 880 return false; 881 882 // If all of the loads and stores that feed the value have the same TBAA tag, 883 // then we can propagate it onto any newly inserted loads. 884 MDNode *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa); 885 886 SmallPtrSet<BasicBlock*, 8> PredsScanned; 887 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 888 AvailablePredsTy AvailablePreds; 889 BasicBlock *OneUnavailablePred = 0; 890 891 // If we got here, the loaded value is transparent through to the start of the 892 // block. Check to see if it is available in any of the predecessor blocks. 893 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 894 PI != PE; ++PI) { 895 BasicBlock *PredBB = *PI; 896 897 // If we already scanned this predecessor, skip it. 898 if (!PredsScanned.insert(PredBB)) 899 continue; 900 901 // Scan the predecessor to see if the value is available in the pred. 902 BBIt = PredBB->end(); 903 MDNode *ThisTBAATag = 0; 904 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6, 905 0, &ThisTBAATag); 906 if (!PredAvailable) { 907 OneUnavailablePred = PredBB; 908 continue; 909 } 910 911 // If tbaa tags disagree or are not present, forget about them. 912 if (TBAATag != ThisTBAATag) TBAATag = 0; 913 914 // If so, this load is partially redundant. Remember this info so that we 915 // can create a PHI node. 916 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 917 } 918 919 // If the loaded value isn't available in any predecessor, it isn't partially 920 // redundant. 921 if (AvailablePreds.empty()) return false; 922 923 // Okay, the loaded value is available in at least one (and maybe all!) 924 // predecessors. If the value is unavailable in more than one unique 925 // predecessor, we want to insert a merge block for those common predecessors. 926 // This ensures that we only have to insert one reload, thus not increasing 927 // code size. 928 BasicBlock *UnavailablePred = 0; 929 930 // If there is exactly one predecessor where the value is unavailable, the 931 // already computed 'OneUnavailablePred' block is it. If it ends in an 932 // unconditional branch, we know that it isn't a critical edge. 933 if (PredsScanned.size() == AvailablePreds.size()+1 && 934 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 935 UnavailablePred = OneUnavailablePred; 936 } else if (PredsScanned.size() != AvailablePreds.size()) { 937 // Otherwise, we had multiple unavailable predecessors or we had a critical 938 // edge from the one. 939 SmallVector<BasicBlock*, 8> PredsToSplit; 940 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 941 942 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 943 AvailablePredSet.insert(AvailablePreds[i].first); 944 945 // Add all the unavailable predecessors to the PredsToSplit list. 946 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 947 PI != PE; ++PI) { 948 BasicBlock *P = *PI; 949 // If the predecessor is an indirect goto, we can't split the edge. 950 if (isa<IndirectBrInst>(P->getTerminator())) 951 return false; 952 953 if (!AvailablePredSet.count(P)) 954 PredsToSplit.push_back(P); 955 } 956 957 // Split them out to their own block. 958 UnavailablePred = 959 SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split", this); 960 } 961 962 // If the value isn't available in all predecessors, then there will be 963 // exactly one where it isn't available. Insert a load on that edge and add 964 // it to the AvailablePreds list. 965 if (UnavailablePred) { 966 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 967 "Can't handle critical edge here!"); 968 LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false, 969 LI->getAlignment(), 970 UnavailablePred->getTerminator()); 971 NewVal->setDebugLoc(LI->getDebugLoc()); 972 if (TBAATag) 973 NewVal->setMetadata(LLVMContext::MD_tbaa, TBAATag); 974 975 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 976 } 977 978 // Now we know that each predecessor of this block has a value in 979 // AvailablePreds, sort them for efficient access as we're walking the preds. 980 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 981 982 // Create a PHI node at the start of the block for the PRE'd load value. 983 pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB); 984 PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "", 985 LoadBB->begin()); 986 PN->takeName(LI); 987 PN->setDebugLoc(LI->getDebugLoc()); 988 989 // Insert new entries into the PHI for each predecessor. A single block may 990 // have multiple entries here. 991 for (pred_iterator PI = PB; PI != PE; ++PI) { 992 BasicBlock *P = *PI; 993 AvailablePredsTy::iterator I = 994 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 995 std::make_pair(P, (Value*)0)); 996 997 assert(I != AvailablePreds.end() && I->first == P && 998 "Didn't find entry for predecessor!"); 999 1000 PN->addIncoming(I->second, I->first); 1001 } 1002 1003 //cerr << "PRE: " << *LI << *PN << "\n"; 1004 1005 LI->replaceAllUsesWith(PN); 1006 LI->eraseFromParent(); 1007 1008 return true; 1009} 1010 1011/// FindMostPopularDest - The specified list contains multiple possible 1012/// threadable destinations. Pick the one that occurs the most frequently in 1013/// the list. 1014static BasicBlock * 1015FindMostPopularDest(BasicBlock *BB, 1016 const SmallVectorImpl<std::pair<BasicBlock*, 1017 BasicBlock*> > &PredToDestList) { 1018 assert(!PredToDestList.empty()); 1019 1020 // Determine popularity. If there are multiple possible destinations, we 1021 // explicitly choose to ignore 'undef' destinations. We prefer to thread 1022 // blocks with known and real destinations to threading undef. We'll handle 1023 // them later if interesting. 1024 DenseMap<BasicBlock*, unsigned> DestPopularity; 1025 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1026 if (PredToDestList[i].second) 1027 DestPopularity[PredToDestList[i].second]++; 1028 1029 // Find the most popular dest. 1030 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin(); 1031 BasicBlock *MostPopularDest = DPI->first; 1032 unsigned Popularity = DPI->second; 1033 SmallVector<BasicBlock*, 4> SamePopularity; 1034 1035 for (++DPI; DPI != DestPopularity.end(); ++DPI) { 1036 // If the popularity of this entry isn't higher than the popularity we've 1037 // seen so far, ignore it. 1038 if (DPI->second < Popularity) 1039 ; // ignore. 1040 else if (DPI->second == Popularity) { 1041 // If it is the same as what we've seen so far, keep track of it. 1042 SamePopularity.push_back(DPI->first); 1043 } else { 1044 // If it is more popular, remember it. 1045 SamePopularity.clear(); 1046 MostPopularDest = DPI->first; 1047 Popularity = DPI->second; 1048 } 1049 } 1050 1051 // Okay, now we know the most popular destination. If there is more than one 1052 // destination, we need to determine one. This is arbitrary, but we need 1053 // to make a deterministic decision. Pick the first one that appears in the 1054 // successor list. 1055 if (!SamePopularity.empty()) { 1056 SamePopularity.push_back(MostPopularDest); 1057 TerminatorInst *TI = BB->getTerminator(); 1058 for (unsigned i = 0; ; ++i) { 1059 assert(i != TI->getNumSuccessors() && "Didn't find any successor!"); 1060 1061 if (std::find(SamePopularity.begin(), SamePopularity.end(), 1062 TI->getSuccessor(i)) == SamePopularity.end()) 1063 continue; 1064 1065 MostPopularDest = TI->getSuccessor(i); 1066 break; 1067 } 1068 } 1069 1070 // Okay, we have finally picked the most popular destination. 1071 return MostPopularDest; 1072} 1073 1074bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB, 1075 ConstantPreference Preference) { 1076 // If threading this would thread across a loop header, don't even try to 1077 // thread the edge. 1078 if (LoopHeaders.count(BB)) 1079 return false; 1080 1081 PredValueInfoTy PredValues; 1082 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference)) 1083 return false; 1084 1085 assert(!PredValues.empty() && 1086 "ComputeValueKnownInPredecessors returned true with no values"); 1087 1088 DEBUG(dbgs() << "IN BB: " << *BB; 1089 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1090 dbgs() << " BB '" << BB->getName() << "': FOUND condition = " 1091 << *PredValues[i].first 1092 << " for pred '" << PredValues[i].second->getName() << "'.\n"; 1093 }); 1094 1095 // Decide what we want to thread through. Convert our list of known values to 1096 // a list of known destinations for each pred. This also discards duplicate 1097 // predecessors and keeps track of the undefined inputs (which are represented 1098 // as a null dest in the PredToDestList). 1099 SmallPtrSet<BasicBlock*, 16> SeenPreds; 1100 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList; 1101 1102 BasicBlock *OnlyDest = 0; 1103 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL; 1104 1105 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1106 BasicBlock *Pred = PredValues[i].second; 1107 if (!SeenPreds.insert(Pred)) 1108 continue; // Duplicate predecessor entry. 1109 1110 // If the predecessor ends with an indirect goto, we can't change its 1111 // destination. 1112 if (isa<IndirectBrInst>(Pred->getTerminator())) 1113 continue; 1114 1115 Constant *Val = PredValues[i].first; 1116 1117 BasicBlock *DestBB; 1118 if (isa<UndefValue>(Val)) 1119 DestBB = 0; 1120 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 1121 DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero()); 1122 else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) { 1123 DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor(); 1124 } else { 1125 assert(isa<IndirectBrInst>(BB->getTerminator()) 1126 && "Unexpected terminator"); 1127 DestBB = cast<BlockAddress>(Val)->getBasicBlock(); 1128 } 1129 1130 // If we have exactly one destination, remember it for efficiency below. 1131 if (PredToDestList.empty()) 1132 OnlyDest = DestBB; 1133 else if (OnlyDest != DestBB) 1134 OnlyDest = MultipleDestSentinel; 1135 1136 PredToDestList.push_back(std::make_pair(Pred, DestBB)); 1137 } 1138 1139 // If all edges were unthreadable, we fail. 1140 if (PredToDestList.empty()) 1141 return false; 1142 1143 // Determine which is the most common successor. If we have many inputs and 1144 // this block is a switch, we want to start by threading the batch that goes 1145 // to the most popular destination first. If we only know about one 1146 // threadable destination (the common case) we can avoid this. 1147 BasicBlock *MostPopularDest = OnlyDest; 1148 1149 if (MostPopularDest == MultipleDestSentinel) 1150 MostPopularDest = FindMostPopularDest(BB, PredToDestList); 1151 1152 // Now that we know what the most popular destination is, factor all 1153 // predecessors that will jump to it into a single predecessor. 1154 SmallVector<BasicBlock*, 16> PredsToFactor; 1155 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1156 if (PredToDestList[i].second == MostPopularDest) { 1157 BasicBlock *Pred = PredToDestList[i].first; 1158 1159 // This predecessor may be a switch or something else that has multiple 1160 // edges to the block. Factor each of these edges by listing them 1161 // according to # occurrences in PredsToFactor. 1162 TerminatorInst *PredTI = Pred->getTerminator(); 1163 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i) 1164 if (PredTI->getSuccessor(i) == BB) 1165 PredsToFactor.push_back(Pred); 1166 } 1167 1168 // If the threadable edges are branching on an undefined value, we get to pick 1169 // the destination that these predecessors should get to. 1170 if (MostPopularDest == 0) 1171 MostPopularDest = BB->getTerminator()-> 1172 getSuccessor(GetBestDestForJumpOnUndef(BB)); 1173 1174 // Ok, try to thread it! 1175 return ThreadEdge(BB, PredsToFactor, MostPopularDest); 1176} 1177 1178/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on 1179/// a PHI node in the current block. See if there are any simplifications we 1180/// can do based on inputs to the phi node. 1181/// 1182bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) { 1183 BasicBlock *BB = PN->getParent(); 1184 1185 // TODO: We could make use of this to do it once for blocks with common PHI 1186 // values. 1187 SmallVector<BasicBlock*, 1> PredBBs; 1188 PredBBs.resize(1); 1189 1190 // If any of the predecessor blocks end in an unconditional branch, we can 1191 // *duplicate* the conditional branch into that block in order to further 1192 // encourage jump threading and to eliminate cases where we have branch on a 1193 // phi of an icmp (branch on icmp is much better). 1194 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1195 BasicBlock *PredBB = PN->getIncomingBlock(i); 1196 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator())) 1197 if (PredBr->isUnconditional()) { 1198 PredBBs[0] = PredBB; 1199 // Try to duplicate BB into PredBB. 1200 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs)) 1201 return true; 1202 } 1203 } 1204 1205 return false; 1206} 1207 1208/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on 1209/// a xor instruction in the current block. See if there are any 1210/// simplifications we can do based on inputs to the xor. 1211/// 1212bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) { 1213 BasicBlock *BB = BO->getParent(); 1214 1215 // If either the LHS or RHS of the xor is a constant, don't do this 1216 // optimization. 1217 if (isa<ConstantInt>(BO->getOperand(0)) || 1218 isa<ConstantInt>(BO->getOperand(1))) 1219 return false; 1220 1221 // If the first instruction in BB isn't a phi, we won't be able to infer 1222 // anything special about any particular predecessor. 1223 if (!isa<PHINode>(BB->front())) 1224 return false; 1225 1226 // If we have a xor as the branch input to this block, and we know that the 1227 // LHS or RHS of the xor in any predecessor is true/false, then we can clone 1228 // the condition into the predecessor and fix that value to true, saving some 1229 // logical ops on that path and encouraging other paths to simplify. 1230 // 1231 // This copies something like this: 1232 // 1233 // BB: 1234 // %X = phi i1 [1], [%X'] 1235 // %Y = icmp eq i32 %A, %B 1236 // %Z = xor i1 %X, %Y 1237 // br i1 %Z, ... 1238 // 1239 // Into: 1240 // BB': 1241 // %Y = icmp ne i32 %A, %B 1242 // br i1 %Z, ... 1243 1244 PredValueInfoTy XorOpValues; 1245 bool isLHS = true; 1246 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues, 1247 WantInteger)) { 1248 assert(XorOpValues.empty()); 1249 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues, 1250 WantInteger)) 1251 return false; 1252 isLHS = false; 1253 } 1254 1255 assert(!XorOpValues.empty() && 1256 "ComputeValueKnownInPredecessors returned true with no values"); 1257 1258 // Scan the information to see which is most popular: true or false. The 1259 // predecessors can be of the set true, false, or undef. 1260 unsigned NumTrue = 0, NumFalse = 0; 1261 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1262 if (isa<UndefValue>(XorOpValues[i].first)) 1263 // Ignore undefs for the count. 1264 continue; 1265 if (cast<ConstantInt>(XorOpValues[i].first)->isZero()) 1266 ++NumFalse; 1267 else 1268 ++NumTrue; 1269 } 1270 1271 // Determine which value to split on, true, false, or undef if neither. 1272 ConstantInt *SplitVal = 0; 1273 if (NumTrue > NumFalse) 1274 SplitVal = ConstantInt::getTrue(BB->getContext()); 1275 else if (NumTrue != 0 || NumFalse != 0) 1276 SplitVal = ConstantInt::getFalse(BB->getContext()); 1277 1278 // Collect all of the blocks that this can be folded into so that we can 1279 // factor this once and clone it once. 1280 SmallVector<BasicBlock*, 8> BlocksToFoldInto; 1281 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1282 if (XorOpValues[i].first != SplitVal && 1283 !isa<UndefValue>(XorOpValues[i].first)) 1284 continue; 1285 1286 BlocksToFoldInto.push_back(XorOpValues[i].second); 1287 } 1288 1289 // If we inferred a value for all of the predecessors, then duplication won't 1290 // help us. However, we can just replace the LHS or RHS with the constant. 1291 if (BlocksToFoldInto.size() == 1292 cast<PHINode>(BB->front()).getNumIncomingValues()) { 1293 if (SplitVal == 0) { 1294 // If all preds provide undef, just nuke the xor, because it is undef too. 1295 BO->replaceAllUsesWith(UndefValue::get(BO->getType())); 1296 BO->eraseFromParent(); 1297 } else if (SplitVal->isZero()) { 1298 // If all preds provide 0, replace the xor with the other input. 1299 BO->replaceAllUsesWith(BO->getOperand(isLHS)); 1300 BO->eraseFromParent(); 1301 } else { 1302 // If all preds provide 1, set the computed value to 1. 1303 BO->setOperand(!isLHS, SplitVal); 1304 } 1305 1306 return true; 1307 } 1308 1309 // Try to duplicate BB into PredBB. 1310 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto); 1311} 1312 1313 1314/// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new 1315/// predecessor to the PHIBB block. If it has PHI nodes, add entries for 1316/// NewPred using the entries from OldPred (suitably mapped). 1317static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, 1318 BasicBlock *OldPred, 1319 BasicBlock *NewPred, 1320 DenseMap<Instruction*, Value*> &ValueMap) { 1321 for (BasicBlock::iterator PNI = PHIBB->begin(); 1322 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) { 1323 // Ok, we have a PHI node. Figure out what the incoming value was for the 1324 // DestBlock. 1325 Value *IV = PN->getIncomingValueForBlock(OldPred); 1326 1327 // Remap the value if necessary. 1328 if (Instruction *Inst = dyn_cast<Instruction>(IV)) { 1329 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst); 1330 if (I != ValueMap.end()) 1331 IV = I->second; 1332 } 1333 1334 PN->addIncoming(IV, NewPred); 1335 } 1336} 1337 1338/// ThreadEdge - We have decided that it is safe and profitable to factor the 1339/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB 1340/// across BB. Transform the IR to reflect this change. 1341bool JumpThreading::ThreadEdge(BasicBlock *BB, 1342 const SmallVectorImpl<BasicBlock*> &PredBBs, 1343 BasicBlock *SuccBB) { 1344 // If threading to the same block as we come from, we would infinite loop. 1345 if (SuccBB == BB) { 1346 DEBUG(dbgs() << " Not threading across BB '" << BB->getName() 1347 << "' - would thread to self!\n"); 1348 return false; 1349 } 1350 1351 // If threading this would thread across a loop header, don't thread the edge. 1352 // See the comments above FindLoopHeaders for justifications and caveats. 1353 if (LoopHeaders.count(BB)) { 1354 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName() 1355 << "' to dest BB '" << SuccBB->getName() 1356 << "' - it might create an irreducible loop!\n"); 1357 return false; 1358 } 1359 1360 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB, Threshold); 1361 if (JumpThreadCost > Threshold) { 1362 DEBUG(dbgs() << " Not threading BB '" << BB->getName() 1363 << "' - Cost is too high: " << JumpThreadCost << "\n"); 1364 return false; 1365 } 1366 1367 // And finally, do it! Start by factoring the predecessors is needed. 1368 BasicBlock *PredBB; 1369 if (PredBBs.size() == 1) 1370 PredBB = PredBBs[0]; 1371 else { 1372 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1373 << " common predecessors.\n"); 1374 PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this); 1375 } 1376 1377 // And finally, do it! 1378 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '" 1379 << SuccBB->getName() << "' with cost: " << JumpThreadCost 1380 << ", across block:\n " 1381 << *BB << "\n"); 1382 1383 LVI->threadEdge(PredBB, BB, SuccBB); 1384 1385 // We are going to have to map operands from the original BB block to the new 1386 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 1387 // account for entry from PredBB. 1388 DenseMap<Instruction*, Value*> ValueMapping; 1389 1390 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), 1391 BB->getName()+".thread", 1392 BB->getParent(), BB); 1393 NewBB->moveAfter(PredBB); 1394 1395 BasicBlock::iterator BI = BB->begin(); 1396 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1397 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1398 1399 // Clone the non-phi instructions of BB into NewBB, keeping track of the 1400 // mapping and using it to remap operands in the cloned instructions. 1401 for (; !isa<TerminatorInst>(BI); ++BI) { 1402 Instruction *New = BI->clone(); 1403 New->setName(BI->getName()); 1404 NewBB->getInstList().push_back(New); 1405 ValueMapping[BI] = New; 1406 1407 // Remap operands to patch up intra-block references. 1408 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1409 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1410 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1411 if (I != ValueMapping.end()) 1412 New->setOperand(i, I->second); 1413 } 1414 } 1415 1416 // We didn't copy the terminator from BB over to NewBB, because there is now 1417 // an unconditional jump to SuccBB. Insert the unconditional jump. 1418 BranchInst *NewBI =BranchInst::Create(SuccBB, NewBB); 1419 NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc()); 1420 1421 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 1422 // PHI nodes for NewBB now. 1423 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping); 1424 1425 // If there were values defined in BB that are used outside the block, then we 1426 // now have to update all uses of the value to use either the original value, 1427 // the cloned value, or some PHI derived value. This can require arbitrary 1428 // PHI insertion, of which we are prepared to do, clean these up now. 1429 SSAUpdater SSAUpdate; 1430 SmallVector<Use*, 16> UsesToRename; 1431 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1432 // Scan all uses of this instruction to see if it is used outside of its 1433 // block, and if so, record them in UsesToRename. 1434 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1435 ++UI) { 1436 Instruction *User = cast<Instruction>(*UI); 1437 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1438 if (UserPN->getIncomingBlock(UI) == BB) 1439 continue; 1440 } else if (User->getParent() == BB) 1441 continue; 1442 1443 UsesToRename.push_back(&UI.getUse()); 1444 } 1445 1446 // If there are no uses outside the block, we're done with this instruction. 1447 if (UsesToRename.empty()) 1448 continue; 1449 1450 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1451 1452 // We found a use of I outside of BB. Rename all uses of I that are outside 1453 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1454 // with the two values we know. 1455 SSAUpdate.Initialize(I->getType(), I->getName()); 1456 SSAUpdate.AddAvailableValue(BB, I); 1457 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]); 1458 1459 while (!UsesToRename.empty()) 1460 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1461 DEBUG(dbgs() << "\n"); 1462 } 1463 1464 1465 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 1466 // NewBB instead of BB. This eliminates predecessors from BB, which requires 1467 // us to simplify any PHI nodes in BB. 1468 TerminatorInst *PredTerm = PredBB->getTerminator(); 1469 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 1470 if (PredTerm->getSuccessor(i) == BB) { 1471 BB->removePredecessor(PredBB, true); 1472 PredTerm->setSuccessor(i, NewBB); 1473 } 1474 1475 // At this point, the IR is fully up to date and consistent. Do a quick scan 1476 // over the new instructions and zap any that are constants or dead. This 1477 // frequently happens because of phi translation. 1478 SimplifyInstructionsInBlock(NewBB, TD, TLI); 1479 1480 // Threaded an edge! 1481 ++NumThreads; 1482 return true; 1483} 1484 1485/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch 1486/// to BB which contains an i1 PHI node and a conditional branch on that PHI. 1487/// If we can duplicate the contents of BB up into PredBB do so now, this 1488/// improves the odds that the branch will be on an analyzable instruction like 1489/// a compare. 1490bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 1491 const SmallVectorImpl<BasicBlock *> &PredBBs) { 1492 assert(!PredBBs.empty() && "Can't handle an empty set"); 1493 1494 // If BB is a loop header, then duplicating this block outside the loop would 1495 // cause us to transform this into an irreducible loop, don't do this. 1496 // See the comments above FindLoopHeaders for justifications and caveats. 1497 if (LoopHeaders.count(BB)) { 1498 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName() 1499 << "' into predecessor block '" << PredBBs[0]->getName() 1500 << "' - it might create an irreducible loop!\n"); 1501 return false; 1502 } 1503 1504 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB, Threshold); 1505 if (DuplicationCost > Threshold) { 1506 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName() 1507 << "' - Cost is too high: " << DuplicationCost << "\n"); 1508 return false; 1509 } 1510 1511 // And finally, do it! Start by factoring the predecessors is needed. 1512 BasicBlock *PredBB; 1513 if (PredBBs.size() == 1) 1514 PredBB = PredBBs[0]; 1515 else { 1516 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1517 << " common predecessors.\n"); 1518 PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this); 1519 } 1520 1521 // Okay, we decided to do this! Clone all the instructions in BB onto the end 1522 // of PredBB. 1523 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '" 1524 << PredBB->getName() << "' to eliminate branch on phi. Cost: " 1525 << DuplicationCost << " block is:" << *BB << "\n"); 1526 1527 // Unless PredBB ends with an unconditional branch, split the edge so that we 1528 // can just clone the bits from BB into the end of the new PredBB. 1529 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator()); 1530 1531 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) { 1532 PredBB = SplitEdge(PredBB, BB, this); 1533 OldPredBranch = cast<BranchInst>(PredBB->getTerminator()); 1534 } 1535 1536 // We are going to have to map operands from the original BB block into the 1537 // PredBB block. Evaluate PHI nodes in BB. 1538 DenseMap<Instruction*, Value*> ValueMapping; 1539 1540 BasicBlock::iterator BI = BB->begin(); 1541 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1542 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1543 1544 // Clone the non-phi instructions of BB into PredBB, keeping track of the 1545 // mapping and using it to remap operands in the cloned instructions. 1546 for (; BI != BB->end(); ++BI) { 1547 Instruction *New = BI->clone(); 1548 1549 // Remap operands to patch up intra-block references. 1550 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1551 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1552 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1553 if (I != ValueMapping.end()) 1554 New->setOperand(i, I->second); 1555 } 1556 1557 // If this instruction can be simplified after the operands are updated, 1558 // just use the simplified value instead. This frequently happens due to 1559 // phi translation. 1560 if (Value *IV = SimplifyInstruction(New, TD)) { 1561 delete New; 1562 ValueMapping[BI] = IV; 1563 } else { 1564 // Otherwise, insert the new instruction into the block. 1565 New->setName(BI->getName()); 1566 PredBB->getInstList().insert(OldPredBranch, New); 1567 ValueMapping[BI] = New; 1568 } 1569 } 1570 1571 // Check to see if the targets of the branch had PHI nodes. If so, we need to 1572 // add entries to the PHI nodes for branch from PredBB now. 1573 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator()); 1574 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB, 1575 ValueMapping); 1576 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB, 1577 ValueMapping); 1578 1579 // If there were values defined in BB that are used outside the block, then we 1580 // now have to update all uses of the value to use either the original value, 1581 // the cloned value, or some PHI derived value. This can require arbitrary 1582 // PHI insertion, of which we are prepared to do, clean these up now. 1583 SSAUpdater SSAUpdate; 1584 SmallVector<Use*, 16> UsesToRename; 1585 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1586 // Scan all uses of this instruction to see if it is used outside of its 1587 // block, and if so, record them in UsesToRename. 1588 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1589 ++UI) { 1590 Instruction *User = cast<Instruction>(*UI); 1591 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1592 if (UserPN->getIncomingBlock(UI) == BB) 1593 continue; 1594 } else if (User->getParent() == BB) 1595 continue; 1596 1597 UsesToRename.push_back(&UI.getUse()); 1598 } 1599 1600 // If there are no uses outside the block, we're done with this instruction. 1601 if (UsesToRename.empty()) 1602 continue; 1603 1604 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1605 1606 // We found a use of I outside of BB. Rename all uses of I that are outside 1607 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1608 // with the two values we know. 1609 SSAUpdate.Initialize(I->getType(), I->getName()); 1610 SSAUpdate.AddAvailableValue(BB, I); 1611 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]); 1612 1613 while (!UsesToRename.empty()) 1614 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1615 DEBUG(dbgs() << "\n"); 1616 } 1617 1618 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge 1619 // that we nuked. 1620 BB->removePredecessor(PredBB, true); 1621 1622 // Remove the unconditional branch at the end of the PredBB block. 1623 OldPredBranch->eraseFromParent(); 1624 1625 ++NumDupes; 1626 return true; 1627} 1628 1629/// TryToUnfoldSelect - Look for blocks of the form 1630/// bb1: 1631/// %a = select 1632/// br bb 1633/// 1634/// bb2: 1635/// %p = phi [%a, %bb] ... 1636/// %c = icmp %p 1637/// br i1 %c 1638/// 1639/// And expand the select into a branch structure if one of its arms allows %c 1640/// to be folded. This later enables threading from bb1 over bb2. 1641bool JumpThreading::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) { 1642 BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); 1643 PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0)); 1644 Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1)); 1645 1646 if (!CondBr || !CondBr->isConditional() || !CondLHS || 1647 CondLHS->getParent() != BB) 1648 return false; 1649 1650 for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) { 1651 BasicBlock *Pred = CondLHS->getIncomingBlock(I); 1652 SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I)); 1653 1654 // Look if one of the incoming values is a select in the corresponding 1655 // predecessor. 1656 if (!SI || SI->getParent() != Pred || !SI->hasOneUse()) 1657 continue; 1658 1659 BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator()); 1660 if (!PredTerm || !PredTerm->isUnconditional()) 1661 continue; 1662 1663 // Now check if one of the select values would allow us to constant fold the 1664 // terminator in BB. We don't do the transform if both sides fold, those 1665 // cases will be threaded in any case. 1666 LazyValueInfo::Tristate LHSFolds = 1667 LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1), 1668 CondRHS, Pred, BB); 1669 LazyValueInfo::Tristate RHSFolds = 1670 LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2), 1671 CondRHS, Pred, BB); 1672 if ((LHSFolds != LazyValueInfo::Unknown || 1673 RHSFolds != LazyValueInfo::Unknown) && 1674 LHSFolds != RHSFolds) { 1675 // Expand the select. 1676 // 1677 // Pred -- 1678 // | v 1679 // | NewBB 1680 // | | 1681 // |----- 1682 // v 1683 // BB 1684 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold", 1685 BB->getParent(), BB); 1686 // Move the unconditional branch to NewBB. 1687 PredTerm->removeFromParent(); 1688 NewBB->getInstList().insert(NewBB->end(), PredTerm); 1689 // Create a conditional branch and update PHI nodes. 1690 BranchInst::Create(NewBB, BB, SI->getCondition(), Pred); 1691 CondLHS->setIncomingValue(I, SI->getFalseValue()); 1692 CondLHS->addIncoming(SI->getTrueValue(), NewBB); 1693 // The select is now dead. 1694 SI->eraseFromParent(); 1695 1696 // Update any other PHI nodes in BB. 1697 for (BasicBlock::iterator BI = BB->begin(); 1698 PHINode *Phi = dyn_cast<PHINode>(BI); ++BI) 1699 if (Phi != CondLHS) 1700 Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB); 1701 return true; 1702 } 1703 } 1704 return false; 1705} 1706