1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements the Jump Threading pass. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #define DEBUG_TYPE "jump-threading" 15 #include "llvm/Transforms/Scalar.h" 16 #include "llvm/IntrinsicInst.h" 17 #include "llvm/LLVMContext.h" 18 #include "llvm/Pass.h" 19 #include "llvm/Analysis/ConstantFolding.h" 20 #include "llvm/Analysis/InstructionSimplify.h" 21 #include "llvm/Analysis/LazyValueInfo.h" 22 #include "llvm/Analysis/Loads.h" 23 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 24 #include "llvm/Transforms/Utils/Local.h" 25 #include "llvm/Transforms/Utils/SSAUpdater.h" 26 #include "llvm/Target/TargetData.h" 27 #include "llvm/Target/TargetLibraryInfo.h" 28 #include "llvm/ADT/DenseMap.h" 29 #include "llvm/ADT/DenseSet.h" 30 #include "llvm/ADT/Statistic.h" 31 #include "llvm/ADT/STLExtras.h" 32 #include "llvm/ADT/SmallPtrSet.h" 33 #include "llvm/ADT/SmallSet.h" 34 #include "llvm/Support/CommandLine.h" 35 #include "llvm/Support/Debug.h" 36 #include "llvm/Support/ValueHandle.h" 37 #include "llvm/Support/raw_ostream.h" 38 using namespace llvm; 39 40 STATISTIC(NumThreads, "Number of jumps threaded"); 41 STATISTIC(NumFolds, "Number of terminators folded"); 42 STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi"); 43 44 static cl::opt<unsigned> 45 Threshold("jump-threading-threshold", 46 cl::desc("Max block size to duplicate for jump threading"), 47 cl::init(6), cl::Hidden); 48 49 namespace { 50 // These are at global scope so static functions can use them too. 51 typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo; 52 typedef SmallVector<std::pair<Constant*, BasicBlock*>, 8> PredValueInfoTy; 53 54 // This is used to keep track of what kind of constant we're currently hoping 55 // to find. 56 enum ConstantPreference { 57 WantInteger, 58 WantBlockAddress 59 }; 60 61 /// This pass performs 'jump threading', which looks at blocks that have 62 /// multiple predecessors and multiple successors. If one or more of the 63 /// predecessors of the block can be proven to always jump to one of the 64 /// successors, we forward the edge from the predecessor to the successor by 65 /// duplicating the contents of this block. 66 /// 67 /// An example of when this can occur is code like this: 68 /// 69 /// if () { ... 70 /// X = 4; 71 /// } 72 /// if (X < 3) { 73 /// 74 /// In this case, the unconditional branch at the end of the first if can be 75 /// revectored to the false side of the second if. 76 /// 77 class JumpThreading : public FunctionPass { 78 TargetData *TD; 79 TargetLibraryInfo *TLI; 80 LazyValueInfo *LVI; 81 #ifdef NDEBUG 82 SmallPtrSet<BasicBlock*, 16> LoopHeaders; 83 #else 84 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; 85 #endif 86 DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet; 87 88 // RAII helper for updating the recursion stack. 89 struct RecursionSetRemover { 90 DenseSet<std::pair<Value*, BasicBlock*> > &TheSet; 91 std::pair<Value*, BasicBlock*> ThePair; 92 93 RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S, 94 std::pair<Value*, BasicBlock*> P) 95 : TheSet(S), ThePair(P) { } 96 97 ~RecursionSetRemover() { 98 TheSet.erase(ThePair); 99 } 100 }; 101 public: 102 static char ID; // Pass identification 103 JumpThreading() : FunctionPass(ID) { 104 initializeJumpThreadingPass(*PassRegistry::getPassRegistry()); 105 } 106 107 bool runOnFunction(Function &F); 108 109 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 110 AU.addRequired<LazyValueInfo>(); 111 AU.addPreserved<LazyValueInfo>(); 112 AU.addRequired<TargetLibraryInfo>(); 113 } 114 115 void FindLoopHeaders(Function &F); 116 bool ProcessBlock(BasicBlock *BB); 117 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs, 118 BasicBlock *SuccBB); 119 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 120 const SmallVectorImpl<BasicBlock *> &PredBBs); 121 122 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, 123 PredValueInfo &Result, 124 ConstantPreference Preference); 125 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB, 126 ConstantPreference Preference); 127 128 bool ProcessBranchOnPHI(PHINode *PN); 129 bool ProcessBranchOnXOR(BinaryOperator *BO); 130 131 bool SimplifyPartiallyRedundantLoad(LoadInst *LI); 132 }; 133 } 134 135 char JumpThreading::ID = 0; 136 INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading", 137 "Jump Threading", false, false) 138 INITIALIZE_PASS_DEPENDENCY(LazyValueInfo) 139 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) 140 INITIALIZE_PASS_END(JumpThreading, "jump-threading", 141 "Jump Threading", false, false) 142 143 // Public interface to the Jump Threading pass 144 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 145 146 /// runOnFunction - Top level algorithm. 147 /// 148 bool JumpThreading::runOnFunction(Function &F) { 149 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n"); 150 TD = getAnalysisIfAvailable<TargetData>(); 151 TLI = &getAnalysis<TargetLibraryInfo>(); 152 LVI = &getAnalysis<LazyValueInfo>(); 153 154 FindLoopHeaders(F); 155 156 bool Changed, EverChanged = false; 157 do { 158 Changed = false; 159 for (Function::iterator I = F.begin(), E = F.end(); I != E;) { 160 BasicBlock *BB = I; 161 // Thread all of the branches we can over this block. 162 while (ProcessBlock(BB)) 163 Changed = true; 164 165 ++I; 166 167 // If the block is trivially dead, zap it. This eliminates the successor 168 // edges which simplifies the CFG. 169 if (pred_begin(BB) == pred_end(BB) && 170 BB != &BB->getParent()->getEntryBlock()) { 171 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName() 172 << "' with terminator: " << *BB->getTerminator() << '\n'); 173 LoopHeaders.erase(BB); 174 LVI->eraseBlock(BB); 175 DeleteDeadBlock(BB); 176 Changed = true; 177 continue; 178 } 179 180 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()); 181 182 // Can't thread an unconditional jump, but if the block is "almost 183 // empty", we can replace uses of it with uses of the successor and make 184 // this dead. 185 if (BI && BI->isUnconditional() && 186 BB != &BB->getParent()->getEntryBlock() && 187 // If the terminator is the only non-phi instruction, try to nuke it. 188 BB->getFirstNonPHIOrDbg()->isTerminator()) { 189 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the 190 // block, we have to make sure it isn't in the LoopHeaders set. We 191 // reinsert afterward if needed. 192 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB); 193 BasicBlock *Succ = BI->getSuccessor(0); 194 195 // FIXME: It is always conservatively correct to drop the info 196 // for a block even if it doesn't get erased. This isn't totally 197 // awesome, but it allows us to use AssertingVH to prevent nasty 198 // dangling pointer issues within LazyValueInfo. 199 LVI->eraseBlock(BB); 200 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) { 201 Changed = true; 202 // If we deleted BB and BB was the header of a loop, then the 203 // successor is now the header of the loop. 204 BB = Succ; 205 } 206 207 if (ErasedFromLoopHeaders) 208 LoopHeaders.insert(BB); 209 } 210 } 211 EverChanged |= Changed; 212 } while (Changed); 213 214 LoopHeaders.clear(); 215 return EverChanged; 216 } 217 218 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 219 /// thread across it. 220 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { 221 /// Ignore PHI nodes, these will be flattened when duplication happens. 222 BasicBlock::const_iterator I = BB->getFirstNonPHI(); 223 224 // FIXME: THREADING will delete values that are just used to compute the 225 // branch, so they shouldn't count against the duplication cost. 226 227 228 // Sum up the cost of each instruction until we get to the terminator. Don't 229 // include the terminator because the copy won't include it. 230 unsigned Size = 0; 231 for (; !isa<TerminatorInst>(I); ++I) { 232 // Debugger intrinsics don't incur code size. 233 if (isa<DbgInfoIntrinsic>(I)) continue; 234 235 // If this is a pointer->pointer bitcast, it is free. 236 if (isa<BitCastInst>(I) && I->getType()->isPointerTy()) 237 continue; 238 239 // All other instructions count for at least one unit. 240 ++Size; 241 242 // Calls are more expensive. If they are non-intrinsic calls, we model them 243 // as having cost of 4. If they are a non-vector intrinsic, we model them 244 // as having cost of 2 total, and if they are a vector intrinsic, we model 245 // them as having cost 1. 246 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 247 if (!isa<IntrinsicInst>(CI)) 248 Size += 3; 249 else if (!CI->getType()->isVectorTy()) 250 Size += 1; 251 } 252 } 253 254 // Threading through a switch statement is particularly profitable. If this 255 // block ends in a switch, decrease its cost to make it more likely to happen. 256 if (isa<SwitchInst>(I)) 257 Size = Size > 6 ? Size-6 : 0; 258 259 // The same holds for indirect branches, but slightly more so. 260 if (isa<IndirectBrInst>(I)) 261 Size = Size > 8 ? Size-8 : 0; 262 263 return Size; 264 } 265 266 /// FindLoopHeaders - We do not want jump threading to turn proper loop 267 /// structures into irreducible loops. Doing this breaks up the loop nesting 268 /// hierarchy and pessimizes later transformations. To prevent this from 269 /// happening, we first have to find the loop headers. Here we approximate this 270 /// by finding targets of backedges in the CFG. 271 /// 272 /// Note that there definitely are cases when we want to allow threading of 273 /// edges across a loop header. For example, threading a jump from outside the 274 /// loop (the preheader) to an exit block of the loop is definitely profitable. 275 /// It is also almost always profitable to thread backedges from within the loop 276 /// to exit blocks, and is often profitable to thread backedges to other blocks 277 /// within the loop (forming a nested loop). This simple analysis is not rich 278 /// enough to track all of these properties and keep it up-to-date as the CFG 279 /// mutates, so we don't allow any of these transformations. 280 /// 281 void JumpThreading::FindLoopHeaders(Function &F) { 282 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; 283 FindFunctionBackedges(F, Edges); 284 285 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 286 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); 287 } 288 289 /// getKnownConstant - Helper method to determine if we can thread over a 290 /// terminator with the given value as its condition, and if so what value to 291 /// use for that. What kind of value this is depends on whether we want an 292 /// integer or a block address, but an undef is always accepted. 293 /// Returns null if Val is null or not an appropriate constant. 294 static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) { 295 if (!Val) 296 return 0; 297 298 // Undef is "known" enough. 299 if (UndefValue *U = dyn_cast<UndefValue>(Val)) 300 return U; 301 302 if (Preference == WantBlockAddress) 303 return dyn_cast<BlockAddress>(Val->stripPointerCasts()); 304 305 return dyn_cast<ConstantInt>(Val); 306 } 307 308 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see 309 /// if we can infer that the value is a known ConstantInt/BlockAddress or undef 310 /// in any of our predecessors. If so, return the known list of value and pred 311 /// BB in the result vector. 312 /// 313 /// This returns true if there were any known values. 314 /// 315 bool JumpThreading:: 316 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result, 317 ConstantPreference Preference) { 318 // This method walks up use-def chains recursively. Because of this, we could 319 // get into an infinite loop going around loops in the use-def chain. To 320 // prevent this, keep track of what (value, block) pairs we've already visited 321 // and terminate the search if we loop back to them 322 if (!RecursionSet.insert(std::make_pair(V, BB)).second) 323 return false; 324 325 // An RAII help to remove this pair from the recursion set once the recursion 326 // stack pops back out again. 327 RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB)); 328 329 // If V is a constant, then it is known in all predecessors. 330 if (Constant *KC = getKnownConstant(V, Preference)) { 331 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 332 Result.push_back(std::make_pair(KC, *PI)); 333 334 return true; 335 } 336 337 // If V is a non-instruction value, or an instruction in a different block, 338 // then it can't be derived from a PHI. 339 Instruction *I = dyn_cast<Instruction>(V); 340 if (I == 0 || I->getParent() != BB) { 341 342 // Okay, if this is a live-in value, see if it has a known value at the end 343 // of any of our predecessors. 344 // 345 // FIXME: This should be an edge property, not a block end property. 346 /// TODO: Per PR2563, we could infer value range information about a 347 /// predecessor based on its terminator. 348 // 349 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if 350 // "I" is a non-local compare-with-a-constant instruction. This would be 351 // able to handle value inequalities better, for example if the compare is 352 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available. 353 // Perhaps getConstantOnEdge should be smart enough to do this? 354 355 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 356 BasicBlock *P = *PI; 357 // If the value is known by LazyValueInfo to be a constant in a 358 // predecessor, use that information to try to thread this block. 359 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB); 360 if (Constant *KC = getKnownConstant(PredCst, Preference)) 361 Result.push_back(std::make_pair(KC, P)); 362 } 363 364 return !Result.empty(); 365 } 366 367 /// If I is a PHI node, then we know the incoming values for any constants. 368 if (PHINode *PN = dyn_cast<PHINode>(I)) { 369 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 370 Value *InVal = PN->getIncomingValue(i); 371 if (Constant *KC = getKnownConstant(InVal, Preference)) { 372 Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i))); 373 } else { 374 Constant *CI = LVI->getConstantOnEdge(InVal, 375 PN->getIncomingBlock(i), BB); 376 if (Constant *KC = getKnownConstant(CI, Preference)) 377 Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i))); 378 } 379 } 380 381 return !Result.empty(); 382 } 383 384 PredValueInfoTy LHSVals, RHSVals; 385 386 // Handle some boolean conditions. 387 if (I->getType()->getPrimitiveSizeInBits() == 1) { 388 assert(Preference == WantInteger && "One-bit non-integer type?"); 389 // X | true -> true 390 // X & false -> false 391 if (I->getOpcode() == Instruction::Or || 392 I->getOpcode() == Instruction::And) { 393 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals, 394 WantInteger); 395 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals, 396 WantInteger); 397 398 if (LHSVals.empty() && RHSVals.empty()) 399 return false; 400 401 ConstantInt *InterestingVal; 402 if (I->getOpcode() == Instruction::Or) 403 InterestingVal = ConstantInt::getTrue(I->getContext()); 404 else 405 InterestingVal = ConstantInt::getFalse(I->getContext()); 406 407 SmallPtrSet<BasicBlock*, 4> LHSKnownBBs; 408 409 // Scan for the sentinel. If we find an undef, force it to the 410 // interesting value: x|undef -> true and x&undef -> false. 411 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) 412 if (LHSVals[i].first == InterestingVal || 413 isa<UndefValue>(LHSVals[i].first)) { 414 Result.push_back(LHSVals[i]); 415 Result.back().first = InterestingVal; 416 LHSKnownBBs.insert(LHSVals[i].second); 417 } 418 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i) 419 if (RHSVals[i].first == InterestingVal || 420 isa<UndefValue>(RHSVals[i].first)) { 421 // If we already inferred a value for this block on the LHS, don't 422 // re-add it. 423 if (!LHSKnownBBs.count(RHSVals[i].second)) { 424 Result.push_back(RHSVals[i]); 425 Result.back().first = InterestingVal; 426 } 427 } 428 429 return !Result.empty(); 430 } 431 432 // Handle the NOT form of XOR. 433 if (I->getOpcode() == Instruction::Xor && 434 isa<ConstantInt>(I->getOperand(1)) && 435 cast<ConstantInt>(I->getOperand(1))->isOne()) { 436 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result, 437 WantInteger); 438 if (Result.empty()) 439 return false; 440 441 // Invert the known values. 442 for (unsigned i = 0, e = Result.size(); i != e; ++i) 443 Result[i].first = ConstantExpr::getNot(Result[i].first); 444 445 return true; 446 } 447 448 // Try to simplify some other binary operator values. 449 } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) { 450 assert(Preference != WantBlockAddress 451 && "A binary operator creating a block address?"); 452 if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) { 453 PredValueInfoTy LHSVals; 454 ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals, 455 WantInteger); 456 457 // Try to use constant folding to simplify the binary operator. 458 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { 459 Constant *V = LHSVals[i].first; 460 Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI); 461 462 if (Constant *KC = getKnownConstant(Folded, WantInteger)) 463 Result.push_back(std::make_pair(KC, LHSVals[i].second)); 464 } 465 } 466 467 return !Result.empty(); 468 } 469 470 // Handle compare with phi operand, where the PHI is defined in this block. 471 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) { 472 assert(Preference == WantInteger && "Compares only produce integers"); 473 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0)); 474 if (PN && PN->getParent() == BB) { 475 // We can do this simplification if any comparisons fold to true or false. 476 // See if any do. 477 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 478 BasicBlock *PredBB = PN->getIncomingBlock(i); 479 Value *LHS = PN->getIncomingValue(i); 480 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB); 481 482 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD); 483 if (Res == 0) { 484 if (!isa<Constant>(RHS)) 485 continue; 486 487 LazyValueInfo::Tristate 488 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS, 489 cast<Constant>(RHS), PredBB, BB); 490 if (ResT == LazyValueInfo::Unknown) 491 continue; 492 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT); 493 } 494 495 if (Constant *KC = getKnownConstant(Res, WantInteger)) 496 Result.push_back(std::make_pair(KC, PredBB)); 497 } 498 499 return !Result.empty(); 500 } 501 502 503 // If comparing a live-in value against a constant, see if we know the 504 // live-in value on any predecessors. 505 if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) { 506 if (!isa<Instruction>(Cmp->getOperand(0)) || 507 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) { 508 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1)); 509 510 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){ 511 BasicBlock *P = *PI; 512 // If the value is known by LazyValueInfo to be a constant in a 513 // predecessor, use that information to try to thread this block. 514 LazyValueInfo::Tristate Res = 515 LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0), 516 RHSCst, P, BB); 517 if (Res == LazyValueInfo::Unknown) 518 continue; 519 520 Constant *ResC = ConstantInt::get(Cmp->getType(), Res); 521 Result.push_back(std::make_pair(ResC, P)); 522 } 523 524 return !Result.empty(); 525 } 526 527 // Try to find a constant value for the LHS of a comparison, 528 // and evaluate it statically if we can. 529 if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) { 530 PredValueInfoTy LHSVals; 531 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals, 532 WantInteger); 533 534 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { 535 Constant *V = LHSVals[i].first; 536 Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(), 537 V, CmpConst); 538 if (Constant *KC = getKnownConstant(Folded, WantInteger)) 539 Result.push_back(std::make_pair(KC, LHSVals[i].second)); 540 } 541 542 return !Result.empty(); 543 } 544 } 545 } 546 547 if (SelectInst *SI = dyn_cast<SelectInst>(I)) { 548 // Handle select instructions where at least one operand is a known constant 549 // and we can figure out the condition value for any predecessor block. 550 Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference); 551 Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference); 552 PredValueInfoTy Conds; 553 if ((TrueVal || FalseVal) && 554 ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds, 555 WantInteger)) { 556 for (unsigned i = 0, e = Conds.size(); i != e; ++i) { 557 Constant *Cond = Conds[i].first; 558 559 // Figure out what value to use for the condition. 560 bool KnownCond; 561 if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) { 562 // A known boolean. 563 KnownCond = CI->isOne(); 564 } else { 565 assert(isa<UndefValue>(Cond) && "Unexpected condition value"); 566 // Either operand will do, so be sure to pick the one that's a known 567 // constant. 568 // FIXME: Do this more cleverly if both values are known constants? 569 KnownCond = (TrueVal != 0); 570 } 571 572 // See if the select has a known constant value for this predecessor. 573 if (Constant *Val = KnownCond ? TrueVal : FalseVal) 574 Result.push_back(std::make_pair(Val, Conds[i].second)); 575 } 576 577 return !Result.empty(); 578 } 579 } 580 581 // If all else fails, see if LVI can figure out a constant value for us. 582 Constant *CI = LVI->getConstant(V, BB); 583 if (Constant *KC = getKnownConstant(CI, Preference)) { 584 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 585 Result.push_back(std::make_pair(KC, *PI)); 586 } 587 588 return !Result.empty(); 589 } 590 591 592 593 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends 594 /// in an undefined jump, decide which block is best to revector to. 595 /// 596 /// Since we can pick an arbitrary destination, we pick the successor with the 597 /// fewest predecessors. This should reduce the in-degree of the others. 598 /// 599 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) { 600 TerminatorInst *BBTerm = BB->getTerminator(); 601 unsigned MinSucc = 0; 602 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); 603 // Compute the successor with the minimum number of predecessors. 604 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 605 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { 606 TestBB = BBTerm->getSuccessor(i); 607 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 608 if (NumPreds < MinNumPreds) { 609 MinSucc = i; 610 MinNumPreds = NumPreds; 611 } 612 } 613 614 return MinSucc; 615 } 616 617 static bool hasAddressTakenAndUsed(BasicBlock *BB) { 618 if (!BB->hasAddressTaken()) return false; 619 620 // If the block has its address taken, it may be a tree of dead constants 621 // hanging off of it. These shouldn't keep the block alive. 622 BlockAddress *BA = BlockAddress::get(BB); 623 BA->removeDeadConstantUsers(); 624 return !BA->use_empty(); 625 } 626 627 /// ProcessBlock - If there are any predecessors whose control can be threaded 628 /// through to a successor, transform them now. 629 bool JumpThreading::ProcessBlock(BasicBlock *BB) { 630 // If the block is trivially dead, just return and let the caller nuke it. 631 // This simplifies other transformations. 632 if (pred_begin(BB) == pred_end(BB) && 633 BB != &BB->getParent()->getEntryBlock()) 634 return false; 635 636 // If this block has a single predecessor, and if that pred has a single 637 // successor, merge the blocks. This encourages recursive jump threading 638 // because now the condition in this block can be threaded through 639 // predecessors of our predecessor block. 640 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) { 641 if (SinglePred->getTerminator()->getNumSuccessors() == 1 && 642 SinglePred != BB && !hasAddressTakenAndUsed(BB)) { 643 // If SinglePred was a loop header, BB becomes one. 644 if (LoopHeaders.erase(SinglePred)) 645 LoopHeaders.insert(BB); 646 647 // Remember if SinglePred was the entry block of the function. If so, we 648 // will need to move BB back to the entry position. 649 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); 650 LVI->eraseBlock(SinglePred); 651 MergeBasicBlockIntoOnlyPred(BB); 652 653 if (isEntry && BB != &BB->getParent()->getEntryBlock()) 654 BB->moveBefore(&BB->getParent()->getEntryBlock()); 655 return true; 656 } 657 } 658 659 // What kind of constant we're looking for. 660 ConstantPreference Preference = WantInteger; 661 662 // Look to see if the terminator is a conditional branch, switch or indirect 663 // branch, if not we can't thread it. 664 Value *Condition; 665 Instruction *Terminator = BB->getTerminator(); 666 if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) { 667 // Can't thread an unconditional jump. 668 if (BI->isUnconditional()) return false; 669 Condition = BI->getCondition(); 670 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) { 671 Condition = SI->getCondition(); 672 } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) { 673 Condition = IB->getAddress()->stripPointerCasts(); 674 Preference = WantBlockAddress; 675 } else { 676 return false; // Must be an invoke. 677 } 678 679 // Run constant folding to see if we can reduce the condition to a simple 680 // constant. 681 if (Instruction *I = dyn_cast<Instruction>(Condition)) { 682 Value *SimpleVal = ConstantFoldInstruction(I, TD, TLI); 683 if (SimpleVal) { 684 I->replaceAllUsesWith(SimpleVal); 685 I->eraseFromParent(); 686 Condition = SimpleVal; 687 } 688 } 689 690 // If the terminator is branching on an undef, we can pick any of the 691 // successors to branch to. Let GetBestDestForJumpOnUndef decide. 692 if (isa<UndefValue>(Condition)) { 693 unsigned BestSucc = GetBestDestForJumpOnUndef(BB); 694 695 // Fold the branch/switch. 696 TerminatorInst *BBTerm = BB->getTerminator(); 697 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { 698 if (i == BestSucc) continue; 699 BBTerm->getSuccessor(i)->removePredecessor(BB, true); 700 } 701 702 DEBUG(dbgs() << " In block '" << BB->getName() 703 << "' folding undef terminator: " << *BBTerm << '\n'); 704 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm); 705 BBTerm->eraseFromParent(); 706 return true; 707 } 708 709 // If the terminator of this block is branching on a constant, simplify the 710 // terminator to an unconditional branch. This can occur due to threading in 711 // other blocks. 712 if (getKnownConstant(Condition, Preference)) { 713 DEBUG(dbgs() << " In block '" << BB->getName() 714 << "' folding terminator: " << *BB->getTerminator() << '\n'); 715 ++NumFolds; 716 ConstantFoldTerminator(BB, true); 717 return true; 718 } 719 720 Instruction *CondInst = dyn_cast<Instruction>(Condition); 721 722 // All the rest of our checks depend on the condition being an instruction. 723 if (CondInst == 0) { 724 // FIXME: Unify this with code below. 725 if (ProcessThreadableEdges(Condition, BB, Preference)) 726 return true; 727 return false; 728 } 729 730 731 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { 732 // For a comparison where the LHS is outside this block, it's possible 733 // that we've branched on it before. Used LVI to see if we can simplify 734 // the branch based on that. 735 BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); 736 Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1)); 737 pred_iterator PI = pred_begin(BB), PE = pred_end(BB); 738 if (CondBr && CondConst && CondBr->isConditional() && PI != PE && 739 (!isa<Instruction>(CondCmp->getOperand(0)) || 740 cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) { 741 // For predecessor edge, determine if the comparison is true or false 742 // on that edge. If they're all true or all false, we can simplify the 743 // branch. 744 // FIXME: We could handle mixed true/false by duplicating code. 745 LazyValueInfo::Tristate Baseline = 746 LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0), 747 CondConst, *PI, BB); 748 if (Baseline != LazyValueInfo::Unknown) { 749 // Check that all remaining incoming values match the first one. 750 while (++PI != PE) { 751 LazyValueInfo::Tristate Ret = 752 LVI->getPredicateOnEdge(CondCmp->getPredicate(), 753 CondCmp->getOperand(0), CondConst, *PI, BB); 754 if (Ret != Baseline) break; 755 } 756 757 // If we terminated early, then one of the values didn't match. 758 if (PI == PE) { 759 unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0; 760 unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1; 761 CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true); 762 BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr); 763 CondBr->eraseFromParent(); 764 return true; 765 } 766 } 767 } 768 } 769 770 // Check for some cases that are worth simplifying. Right now we want to look 771 // for loads that are used by a switch or by the condition for the branch. If 772 // we see one, check to see if it's partially redundant. If so, insert a PHI 773 // which can then be used to thread the values. 774 // 775 Value *SimplifyValue = CondInst; 776 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 777 if (isa<Constant>(CondCmp->getOperand(1))) 778 SimplifyValue = CondCmp->getOperand(0); 779 780 // TODO: There are other places where load PRE would be profitable, such as 781 // more complex comparisons. 782 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 783 if (SimplifyPartiallyRedundantLoad(LI)) 784 return true; 785 786 787 // Handle a variety of cases where we are branching on something derived from 788 // a PHI node in the current block. If we can prove that any predecessors 789 // compute a predictable value based on a PHI node, thread those predecessors. 790 // 791 if (ProcessThreadableEdges(CondInst, BB, Preference)) 792 return true; 793 794 // If this is an otherwise-unfoldable branch on a phi node in the current 795 // block, see if we can simplify. 796 if (PHINode *PN = dyn_cast<PHINode>(CondInst)) 797 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 798 return ProcessBranchOnPHI(PN); 799 800 801 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify. 802 if (CondInst->getOpcode() == Instruction::Xor && 803 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator())) 804 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst)); 805 806 807 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 808 // "(X == 4)", thread through this block. 809 810 return false; 811 } 812 813 814 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 815 /// load instruction, eliminate it by replacing it with a PHI node. This is an 816 /// important optimization that encourages jump threading, and needs to be run 817 /// interlaced with other jump threading tasks. 818 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 819 // Don't hack volatile/atomic loads. 820 if (!LI->isSimple()) return false; 821 822 // If the load is defined in a block with exactly one predecessor, it can't be 823 // partially redundant. 824 BasicBlock *LoadBB = LI->getParent(); 825 if (LoadBB->getSinglePredecessor()) 826 return false; 827 828 Value *LoadedPtr = LI->getOperand(0); 829 830 // If the loaded operand is defined in the LoadBB, it can't be available. 831 // TODO: Could do simple PHI translation, that would be fun :) 832 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 833 if (PtrOp->getParent() == LoadBB) 834 return false; 835 836 // Scan a few instructions up from the load, to see if it is obviously live at 837 // the entry to its block. 838 BasicBlock::iterator BBIt = LI; 839 840 if (Value *AvailableVal = 841 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) { 842 // If the value if the load is locally available within the block, just use 843 // it. This frequently occurs for reg2mem'd allocas. 844 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 845 846 // If the returned value is the load itself, replace with an undef. This can 847 // only happen in dead loops. 848 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); 849 LI->replaceAllUsesWith(AvailableVal); 850 LI->eraseFromParent(); 851 return true; 852 } 853 854 // Otherwise, if we scanned the whole block and got to the top of the block, 855 // we know the block is locally transparent to the load. If not, something 856 // might clobber its value. 857 if (BBIt != LoadBB->begin()) 858 return false; 859 860 // If all of the loads and stores that feed the value have the same TBAA tag, 861 // then we can propagate it onto any newly inserted loads. 862 MDNode *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa); 863 864 SmallPtrSet<BasicBlock*, 8> PredsScanned; 865 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 866 AvailablePredsTy AvailablePreds; 867 BasicBlock *OneUnavailablePred = 0; 868 869 // If we got here, the loaded value is transparent through to the start of the 870 // block. Check to see if it is available in any of the predecessor blocks. 871 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 872 PI != PE; ++PI) { 873 BasicBlock *PredBB = *PI; 874 875 // If we already scanned this predecessor, skip it. 876 if (!PredsScanned.insert(PredBB)) 877 continue; 878 879 // Scan the predecessor to see if the value is available in the pred. 880 BBIt = PredBB->end(); 881 MDNode *ThisTBAATag = 0; 882 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6, 883 0, &ThisTBAATag); 884 if (!PredAvailable) { 885 OneUnavailablePred = PredBB; 886 continue; 887 } 888 889 // If tbaa tags disagree or are not present, forget about them. 890 if (TBAATag != ThisTBAATag) TBAATag = 0; 891 892 // If so, this load is partially redundant. Remember this info so that we 893 // can create a PHI node. 894 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 895 } 896 897 // If the loaded value isn't available in any predecessor, it isn't partially 898 // redundant. 899 if (AvailablePreds.empty()) return false; 900 901 // Okay, the loaded value is available in at least one (and maybe all!) 902 // predecessors. If the value is unavailable in more than one unique 903 // predecessor, we want to insert a merge block for those common predecessors. 904 // This ensures that we only have to insert one reload, thus not increasing 905 // code size. 906 BasicBlock *UnavailablePred = 0; 907 908 // If there is exactly one predecessor where the value is unavailable, the 909 // already computed 'OneUnavailablePred' block is it. If it ends in an 910 // unconditional branch, we know that it isn't a critical edge. 911 if (PredsScanned.size() == AvailablePreds.size()+1 && 912 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 913 UnavailablePred = OneUnavailablePred; 914 } else if (PredsScanned.size() != AvailablePreds.size()) { 915 // Otherwise, we had multiple unavailable predecessors or we had a critical 916 // edge from the one. 917 SmallVector<BasicBlock*, 8> PredsToSplit; 918 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 919 920 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 921 AvailablePredSet.insert(AvailablePreds[i].first); 922 923 // Add all the unavailable predecessors to the PredsToSplit list. 924 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 925 PI != PE; ++PI) { 926 BasicBlock *P = *PI; 927 // If the predecessor is an indirect goto, we can't split the edge. 928 if (isa<IndirectBrInst>(P->getTerminator())) 929 return false; 930 931 if (!AvailablePredSet.count(P)) 932 PredsToSplit.push_back(P); 933 } 934 935 // Split them out to their own block. 936 UnavailablePred = 937 SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split", this); 938 } 939 940 // If the value isn't available in all predecessors, then there will be 941 // exactly one where it isn't available. Insert a load on that edge and add 942 // it to the AvailablePreds list. 943 if (UnavailablePred) { 944 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 945 "Can't handle critical edge here!"); 946 LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false, 947 LI->getAlignment(), 948 UnavailablePred->getTerminator()); 949 NewVal->setDebugLoc(LI->getDebugLoc()); 950 if (TBAATag) 951 NewVal->setMetadata(LLVMContext::MD_tbaa, TBAATag); 952 953 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 954 } 955 956 // Now we know that each predecessor of this block has a value in 957 // AvailablePreds, sort them for efficient access as we're walking the preds. 958 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 959 960 // Create a PHI node at the start of the block for the PRE'd load value. 961 pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB); 962 PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "", 963 LoadBB->begin()); 964 PN->takeName(LI); 965 PN->setDebugLoc(LI->getDebugLoc()); 966 967 // Insert new entries into the PHI for each predecessor. A single block may 968 // have multiple entries here. 969 for (pred_iterator PI = PB; PI != PE; ++PI) { 970 BasicBlock *P = *PI; 971 AvailablePredsTy::iterator I = 972 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 973 std::make_pair(P, (Value*)0)); 974 975 assert(I != AvailablePreds.end() && I->first == P && 976 "Didn't find entry for predecessor!"); 977 978 PN->addIncoming(I->second, I->first); 979 } 980 981 //cerr << "PRE: " << *LI << *PN << "\n"; 982 983 LI->replaceAllUsesWith(PN); 984 LI->eraseFromParent(); 985 986 return true; 987 } 988 989 /// FindMostPopularDest - The specified list contains multiple possible 990 /// threadable destinations. Pick the one that occurs the most frequently in 991 /// the list. 992 static BasicBlock * 993 FindMostPopularDest(BasicBlock *BB, 994 const SmallVectorImpl<std::pair<BasicBlock*, 995 BasicBlock*> > &PredToDestList) { 996 assert(!PredToDestList.empty()); 997 998 // Determine popularity. If there are multiple possible destinations, we 999 // explicitly choose to ignore 'undef' destinations. We prefer to thread 1000 // blocks with known and real destinations to threading undef. We'll handle 1001 // them later if interesting. 1002 DenseMap<BasicBlock*, unsigned> DestPopularity; 1003 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1004 if (PredToDestList[i].second) 1005 DestPopularity[PredToDestList[i].second]++; 1006 1007 // Find the most popular dest. 1008 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin(); 1009 BasicBlock *MostPopularDest = DPI->first; 1010 unsigned Popularity = DPI->second; 1011 SmallVector<BasicBlock*, 4> SamePopularity; 1012 1013 for (++DPI; DPI != DestPopularity.end(); ++DPI) { 1014 // If the popularity of this entry isn't higher than the popularity we've 1015 // seen so far, ignore it. 1016 if (DPI->second < Popularity) 1017 ; // ignore. 1018 else if (DPI->second == Popularity) { 1019 // If it is the same as what we've seen so far, keep track of it. 1020 SamePopularity.push_back(DPI->first); 1021 } else { 1022 // If it is more popular, remember it. 1023 SamePopularity.clear(); 1024 MostPopularDest = DPI->first; 1025 Popularity = DPI->second; 1026 } 1027 } 1028 1029 // Okay, now we know the most popular destination. If there is more than one 1030 // destination, we need to determine one. This is arbitrary, but we need 1031 // to make a deterministic decision. Pick the first one that appears in the 1032 // successor list. 1033 if (!SamePopularity.empty()) { 1034 SamePopularity.push_back(MostPopularDest); 1035 TerminatorInst *TI = BB->getTerminator(); 1036 for (unsigned i = 0; ; ++i) { 1037 assert(i != TI->getNumSuccessors() && "Didn't find any successor!"); 1038 1039 if (std::find(SamePopularity.begin(), SamePopularity.end(), 1040 TI->getSuccessor(i)) == SamePopularity.end()) 1041 continue; 1042 1043 MostPopularDest = TI->getSuccessor(i); 1044 break; 1045 } 1046 } 1047 1048 // Okay, we have finally picked the most popular destination. 1049 return MostPopularDest; 1050 } 1051 1052 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB, 1053 ConstantPreference Preference) { 1054 // If threading this would thread across a loop header, don't even try to 1055 // thread the edge. 1056 if (LoopHeaders.count(BB)) 1057 return false; 1058 1059 PredValueInfoTy PredValues; 1060 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference)) 1061 return false; 1062 1063 assert(!PredValues.empty() && 1064 "ComputeValueKnownInPredecessors returned true with no values"); 1065 1066 DEBUG(dbgs() << "IN BB: " << *BB; 1067 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1068 dbgs() << " BB '" << BB->getName() << "': FOUND condition = " 1069 << *PredValues[i].first 1070 << " for pred '" << PredValues[i].second->getName() << "'.\n"; 1071 }); 1072 1073 // Decide what we want to thread through. Convert our list of known values to 1074 // a list of known destinations for each pred. This also discards duplicate 1075 // predecessors and keeps track of the undefined inputs (which are represented 1076 // as a null dest in the PredToDestList). 1077 SmallPtrSet<BasicBlock*, 16> SeenPreds; 1078 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList; 1079 1080 BasicBlock *OnlyDest = 0; 1081 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL; 1082 1083 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { 1084 BasicBlock *Pred = PredValues[i].second; 1085 if (!SeenPreds.insert(Pred)) 1086 continue; // Duplicate predecessor entry. 1087 1088 // If the predecessor ends with an indirect goto, we can't change its 1089 // destination. 1090 if (isa<IndirectBrInst>(Pred->getTerminator())) 1091 continue; 1092 1093 Constant *Val = PredValues[i].first; 1094 1095 BasicBlock *DestBB; 1096 if (isa<UndefValue>(Val)) 1097 DestBB = 0; 1098 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 1099 DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero()); 1100 else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) { 1101 DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor(); 1102 } else { 1103 assert(isa<IndirectBrInst>(BB->getTerminator()) 1104 && "Unexpected terminator"); 1105 DestBB = cast<BlockAddress>(Val)->getBasicBlock(); 1106 } 1107 1108 // If we have exactly one destination, remember it for efficiency below. 1109 if (PredToDestList.empty()) 1110 OnlyDest = DestBB; 1111 else if (OnlyDest != DestBB) 1112 OnlyDest = MultipleDestSentinel; 1113 1114 PredToDestList.push_back(std::make_pair(Pred, DestBB)); 1115 } 1116 1117 // If all edges were unthreadable, we fail. 1118 if (PredToDestList.empty()) 1119 return false; 1120 1121 // Determine which is the most common successor. If we have many inputs and 1122 // this block is a switch, we want to start by threading the batch that goes 1123 // to the most popular destination first. If we only know about one 1124 // threadable destination (the common case) we can avoid this. 1125 BasicBlock *MostPopularDest = OnlyDest; 1126 1127 if (MostPopularDest == MultipleDestSentinel) 1128 MostPopularDest = FindMostPopularDest(BB, PredToDestList); 1129 1130 // Now that we know what the most popular destination is, factor all 1131 // predecessors that will jump to it into a single predecessor. 1132 SmallVector<BasicBlock*, 16> PredsToFactor; 1133 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) 1134 if (PredToDestList[i].second == MostPopularDest) { 1135 BasicBlock *Pred = PredToDestList[i].first; 1136 1137 // This predecessor may be a switch or something else that has multiple 1138 // edges to the block. Factor each of these edges by listing them 1139 // according to # occurrences in PredsToFactor. 1140 TerminatorInst *PredTI = Pred->getTerminator(); 1141 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i) 1142 if (PredTI->getSuccessor(i) == BB) 1143 PredsToFactor.push_back(Pred); 1144 } 1145 1146 // If the threadable edges are branching on an undefined value, we get to pick 1147 // the destination that these predecessors should get to. 1148 if (MostPopularDest == 0) 1149 MostPopularDest = BB->getTerminator()-> 1150 getSuccessor(GetBestDestForJumpOnUndef(BB)); 1151 1152 // Ok, try to thread it! 1153 return ThreadEdge(BB, PredsToFactor, MostPopularDest); 1154 } 1155 1156 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on 1157 /// a PHI node in the current block. See if there are any simplifications we 1158 /// can do based on inputs to the phi node. 1159 /// 1160 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) { 1161 BasicBlock *BB = PN->getParent(); 1162 1163 // TODO: We could make use of this to do it once for blocks with common PHI 1164 // values. 1165 SmallVector<BasicBlock*, 1> PredBBs; 1166 PredBBs.resize(1); 1167 1168 // If any of the predecessor blocks end in an unconditional branch, we can 1169 // *duplicate* the conditional branch into that block in order to further 1170 // encourage jump threading and to eliminate cases where we have branch on a 1171 // phi of an icmp (branch on icmp is much better). 1172 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1173 BasicBlock *PredBB = PN->getIncomingBlock(i); 1174 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator())) 1175 if (PredBr->isUnconditional()) { 1176 PredBBs[0] = PredBB; 1177 // Try to duplicate BB into PredBB. 1178 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs)) 1179 return true; 1180 } 1181 } 1182 1183 return false; 1184 } 1185 1186 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on 1187 /// a xor instruction in the current block. See if there are any 1188 /// simplifications we can do based on inputs to the xor. 1189 /// 1190 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) { 1191 BasicBlock *BB = BO->getParent(); 1192 1193 // If either the LHS or RHS of the xor is a constant, don't do this 1194 // optimization. 1195 if (isa<ConstantInt>(BO->getOperand(0)) || 1196 isa<ConstantInt>(BO->getOperand(1))) 1197 return false; 1198 1199 // If the first instruction in BB isn't a phi, we won't be able to infer 1200 // anything special about any particular predecessor. 1201 if (!isa<PHINode>(BB->front())) 1202 return false; 1203 1204 // If we have a xor as the branch input to this block, and we know that the 1205 // LHS or RHS of the xor in any predecessor is true/false, then we can clone 1206 // the condition into the predecessor and fix that value to true, saving some 1207 // logical ops on that path and encouraging other paths to simplify. 1208 // 1209 // This copies something like this: 1210 // 1211 // BB: 1212 // %X = phi i1 [1], [%X'] 1213 // %Y = icmp eq i32 %A, %B 1214 // %Z = xor i1 %X, %Y 1215 // br i1 %Z, ... 1216 // 1217 // Into: 1218 // BB': 1219 // %Y = icmp ne i32 %A, %B 1220 // br i1 %Z, ... 1221 1222 PredValueInfoTy XorOpValues; 1223 bool isLHS = true; 1224 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues, 1225 WantInteger)) { 1226 assert(XorOpValues.empty()); 1227 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues, 1228 WantInteger)) 1229 return false; 1230 isLHS = false; 1231 } 1232 1233 assert(!XorOpValues.empty() && 1234 "ComputeValueKnownInPredecessors returned true with no values"); 1235 1236 // Scan the information to see which is most popular: true or false. The 1237 // predecessors can be of the set true, false, or undef. 1238 unsigned NumTrue = 0, NumFalse = 0; 1239 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1240 if (isa<UndefValue>(XorOpValues[i].first)) 1241 // Ignore undefs for the count. 1242 continue; 1243 if (cast<ConstantInt>(XorOpValues[i].first)->isZero()) 1244 ++NumFalse; 1245 else 1246 ++NumTrue; 1247 } 1248 1249 // Determine which value to split on, true, false, or undef if neither. 1250 ConstantInt *SplitVal = 0; 1251 if (NumTrue > NumFalse) 1252 SplitVal = ConstantInt::getTrue(BB->getContext()); 1253 else if (NumTrue != 0 || NumFalse != 0) 1254 SplitVal = ConstantInt::getFalse(BB->getContext()); 1255 1256 // Collect all of the blocks that this can be folded into so that we can 1257 // factor this once and clone it once. 1258 SmallVector<BasicBlock*, 8> BlocksToFoldInto; 1259 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { 1260 if (XorOpValues[i].first != SplitVal && 1261 !isa<UndefValue>(XorOpValues[i].first)) 1262 continue; 1263 1264 BlocksToFoldInto.push_back(XorOpValues[i].second); 1265 } 1266 1267 // If we inferred a value for all of the predecessors, then duplication won't 1268 // help us. However, we can just replace the LHS or RHS with the constant. 1269 if (BlocksToFoldInto.size() == 1270 cast<PHINode>(BB->front()).getNumIncomingValues()) { 1271 if (SplitVal == 0) { 1272 // If all preds provide undef, just nuke the xor, because it is undef too. 1273 BO->replaceAllUsesWith(UndefValue::get(BO->getType())); 1274 BO->eraseFromParent(); 1275 } else if (SplitVal->isZero()) { 1276 // If all preds provide 0, replace the xor with the other input. 1277 BO->replaceAllUsesWith(BO->getOperand(isLHS)); 1278 BO->eraseFromParent(); 1279 } else { 1280 // If all preds provide 1, set the computed value to 1. 1281 BO->setOperand(!isLHS, SplitVal); 1282 } 1283 1284 return true; 1285 } 1286 1287 // Try to duplicate BB into PredBB. 1288 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto); 1289 } 1290 1291 1292 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new 1293 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for 1294 /// NewPred using the entries from OldPred (suitably mapped). 1295 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, 1296 BasicBlock *OldPred, 1297 BasicBlock *NewPred, 1298 DenseMap<Instruction*, Value*> &ValueMap) { 1299 for (BasicBlock::iterator PNI = PHIBB->begin(); 1300 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) { 1301 // Ok, we have a PHI node. Figure out what the incoming value was for the 1302 // DestBlock. 1303 Value *IV = PN->getIncomingValueForBlock(OldPred); 1304 1305 // Remap the value if necessary. 1306 if (Instruction *Inst = dyn_cast<Instruction>(IV)) { 1307 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst); 1308 if (I != ValueMap.end()) 1309 IV = I->second; 1310 } 1311 1312 PN->addIncoming(IV, NewPred); 1313 } 1314 } 1315 1316 /// ThreadEdge - We have decided that it is safe and profitable to factor the 1317 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB 1318 /// across BB. Transform the IR to reflect this change. 1319 bool JumpThreading::ThreadEdge(BasicBlock *BB, 1320 const SmallVectorImpl<BasicBlock*> &PredBBs, 1321 BasicBlock *SuccBB) { 1322 // If threading to the same block as we come from, we would infinite loop. 1323 if (SuccBB == BB) { 1324 DEBUG(dbgs() << " Not threading across BB '" << BB->getName() 1325 << "' - would thread to self!\n"); 1326 return false; 1327 } 1328 1329 // If threading this would thread across a loop header, don't thread the edge. 1330 // See the comments above FindLoopHeaders for justifications and caveats. 1331 if (LoopHeaders.count(BB)) { 1332 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName() 1333 << "' to dest BB '" << SuccBB->getName() 1334 << "' - it might create an irreducible loop!\n"); 1335 return false; 1336 } 1337 1338 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 1339 if (JumpThreadCost > Threshold) { 1340 DEBUG(dbgs() << " Not threading BB '" << BB->getName() 1341 << "' - Cost is too high: " << JumpThreadCost << "\n"); 1342 return false; 1343 } 1344 1345 // And finally, do it! Start by factoring the predecessors is needed. 1346 BasicBlock *PredBB; 1347 if (PredBBs.size() == 1) 1348 PredBB = PredBBs[0]; 1349 else { 1350 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1351 << " common predecessors.\n"); 1352 PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this); 1353 } 1354 1355 // And finally, do it! 1356 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '" 1357 << SuccBB->getName() << "' with cost: " << JumpThreadCost 1358 << ", across block:\n " 1359 << *BB << "\n"); 1360 1361 LVI->threadEdge(PredBB, BB, SuccBB); 1362 1363 // We are going to have to map operands from the original BB block to the new 1364 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 1365 // account for entry from PredBB. 1366 DenseMap<Instruction*, Value*> ValueMapping; 1367 1368 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), 1369 BB->getName()+".thread", 1370 BB->getParent(), BB); 1371 NewBB->moveAfter(PredBB); 1372 1373 BasicBlock::iterator BI = BB->begin(); 1374 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1375 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1376 1377 // Clone the non-phi instructions of BB into NewBB, keeping track of the 1378 // mapping and using it to remap operands in the cloned instructions. 1379 for (; !isa<TerminatorInst>(BI); ++BI) { 1380 Instruction *New = BI->clone(); 1381 New->setName(BI->getName()); 1382 NewBB->getInstList().push_back(New); 1383 ValueMapping[BI] = New; 1384 1385 // Remap operands to patch up intra-block references. 1386 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1387 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1388 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1389 if (I != ValueMapping.end()) 1390 New->setOperand(i, I->second); 1391 } 1392 } 1393 1394 // We didn't copy the terminator from BB over to NewBB, because there is now 1395 // an unconditional jump to SuccBB. Insert the unconditional jump. 1396 BranchInst *NewBI =BranchInst::Create(SuccBB, NewBB); 1397 NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc()); 1398 1399 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 1400 // PHI nodes for NewBB now. 1401 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping); 1402 1403 // If there were values defined in BB that are used outside the block, then we 1404 // now have to update all uses of the value to use either the original value, 1405 // the cloned value, or some PHI derived value. This can require arbitrary 1406 // PHI insertion, of which we are prepared to do, clean these up now. 1407 SSAUpdater SSAUpdate; 1408 SmallVector<Use*, 16> UsesToRename; 1409 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1410 // Scan all uses of this instruction to see if it is used outside of its 1411 // block, and if so, record them in UsesToRename. 1412 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1413 ++UI) { 1414 Instruction *User = cast<Instruction>(*UI); 1415 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1416 if (UserPN->getIncomingBlock(UI) == BB) 1417 continue; 1418 } else if (User->getParent() == BB) 1419 continue; 1420 1421 UsesToRename.push_back(&UI.getUse()); 1422 } 1423 1424 // If there are no uses outside the block, we're done with this instruction. 1425 if (UsesToRename.empty()) 1426 continue; 1427 1428 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1429 1430 // We found a use of I outside of BB. Rename all uses of I that are outside 1431 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1432 // with the two values we know. 1433 SSAUpdate.Initialize(I->getType(), I->getName()); 1434 SSAUpdate.AddAvailableValue(BB, I); 1435 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]); 1436 1437 while (!UsesToRename.empty()) 1438 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1439 DEBUG(dbgs() << "\n"); 1440 } 1441 1442 1443 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 1444 // NewBB instead of BB. This eliminates predecessors from BB, which requires 1445 // us to simplify any PHI nodes in BB. 1446 TerminatorInst *PredTerm = PredBB->getTerminator(); 1447 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 1448 if (PredTerm->getSuccessor(i) == BB) { 1449 BB->removePredecessor(PredBB, true); 1450 PredTerm->setSuccessor(i, NewBB); 1451 } 1452 1453 // At this point, the IR is fully up to date and consistent. Do a quick scan 1454 // over the new instructions and zap any that are constants or dead. This 1455 // frequently happens because of phi translation. 1456 SimplifyInstructionsInBlock(NewBB, TD); 1457 1458 // Threaded an edge! 1459 ++NumThreads; 1460 return true; 1461 } 1462 1463 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch 1464 /// to BB which contains an i1 PHI node and a conditional branch on that PHI. 1465 /// If we can duplicate the contents of BB up into PredBB do so now, this 1466 /// improves the odds that the branch will be on an analyzable instruction like 1467 /// a compare. 1468 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, 1469 const SmallVectorImpl<BasicBlock *> &PredBBs) { 1470 assert(!PredBBs.empty() && "Can't handle an empty set"); 1471 1472 // If BB is a loop header, then duplicating this block outside the loop would 1473 // cause us to transform this into an irreducible loop, don't do this. 1474 // See the comments above FindLoopHeaders for justifications and caveats. 1475 if (LoopHeaders.count(BB)) { 1476 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName() 1477 << "' into predecessor block '" << PredBBs[0]->getName() 1478 << "' - it might create an irreducible loop!\n"); 1479 return false; 1480 } 1481 1482 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB); 1483 if (DuplicationCost > Threshold) { 1484 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName() 1485 << "' - Cost is too high: " << DuplicationCost << "\n"); 1486 return false; 1487 } 1488 1489 // And finally, do it! Start by factoring the predecessors is needed. 1490 BasicBlock *PredBB; 1491 if (PredBBs.size() == 1) 1492 PredBB = PredBBs[0]; 1493 else { 1494 DEBUG(dbgs() << " Factoring out " << PredBBs.size() 1495 << " common predecessors.\n"); 1496 PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this); 1497 } 1498 1499 // Okay, we decided to do this! Clone all the instructions in BB onto the end 1500 // of PredBB. 1501 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '" 1502 << PredBB->getName() << "' to eliminate branch on phi. Cost: " 1503 << DuplicationCost << " block is:" << *BB << "\n"); 1504 1505 // Unless PredBB ends with an unconditional branch, split the edge so that we 1506 // can just clone the bits from BB into the end of the new PredBB. 1507 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator()); 1508 1509 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) { 1510 PredBB = SplitEdge(PredBB, BB, this); 1511 OldPredBranch = cast<BranchInst>(PredBB->getTerminator()); 1512 } 1513 1514 // We are going to have to map operands from the original BB block into the 1515 // PredBB block. Evaluate PHI nodes in BB. 1516 DenseMap<Instruction*, Value*> ValueMapping; 1517 1518 BasicBlock::iterator BI = BB->begin(); 1519 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 1520 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 1521 1522 // Clone the non-phi instructions of BB into PredBB, keeping track of the 1523 // mapping and using it to remap operands in the cloned instructions. 1524 for (; BI != BB->end(); ++BI) { 1525 Instruction *New = BI->clone(); 1526 1527 // Remap operands to patch up intra-block references. 1528 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 1529 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 1530 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 1531 if (I != ValueMapping.end()) 1532 New->setOperand(i, I->second); 1533 } 1534 1535 // If this instruction can be simplified after the operands are updated, 1536 // just use the simplified value instead. This frequently happens due to 1537 // phi translation. 1538 if (Value *IV = SimplifyInstruction(New, TD)) { 1539 delete New; 1540 ValueMapping[BI] = IV; 1541 } else { 1542 // Otherwise, insert the new instruction into the block. 1543 New->setName(BI->getName()); 1544 PredBB->getInstList().insert(OldPredBranch, New); 1545 ValueMapping[BI] = New; 1546 } 1547 } 1548 1549 // Check to see if the targets of the branch had PHI nodes. If so, we need to 1550 // add entries to the PHI nodes for branch from PredBB now. 1551 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator()); 1552 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB, 1553 ValueMapping); 1554 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB, 1555 ValueMapping); 1556 1557 // If there were values defined in BB that are used outside the block, then we 1558 // now have to update all uses of the value to use either the original value, 1559 // the cloned value, or some PHI derived value. This can require arbitrary 1560 // PHI insertion, of which we are prepared to do, clean these up now. 1561 SSAUpdater SSAUpdate; 1562 SmallVector<Use*, 16> UsesToRename; 1563 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 1564 // Scan all uses of this instruction to see if it is used outside of its 1565 // block, and if so, record them in UsesToRename. 1566 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 1567 ++UI) { 1568 Instruction *User = cast<Instruction>(*UI); 1569 if (PHINode *UserPN = dyn_cast<PHINode>(User)) { 1570 if (UserPN->getIncomingBlock(UI) == BB) 1571 continue; 1572 } else if (User->getParent() == BB) 1573 continue; 1574 1575 UsesToRename.push_back(&UI.getUse()); 1576 } 1577 1578 // If there are no uses outside the block, we're done with this instruction. 1579 if (UsesToRename.empty()) 1580 continue; 1581 1582 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); 1583 1584 // We found a use of I outside of BB. Rename all uses of I that are outside 1585 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks 1586 // with the two values we know. 1587 SSAUpdate.Initialize(I->getType(), I->getName()); 1588 SSAUpdate.AddAvailableValue(BB, I); 1589 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]); 1590 1591 while (!UsesToRename.empty()) 1592 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); 1593 DEBUG(dbgs() << "\n"); 1594 } 1595 1596 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge 1597 // that we nuked. 1598 BB->removePredecessor(PredBB, true); 1599 1600 // Remove the unconditional branch at the end of the PredBB block. 1601 OldPredBranch->eraseFromParent(); 1602 1603 ++NumDupes; 1604 return true; 1605 } 1606 1607 1608