JumpThreading.cpp revision a3522000ab9c821f48856d0c25ada8297c1c2914
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/Pass.h" 18#include "llvm/ADT/DenseMap.h" 19#include "llvm/ADT/Statistic.h" 20#include "llvm/ADT/STLExtras.h" 21#include "llvm/Analysis/ConstantFolding.h" 22#include "llvm/Transforms/Utils/BasicBlockUtils.h" 23#include "llvm/Transforms/Utils/Local.h" 24#include "llvm/Target/TargetData.h" 25#include "llvm/Support/CommandLine.h" 26#include "llvm/Support/Compiler.h" 27#include "llvm/Support/Debug.h" 28#include "llvm/ADT/SmallPtrSet.h" 29using namespace llvm; 30 31STATISTIC(NumThreads, "Number of jumps threaded"); 32STATISTIC(NumFolds, "Number of terminators folded"); 33 34static cl::opt<unsigned> 35Threshold("jump-threading-threshold", 36 cl::desc("Max block size to duplicate for jump threading"), 37 cl::init(6), cl::Hidden); 38 39namespace { 40 /// This pass performs 'jump threading', which looks at blocks that have 41 /// multiple predecessors and multiple successors. If one or more of the 42 /// predecessors of the block can be proven to always jump to one of the 43 /// successors, we forward the edge from the predecessor to the successor by 44 /// duplicating the contents of this block. 45 /// 46 /// An example of when this can occur is code like this: 47 /// 48 /// if () { ... 49 /// X = 4; 50 /// } 51 /// if (X < 3) { 52 /// 53 /// In this case, the unconditional branch at the end of the first if can be 54 /// revectored to the false side of the second if. 55 /// 56 class VISIBILITY_HIDDEN JumpThreading : public FunctionPass { 57 TargetData *TD; 58 public: 59 static char ID; // Pass identification 60 JumpThreading() : FunctionPass(&ID) {} 61 62 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 63 AU.addRequired<TargetData>(); 64 } 65 66 bool runOnFunction(Function &F); 67 bool ProcessBlock(BasicBlock *BB); 68 void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB); 69 BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal); 70 71 bool ProcessJumpOnPHI(PHINode *PN); 72 bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); 73 bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); 74 75 bool SimplifyPartiallyRedundantLoad(LoadInst *LI); 76 }; 77} 78 79char JumpThreading::ID = 0; 80static RegisterPass<JumpThreading> 81X("jump-threading", "Jump Threading"); 82 83// Public interface to the Jump Threading pass 84FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 85 86/// runOnFunction - Top level algorithm. 87/// 88bool JumpThreading::runOnFunction(Function &F) { 89 DOUT << "Jump threading on function '" << F.getNameStart() << "'\n"; 90 TD = &getAnalysis<TargetData>(); 91 92 bool AnotherIteration = true, EverChanged = false; 93 while (AnotherIteration) { 94 AnotherIteration = false; 95 bool Changed = false; 96 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) 97 while (ProcessBlock(I)) 98 Changed = true; 99 AnotherIteration = Changed; 100 EverChanged |= Changed; 101 } 102 return EverChanged; 103} 104 105/// FactorCommonPHIPreds - If there are multiple preds with the same incoming 106/// value for the PHI, factor them together so we get one block to thread for 107/// the whole group. 108/// This is important for things like "phi i1 [true, true, false, true, x]" 109/// where we only need to clone the block for the true blocks once. 110/// 111BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Constant *CstVal) { 112 SmallVector<BasicBlock*, 16> CommonPreds; 113 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 114 if (PN->getIncomingValue(i) == CstVal) 115 CommonPreds.push_back(PN->getIncomingBlock(i)); 116 117 if (CommonPreds.size() == 1) 118 return CommonPreds[0]; 119 120 DOUT << " Factoring out " << CommonPreds.size() 121 << " common predecessors.\n"; 122 return SplitBlockPredecessors(PN->getParent(), 123 &CommonPreds[0], CommonPreds.size(), 124 ".thr_comm", this); 125} 126 127 128/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 129/// thread across it. 130static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { 131 /// Ignore PHI nodes, these will be flattened when duplication happens. 132 BasicBlock::const_iterator I = BB->getFirstNonPHI(); 133 134 // Sum up the cost of each instruction until we get to the terminator. Don't 135 // include the terminator because the copy won't include it. 136 unsigned Size = 0; 137 for (; !isa<TerminatorInst>(I); ++I) { 138 // Debugger intrinsics don't incur code size. 139 if (isa<DbgInfoIntrinsic>(I)) continue; 140 141 // If this is a pointer->pointer bitcast, it is free. 142 if (isa<BitCastInst>(I) && isa<PointerType>(I->getType())) 143 continue; 144 145 // All other instructions count for at least one unit. 146 ++Size; 147 148 // Calls are more expensive. If they are non-intrinsic calls, we model them 149 // as having cost of 4. If they are a non-vector intrinsic, we model them 150 // as having cost of 2 total, and if they are a vector intrinsic, we model 151 // them as having cost 1. 152 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 153 if (!isa<IntrinsicInst>(CI)) 154 Size += 3; 155 else if (isa<VectorType>(CI->getType())) 156 Size += 1; 157 } 158 } 159 160 // Threading through a switch statement is particularly profitable. If this 161 // block ends in a switch, decrease its cost to make it more likely to happen. 162 if (isa<SwitchInst>(I)) 163 Size = Size > 6 ? Size-6 : 0; 164 165 return Size; 166} 167 168/// ProcessBlock - If there are any predecessors whose control can be threaded 169/// through to a successor, transform them now. 170bool JumpThreading::ProcessBlock(BasicBlock *BB) { 171 // If this block has a single predecessor, and if that pred has a single 172 // successor, merge the blocks. This encourages recursive jump threading 173 // because now the condition in this block can be threaded through 174 // predecessors of our predecessor block. 175 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) 176 if (SinglePred->getTerminator()->getNumSuccessors() == 1 && 177 SinglePred != BB) { 178 // Remember if SinglePred was the entry block of the function. If so, we 179 // will need to move BB back to the entry position. 180 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); 181 MergeBasicBlockIntoOnlyPred(BB); 182 183 if (isEntry && BB != &BB->getParent()->getEntryBlock()) 184 BB->moveBefore(&BB->getParent()->getEntryBlock()); 185 return true; 186 } 187 188 // See if this block ends with a branch or switch. If so, see if the 189 // condition is a phi node. If so, and if an entry of the phi node is a 190 // constant, we can thread the block. 191 Value *Condition; 192 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 193 // Can't thread an unconditional jump. 194 if (BI->isUnconditional()) return false; 195 Condition = BI->getCondition(); 196 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) 197 Condition = SI->getCondition(); 198 else 199 return false; // Must be an invoke. 200 201 // If the terminator of this block is branching on a constant, simplify the 202 // terminator to an unconditional branch. This can occur due to threading in 203 // other blocks. 204 if (isa<ConstantInt>(Condition)) { 205 DOUT << " In block '" << BB->getNameStart() 206 << "' folding terminator: " << *BB->getTerminator(); 207 ++NumFolds; 208 ConstantFoldTerminator(BB); 209 return true; 210 } 211 212 // If there is only a single predecessor of this block, nothing to fold. 213 if (BB->getSinglePredecessor()) 214 return false; 215 216 // See if this is a phi node in the current block. 217 PHINode *PN = dyn_cast<PHINode>(Condition); 218 if (PN && PN->getParent() == BB) 219 return ProcessJumpOnPHI(PN); 220 221 // If this is a conditional branch whose condition is and/or of a phi, try to 222 // simplify it. 223 if (BinaryOperator *CondI = dyn_cast<BinaryOperator>(Condition)) { 224 if ((CondI->getOpcode() == Instruction::And || 225 CondI->getOpcode() == Instruction::Or) && 226 isa<BranchInst>(BB->getTerminator()) && 227 ProcessBranchOnLogical(CondI, BB, 228 CondI->getOpcode() == Instruction::And)) 229 return true; 230 } 231 232 // If we have "br (phi != 42)" and the phi node has any constant values as 233 // operands, we can thread through this block. 234 if (CmpInst *CondCmp = dyn_cast<CmpInst>(Condition)) 235 if (isa<PHINode>(CondCmp->getOperand(0)) && 236 isa<Constant>(CondCmp->getOperand(1)) && 237 ProcessBranchOnCompare(CondCmp, BB)) 238 return true; 239 240 // Check for some cases that are worth simplifying. Right now we want to look 241 // for loads that are used by a switch or by the condition for the branch. If 242 // we see one, check to see if it's partially redundant. If so, insert a PHI 243 // which can then be used to thread the values. 244 // 245 // This is particularly important because reg2mem inserts loads and stores all 246 // over the place, and this blocks jump threading if we don't zap them. 247 Value *SimplifyValue = Condition; 248 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 249 if (isa<Constant>(CondCmp->getOperand(1))) 250 SimplifyValue = CondCmp->getOperand(0); 251 252 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 253 if (SimplifyPartiallyRedundantLoad(LI)) 254 return true; 255 256 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 257 // "(X == 4)" thread through this block. 258 259 return false; 260} 261 262/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 263/// load instruction, eliminate it by replacing it with a PHI node. This is an 264/// important optimization that encourages jump threading, and needs to be run 265/// interlaced with other jump threading tasks. 266bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 267 // Don't hack volatile loads. 268 if (LI->isVolatile()) return false; 269 270 // If the load is defined in a block with exactly one predecessor, it can't be 271 // partially redundant. 272 BasicBlock *LoadBB = LI->getParent(); 273 if (LoadBB->getSinglePredecessor()) 274 return false; 275 276 Value *LoadedPtr = LI->getOperand(0); 277 278 // If the loaded operand is defined in the LoadBB, it can't be available. 279 // FIXME: Could do PHI translation, that would be fun :) 280 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 281 if (PtrOp->getParent() == LoadBB) 282 return false; 283 284 // Scan a few instructions up from the load, to see if it is obviously live at 285 // the entry to its block. 286 BasicBlock::iterator BBIt = LI; 287 288 if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB, 289 BBIt, 6)) { 290 // If the value if the load is locally available within the block, just use 291 // it. This frequently occurs for reg2mem'd allocas. 292 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 293 LI->replaceAllUsesWith(AvailableVal); 294 LI->eraseFromParent(); 295 return true; 296 } 297 298 // Otherwise, if we scanned the whole block and got to the top of the block, 299 // we know the block is locally transparent to the load. If not, something 300 // might clobber its value. 301 if (BBIt != LoadBB->begin()) 302 return false; 303 304 305 SmallPtrSet<BasicBlock*, 8> PredsScanned; 306 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 307 AvailablePredsTy AvailablePreds; 308 BasicBlock *OneUnavailablePred = 0; 309 310 // If we got here, the loaded value is transparent through to the start of the 311 // block. Check to see if it is available in any of the predecessor blocks. 312 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 313 PI != PE; ++PI) { 314 BasicBlock *PredBB = *PI; 315 316 // If we already scanned this predecessor, skip it. 317 if (!PredsScanned.insert(PredBB)) 318 continue; 319 320 // Scan the predecessor to see if the value is available in the pred. 321 BBIt = PredBB->end(); 322 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); 323 if (!PredAvailable) { 324 OneUnavailablePred = PredBB; 325 continue; 326 } 327 328 // If so, this load is partially redundant. Remember this info so that we 329 // can create a PHI node. 330 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 331 } 332 333 // If the loaded value isn't available in any predecessor, it isn't partially 334 // redundant. 335 if (AvailablePreds.empty()) return false; 336 337 // Okay, the loaded value is available in at least one (and maybe all!) 338 // predecessors. If the value is unavailable in more than one unique 339 // predecessor, we want to insert a merge block for those common predecessors. 340 // This ensures that we only have to insert one reload, thus not increasing 341 // code size. 342 BasicBlock *UnavailablePred = 0; 343 344 // If there is exactly one predecessor where the value is unavailable, the 345 // already computed 'OneUnavailablePred' block is it. If it ends in an 346 // unconditional branch, we know that it isn't a critical edge. 347 if (PredsScanned.size() == AvailablePreds.size()+1 && 348 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 349 UnavailablePred = OneUnavailablePred; 350 } else if (PredsScanned.size() != AvailablePreds.size()) { 351 // Otherwise, we had multiple unavailable predecessors or we had a critical 352 // edge from the one. 353 SmallVector<BasicBlock*, 8> PredsToSplit; 354 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 355 356 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 357 AvailablePredSet.insert(AvailablePreds[i].first); 358 359 // Add all the unavailable predecessors to the PredsToSplit list. 360 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 361 PI != PE; ++PI) 362 if (!AvailablePredSet.count(*PI)) 363 PredsToSplit.push_back(*PI); 364 365 // Split them out to their own block. 366 UnavailablePred = 367 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), 368 "thread-split", this); 369 } 370 371 // If the value isn't available in all predecessors, then there will be 372 // exactly one where it isn't available. Insert a load on that edge and add 373 // it to the AvailablePreds list. 374 if (UnavailablePred) { 375 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 376 "Can't handle critical edge here!"); 377 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", 378 UnavailablePred->getTerminator()); 379 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 380 } 381 382 // Now we know that each predecessor of this block has a value in 383 // AvailablePreds, sort them for efficient access as we're walking the preds. 384 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 385 386 // Create a PHI node at the start of the block for the PRE'd load value. 387 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); 388 PN->takeName(LI); 389 390 // Insert new entries into the PHI for each predecessor. A single block may 391 // have multiple entries here. 392 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; 393 ++PI) { 394 AvailablePredsTy::iterator I = 395 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 396 std::make_pair(*PI, (Value*)0)); 397 398 assert(I != AvailablePreds.end() && I->first == *PI && 399 "Didn't find entry for predecessor!"); 400 401 PN->addIncoming(I->second, I->first); 402 } 403 404 //cerr << "PRE: " << *LI << *PN << "\n"; 405 406 LI->replaceAllUsesWith(PN); 407 LI->eraseFromParent(); 408 409 return true; 410} 411 412 413/// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in 414/// the current block. See if there are any simplifications we can do based on 415/// inputs to the phi node. 416/// 417bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { 418 // See if the phi node has any constant values. If so, we can determine where 419 // the corresponding predecessor will branch. 420 ConstantInt *PredCst = 0; 421 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 422 if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) 423 break; 424 425 // If no incoming value has a constant, we don't know the destination of any 426 // predecessors. 427 if (PredCst == 0) 428 return false; 429 430 // See if the cost of duplicating this block is low enough. 431 BasicBlock *BB = PN->getParent(); 432 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 433 if (JumpThreadCost > Threshold) { 434 DOUT << " Not threading BB '" << BB->getNameStart() 435 << "' - Cost is too high: " << JumpThreadCost << "\n"; 436 return false; 437 } 438 439 // If so, we can actually do this threading. Merge any common predecessors 440 // that will act the same. 441 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 442 443 // Next, figure out which successor we are threading to. 444 BasicBlock *SuccBB; 445 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 446 SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse()); 447 else { 448 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); 449 SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst)); 450 } 451 452 // If threading to the same block as we come from, we would infinite loop. 453 if (SuccBB == BB) { 454 DOUT << " Not threading BB '" << BB->getNameStart() 455 << "' - would thread to self!\n"; 456 return false; 457 } 458 459 // And finally, do it! 460 DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '" 461 << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost 462 << ", across block:\n " 463 << *BB << "\n"; 464 465 ThreadEdge(BB, PredBB, SuccBB); 466 ++NumThreads; 467 return true; 468} 469 470/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch 471/// whose condition is an AND/OR where one side is PN. If PN has constant 472/// operands that permit us to evaluate the condition for some operand, thread 473/// through the block. For example with: 474/// br (and X, phi(Y, Z, false)) 475/// the predecessor corresponding to the 'false' will always jump to the false 476/// destination of the branch. 477/// 478bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB, 479 bool isAnd) { 480 // If this is a binary operator tree of the same AND/OR opcode, check the 481 // LHS/RHS. 482 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) 483 if ((isAnd && BO->getOpcode() == Instruction::And) || 484 (!isAnd && BO->getOpcode() == Instruction::Or)) { 485 if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd)) 486 return true; 487 if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd)) 488 return true; 489 } 490 491 // If this isn't a PHI node, we can't handle it. 492 PHINode *PN = dyn_cast<PHINode>(V); 493 if (!PN || PN->getParent() != BB) return false; 494 495 // We can only do the simplification for phi nodes of 'false' with AND or 496 // 'true' with OR. See if we have any entries in the phi for this. 497 unsigned PredNo = ~0U; 498 ConstantInt *PredCst = ConstantInt::get(Type::Int1Ty, !isAnd); 499 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 500 if (PN->getIncomingValue(i) == PredCst) { 501 PredNo = i; 502 break; 503 } 504 } 505 506 // If no match, bail out. 507 if (PredNo == ~0U) 508 return false; 509 510 // See if the cost of duplicating this block is low enough. 511 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 512 if (JumpThreadCost > Threshold) { 513 DOUT << " Not threading BB '" << BB->getNameStart() 514 << "' - Cost is too high: " << JumpThreadCost << "\n"; 515 return false; 516 } 517 518 // If so, we can actually do this threading. Merge any common predecessors 519 // that will act the same. 520 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 521 522 // Next, figure out which successor we are threading to. If this was an AND, 523 // the constant must be FALSE, and we must be targeting the 'false' block. 524 // If this is an OR, the constant must be TRUE, and we must be targeting the 525 // 'true' block. 526 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd); 527 528 // If threading to the same block as we come from, we would infinite loop. 529 if (SuccBB == BB) { 530 DOUT << " Not threading BB '" << BB->getNameStart() 531 << "' - would thread to self!\n"; 532 return false; 533 } 534 535 // And finally, do it! 536 DOUT << " Threading edge through bool from '" << PredBB->getNameStart() 537 << "' to '" << SuccBB->getNameStart() << "' with cost: " 538 << JumpThreadCost << ", across block:\n " 539 << *BB << "\n"; 540 541 ThreadEdge(BB, PredBB, SuccBB); 542 ++NumThreads; 543 return true; 544} 545 546/// ProcessBranchOnCompare - We found a branch on a comparison between a phi 547/// node and a constant. If the PHI node contains any constants as inputs, we 548/// can fold the compare for that edge and thread through it. 549bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) { 550 PHINode *PN = cast<PHINode>(Cmp->getOperand(0)); 551 Constant *RHS = cast<Constant>(Cmp->getOperand(1)); 552 553 // If the phi isn't in the current block, an incoming edge to this block 554 // doesn't control the destination. 555 if (PN->getParent() != BB) 556 return false; 557 558 // We can do this simplification if any comparisons fold to true or false. 559 // See if any do. 560 Constant *PredCst = 0; 561 bool TrueDirection = false; 562 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 563 PredCst = dyn_cast<Constant>(PN->getIncomingValue(i)); 564 if (PredCst == 0) continue; 565 566 Constant *Res; 567 if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp)) 568 Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS); 569 else 570 Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(), 571 PredCst, RHS); 572 // If this folded to a constant expr, we can't do anything. 573 if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) { 574 TrueDirection = ResC->getZExtValue(); 575 break; 576 } 577 // If this folded to undef, just go the false way. 578 if (isa<UndefValue>(Res)) { 579 TrueDirection = false; 580 break; 581 } 582 583 // Otherwise, we can't fold this input. 584 PredCst = 0; 585 } 586 587 // If no match, bail out. 588 if (PredCst == 0) 589 return false; 590 591 // See if the cost of duplicating this block is low enough. 592 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 593 if (JumpThreadCost > Threshold) { 594 DOUT << " Not threading BB '" << BB->getNameStart() 595 << "' - Cost is too high: " << JumpThreadCost << "\n"; 596 return false; 597 } 598 599 // If so, we can actually do this threading. Merge any common predecessors 600 // that will act the same. 601 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 602 603 // Next, get our successor. 604 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection); 605 606 // If threading to the same block as we come from, we would infinite loop. 607 if (SuccBB == BB) { 608 DOUT << " Not threading BB '" << BB->getNameStart() 609 << "' - would thread to self!\n"; 610 return false; 611 } 612 613 614 // And finally, do it! 615 DOUT << " Threading edge through bool from '" << PredBB->getNameStart() 616 << "' to '" << SuccBB->getNameStart() << "' with cost: " 617 << JumpThreadCost << ", across block:\n " 618 << *BB << "\n"; 619 620 ThreadEdge(BB, PredBB, SuccBB); 621 ++NumThreads; 622 return true; 623} 624 625 626/// ThreadEdge - We have decided that it is safe and profitable to thread an 627/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this 628/// change. 629void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, 630 BasicBlock *SuccBB) { 631 632 // Jump Threading can not update SSA properties correctly if the values 633 // defined in the duplicated block are used outside of the block itself. For 634 // this reason, we spill all values that are used outside of BB to the stack. 635 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 636 if (!I->isUsedOutsideOfBlock(BB)) 637 continue; 638 639 // We found a use of I outside of BB. Create a new stack slot to 640 // break this inter-block usage pattern. 641 DemoteRegToStack(*I); 642 } 643 644 // We are going to have to map operands from the original BB block to the new 645 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 646 // account for entry from PredBB. 647 DenseMap<Instruction*, Value*> ValueMapping; 648 649 BasicBlock *NewBB = 650 BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB); 651 NewBB->moveAfter(PredBB); 652 653 BasicBlock::iterator BI = BB->begin(); 654 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 655 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 656 657 // Clone the non-phi instructions of BB into NewBB, keeping track of the 658 // mapping and using it to remap operands in the cloned instructions. 659 for (; !isa<TerminatorInst>(BI); ++BI) { 660 Instruction *New = BI->clone(); 661 New->setName(BI->getNameStart()); 662 NewBB->getInstList().push_back(New); 663 ValueMapping[BI] = New; 664 665 // Remap operands to patch up intra-block references. 666 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 667 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) 668 if (Value *Remapped = ValueMapping[Inst]) 669 New->setOperand(i, Remapped); 670 } 671 672 // We didn't copy the terminator from BB over to NewBB, because there is now 673 // an unconditional jump to SuccBB. Insert the unconditional jump. 674 BranchInst::Create(SuccBB, NewBB); 675 676 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 677 // PHI nodes for NewBB now. 678 for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) { 679 PHINode *PN = cast<PHINode>(PNI); 680 // Ok, we have a PHI node. Figure out what the incoming value was for the 681 // DestBlock. 682 Value *IV = PN->getIncomingValueForBlock(BB); 683 684 // Remap the value if necessary. 685 if (Instruction *Inst = dyn_cast<Instruction>(IV)) 686 if (Value *MappedIV = ValueMapping[Inst]) 687 IV = MappedIV; 688 PN->addIncoming(IV, NewBB); 689 } 690 691 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 692 // NewBB instead of BB. This eliminates predecessors from BB, which requires 693 // us to simplify any PHI nodes in BB. 694 TerminatorInst *PredTerm = PredBB->getTerminator(); 695 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 696 if (PredTerm->getSuccessor(i) == BB) { 697 BB->removePredecessor(PredBB); 698 PredTerm->setSuccessor(i, NewBB); 699 } 700 701 // At this point, the IR is fully up to date and consistent. Do a quick scan 702 // over the new instructions and zap any that are constants or dead. This 703 // frequently happens because of phi translation. 704 BI = NewBB->begin(); 705 for (BasicBlock::iterator E = NewBB->end(); BI != E; ) { 706 Instruction *Inst = BI++; 707 if (Constant *C = ConstantFoldInstruction(Inst, TD)) { 708 Inst->replaceAllUsesWith(C); 709 Inst->eraseFromParent(); 710 continue; 711 } 712 713 RecursivelyDeleteTriviallyDeadInstructions(Inst); 714 } 715} 716