JumpThreading.cpp revision 3cda3cdab118fd2b185b3e6e05e2b19a4a70c1ad
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 39static cl::opt<int> 40DebugIterations("jump-threading-debug", 41 cl::desc("Stop jump threading after N iterations"), 42 cl::init(-1), cl::Hidden); 43 44namespace { 45 /// This pass performs 'jump threading', which looks at blocks that have 46 /// multiple predecessors and multiple successors. If one or more of the 47 /// predecessors of the block can be proven to always jump to one of the 48 /// successors, we forward the edge from the predecessor to the successor by 49 /// duplicating the contents of this block. 50 /// 51 /// An example of when this can occur is code like this: 52 /// 53 /// if () { ... 54 /// X = 4; 55 /// } 56 /// if (X < 3) { 57 /// 58 /// In this case, the unconditional branch at the end of the first if can be 59 /// revectored to the false side of the second if. 60 /// 61 class VISIBILITY_HIDDEN JumpThreading : public FunctionPass { 62 TargetData *TD; 63 public: 64 static char ID; // Pass identification 65 JumpThreading() : FunctionPass(&ID) {} 66 67 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 68 AU.addRequired<TargetData>(); 69 } 70 71 bool runOnFunction(Function &F); 72 bool ProcessBlock(BasicBlock *BB); 73 void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB); 74 BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal); 75 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 76 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 77 78 bool ProcessJumpOnPHI(PHINode *PN); 79 bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); 80 bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); 81 82 bool SimplifyPartiallyRedundantLoad(LoadInst *LI); 83 }; 84} 85 86char JumpThreading::ID = 0; 87static RegisterPass<JumpThreading> 88X("jump-threading", "Jump Threading"); 89 90// Public interface to the Jump Threading pass 91FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 92 93/// runOnFunction - Top level algorithm. 94/// 95bool JumpThreading::runOnFunction(Function &F) { 96 DOUT << "Jump threading on function '" << F.getNameStart() << "'\n"; 97 TD = &getAnalysis<TargetData>(); 98 99 bool AnotherIteration = true, EverChanged = false; 100 while (AnotherIteration) { 101 AnotherIteration = false; 102 bool Changed = false; 103 for (Function::iterator I = F.begin(), E = F.end(); I != E;) { 104 BasicBlock *BB = I; 105 while (ProcessBlock(BB)) 106 Changed = true; 107 108 ++I; 109 110 // If the block is trivially dead, zap it. This eliminates the successor 111 // edges which simplifies the CFG. 112 if (pred_begin(BB) == pred_end(BB) && 113 BB != &BB->getParent()->getEntryBlock() && 114 DebugIterations != 0) { 115 DOUT << " JT: Deleting dead block '" << BB->getNameStart() 116 << "' with terminator: " << *BB->getTerminator(); 117 DeleteDeadBlock(BB); 118 Changed = true; 119 120 if (DebugIterations != -1) 121 DebugIterations = DebugIterations-1; 122 } 123 } 124 AnotherIteration = Changed; 125 EverChanged |= Changed; 126 } 127 return EverChanged; 128} 129 130/// FactorCommonPHIPreds - If there are multiple preds with the same incoming 131/// value for the PHI, factor them together so we get one block to thread for 132/// the whole group. 133/// This is important for things like "phi i1 [true, true, false, true, x]" 134/// where we only need to clone the block for the true blocks once. 135/// 136BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Constant *CstVal) { 137 SmallVector<BasicBlock*, 16> CommonPreds; 138 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 139 if (PN->getIncomingValue(i) == CstVal) 140 CommonPreds.push_back(PN->getIncomingBlock(i)); 141 142 if (CommonPreds.size() == 1) 143 return CommonPreds[0]; 144 145 DOUT << " Factoring out " << CommonPreds.size() 146 << " common predecessors.\n"; 147 return SplitBlockPredecessors(PN->getParent(), 148 &CommonPreds[0], CommonPreds.size(), 149 ".thr_comm", this); 150} 151 152 153/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 154/// thread across it. 155static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { 156 /// Ignore PHI nodes, these will be flattened when duplication happens. 157 BasicBlock::const_iterator I = BB->getFirstNonPHI(); 158 159 // Sum up the cost of each instruction until we get to the terminator. Don't 160 // include the terminator because the copy won't include it. 161 unsigned Size = 0; 162 for (; !isa<TerminatorInst>(I); ++I) { 163 // Debugger intrinsics don't incur code size. 164 if (isa<DbgInfoIntrinsic>(I)) continue; 165 166 // If this is a pointer->pointer bitcast, it is free. 167 if (isa<BitCastInst>(I) && isa<PointerType>(I->getType())) 168 continue; 169 170 // All other instructions count for at least one unit. 171 ++Size; 172 173 // Calls are more expensive. If they are non-intrinsic calls, we model them 174 // as having cost of 4. If they are a non-vector intrinsic, we model them 175 // as having cost of 2 total, and if they are a vector intrinsic, we model 176 // them as having cost 1. 177 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 178 if (!isa<IntrinsicInst>(CI)) 179 Size += 3; 180 else if (isa<VectorType>(CI->getType())) 181 Size += 1; 182 } 183 } 184 185 // Threading through a switch statement is particularly profitable. If this 186 // block ends in a switch, decrease its cost to make it more likely to happen. 187 if (isa<SwitchInst>(I)) 188 Size = Size > 6 ? Size-6 : 0; 189 190 return Size; 191} 192 193/// ProcessBlock - If there are any predecessors whose control can be threaded 194/// through to a successor, transform them now. 195bool JumpThreading::ProcessBlock(BasicBlock *BB) { 196 if (DebugIterations == 0) return false; 197 if (DebugIterations != -1) 198 DebugIterations = DebugIterations-1; 199 200 // If this block has a single predecessor, and if that pred has a single 201 // successor, merge the blocks. This encourages recursive jump threading 202 // because now the condition in this block can be threaded through 203 // predecessors of our predecessor block. 204 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) 205 if (SinglePred->getTerminator()->getNumSuccessors() == 1 && 206 SinglePred != BB) { 207 // Remember if SinglePred was the entry block of the function. If so, we 208 // will need to move BB back to the entry position. 209 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); 210 MergeBasicBlockIntoOnlyPred(BB); 211 212 if (isEntry && BB != &BB->getParent()->getEntryBlock()) 213 BB->moveBefore(&BB->getParent()->getEntryBlock()); 214 return true; 215 } 216 217 // See if this block ends with a branch or switch. If so, see if the 218 // condition is a phi node. If so, and if an entry of the phi node is a 219 // constant, we can thread the block. 220 Value *Condition; 221 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 222 // Can't thread an unconditional jump. 223 if (BI->isUnconditional()) return false; 224 Condition = BI->getCondition(); 225 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) 226 Condition = SI->getCondition(); 227 else 228 return false; // Must be an invoke. 229 230 // If the terminator of this block is branching on a constant, simplify the 231 // terminator to an unconditional branch. This can occur due to threading in 232 // other blocks. 233 if (isa<ConstantInt>(Condition)) { 234 DOUT << " In block '" << BB->getNameStart() 235 << "' folding terminator: " << *BB->getTerminator(); 236 ++NumFolds; 237 ConstantFoldTerminator(BB); 238 return true; 239 } 240 241 // If the terminator is branching on an undef, we can pick any of the 242 // successors to branch to. Since this is arbitrary, we pick the successor 243 // with the fewest predecessors. This should reduce the in-degree of the 244 // others. 245 if (isa<UndefValue>(Condition)) { 246 TerminatorInst *BBTerm = BB->getTerminator(); 247 unsigned MinSucc = 0; 248 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); 249 // Compute the successor with the minimum number of predecessors. 250 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 251 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { 252 TestBB = BBTerm->getSuccessor(i); 253 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 254 if (NumPreds < MinNumPreds) 255 MinSucc = i; 256 } 257 258 // Fold the branch/switch. 259 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { 260 if (i == MinSucc) continue; 261 BBTerm->getSuccessor(i)->removePredecessor(BB); 262 } 263 264 DOUT << " In block '" << BB->getNameStart() 265 << "' folding undef terminator: " << *BBTerm; 266 BranchInst::Create(BBTerm->getSuccessor(MinSucc), BBTerm); 267 BBTerm->eraseFromParent(); 268 return true; 269 } 270 271 Instruction *CondInst = dyn_cast<Instruction>(Condition); 272 273 // If the condition is an instruction defined in another block, see if a 274 // predecessor has the same condition: 275 // br COND, BBX, BBY 276 // BBX: 277 // br COND, BBZ, BBW 278 if (!Condition->hasOneUse() && // Multiple uses. 279 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition. 280 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 281 if (isa<BranchInst>(BB->getTerminator())) { 282 for (; PI != E; ++PI) 283 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 284 if (PBI->isConditional() && PBI->getCondition() == Condition && 285 ProcessBranchOnDuplicateCond(*PI, BB)) 286 return true; 287 } else { 288 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator"); 289 for (; PI != E; ++PI) 290 if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator())) 291 if (PSI->getCondition() == Condition && 292 ProcessSwitchOnDuplicateCond(*PI, BB)) 293 return true; 294 } 295 } 296 297 // If there is only a single predecessor of this block, nothing to fold. 298 if (BB->getSinglePredecessor()) 299 return false; 300 301 // All the rest of our checks depend on the condition being an instruction. 302 if (CondInst == 0) 303 return false; 304 305 // See if this is a phi node in the current block. 306 if (PHINode *PN = dyn_cast<PHINode>(CondInst)) 307 if (PN->getParent() == BB) 308 return ProcessJumpOnPHI(PN); 309 310 // If this is a conditional branch whose condition is and/or of a phi, try to 311 // simplify it. 312 if ((CondInst->getOpcode() == Instruction::And || 313 CondInst->getOpcode() == Instruction::Or) && 314 isa<BranchInst>(BB->getTerminator()) && 315 ProcessBranchOnLogical(CondInst, BB, 316 CondInst->getOpcode() == Instruction::And)) 317 return true; 318 319 // If we have "br (phi != 42)" and the phi node has any constant values as 320 // operands, we can thread through this block. 321 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) 322 if (isa<PHINode>(CondCmp->getOperand(0)) && 323 isa<Constant>(CondCmp->getOperand(1)) && 324 ProcessBranchOnCompare(CondCmp, BB)) 325 return true; 326 327 // Check for some cases that are worth simplifying. Right now we want to look 328 // for loads that are used by a switch or by the condition for the branch. If 329 // we see one, check to see if it's partially redundant. If so, insert a PHI 330 // which can then be used to thread the values. 331 // 332 // This is particularly important because reg2mem inserts loads and stores all 333 // over the place, and this blocks jump threading if we don't zap them. 334 Value *SimplifyValue = CondInst; 335 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 336 if (isa<Constant>(CondCmp->getOperand(1))) 337 SimplifyValue = CondCmp->getOperand(0); 338 339 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 340 if (SimplifyPartiallyRedundantLoad(LI)) 341 return true; 342 343 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 344 // "(X == 4)" thread through this block. 345 346 return false; 347} 348 349/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that 350/// block that jump on exactly the same condition. This means that we almost 351/// always know the direction of the edge in the DESTBB: 352/// PREDBB: 353/// br COND, DESTBB, BBY 354/// DESTBB: 355/// br COND, BBZ, BBW 356/// 357/// If DESTBB has multiple predecessors, we can't just constant fold the branch 358/// in DESTBB, we have to thread over it. 359bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, 360 BasicBlock *BB) { 361 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); 362 363 // If both successors of PredBB go to DESTBB, we don't know anything. We can 364 // fold the branch to an unconditional one, which allows other recursive 365 // simplifications. 366 bool BranchDir; 367 if (PredBI->getSuccessor(1) != BB) 368 BranchDir = true; 369 else if (PredBI->getSuccessor(0) != BB) 370 BranchDir = false; 371 else { 372 DOUT << " In block '" << PredBB->getNameStart() 373 << "' folding terminator: " << *PredBB->getTerminator(); 374 ++NumFolds; 375 ConstantFoldTerminator(PredBB); 376 return true; 377 } 378 379 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); 380 381 // If the dest block has one predecessor, just fix the branch condition to a 382 // constant and fold it. 383 if (BB->getSinglePredecessor()) { 384 DOUT << " In block '" << BB->getNameStart() 385 << "' folding condition to '" << BranchDir << "': " 386 << *BB->getTerminator(); 387 ++NumFolds; 388 DestBI->setCondition(ConstantInt::get(Type::Int1Ty, BranchDir)); 389 ConstantFoldTerminator(BB); 390 return true; 391 } 392 393 // Otherwise we need to thread from PredBB to DestBB's successor which 394 // involves code duplication. Check to see if it is worth it. 395 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 396 if (JumpThreadCost > Threshold) { 397 DOUT << " Not threading BB '" << BB->getNameStart() 398 << "' - Cost is too high: " << JumpThreadCost << "\n"; 399 return false; 400 } 401 402 // Next, figure out which successor we are threading to. 403 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); 404 405 // If threading to the same block as we come from, we would infinite loop. 406 if (SuccBB == BB) { 407 DOUT << " Not threading BB '" << BB->getNameStart() 408 << "' - would thread to self!\n"; 409 return false; 410 } 411 412 // And finally, do it! 413 DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '" 414 << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost 415 << ", across block:\n " 416 << *BB << "\n"; 417 418 ThreadEdge(BB, PredBB, SuccBB); 419 ++NumThreads; 420 return true; 421} 422 423/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that 424/// block that switch on exactly the same condition. This means that we almost 425/// always know the direction of the edge in the DESTBB: 426/// PREDBB: 427/// switch COND [... DESTBB, BBY ... ] 428/// DESTBB: 429/// switch COND [... BBZ, BBW ] 430/// 431/// Optimizing switches like this is very important, because simplifycfg builds 432/// switches out of repeated 'if' conditions. 433bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, 434 BasicBlock *DestBB) { 435 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); 436 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); 437 438 // There are a variety of optimizations that we can potentially do on these 439 // blocks: we order them from most to least preferable. 440 441 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB 442 // directly to their destination. This does not introduce *any* code size 443 // growth. 444 445 // FIXME: Thread if it just contains a PHI. 446 if (isa<SwitchInst>(DestBB->begin())) { 447 bool MadeChange = false; 448 // Ignore the default edge for now. 449 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { 450 ConstantInt *DestVal = DestSI->getCaseValue(i); 451 BasicBlock *DestSucc = DestSI->getSuccessor(i); 452 453 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if 454 // PredSI has an explicit case for it. If so, forward. If it is covered 455 // by the default case, we can't update PredSI. 456 unsigned PredCase = PredSI->findCaseValue(DestVal); 457 if (PredCase == 0) continue; 458 459 // If PredSI doesn't go to DestBB on this value, then it won't reach the 460 // case on this condition. 461 if (PredSI->getSuccessor(PredCase) != DestBB && 462 DestSI->getSuccessor(i) != DestBB) 463 continue; 464 465 // Otherwise, we're safe to make the change. Make sure that the edge from 466 // DestSI to DestSucc is not critical and has no PHI nodes. 467 DOUT << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI; 468 DOUT << "THROUGH: " << *DestSI; 469 470 // If the destination has PHI nodes, just split the edge for updating 471 // simplicity. 472 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ 473 SplitCriticalEdge(DestSI, i, this); 474 DestSucc = DestSI->getSuccessor(i); 475 } 476 FoldSingleEntryPHINodes(DestSucc); 477 PredSI->setSuccessor(PredCase, DestSucc); 478 MadeChange = true; 479 } 480 481 if (MadeChange) 482 return true; 483 } 484 485 return false; 486} 487 488 489/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 490/// load instruction, eliminate it by replacing it with a PHI node. This is an 491/// important optimization that encourages jump threading, and needs to be run 492/// interlaced with other jump threading tasks. 493bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 494 // Don't hack volatile loads. 495 if (LI->isVolatile()) return false; 496 497 // If the load is defined in a block with exactly one predecessor, it can't be 498 // partially redundant. 499 BasicBlock *LoadBB = LI->getParent(); 500 if (LoadBB->getSinglePredecessor()) 501 return false; 502 503 Value *LoadedPtr = LI->getOperand(0); 504 505 // If the loaded operand is defined in the LoadBB, it can't be available. 506 // FIXME: Could do PHI translation, that would be fun :) 507 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 508 if (PtrOp->getParent() == LoadBB) 509 return false; 510 511 // Scan a few instructions up from the load, to see if it is obviously live at 512 // the entry to its block. 513 BasicBlock::iterator BBIt = LI; 514 515 if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB, 516 BBIt, 6)) { 517 // If the value if the load is locally available within the block, just use 518 // it. This frequently occurs for reg2mem'd allocas. 519 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 520 LI->replaceAllUsesWith(AvailableVal); 521 LI->eraseFromParent(); 522 return true; 523 } 524 525 // Otherwise, if we scanned the whole block and got to the top of the block, 526 // we know the block is locally transparent to the load. If not, something 527 // might clobber its value. 528 if (BBIt != LoadBB->begin()) 529 return false; 530 531 532 SmallPtrSet<BasicBlock*, 8> PredsScanned; 533 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 534 AvailablePredsTy AvailablePreds; 535 BasicBlock *OneUnavailablePred = 0; 536 537 // If we got here, the loaded value is transparent through to the start of the 538 // block. Check to see if it is available in any of the predecessor blocks. 539 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 540 PI != PE; ++PI) { 541 BasicBlock *PredBB = *PI; 542 543 // If we already scanned this predecessor, skip it. 544 if (!PredsScanned.insert(PredBB)) 545 continue; 546 547 // Scan the predecessor to see if the value is available in the pred. 548 BBIt = PredBB->end(); 549 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); 550 if (!PredAvailable) { 551 OneUnavailablePred = PredBB; 552 continue; 553 } 554 555 // If so, this load is partially redundant. Remember this info so that we 556 // can create a PHI node. 557 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 558 } 559 560 // If the loaded value isn't available in any predecessor, it isn't partially 561 // redundant. 562 if (AvailablePreds.empty()) return false; 563 564 // Okay, the loaded value is available in at least one (and maybe all!) 565 // predecessors. If the value is unavailable in more than one unique 566 // predecessor, we want to insert a merge block for those common predecessors. 567 // This ensures that we only have to insert one reload, thus not increasing 568 // code size. 569 BasicBlock *UnavailablePred = 0; 570 571 // If there is exactly one predecessor where the value is unavailable, the 572 // already computed 'OneUnavailablePred' block is it. If it ends in an 573 // unconditional branch, we know that it isn't a critical edge. 574 if (PredsScanned.size() == AvailablePreds.size()+1 && 575 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 576 UnavailablePred = OneUnavailablePred; 577 } else if (PredsScanned.size() != AvailablePreds.size()) { 578 // Otherwise, we had multiple unavailable predecessors or we had a critical 579 // edge from the one. 580 SmallVector<BasicBlock*, 8> PredsToSplit; 581 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 582 583 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 584 AvailablePredSet.insert(AvailablePreds[i].first); 585 586 // Add all the unavailable predecessors to the PredsToSplit list. 587 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 588 PI != PE; ++PI) 589 if (!AvailablePredSet.count(*PI)) 590 PredsToSplit.push_back(*PI); 591 592 // Split them out to their own block. 593 UnavailablePred = 594 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), 595 "thread-split", this); 596 } 597 598 // If the value isn't available in all predecessors, then there will be 599 // exactly one where it isn't available. Insert a load on that edge and add 600 // it to the AvailablePreds list. 601 if (UnavailablePred) { 602 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 603 "Can't handle critical edge here!"); 604 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", 605 UnavailablePred->getTerminator()); 606 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 607 } 608 609 // Now we know that each predecessor of this block has a value in 610 // AvailablePreds, sort them for efficient access as we're walking the preds. 611 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 612 613 // Create a PHI node at the start of the block for the PRE'd load value. 614 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); 615 PN->takeName(LI); 616 617 // Insert new entries into the PHI for each predecessor. A single block may 618 // have multiple entries here. 619 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; 620 ++PI) { 621 AvailablePredsTy::iterator I = 622 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 623 std::make_pair(*PI, (Value*)0)); 624 625 assert(I != AvailablePreds.end() && I->first == *PI && 626 "Didn't find entry for predecessor!"); 627 628 PN->addIncoming(I->second, I->first); 629 } 630 631 //cerr << "PRE: " << *LI << *PN << "\n"; 632 633 LI->replaceAllUsesWith(PN); 634 LI->eraseFromParent(); 635 636 return true; 637} 638 639 640/// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in 641/// the current block. See if there are any simplifications we can do based on 642/// inputs to the phi node. 643/// 644bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { 645 // See if the phi node has any constant values. If so, we can determine where 646 // the corresponding predecessor will branch. 647 ConstantInt *PredCst = 0; 648 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 649 if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) 650 break; 651 652 // If no incoming value has a constant, we don't know the destination of any 653 // predecessors. 654 if (PredCst == 0) 655 return false; 656 657 // See if the cost of duplicating this block is low enough. 658 BasicBlock *BB = PN->getParent(); 659 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 660 if (JumpThreadCost > Threshold) { 661 DOUT << " Not threading BB '" << BB->getNameStart() 662 << "' - Cost is too high: " << JumpThreadCost << "\n"; 663 return false; 664 } 665 666 // If so, we can actually do this threading. Merge any common predecessors 667 // that will act the same. 668 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 669 670 // Next, figure out which successor we are threading to. 671 BasicBlock *SuccBB; 672 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 673 SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse()); 674 else { 675 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); 676 SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst)); 677 } 678 679 // If threading to the same block as we come from, we would infinite loop. 680 if (SuccBB == BB) { 681 DOUT << " Not threading BB '" << BB->getNameStart() 682 << "' - would thread to self!\n"; 683 return false; 684 } 685 686 // And finally, do it! 687 DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '" 688 << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost 689 << ", across block:\n " 690 << *BB << "\n"; 691 692 ThreadEdge(BB, PredBB, SuccBB); 693 ++NumThreads; 694 return true; 695} 696 697/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch 698/// whose condition is an AND/OR where one side is PN. If PN has constant 699/// operands that permit us to evaluate the condition for some operand, thread 700/// through the block. For example with: 701/// br (and X, phi(Y, Z, false)) 702/// the predecessor corresponding to the 'false' will always jump to the false 703/// destination of the branch. 704/// 705bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB, 706 bool isAnd) { 707 // If this is a binary operator tree of the same AND/OR opcode, check the 708 // LHS/RHS. 709 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) 710 if ((isAnd && BO->getOpcode() == Instruction::And) || 711 (!isAnd && BO->getOpcode() == Instruction::Or)) { 712 if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd)) 713 return true; 714 if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd)) 715 return true; 716 } 717 718 // If this isn't a PHI node, we can't handle it. 719 PHINode *PN = dyn_cast<PHINode>(V); 720 if (!PN || PN->getParent() != BB) return false; 721 722 // We can only do the simplification for phi nodes of 'false' with AND or 723 // 'true' with OR. See if we have any entries in the phi for this. 724 unsigned PredNo = ~0U; 725 ConstantInt *PredCst = ConstantInt::get(Type::Int1Ty, !isAnd); 726 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 727 if (PN->getIncomingValue(i) == PredCst) { 728 PredNo = i; 729 break; 730 } 731 } 732 733 // If no match, bail out. 734 if (PredNo == ~0U) 735 return false; 736 737 // See if the cost of duplicating this block is low enough. 738 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 739 if (JumpThreadCost > Threshold) { 740 DOUT << " Not threading BB '" << BB->getNameStart() 741 << "' - Cost is too high: " << JumpThreadCost << "\n"; 742 return false; 743 } 744 745 // If so, we can actually do this threading. Merge any common predecessors 746 // that will act the same. 747 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 748 749 // Next, figure out which successor we are threading to. If this was an AND, 750 // the constant must be FALSE, and we must be targeting the 'false' block. 751 // If this is an OR, the constant must be TRUE, and we must be targeting the 752 // 'true' block. 753 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd); 754 755 // If threading to the same block as we come from, we would infinite loop. 756 if (SuccBB == BB) { 757 DOUT << " Not threading BB '" << BB->getNameStart() 758 << "' - would thread to self!\n"; 759 return false; 760 } 761 762 // And finally, do it! 763 DOUT << " Threading edge through bool from '" << PredBB->getNameStart() 764 << "' to '" << SuccBB->getNameStart() << "' with cost: " 765 << JumpThreadCost << ", across block:\n " 766 << *BB << "\n"; 767 768 ThreadEdge(BB, PredBB, SuccBB); 769 ++NumThreads; 770 return true; 771} 772 773/// ProcessBranchOnCompare - We found a branch on a comparison between a phi 774/// node and a constant. If the PHI node contains any constants as inputs, we 775/// can fold the compare for that edge and thread through it. 776bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) { 777 PHINode *PN = cast<PHINode>(Cmp->getOperand(0)); 778 Constant *RHS = cast<Constant>(Cmp->getOperand(1)); 779 780 // If the phi isn't in the current block, an incoming edge to this block 781 // doesn't control the destination. 782 if (PN->getParent() != BB) 783 return false; 784 785 // We can do this simplification if any comparisons fold to true or false. 786 // See if any do. 787 Constant *PredCst = 0; 788 bool TrueDirection = false; 789 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 790 PredCst = dyn_cast<Constant>(PN->getIncomingValue(i)); 791 if (PredCst == 0) continue; 792 793 Constant *Res; 794 if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp)) 795 Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS); 796 else 797 Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(), 798 PredCst, RHS); 799 // If this folded to a constant expr, we can't do anything. 800 if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) { 801 TrueDirection = ResC->getZExtValue(); 802 break; 803 } 804 // If this folded to undef, just go the false way. 805 if (isa<UndefValue>(Res)) { 806 TrueDirection = false; 807 break; 808 } 809 810 // Otherwise, we can't fold this input. 811 PredCst = 0; 812 } 813 814 // If no match, bail out. 815 if (PredCst == 0) 816 return false; 817 818 // See if the cost of duplicating this block is low enough. 819 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 820 if (JumpThreadCost > Threshold) { 821 DOUT << " Not threading BB '" << BB->getNameStart() 822 << "' - Cost is too high: " << JumpThreadCost << "\n"; 823 return false; 824 } 825 826 // If so, we can actually do this threading. Merge any common predecessors 827 // that will act the same. 828 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 829 830 // Next, get our successor. 831 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection); 832 833 // If threading to the same block as we come from, we would infinite loop. 834 if (SuccBB == BB) { 835 DOUT << " Not threading BB '" << BB->getNameStart() 836 << "' - would thread to self!\n"; 837 return false; 838 } 839 840 841 // And finally, do it! 842 DOUT << " Threading edge through bool from '" << PredBB->getNameStart() 843 << "' to '" << SuccBB->getNameStart() << "' with cost: " 844 << JumpThreadCost << ", across block:\n " 845 << *BB << "\n"; 846 847 ThreadEdge(BB, PredBB, SuccBB); 848 ++NumThreads; 849 return true; 850} 851 852 853/// ThreadEdge - We have decided that it is safe and profitable to thread an 854/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this 855/// change. 856void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, 857 BasicBlock *SuccBB) { 858 859 // Jump Threading can not update SSA properties correctly if the values 860 // defined in the duplicated block are used outside of the block itself. For 861 // this reason, we spill all values that are used outside of BB to the stack. 862 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 863 if (!I->isUsedOutsideOfBlock(BB)) 864 continue; 865 866 // We found a use of I outside of BB. Create a new stack slot to 867 // break this inter-block usage pattern. 868 DemoteRegToStack(*I); 869 } 870 871 // We are going to have to map operands from the original BB block to the new 872 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 873 // account for entry from PredBB. 874 DenseMap<Instruction*, Value*> ValueMapping; 875 876 BasicBlock *NewBB = 877 BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB); 878 NewBB->moveAfter(PredBB); 879 880 BasicBlock::iterator BI = BB->begin(); 881 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 882 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 883 884 // Clone the non-phi instructions of BB into NewBB, keeping track of the 885 // mapping and using it to remap operands in the cloned instructions. 886 for (; !isa<TerminatorInst>(BI); ++BI) { 887 Instruction *New = BI->clone(); 888 New->setName(BI->getNameStart()); 889 NewBB->getInstList().push_back(New); 890 ValueMapping[BI] = New; 891 892 // Remap operands to patch up intra-block references. 893 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 894 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) 895 if (Value *Remapped = ValueMapping[Inst]) 896 New->setOperand(i, Remapped); 897 } 898 899 // We didn't copy the terminator from BB over to NewBB, because there is now 900 // an unconditional jump to SuccBB. Insert the unconditional jump. 901 BranchInst::Create(SuccBB, NewBB); 902 903 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 904 // PHI nodes for NewBB now. 905 for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) { 906 PHINode *PN = cast<PHINode>(PNI); 907 // Ok, we have a PHI node. Figure out what the incoming value was for the 908 // DestBlock. 909 Value *IV = PN->getIncomingValueForBlock(BB); 910 911 // Remap the value if necessary. 912 if (Instruction *Inst = dyn_cast<Instruction>(IV)) 913 if (Value *MappedIV = ValueMapping[Inst]) 914 IV = MappedIV; 915 PN->addIncoming(IV, NewBB); 916 } 917 918 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 919 // NewBB instead of BB. This eliminates predecessors from BB, which requires 920 // us to simplify any PHI nodes in BB. 921 TerminatorInst *PredTerm = PredBB->getTerminator(); 922 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 923 if (PredTerm->getSuccessor(i) == BB) { 924 BB->removePredecessor(PredBB); 925 PredTerm->setSuccessor(i, NewBB); 926 } 927 928 // At this point, the IR is fully up to date and consistent. Do a quick scan 929 // over the new instructions and zap any that are constants or dead. This 930 // frequently happens because of phi translation. 931 BI = NewBB->begin(); 932 for (BasicBlock::iterator E = NewBB->end(); BI != E; ) { 933 Instruction *Inst = BI++; 934 if (Constant *C = ConstantFoldInstruction(Inst, TD)) { 935 Inst->replaceAllUsesWith(C); 936 Inst->eraseFromParent(); 937 continue; 938 } 939 940 RecursivelyDeleteTriviallyDeadInstructions(Inst); 941 } 942} 943