JumpThreading.cpp revision 460f656475738d1a95a6be95346908ce1597df25
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/Transforms/Utils/BasicBlockUtils.h" 21#include "llvm/Transforms/Utils/Local.h" 22#include "llvm/Target/TargetData.h" 23#include "llvm/ADT/DenseMap.h" 24#include "llvm/ADT/Statistic.h" 25#include "llvm/ADT/STLExtras.h" 26#include "llvm/ADT/SmallPtrSet.h" 27#include "llvm/ADT/SmallSet.h" 28#include "llvm/Support/CommandLine.h" 29#include "llvm/Support/Compiler.h" 30#include "llvm/Support/Debug.h" 31#include "llvm/Support/ValueHandle.h" 32#include "llvm/Support/raw_ostream.h" 33using namespace llvm; 34 35STATISTIC(NumThreads, "Number of jumps threaded"); 36STATISTIC(NumFolds, "Number of terminators folded"); 37 38static cl::opt<unsigned> 39Threshold("jump-threading-threshold", 40 cl::desc("Max block size to duplicate for jump threading"), 41 cl::init(6), cl::Hidden); 42 43namespace { 44 /// This pass performs 'jump threading', which looks at blocks that have 45 /// multiple predecessors and multiple successors. If one or more of the 46 /// predecessors of the block can be proven to always jump to one of the 47 /// successors, we forward the edge from the predecessor to the successor by 48 /// duplicating the contents of this block. 49 /// 50 /// An example of when this can occur is code like this: 51 /// 52 /// if () { ... 53 /// X = 4; 54 /// } 55 /// if (X < 3) { 56 /// 57 /// In this case, the unconditional branch at the end of the first if can be 58 /// revectored to the false side of the second if. 59 /// 60 class VISIBILITY_HIDDEN JumpThreading : public FunctionPass { 61 TargetData *TD; 62#ifdef NDEBUG 63 SmallPtrSet<BasicBlock*, 16> LoopHeaders; 64#else 65 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; 66#endif 67 public: 68 static char ID; // Pass identification 69 JumpThreading() : FunctionPass(&ID) {} 70 71 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 72 } 73 74 bool runOnFunction(Function &F); 75 void FindLoopHeaders(Function &F); 76 77 bool ProcessBlock(BasicBlock *BB); 78 bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB, 79 unsigned JumpThreadCost); 80 BasicBlock *FactorCommonPHIPreds(PHINode *PN, Value *Val); 81 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 82 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); 83 84 bool ProcessJumpOnPHI(PHINode *PN); 85 bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); 86 bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); 87 88 bool SimplifyPartiallyRedundantLoad(LoadInst *LI); 89 }; 90} 91 92char JumpThreading::ID = 0; 93static RegisterPass<JumpThreading> 94X("jump-threading", "Jump Threading"); 95 96// Public interface to the Jump Threading pass 97FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 98 99/// runOnFunction - Top level algorithm. 100/// 101bool JumpThreading::runOnFunction(Function &F) { 102 DEBUG(errs() << "Jump threading on function '" << F.getName() << "'\n"); 103 TD = getAnalysisIfAvailable<TargetData>(); 104 105 FindLoopHeaders(F); 106 107 bool AnotherIteration = true, EverChanged = false; 108 while (AnotherIteration) { 109 AnotherIteration = false; 110 bool Changed = false; 111 for (Function::iterator I = F.begin(), E = F.end(); I != E;) { 112 BasicBlock *BB = I; 113 while (ProcessBlock(BB)) 114 Changed = true; 115 116 ++I; 117 118 // If the block is trivially dead, zap it. This eliminates the successor 119 // edges which simplifies the CFG. 120 if (pred_begin(BB) == pred_end(BB) && 121 BB != &BB->getParent()->getEntryBlock()) { 122 DEBUG(errs() << " JT: Deleting dead block '" << BB->getName() 123 << "' with terminator: " << *BB->getTerminator()); 124 LoopHeaders.erase(BB); 125 DeleteDeadBlock(BB); 126 Changed = true; 127 } 128 } 129 AnotherIteration = Changed; 130 EverChanged |= Changed; 131 } 132 133 LoopHeaders.clear(); 134 return EverChanged; 135} 136 137/// FindLoopHeaders - We do not want jump threading to turn proper loop 138/// structures into irreducible loops. Doing this breaks up the loop nesting 139/// hierarchy and pessimizes later transformations. To prevent this from 140/// happening, we first have to find the loop headers. Here we approximate this 141/// by finding targets of backedges in the CFG. 142/// 143/// Note that there definitely are cases when we want to allow threading of 144/// edges across a loop header. For example, threading a jump from outside the 145/// loop (the preheader) to an exit block of the loop is definitely profitable. 146/// It is also almost always profitable to thread backedges from within the loop 147/// to exit blocks, and is often profitable to thread backedges to other blocks 148/// within the loop (forming a nested loop). This simple analysis is not rich 149/// enough to track all of these properties and keep it up-to-date as the CFG 150/// mutates, so we don't allow any of these transformations. 151/// 152void JumpThreading::FindLoopHeaders(Function &F) { 153 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; 154 FindFunctionBackedges(F, Edges); 155 156 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 157 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); 158} 159 160 161/// FactorCommonPHIPreds - If there are multiple preds with the same incoming 162/// value for the PHI, factor them together so we get one block to thread for 163/// the whole group. 164/// This is important for things like "phi i1 [true, true, false, true, x]" 165/// where we only need to clone the block for the true blocks once. 166/// 167BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Value *Val) { 168 SmallVector<BasicBlock*, 16> CommonPreds; 169 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 170 if (PN->getIncomingValue(i) == Val) 171 CommonPreds.push_back(PN->getIncomingBlock(i)); 172 173 if (CommonPreds.size() == 1) 174 return CommonPreds[0]; 175 176 DEBUG(errs() << " Factoring out " << CommonPreds.size() 177 << " common predecessors.\n"); 178 return SplitBlockPredecessors(PN->getParent(), 179 &CommonPreds[0], CommonPreds.size(), 180 ".thr_comm", this); 181} 182 183 184/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 185/// thread across it. 186static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { 187 /// Ignore PHI nodes, these will be flattened when duplication happens. 188 BasicBlock::const_iterator I = BB->getFirstNonPHI(); 189 190 // Sum up the cost of each instruction until we get to the terminator. Don't 191 // include the terminator because the copy won't include it. 192 unsigned Size = 0; 193 for (; !isa<TerminatorInst>(I); ++I) { 194 // Debugger intrinsics don't incur code size. 195 if (isa<DbgInfoIntrinsic>(I)) continue; 196 197 // If this is a pointer->pointer bitcast, it is free. 198 if (isa<BitCastInst>(I) && isa<PointerType>(I->getType())) 199 continue; 200 201 // All other instructions count for at least one unit. 202 ++Size; 203 204 // Calls are more expensive. If they are non-intrinsic calls, we model them 205 // as having cost of 4. If they are a non-vector intrinsic, we model them 206 // as having cost of 2 total, and if they are a vector intrinsic, we model 207 // them as having cost 1. 208 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 209 if (!isa<IntrinsicInst>(CI)) 210 Size += 3; 211 else if (!isa<VectorType>(CI->getType())) 212 Size += 1; 213 } 214 } 215 216 // Threading through a switch statement is particularly profitable. If this 217 // block ends in a switch, decrease its cost to make it more likely to happen. 218 if (isa<SwitchInst>(I)) 219 Size = Size > 6 ? Size-6 : 0; 220 221 return Size; 222} 223 224/// ProcessBlock - If there are any predecessors whose control can be threaded 225/// through to a successor, transform them now. 226bool JumpThreading::ProcessBlock(BasicBlock *BB) { 227 // If this block has a single predecessor, and if that pred has a single 228 // successor, merge the blocks. This encourages recursive jump threading 229 // because now the condition in this block can be threaded through 230 // predecessors of our predecessor block. 231 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) 232 if (SinglePred->getTerminator()->getNumSuccessors() == 1 && 233 SinglePred != BB) { 234 // If SinglePred was a loop header, BB becomes one. 235 if (LoopHeaders.erase(SinglePred)) 236 LoopHeaders.insert(BB); 237 238 // Remember if SinglePred was the entry block of the function. If so, we 239 // will need to move BB back to the entry position. 240 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); 241 MergeBasicBlockIntoOnlyPred(BB); 242 243 if (isEntry && BB != &BB->getParent()->getEntryBlock()) 244 BB->moveBefore(&BB->getParent()->getEntryBlock()); 245 return true; 246 } 247 248 // See if this block ends with a branch or switch. If so, see if the 249 // condition is a phi node. If so, and if an entry of the phi node is a 250 // constant, we can thread the block. 251 Value *Condition; 252 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 253 // Can't thread an unconditional jump. 254 if (BI->isUnconditional()) return false; 255 Condition = BI->getCondition(); 256 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) 257 Condition = SI->getCondition(); 258 else 259 return false; // Must be an invoke. 260 261 // If the terminator of this block is branching on a constant, simplify the 262 // terminator to an unconditional branch. This can occur due to threading in 263 // other blocks. 264 if (isa<ConstantInt>(Condition)) { 265 DEBUG(errs() << " In block '" << BB->getName() 266 << "' folding terminator: " << *BB->getTerminator()); 267 ++NumFolds; 268 ConstantFoldTerminator(BB); 269 return true; 270 } 271 272 // If the terminator is branching on an undef, we can pick any of the 273 // successors to branch to. Since this is arbitrary, we pick the successor 274 // with the fewest predecessors. This should reduce the in-degree of the 275 // others. 276 if (isa<UndefValue>(Condition)) { 277 TerminatorInst *BBTerm = BB->getTerminator(); 278 unsigned MinSucc = 0; 279 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); 280 // Compute the successor with the minimum number of predecessors. 281 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 282 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { 283 TestBB = BBTerm->getSuccessor(i); 284 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); 285 if (NumPreds < MinNumPreds) 286 MinSucc = i; 287 } 288 289 // Fold the branch/switch. 290 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { 291 if (i == MinSucc) continue; 292 BBTerm->getSuccessor(i)->removePredecessor(BB); 293 } 294 295 DEBUG(errs() << " In block '" << BB->getName() 296 << "' folding undef terminator: " << *BBTerm); 297 BranchInst::Create(BBTerm->getSuccessor(MinSucc), BBTerm); 298 BBTerm->eraseFromParent(); 299 return true; 300 } 301 302 Instruction *CondInst = dyn_cast<Instruction>(Condition); 303 304 // If the condition is an instruction defined in another block, see if a 305 // predecessor has the same condition: 306 // br COND, BBX, BBY 307 // BBX: 308 // br COND, BBZ, BBW 309 if (!Condition->hasOneUse() && // Multiple uses. 310 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition. 311 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 312 if (isa<BranchInst>(BB->getTerminator())) { 313 for (; PI != E; ++PI) 314 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 315 if (PBI->isConditional() && PBI->getCondition() == Condition && 316 ProcessBranchOnDuplicateCond(*PI, BB)) 317 return true; 318 } else { 319 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator"); 320 for (; PI != E; ++PI) 321 if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator())) 322 if (PSI->getCondition() == Condition && 323 ProcessSwitchOnDuplicateCond(*PI, BB)) 324 return true; 325 } 326 } 327 328 // All the rest of our checks depend on the condition being an instruction. 329 if (CondInst == 0) 330 return false; 331 332 // See if this is a phi node in the current block. 333 if (PHINode *PN = dyn_cast<PHINode>(CondInst)) 334 if (PN->getParent() == BB) 335 return ProcessJumpOnPHI(PN); 336 337 // If this is a conditional branch whose condition is and/or of a phi, try to 338 // simplify it. 339 if ((CondInst->getOpcode() == Instruction::And || 340 CondInst->getOpcode() == Instruction::Or) && 341 isa<BranchInst>(BB->getTerminator()) && 342 ProcessBranchOnLogical(CondInst, BB, 343 CondInst->getOpcode() == Instruction::And)) 344 return true; 345 346 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { 347 if (isa<PHINode>(CondCmp->getOperand(0))) { 348 // If we have "br (phi != 42)" and the phi node has any constant values 349 // as operands, we can thread through this block. 350 // 351 // If we have "br (cmp phi, x)" and the phi node contains x such that the 352 // comparison uniquely identifies the branch target, we can thread 353 // through this block. 354 355 if (ProcessBranchOnCompare(CondCmp, BB)) 356 return true; 357 } 358 359 // If we have a comparison, loop over the predecessors to see if there is 360 // a condition with the same value. 361 pred_iterator PI = pred_begin(BB), E = pred_end(BB); 362 for (; PI != E; ++PI) 363 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 364 if (PBI->isConditional() && *PI != BB) { 365 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) { 366 if (CI->getOperand(0) == CondCmp->getOperand(0) && 367 CI->getOperand(1) == CondCmp->getOperand(1) && 368 CI->getPredicate() == CondCmp->getPredicate()) { 369 // TODO: Could handle things like (x != 4) --> (x == 17) 370 if (ProcessBranchOnDuplicateCond(*PI, BB)) 371 return true; 372 } 373 } 374 } 375 } 376 377 // Check for some cases that are worth simplifying. Right now we want to look 378 // for loads that are used by a switch or by the condition for the branch. If 379 // we see one, check to see if it's partially redundant. If so, insert a PHI 380 // which can then be used to thread the values. 381 // 382 // This is particularly important because reg2mem inserts loads and stores all 383 // over the place, and this blocks jump threading if we don't zap them. 384 Value *SimplifyValue = CondInst; 385 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) 386 if (isa<Constant>(CondCmp->getOperand(1))) 387 SimplifyValue = CondCmp->getOperand(0); 388 389 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) 390 if (SimplifyPartiallyRedundantLoad(LI)) 391 return true; 392 393 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know 394 // "(X == 4)" thread through this block. 395 396 return false; 397} 398 399/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that 400/// block that jump on exactly the same condition. This means that we almost 401/// always know the direction of the edge in the DESTBB: 402/// PREDBB: 403/// br COND, DESTBB, BBY 404/// DESTBB: 405/// br COND, BBZ, BBW 406/// 407/// If DESTBB has multiple predecessors, we can't just constant fold the branch 408/// in DESTBB, we have to thread over it. 409bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, 410 BasicBlock *BB) { 411 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); 412 413 // If both successors of PredBB go to DESTBB, we don't know anything. We can 414 // fold the branch to an unconditional one, which allows other recursive 415 // simplifications. 416 bool BranchDir; 417 if (PredBI->getSuccessor(1) != BB) 418 BranchDir = true; 419 else if (PredBI->getSuccessor(0) != BB) 420 BranchDir = false; 421 else { 422 DEBUG(errs() << " In block '" << PredBB->getName() 423 << "' folding terminator: " << *PredBB->getTerminator()); 424 ++NumFolds; 425 ConstantFoldTerminator(PredBB); 426 return true; 427 } 428 429 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); 430 431 // If the dest block has one predecessor, just fix the branch condition to a 432 // constant and fold it. 433 if (BB->getSinglePredecessor()) { 434 DEBUG(errs() << " In block '" << BB->getName() 435 << "' folding condition to '" << BranchDir << "': " 436 << *BB->getTerminator()); 437 ++NumFolds; 438 DestBI->setCondition(ConstantInt::get(Type::Int1Ty, BranchDir)); 439 ConstantFoldTerminator(BB); 440 return true; 441 } 442 443 // Otherwise we need to thread from PredBB to DestBB's successor which 444 // involves code duplication. Check to see if it is worth it. 445 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 446 if (JumpThreadCost > Threshold) { 447 DEBUG(errs() << " Not threading BB '" << BB->getName() 448 << "' - Cost is too high: " << JumpThreadCost << "\n"); 449 return false; 450 } 451 452 // Next, figure out which successor we are threading to. 453 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); 454 455 // Ok, try to thread it! 456 return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost); 457} 458 459/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that 460/// block that switch on exactly the same condition. This means that we almost 461/// always know the direction of the edge in the DESTBB: 462/// PREDBB: 463/// switch COND [... DESTBB, BBY ... ] 464/// DESTBB: 465/// switch COND [... BBZ, BBW ] 466/// 467/// Optimizing switches like this is very important, because simplifycfg builds 468/// switches out of repeated 'if' conditions. 469bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, 470 BasicBlock *DestBB) { 471 // Can't thread edge to self. 472 if (PredBB == DestBB) 473 return false; 474 475 476 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); 477 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); 478 479 // There are a variety of optimizations that we can potentially do on these 480 // blocks: we order them from most to least preferable. 481 482 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB 483 // directly to their destination. This does not introduce *any* code size 484 // growth. Skip debug info first. 485 BasicBlock::iterator BBI = DestBB->begin(); 486 while (isa<DbgInfoIntrinsic>(BBI)) 487 BBI++; 488 489 // FIXME: Thread if it just contains a PHI. 490 if (isa<SwitchInst>(BBI)) { 491 bool MadeChange = false; 492 // Ignore the default edge for now. 493 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { 494 ConstantInt *DestVal = DestSI->getCaseValue(i); 495 BasicBlock *DestSucc = DestSI->getSuccessor(i); 496 497 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if 498 // PredSI has an explicit case for it. If so, forward. If it is covered 499 // by the default case, we can't update PredSI. 500 unsigned PredCase = PredSI->findCaseValue(DestVal); 501 if (PredCase == 0) continue; 502 503 // If PredSI doesn't go to DestBB on this value, then it won't reach the 504 // case on this condition. 505 if (PredSI->getSuccessor(PredCase) != DestBB && 506 DestSI->getSuccessor(i) != DestBB) 507 continue; 508 509 // Otherwise, we're safe to make the change. Make sure that the edge from 510 // DestSI to DestSucc is not critical and has no PHI nodes. 511 DEBUG(errs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI); 512 DEBUG(errs() << "THROUGH: " << *DestSI); 513 514 // If the destination has PHI nodes, just split the edge for updating 515 // simplicity. 516 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ 517 SplitCriticalEdge(DestSI, i, this); 518 DestSucc = DestSI->getSuccessor(i); 519 } 520 FoldSingleEntryPHINodes(DestSucc); 521 PredSI->setSuccessor(PredCase, DestSucc); 522 MadeChange = true; 523 } 524 525 if (MadeChange) 526 return true; 527 } 528 529 return false; 530} 531 532 533/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant 534/// load instruction, eliminate it by replacing it with a PHI node. This is an 535/// important optimization that encourages jump threading, and needs to be run 536/// interlaced with other jump threading tasks. 537bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { 538 // Don't hack volatile loads. 539 if (LI->isVolatile()) return false; 540 541 // If the load is defined in a block with exactly one predecessor, it can't be 542 // partially redundant. 543 BasicBlock *LoadBB = LI->getParent(); 544 if (LoadBB->getSinglePredecessor()) 545 return false; 546 547 Value *LoadedPtr = LI->getOperand(0); 548 549 // If the loaded operand is defined in the LoadBB, it can't be available. 550 // FIXME: Could do PHI translation, that would be fun :) 551 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) 552 if (PtrOp->getParent() == LoadBB) 553 return false; 554 555 // Scan a few instructions up from the load, to see if it is obviously live at 556 // the entry to its block. 557 BasicBlock::iterator BBIt = LI; 558 559 if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB, 560 BBIt, 6)) { 561 // If the value if the load is locally available within the block, just use 562 // it. This frequently occurs for reg2mem'd allocas. 563 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; 564 565 // If the returned value is the load itself, replace with an undef. This can 566 // only happen in dead loops. 567 if (AvailableVal == LI) AvailableVal = 568 AvailableVal->getContext().getUndef(LI->getType()); 569 LI->replaceAllUsesWith(AvailableVal); 570 LI->eraseFromParent(); 571 return true; 572 } 573 574 // Otherwise, if we scanned the whole block and got to the top of the block, 575 // we know the block is locally transparent to the load. If not, something 576 // might clobber its value. 577 if (BBIt != LoadBB->begin()) 578 return false; 579 580 581 SmallPtrSet<BasicBlock*, 8> PredsScanned; 582 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; 583 AvailablePredsTy AvailablePreds; 584 BasicBlock *OneUnavailablePred = 0; 585 586 // If we got here, the loaded value is transparent through to the start of the 587 // block. Check to see if it is available in any of the predecessor blocks. 588 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 589 PI != PE; ++PI) { 590 BasicBlock *PredBB = *PI; 591 592 // If we already scanned this predecessor, skip it. 593 if (!PredsScanned.insert(PredBB)) 594 continue; 595 596 // Scan the predecessor to see if the value is available in the pred. 597 BBIt = PredBB->end(); 598 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); 599 if (!PredAvailable) { 600 OneUnavailablePred = PredBB; 601 continue; 602 } 603 604 // If so, this load is partially redundant. Remember this info so that we 605 // can create a PHI node. 606 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); 607 } 608 609 // If the loaded value isn't available in any predecessor, it isn't partially 610 // redundant. 611 if (AvailablePreds.empty()) return false; 612 613 // Okay, the loaded value is available in at least one (and maybe all!) 614 // predecessors. If the value is unavailable in more than one unique 615 // predecessor, we want to insert a merge block for those common predecessors. 616 // This ensures that we only have to insert one reload, thus not increasing 617 // code size. 618 BasicBlock *UnavailablePred = 0; 619 620 // If there is exactly one predecessor where the value is unavailable, the 621 // already computed 'OneUnavailablePred' block is it. If it ends in an 622 // unconditional branch, we know that it isn't a critical edge. 623 if (PredsScanned.size() == AvailablePreds.size()+1 && 624 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { 625 UnavailablePred = OneUnavailablePred; 626 } else if (PredsScanned.size() != AvailablePreds.size()) { 627 // Otherwise, we had multiple unavailable predecessors or we had a critical 628 // edge from the one. 629 SmallVector<BasicBlock*, 8> PredsToSplit; 630 SmallPtrSet<BasicBlock*, 8> AvailablePredSet; 631 632 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) 633 AvailablePredSet.insert(AvailablePreds[i].first); 634 635 // Add all the unavailable predecessors to the PredsToSplit list. 636 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); 637 PI != PE; ++PI) 638 if (!AvailablePredSet.count(*PI)) 639 PredsToSplit.push_back(*PI); 640 641 // Split them out to their own block. 642 UnavailablePred = 643 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), 644 "thread-split", this); 645 } 646 647 // If the value isn't available in all predecessors, then there will be 648 // exactly one where it isn't available. Insert a load on that edge and add 649 // it to the AvailablePreds list. 650 if (UnavailablePred) { 651 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && 652 "Can't handle critical edge here!"); 653 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", 654 UnavailablePred->getTerminator()); 655 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); 656 } 657 658 // Now we know that each predecessor of this block has a value in 659 // AvailablePreds, sort them for efficient access as we're walking the preds. 660 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); 661 662 // Create a PHI node at the start of the block for the PRE'd load value. 663 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); 664 PN->takeName(LI); 665 666 // Insert new entries into the PHI for each predecessor. A single block may 667 // have multiple entries here. 668 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; 669 ++PI) { 670 AvailablePredsTy::iterator I = 671 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), 672 std::make_pair(*PI, (Value*)0)); 673 674 assert(I != AvailablePreds.end() && I->first == *PI && 675 "Didn't find entry for predecessor!"); 676 677 PN->addIncoming(I->second, I->first); 678 } 679 680 //cerr << "PRE: " << *LI << *PN << "\n"; 681 682 LI->replaceAllUsesWith(PN); 683 LI->eraseFromParent(); 684 685 return true; 686} 687 688 689/// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in 690/// the current block. See if there are any simplifications we can do based on 691/// inputs to the phi node. 692/// 693bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { 694 // See if the phi node has any constant values. If so, we can determine where 695 // the corresponding predecessor will branch. 696 ConstantInt *PredCst = 0; 697 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 698 if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) 699 break; 700 701 // If no incoming value has a constant, we don't know the destination of any 702 // predecessors. 703 if (PredCst == 0) 704 return false; 705 706 // See if the cost of duplicating this block is low enough. 707 BasicBlock *BB = PN->getParent(); 708 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 709 if (JumpThreadCost > Threshold) { 710 DEBUG(errs() << " Not threading BB '" << BB->getName() 711 << "' - Cost is too high: " << JumpThreadCost << "\n"); 712 return false; 713 } 714 715 // If so, we can actually do this threading. Merge any common predecessors 716 // that will act the same. 717 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 718 719 // Next, figure out which successor we are threading to. 720 BasicBlock *SuccBB; 721 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 722 SuccBB = BI->getSuccessor(PredCst == PredBB->getContext().getFalse()); 723 else { 724 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); 725 SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst)); 726 } 727 728 // Ok, try to thread it! 729 return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost); 730} 731 732/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch 733/// whose condition is an AND/OR where one side is PN. If PN has constant 734/// operands that permit us to evaluate the condition for some operand, thread 735/// through the block. For example with: 736/// br (and X, phi(Y, Z, false)) 737/// the predecessor corresponding to the 'false' will always jump to the false 738/// destination of the branch. 739/// 740bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB, 741 bool isAnd) { 742 // If this is a binary operator tree of the same AND/OR opcode, check the 743 // LHS/RHS. 744 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) 745 if ((isAnd && BO->getOpcode() == Instruction::And) || 746 (!isAnd && BO->getOpcode() == Instruction::Or)) { 747 if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd)) 748 return true; 749 if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd)) 750 return true; 751 } 752 753 // If this isn't a PHI node, we can't handle it. 754 PHINode *PN = dyn_cast<PHINode>(V); 755 if (!PN || PN->getParent() != BB) return false; 756 757 // We can only do the simplification for phi nodes of 'false' with AND or 758 // 'true' with OR. See if we have any entries in the phi for this. 759 unsigned PredNo = ~0U; 760 ConstantInt *PredCst = ConstantInt::get(Type::Int1Ty, !isAnd); 761 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 762 if (PN->getIncomingValue(i) == PredCst) { 763 PredNo = i; 764 break; 765 } 766 } 767 768 // If no match, bail out. 769 if (PredNo == ~0U) 770 return false; 771 772 // See if the cost of duplicating this block is low enough. 773 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 774 if (JumpThreadCost > Threshold) { 775 DEBUG(errs() << " Not threading BB '" << BB->getName() 776 << "' - Cost is too high: " << JumpThreadCost << "\n"); 777 return false; 778 } 779 780 // If so, we can actually do this threading. Merge any common predecessors 781 // that will act the same. 782 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 783 784 // Next, figure out which successor we are threading to. If this was an AND, 785 // the constant must be FALSE, and we must be targeting the 'false' block. 786 // If this is an OR, the constant must be TRUE, and we must be targeting the 787 // 'true' block. 788 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd); 789 790 // Ok, try to thread it! 791 return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost); 792} 793 794/// GetResultOfComparison - Given an icmp/fcmp predicate and the left and right 795/// hand sides of the compare instruction, try to determine the result. If the 796/// result can not be determined, a null pointer is returned. 797static Constant *GetResultOfComparison(CmpInst::Predicate pred, 798 Value *LHS, Value *RHS, 799 LLVMContext &Context) { 800 if (Constant *CLHS = dyn_cast<Constant>(LHS)) 801 if (Constant *CRHS = dyn_cast<Constant>(RHS)) 802 return Context.getConstantExprCompare(pred, CLHS, CRHS); 803 804 if (LHS == RHS) 805 if (isa<IntegerType>(LHS->getType()) || isa<PointerType>(LHS->getType())) 806 return ICmpInst::isTrueWhenEqual(pred) ? 807 Context.getTrue() : Context.getFalse(); 808 809 return 0; 810} 811 812/// ProcessBranchOnCompare - We found a branch on a comparison between a phi 813/// node and a value. If we can identify when the comparison is true between 814/// the phi inputs and the value, we can fold the compare for that edge and 815/// thread through it. 816bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) { 817 PHINode *PN = cast<PHINode>(Cmp->getOperand(0)); 818 Value *RHS = Cmp->getOperand(1); 819 820 // If the phi isn't in the current block, an incoming edge to this block 821 // doesn't control the destination. 822 if (PN->getParent() != BB) 823 return false; 824 825 // We can do this simplification if any comparisons fold to true or false. 826 // See if any do. 827 Value *PredVal = 0; 828 bool TrueDirection = false; 829 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 830 PredVal = PN->getIncomingValue(i); 831 832 Constant *Res = GetResultOfComparison(Cmp->getPredicate(), PredVal, 833 RHS, Cmp->getContext()); 834 if (!Res) { 835 PredVal = 0; 836 continue; 837 } 838 839 // If this folded to a constant expr, we can't do anything. 840 if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) { 841 TrueDirection = ResC->getZExtValue(); 842 break; 843 } 844 // If this folded to undef, just go the false way. 845 if (isa<UndefValue>(Res)) { 846 TrueDirection = false; 847 break; 848 } 849 850 // Otherwise, we can't fold this input. 851 PredVal = 0; 852 } 853 854 // If no match, bail out. 855 if (PredVal == 0) 856 return false; 857 858 // See if the cost of duplicating this block is low enough. 859 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 860 if (JumpThreadCost > Threshold) { 861 DEBUG(errs() << " Not threading BB '" << BB->getName() 862 << "' - Cost is too high: " << JumpThreadCost << "\n"); 863 return false; 864 } 865 866 // If so, we can actually do this threading. Merge any common predecessors 867 // that will act the same. 868 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredVal); 869 870 // Next, get our successor. 871 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection); 872 873 // Ok, try to thread it! 874 return ThreadEdge(BB, PredBB, SuccBB, JumpThreadCost); 875} 876 877 878/// ThreadEdge - We have decided that it is safe and profitable to thread an 879/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this 880/// change. 881bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, 882 BasicBlock *SuccBB, unsigned JumpThreadCost) { 883 884 // If threading to the same block as we come from, we would infinite loop. 885 if (SuccBB == BB) { 886 DEBUG(errs() << " Not threading across BB '" << BB->getName() 887 << "' - would thread to self!\n"); 888 return false; 889 } 890 891 // If threading this would thread across a loop header, don't thread the edge. 892 // See the comments above FindLoopHeaders for justifications and caveats. 893 if (LoopHeaders.count(BB)) { 894 DEBUG(errs() << " Not threading from '" << PredBB->getName() 895 << "' across loop header BB '" << BB->getName() 896 << "' to dest BB '" << SuccBB->getName() 897 << "' - it might create an irreducible loop!\n"); 898 return false; 899 } 900 901 // And finally, do it! 902 DEBUG(errs() << " Threading edge from '" << PredBB->getName() << "' to '" 903 << SuccBB->getName() << "' with cost: " << JumpThreadCost 904 << ", across block:\n " 905 << *BB << "\n"); 906 907 // Jump Threading can not update SSA properties correctly if the values 908 // defined in the duplicated block are used outside of the block itself. For 909 // this reason, we spill all values that are used outside of BB to the stack. 910 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { 911 if (!I->isUsedOutsideOfBlock(BB)) 912 continue; 913 914 // We found a use of I outside of BB. Create a new stack slot to 915 // break this inter-block usage pattern. 916 DemoteRegToStack(*I); 917 } 918 919 // We are going to have to map operands from the original BB block to the new 920 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 921 // account for entry from PredBB. 922 DenseMap<Instruction*, Value*> ValueMapping; 923 924 BasicBlock *NewBB = 925 BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB); 926 NewBB->moveAfter(PredBB); 927 928 BasicBlock::iterator BI = BB->begin(); 929 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 930 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 931 932 // Clone the non-phi instructions of BB into NewBB, keeping track of the 933 // mapping and using it to remap operands in the cloned instructions. 934 for (; !isa<TerminatorInst>(BI); ++BI) { 935 Instruction *New = BI->clone(BI->getContext()); 936 New->setName(BI->getName()); 937 NewBB->getInstList().push_back(New); 938 ValueMapping[BI] = New; 939 940 // Remap operands to patch up intra-block references. 941 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 942 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { 943 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 944 if (I != ValueMapping.end()) 945 New->setOperand(i, I->second); 946 } 947 } 948 949 // We didn't copy the terminator from BB over to NewBB, because there is now 950 // an unconditional jump to SuccBB. Insert the unconditional jump. 951 BranchInst::Create(SuccBB, NewBB); 952 953 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 954 // PHI nodes for NewBB now. 955 for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) { 956 PHINode *PN = cast<PHINode>(PNI); 957 // Ok, we have a PHI node. Figure out what the incoming value was for the 958 // DestBlock. 959 Value *IV = PN->getIncomingValueForBlock(BB); 960 961 // Remap the value if necessary. 962 if (Instruction *Inst = dyn_cast<Instruction>(IV)) { 963 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); 964 if (I != ValueMapping.end()) 965 IV = I->second; 966 } 967 PN->addIncoming(IV, NewBB); 968 } 969 970 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to 971 // NewBB instead of BB. This eliminates predecessors from BB, which requires 972 // us to simplify any PHI nodes in BB. 973 TerminatorInst *PredTerm = PredBB->getTerminator(); 974 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 975 if (PredTerm->getSuccessor(i) == BB) { 976 BB->removePredecessor(PredBB); 977 PredTerm->setSuccessor(i, NewBB); 978 } 979 980 // At this point, the IR is fully up to date and consistent. Do a quick scan 981 // over the new instructions and zap any that are constants or dead. This 982 // frequently happens because of phi translation. 983 BI = NewBB->begin(); 984 for (BasicBlock::iterator E = NewBB->end(); BI != E; ) { 985 Instruction *Inst = BI++; 986 if (Constant *C = ConstantFoldInstruction(Inst, BB->getContext(), TD)) { 987 Inst->replaceAllUsesWith(C); 988 Inst->eraseFromParent(); 989 continue; 990 } 991 992 RecursivelyDeleteTriviallyDeadInstructions(Inst); 993 } 994 995 // Threaded an edge! 996 ++NumThreads; 997 return true; 998} 999