1 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===// 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 pass transforms loops that contain branches on loop-invariant conditions 11 // to have multiple loops. For example, it turns the left into the right code: 12 // 13 // for (...) if (lic) 14 // A for (...) 15 // if (lic) A; B; C 16 // B else 17 // C for (...) 18 // A; C 19 // 20 // This can increase the size of the code exponentially (doubling it every time 21 // a loop is unswitched) so we only unswitch if the resultant code will be 22 // smaller than a threshold. 23 // 24 // This pass expects LICM to be run before it to hoist invariant conditions out 25 // of the loop, to make the unswitching opportunity obvious. 26 // 27 //===----------------------------------------------------------------------===// 28 29 #define DEBUG_TYPE "loop-unswitch" 30 #include "llvm/Transforms/Scalar.h" 31 #include "llvm/Constants.h" 32 #include "llvm/DerivedTypes.h" 33 #include "llvm/Function.h" 34 #include "llvm/Instructions.h" 35 #include "llvm/Analysis/CodeMetrics.h" 36 #include "llvm/Analysis/InstructionSimplify.h" 37 #include "llvm/Analysis/LoopInfo.h" 38 #include "llvm/Analysis/LoopPass.h" 39 #include "llvm/Analysis/Dominators.h" 40 #include "llvm/Analysis/ScalarEvolution.h" 41 #include "llvm/Transforms/Utils/Cloning.h" 42 #include "llvm/Transforms/Utils/Local.h" 43 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 44 #include "llvm/ADT/Statistic.h" 45 #include "llvm/ADT/SmallPtrSet.h" 46 #include "llvm/ADT/STLExtras.h" 47 #include "llvm/Support/CommandLine.h" 48 #include "llvm/Support/Debug.h" 49 #include "llvm/Support/raw_ostream.h" 50 #include <algorithm> 51 #include <map> 52 #include <set> 53 using namespace llvm; 54 55 STATISTIC(NumBranches, "Number of branches unswitched"); 56 STATISTIC(NumSwitches, "Number of switches unswitched"); 57 STATISTIC(NumSelects , "Number of selects unswitched"); 58 STATISTIC(NumTrivial , "Number of unswitches that are trivial"); 59 STATISTIC(NumSimplify, "Number of simplifications of unswitched code"); 60 STATISTIC(TotalInsts, "Total number of instructions analyzed"); 61 62 // The specific value of 100 here was chosen based only on intuition and a 63 // few specific examples. 64 static cl::opt<unsigned> 65 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), 66 cl::init(100), cl::Hidden); 67 68 namespace { 69 70 class LUAnalysisCache { 71 72 typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> > 73 UnswitchedValsMap; 74 75 typedef UnswitchedValsMap::iterator UnswitchedValsIt; 76 77 struct LoopProperties { 78 unsigned CanBeUnswitchedCount; 79 unsigned SizeEstimation; 80 UnswitchedValsMap UnswitchedVals; 81 }; 82 83 // Here we use std::map instead of DenseMap, since we need to keep valid 84 // LoopProperties pointer for current loop for better performance. 85 typedef std::map<const Loop*, LoopProperties> LoopPropsMap; 86 typedef LoopPropsMap::iterator LoopPropsMapIt; 87 88 LoopPropsMap LoopsProperties; 89 UnswitchedValsMap* CurLoopInstructions; 90 LoopProperties* CurrentLoopProperties; 91 92 // Max size of code we can produce on remained iterations. 93 unsigned MaxSize; 94 95 public: 96 97 LUAnalysisCache() : 98 CurLoopInstructions(NULL), CurrentLoopProperties(NULL), 99 MaxSize(Threshold) 100 {} 101 102 // Analyze loop. Check its size, calculate is it possible to unswitch 103 // it. Returns true if we can unswitch this loop. 104 bool countLoop(const Loop* L); 105 106 // Clean all data related to given loop. 107 void forgetLoop(const Loop* L); 108 109 // Mark case value as unswitched. 110 // Since SI instruction can be partly unswitched, in order to avoid 111 // extra unswitching in cloned loops keep track all unswitched values. 112 void setUnswitched(const SwitchInst* SI, const Value* V); 113 114 // Check was this case value unswitched before or not. 115 bool isUnswitched(const SwitchInst* SI, const Value* V); 116 117 // Clone all loop-unswitch related loop properties. 118 // Redistribute unswitching quotas. 119 // Note, that new loop data is stored inside the VMap. 120 void cloneData(const Loop* NewLoop, const Loop* OldLoop, 121 const ValueToValueMapTy& VMap); 122 }; 123 124 class LoopUnswitch : public LoopPass { 125 LoopInfo *LI; // Loop information 126 LPPassManager *LPM; 127 128 // LoopProcessWorklist - Used to check if second loop needs processing 129 // after RewriteLoopBodyWithConditionConstant rewrites first loop. 130 std::vector<Loop*> LoopProcessWorklist; 131 132 LUAnalysisCache BranchesInfo; 133 134 bool OptimizeForSize; 135 bool redoLoop; 136 137 Loop *currentLoop; 138 DominatorTree *DT; 139 BasicBlock *loopHeader; 140 BasicBlock *loopPreheader; 141 142 // LoopBlocks contains all of the basic blocks of the loop, including the 143 // preheader of the loop, the body of the loop, and the exit blocks of the 144 // loop, in that order. 145 std::vector<BasicBlock*> LoopBlocks; 146 // NewBlocks contained cloned copy of basic blocks from LoopBlocks. 147 std::vector<BasicBlock*> NewBlocks; 148 149 public: 150 static char ID; // Pass ID, replacement for typeid 151 explicit LoopUnswitch(bool Os = false) : 152 LoopPass(ID), OptimizeForSize(Os), redoLoop(false), 153 currentLoop(NULL), DT(NULL), loopHeader(NULL), 154 loopPreheader(NULL) { 155 initializeLoopUnswitchPass(*PassRegistry::getPassRegistry()); 156 } 157 158 bool runOnLoop(Loop *L, LPPassManager &LPM); 159 bool processCurrentLoop(); 160 161 /// This transformation requires natural loop information & requires that 162 /// loop preheaders be inserted into the CFG. 163 /// 164 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 165 AU.addRequiredID(LoopSimplifyID); 166 AU.addPreservedID(LoopSimplifyID); 167 AU.addRequired<LoopInfo>(); 168 AU.addPreserved<LoopInfo>(); 169 AU.addRequiredID(LCSSAID); 170 AU.addPreservedID(LCSSAID); 171 AU.addPreserved<DominatorTree>(); 172 AU.addPreserved<ScalarEvolution>(); 173 } 174 175 private: 176 177 virtual void releaseMemory() { 178 BranchesInfo.forgetLoop(currentLoop); 179 } 180 181 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist, 182 /// remove it. 183 void RemoveLoopFromWorklist(Loop *L) { 184 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(), 185 LoopProcessWorklist.end(), L); 186 if (I != LoopProcessWorklist.end()) 187 LoopProcessWorklist.erase(I); 188 } 189 190 void initLoopData() { 191 loopHeader = currentLoop->getHeader(); 192 loopPreheader = currentLoop->getLoopPreheader(); 193 } 194 195 /// Split all of the edges from inside the loop to their exit blocks. 196 /// Update the appropriate Phi nodes as we do so. 197 void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks); 198 199 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val); 200 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 201 BasicBlock *ExitBlock); 202 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L); 203 204 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 205 Constant *Val, bool isEqual); 206 207 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 208 BasicBlock *TrueDest, 209 BasicBlock *FalseDest, 210 Instruction *InsertPt); 211 212 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); 213 void RemoveBlockIfDead(BasicBlock *BB, 214 std::vector<Instruction*> &Worklist, Loop *l); 215 void RemoveLoopFromHierarchy(Loop *L); 216 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0, 217 BasicBlock **LoopExit = 0); 218 219 }; 220 } 221 222 // Analyze loop. Check its size, calculate is it possible to unswitch 223 // it. Returns true if we can unswitch this loop. 224 bool LUAnalysisCache::countLoop(const Loop* L) { 225 226 std::pair<LoopPropsMapIt, bool> InsertRes = 227 LoopsProperties.insert(std::make_pair(L, LoopProperties())); 228 229 LoopProperties& Props = InsertRes.first->second; 230 231 if (InsertRes.second) { 232 // New loop. 233 234 // Limit the number of instructions to avoid causing significant code 235 // expansion, and the number of basic blocks, to avoid loops with 236 // large numbers of branches which cause loop unswitching to go crazy. 237 // This is a very ad-hoc heuristic. 238 239 // FIXME: This is overly conservative because it does not take into 240 // consideration code simplification opportunities and code that can 241 // be shared by the resultant unswitched loops. 242 CodeMetrics Metrics; 243 for (Loop::block_iterator I = L->block_begin(), 244 E = L->block_end(); 245 I != E; ++I) 246 Metrics.analyzeBasicBlock(*I); 247 248 Props.SizeEstimation = std::min(Metrics.NumInsts, Metrics.NumBlocks * 5); 249 Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation); 250 MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount; 251 } 252 253 if (!Props.CanBeUnswitchedCount) { 254 DEBUG(dbgs() << "NOT unswitching loop %" 255 << L->getHeader()->getName() << ", cost too high: " 256 << L->getBlocks().size() << "\n"); 257 258 return false; 259 } 260 261 // Be careful. This links are good only before new loop addition. 262 CurrentLoopProperties = &Props; 263 CurLoopInstructions = &Props.UnswitchedVals; 264 265 return true; 266 } 267 268 // Clean all data related to given loop. 269 void LUAnalysisCache::forgetLoop(const Loop* L) { 270 271 LoopPropsMapIt LIt = LoopsProperties.find(L); 272 273 if (LIt != LoopsProperties.end()) { 274 LoopProperties& Props = LIt->second; 275 MaxSize += Props.CanBeUnswitchedCount * Props.SizeEstimation; 276 LoopsProperties.erase(LIt); 277 } 278 279 CurrentLoopProperties = NULL; 280 CurLoopInstructions = NULL; 281 } 282 283 // Mark case value as unswitched. 284 // Since SI instruction can be partly unswitched, in order to avoid 285 // extra unswitching in cloned loops keep track all unswitched values. 286 void LUAnalysisCache::setUnswitched(const SwitchInst* SI, const Value* V) { 287 (*CurLoopInstructions)[SI].insert(V); 288 } 289 290 // Check was this case value unswitched before or not. 291 bool LUAnalysisCache::isUnswitched(const SwitchInst* SI, const Value* V) { 292 return (*CurLoopInstructions)[SI].count(V); 293 } 294 295 // Clone all loop-unswitch related loop properties. 296 // Redistribute unswitching quotas. 297 // Note, that new loop data is stored inside the VMap. 298 void LUAnalysisCache::cloneData(const Loop* NewLoop, const Loop* OldLoop, 299 const ValueToValueMapTy& VMap) { 300 301 LoopProperties& NewLoopProps = LoopsProperties[NewLoop]; 302 LoopProperties& OldLoopProps = *CurrentLoopProperties; 303 UnswitchedValsMap& Insts = OldLoopProps.UnswitchedVals; 304 305 // Reallocate "can-be-unswitched quota" 306 307 --OldLoopProps.CanBeUnswitchedCount; 308 unsigned Quota = OldLoopProps.CanBeUnswitchedCount; 309 NewLoopProps.CanBeUnswitchedCount = Quota / 2; 310 OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2; 311 312 NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation; 313 314 // Clone unswitched values info: 315 // for new loop switches we clone info about values that was 316 // already unswitched and has redundant successors. 317 for (UnswitchedValsIt I = Insts.begin(); I != Insts.end(); ++I) { 318 const SwitchInst* OldInst = I->first; 319 Value* NewI = VMap.lookup(OldInst); 320 const SwitchInst* NewInst = cast_or_null<SwitchInst>(NewI); 321 assert(NewInst && "All instructions that are in SrcBB must be in VMap."); 322 323 NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst]; 324 } 325 } 326 327 char LoopUnswitch::ID = 0; 328 INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops", 329 false, false) 330 INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 331 INITIALIZE_PASS_DEPENDENCY(LoopInfo) 332 INITIALIZE_PASS_DEPENDENCY(LCSSA) 333 INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops", 334 false, false) 335 336 Pass *llvm::createLoopUnswitchPass(bool Os) { 337 return new LoopUnswitch(Os); 338 } 339 340 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is 341 /// invariant in the loop, or has an invariant piece, return the invariant. 342 /// Otherwise, return null. 343 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { 344 345 // We started analyze new instruction, increment scanned instructions counter. 346 ++TotalInsts; 347 348 // We can never unswitch on vector conditions. 349 if (Cond->getType()->isVectorTy()) 350 return 0; 351 352 // Constants should be folded, not unswitched on! 353 if (isa<Constant>(Cond)) return 0; 354 355 // TODO: Handle: br (VARIANT|INVARIANT). 356 357 // Hoist simple values out. 358 if (L->makeLoopInvariant(Cond, Changed)) 359 return Cond; 360 361 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 362 if (BO->getOpcode() == Instruction::And || 363 BO->getOpcode() == Instruction::Or) { 364 // If either the left or right side is invariant, we can unswitch on this, 365 // which will cause the branch to go away in one loop and the condition to 366 // simplify in the other one. 367 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) 368 return LHS; 369 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) 370 return RHS; 371 } 372 373 return 0; 374 } 375 376 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) { 377 LI = &getAnalysis<LoopInfo>(); 378 LPM = &LPM_Ref; 379 DT = getAnalysisIfAvailable<DominatorTree>(); 380 currentLoop = L; 381 Function *F = currentLoop->getHeader()->getParent(); 382 bool Changed = false; 383 do { 384 assert(currentLoop->isLCSSAForm(*DT)); 385 redoLoop = false; 386 Changed |= processCurrentLoop(); 387 } while(redoLoop); 388 389 if (Changed) { 390 // FIXME: Reconstruct dom info, because it is not preserved properly. 391 if (DT) 392 DT->runOnFunction(*F); 393 } 394 return Changed; 395 } 396 397 /// processCurrentLoop - Do actual work and unswitch loop if possible 398 /// and profitable. 399 bool LoopUnswitch::processCurrentLoop() { 400 bool Changed = false; 401 402 initLoopData(); 403 404 // If LoopSimplify was unable to form a preheader, don't do any unswitching. 405 if (!loopPreheader) 406 return false; 407 408 // Loops with indirectbr cannot be cloned. 409 if (!currentLoop->isSafeToClone()) 410 return false; 411 412 // Without dedicated exits, splitting the exit edge may fail. 413 if (!currentLoop->hasDedicatedExits()) 414 return false; 415 416 LLVMContext &Context = loopHeader->getContext(); 417 418 // Probably we reach the quota of branches for this loop. If so 419 // stop unswitching. 420 if (!BranchesInfo.countLoop(currentLoop)) 421 return false; 422 423 // Loop over all of the basic blocks in the loop. If we find an interior 424 // block that is branching on a loop-invariant condition, we can unswitch this 425 // loop. 426 for (Loop::block_iterator I = currentLoop->block_begin(), 427 E = currentLoop->block_end(); I != E; ++I) { 428 TerminatorInst *TI = (*I)->getTerminator(); 429 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 430 // If this isn't branching on an invariant condition, we can't unswitch 431 // it. 432 if (BI->isConditional()) { 433 // See if this, or some part of it, is loop invariant. If so, we can 434 // unswitch on it if we desire. 435 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), 436 currentLoop, Changed); 437 if (LoopCond && UnswitchIfProfitable(LoopCond, 438 ConstantInt::getTrue(Context))) { 439 ++NumBranches; 440 return true; 441 } 442 } 443 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 444 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 445 currentLoop, Changed); 446 unsigned NumCases = SI->getNumCases(); 447 if (LoopCond && NumCases) { 448 // Find a value to unswitch on: 449 // FIXME: this should chose the most expensive case! 450 // FIXME: scan for a case with a non-critical edge? 451 Constant *UnswitchVal = NULL; 452 453 // Do not process same value again and again. 454 // At this point we have some cases already unswitched and 455 // some not yet unswitched. Let's find the first not yet unswitched one. 456 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); 457 i != e; ++i) { 458 Constant* UnswitchValCandidate = i.getCaseValue(); 459 if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) { 460 UnswitchVal = UnswitchValCandidate; 461 break; 462 } 463 } 464 465 if (!UnswitchVal) 466 continue; 467 468 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) { 469 ++NumSwitches; 470 return true; 471 } 472 } 473 } 474 475 // Scan the instructions to check for unswitchable values. 476 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 477 BBI != E; ++BBI) 478 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 479 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 480 currentLoop, Changed); 481 if (LoopCond && UnswitchIfProfitable(LoopCond, 482 ConstantInt::getTrue(Context))) { 483 ++NumSelects; 484 return true; 485 } 486 } 487 } 488 return Changed; 489 } 490 491 /// isTrivialLoopExitBlock - Check to see if all paths from BB exit the 492 /// loop with no side effects (including infinite loops). 493 /// 494 /// If true, we return true and set ExitBB to the block we 495 /// exit through. 496 /// 497 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, 498 BasicBlock *&ExitBB, 499 std::set<BasicBlock*> &Visited) { 500 if (!Visited.insert(BB).second) { 501 // Already visited. Without more analysis, this could indicate an infinite 502 // loop. 503 return false; 504 } else if (!L->contains(BB)) { 505 // Otherwise, this is a loop exit, this is fine so long as this is the 506 // first exit. 507 if (ExitBB != 0) return false; 508 ExitBB = BB; 509 return true; 510 } 511 512 // Otherwise, this is an unvisited intra-loop node. Check all successors. 513 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { 514 // Check to see if the successor is a trivial loop exit. 515 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) 516 return false; 517 } 518 519 // Okay, everything after this looks good, check to make sure that this block 520 // doesn't include any side effects. 521 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 522 if (I->mayHaveSideEffects()) 523 return false; 524 525 return true; 526 } 527 528 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally 529 /// leads to an exit from the specified loop, and has no side-effects in the 530 /// process. If so, return the block that is exited to, otherwise return null. 531 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { 532 std::set<BasicBlock*> Visited; 533 Visited.insert(L->getHeader()); // Branches to header make infinite loops. 534 BasicBlock *ExitBB = 0; 535 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) 536 return ExitBB; 537 return 0; 538 } 539 540 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is 541 /// trivial: that is, that the condition controls whether or not the loop does 542 /// anything at all. If this is a trivial condition, unswitching produces no 543 /// code duplications (equivalently, it produces a simpler loop and a new empty 544 /// loop, which gets deleted). 545 /// 546 /// If this is a trivial condition, return true, otherwise return false. When 547 /// returning true, this sets Cond and Val to the condition that controls the 548 /// trivial condition: when Cond dynamically equals Val, the loop is known to 549 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when 550 /// Cond == Val. 551 /// 552 bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val, 553 BasicBlock **LoopExit) { 554 BasicBlock *Header = currentLoop->getHeader(); 555 TerminatorInst *HeaderTerm = Header->getTerminator(); 556 LLVMContext &Context = Header->getContext(); 557 558 BasicBlock *LoopExitBB = 0; 559 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) { 560 // If the header block doesn't end with a conditional branch on Cond, we 561 // can't handle it. 562 if (!BI->isConditional() || BI->getCondition() != Cond) 563 return false; 564 565 // Check to see if a successor of the branch is guaranteed to 566 // exit through a unique exit block without having any 567 // side-effects. If so, determine the value of Cond that causes it to do 568 // this. 569 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 570 BI->getSuccessor(0)))) { 571 if (Val) *Val = ConstantInt::getTrue(Context); 572 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 573 BI->getSuccessor(1)))) { 574 if (Val) *Val = ConstantInt::getFalse(Context); 575 } 576 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) { 577 // If this isn't a switch on Cond, we can't handle it. 578 if (SI->getCondition() != Cond) return false; 579 580 // Check to see if a successor of the switch is guaranteed to go to the 581 // latch block or exit through a one exit block without having any 582 // side-effects. If so, determine the value of Cond that causes it to do 583 // this. 584 // Note that we can't trivially unswitch on the default case or 585 // on already unswitched cases. 586 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); 587 i != e; ++i) { 588 BasicBlock* LoopExitCandidate; 589 if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop, 590 i.getCaseSuccessor()))) { 591 // Okay, we found a trivial case, remember the value that is trivial. 592 ConstantInt* CaseVal = i.getCaseValue(); 593 594 // Check that it was not unswitched before, since already unswitched 595 // trivial vals are looks trivial too. 596 if (BranchesInfo.isUnswitched(SI, CaseVal)) 597 continue; 598 LoopExitBB = LoopExitCandidate; 599 if (Val) *Val = CaseVal; 600 break; 601 } 602 } 603 } 604 605 // If we didn't find a single unique LoopExit block, or if the loop exit block 606 // contains phi nodes, this isn't trivial. 607 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) 608 return false; // Can't handle this. 609 610 if (LoopExit) *LoopExit = LoopExitBB; 611 612 // We already know that nothing uses any scalar values defined inside of this 613 // loop. As such, we just have to check to see if this loop will execute any 614 // side-effecting instructions (e.g. stores, calls, volatile loads) in the 615 // part of the loop that the code *would* execute. We already checked the 616 // tail, check the header now. 617 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 618 if (I->mayHaveSideEffects()) 619 return false; 620 return true; 621 } 622 623 /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when 624 /// LoopCond == Val to simplify the loop. If we decide that this is profitable, 625 /// unswitch the loop, reprocess the pieces, then return true. 626 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val) { 627 Function *F = loopHeader->getParent(); 628 Constant *CondVal = 0; 629 BasicBlock *ExitBlock = 0; 630 631 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) { 632 // If the condition is trivial, always unswitch. There is no code growth 633 // for this case. 634 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock); 635 return true; 636 } 637 638 // Check to see if it would be profitable to unswitch current loop. 639 640 // Do not do non-trivial unswitch while optimizing for size. 641 if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize)) 642 return false; 643 644 UnswitchNontrivialCondition(LoopCond, Val, currentLoop); 645 return true; 646 } 647 648 /// CloneLoop - Recursively clone the specified loop and all of its children, 649 /// mapping the blocks with the specified map. 650 static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM, 651 LoopInfo *LI, LPPassManager *LPM) { 652 Loop *New = new Loop(); 653 LPM->insertLoop(New, PL); 654 655 // Add all of the blocks in L to the new loop. 656 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 657 I != E; ++I) 658 if (LI->getLoopFor(*I) == L) 659 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase()); 660 661 // Add all of the subloops to the new loop. 662 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 663 CloneLoop(*I, New, VM, LI, LPM); 664 665 return New; 666 } 667 668 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values 669 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the 670 /// code immediately before InsertPt. 671 void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 672 BasicBlock *TrueDest, 673 BasicBlock *FalseDest, 674 Instruction *InsertPt) { 675 // Insert a conditional branch on LIC to the two preheaders. The original 676 // code is the true version and the new code is the false version. 677 Value *BranchVal = LIC; 678 if (!isa<ConstantInt>(Val) || 679 Val->getType() != Type::getInt1Ty(LIC->getContext())) 680 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val); 681 else if (Val != ConstantInt::getTrue(Val->getContext())) 682 // We want to enter the new loop when the condition is true. 683 std::swap(TrueDest, FalseDest); 684 685 // Insert the new branch. 686 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt); 687 688 // If either edge is critical, split it. This helps preserve LoopSimplify 689 // form for enclosing loops. 690 SplitCriticalEdge(BI, 0, this, false, false, true); 691 SplitCriticalEdge(BI, 1, this, false, false, true); 692 } 693 694 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable 695 /// condition in it (a cond branch from its header block to its latch block, 696 /// where the path through the loop that doesn't execute its body has no 697 /// side-effects), unswitch it. This doesn't involve any code duplication, just 698 /// moving the conditional branch outside of the loop and updating loop info. 699 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, 700 Constant *Val, 701 BasicBlock *ExitBlock) { 702 DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %" 703 << loopHeader->getName() << " [" << L->getBlocks().size() 704 << " blocks] in Function " << L->getHeader()->getParent()->getName() 705 << " on cond: " << *Val << " == " << *Cond << "\n"); 706 707 // First step, split the preheader, so that we know that there is a safe place 708 // to insert the conditional branch. We will change loopPreheader to have a 709 // conditional branch on Cond. 710 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this); 711 712 // Now that we have a place to insert the conditional branch, create a place 713 // to branch to: this is the exit block out of the loop that we should 714 // short-circuit to. 715 716 // Split this block now, so that the loop maintains its exit block, and so 717 // that the jump from the preheader can execute the contents of the exit block 718 // without actually branching to it (the exit block should be dominated by the 719 // loop header, not the preheader). 720 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 721 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this); 722 723 // Okay, now we have a position to branch from and a position to branch to, 724 // insert the new conditional branch. 725 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, 726 loopPreheader->getTerminator()); 727 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L); 728 loopPreheader->getTerminator()->eraseFromParent(); 729 730 // We need to reprocess this loop, it could be unswitched again. 731 redoLoop = true; 732 733 // Now that we know that the loop is never entered when this condition is a 734 // particular value, rewrite the loop with this info. We know that this will 735 // at least eliminate the old branch. 736 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false); 737 ++NumTrivial; 738 } 739 740 /// SplitExitEdges - Split all of the edges from inside the loop to their exit 741 /// blocks. Update the appropriate Phi nodes as we do so. 742 void LoopUnswitch::SplitExitEdges(Loop *L, 743 const SmallVector<BasicBlock *, 8> &ExitBlocks){ 744 745 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 746 BasicBlock *ExitBlock = ExitBlocks[i]; 747 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock), 748 pred_end(ExitBlock)); 749 750 // Although SplitBlockPredecessors doesn't preserve loop-simplify in 751 // general, if we call it on all predecessors of all exits then it does. 752 if (!ExitBlock->isLandingPad()) { 753 SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", this); 754 } else { 755 SmallVector<BasicBlock*, 2> NewBBs; 756 SplitLandingPadPredecessors(ExitBlock, Preds, ".us-lcssa", ".us-lcssa", 757 this, NewBBs); 758 } 759 } 760 } 761 762 /// UnswitchNontrivialCondition - We determined that the loop is profitable 763 /// to unswitch when LIC equal Val. Split it into loop versions and test the 764 /// condition outside of either loop. Return the loops created as Out1/Out2. 765 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, 766 Loop *L) { 767 Function *F = loopHeader->getParent(); 768 DEBUG(dbgs() << "loop-unswitch: Unswitching loop %" 769 << loopHeader->getName() << " [" << L->getBlocks().size() 770 << " blocks] in Function " << F->getName() 771 << " when '" << *Val << "' == " << *LIC << "\n"); 772 773 if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>()) 774 SE->forgetLoop(L); 775 776 LoopBlocks.clear(); 777 NewBlocks.clear(); 778 779 // First step, split the preheader and exit blocks, and add these blocks to 780 // the LoopBlocks list. 781 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this); 782 LoopBlocks.push_back(NewPreheader); 783 784 // We want the loop to come after the preheader, but before the exit blocks. 785 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 786 787 SmallVector<BasicBlock*, 8> ExitBlocks; 788 L->getUniqueExitBlocks(ExitBlocks); 789 790 // Split all of the edges from inside the loop to their exit blocks. Update 791 // the appropriate Phi nodes as we do so. 792 SplitExitEdges(L, ExitBlocks); 793 794 // The exit blocks may have been changed due to edge splitting, recompute. 795 ExitBlocks.clear(); 796 L->getUniqueExitBlocks(ExitBlocks); 797 798 // Add exit blocks to the loop blocks. 799 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 800 801 // Next step, clone all of the basic blocks that make up the loop (including 802 // the loop preheader and exit blocks), keeping track of the mapping between 803 // the instructions and blocks. 804 NewBlocks.reserve(LoopBlocks.size()); 805 ValueToValueMapTy VMap; 806 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 807 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F); 808 809 NewBlocks.push_back(NewBB); 810 VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping. 811 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L); 812 } 813 814 // Splice the newly inserted blocks into the function right before the 815 // original preheader. 816 F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(), 817 NewBlocks[0], F->end()); 818 819 // Now we create the new Loop object for the versioned loop. 820 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM); 821 822 // Recalculate unswitching quota, inherit simplified switches info for NewBB, 823 // Probably clone more loop-unswitch related loop properties. 824 BranchesInfo.cloneData(NewLoop, L, VMap); 825 826 Loop *ParentLoop = L->getParentLoop(); 827 if (ParentLoop) { 828 // Make sure to add the cloned preheader and exit blocks to the parent loop 829 // as well. 830 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase()); 831 } 832 833 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 834 BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]); 835 // The new exit block should be in the same loop as the old one. 836 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i])) 837 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase()); 838 839 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 840 "Exit block should have been split to have one successor!"); 841 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 842 843 // If the successor of the exit block had PHI nodes, add an entry for 844 // NewExit. 845 PHINode *PN; 846 for (BasicBlock::iterator I = ExitSucc->begin(); isa<PHINode>(I); ++I) { 847 PN = cast<PHINode>(I); 848 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); 849 ValueToValueMapTy::iterator It = VMap.find(V); 850 if (It != VMap.end()) V = It->second; 851 PN->addIncoming(V, NewExit); 852 } 853 854 if (LandingPadInst *LPad = NewExit->getLandingPadInst()) { 855 PN = PHINode::Create(LPad->getType(), 0, "", 856 ExitSucc->getFirstInsertionPt()); 857 858 for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc); 859 I != E; ++I) { 860 BasicBlock *BB = *I; 861 LandingPadInst *LPI = BB->getLandingPadInst(); 862 LPI->replaceAllUsesWith(PN); 863 PN->addIncoming(LPI, BB); 864 } 865 } 866 } 867 868 // Rewrite the code to refer to itself. 869 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) 870 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 871 E = NewBlocks[i]->end(); I != E; ++I) 872 RemapInstruction(I, VMap,RF_NoModuleLevelChanges|RF_IgnoreMissingEntries); 873 874 // Rewrite the original preheader to select between versions of the loop. 875 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator()); 876 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 877 "Preheader splitting did not work correctly!"); 878 879 // Emit the new branch that selects between the two versions of this loop. 880 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); 881 LPM->deleteSimpleAnalysisValue(OldBR, L); 882 OldBR->eraseFromParent(); 883 884 LoopProcessWorklist.push_back(NewLoop); 885 redoLoop = true; 886 887 // Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody 888 // deletes the instruction (for example by simplifying a PHI that feeds into 889 // the condition that we're unswitching on), we don't rewrite the second 890 // iteration. 891 WeakVH LICHandle(LIC); 892 893 // Now we rewrite the original code to know that the condition is true and the 894 // new code to know that the condition is false. 895 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false); 896 897 // It's possible that simplifying one loop could cause the other to be 898 // changed to another value or a constant. If its a constant, don't simplify 899 // it. 900 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop && 901 LICHandle && !isa<Constant>(LICHandle)) 902 RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true); 903 } 904 905 /// RemoveFromWorklist - Remove all instances of I from the worklist vector 906 /// specified. 907 static void RemoveFromWorklist(Instruction *I, 908 std::vector<Instruction*> &Worklist) { 909 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(), 910 Worklist.end(), I); 911 while (WI != Worklist.end()) { 912 unsigned Offset = WI-Worklist.begin(); 913 Worklist.erase(WI); 914 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I); 915 } 916 } 917 918 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the 919 /// program, replacing all uses with V and update the worklist. 920 static void ReplaceUsesOfWith(Instruction *I, Value *V, 921 std::vector<Instruction*> &Worklist, 922 Loop *L, LPPassManager *LPM) { 923 DEBUG(dbgs() << "Replace with '" << *V << "': " << *I); 924 925 // Add uses to the worklist, which may be dead now. 926 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 927 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 928 Worklist.push_back(Use); 929 930 // Add users to the worklist which may be simplified now. 931 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 932 UI != E; ++UI) 933 Worklist.push_back(cast<Instruction>(*UI)); 934 LPM->deleteSimpleAnalysisValue(I, L); 935 RemoveFromWorklist(I, Worklist); 936 I->replaceAllUsesWith(V); 937 I->eraseFromParent(); 938 ++NumSimplify; 939 } 940 941 /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop 942 /// information, and remove any dead successors it has. 943 /// 944 void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB, 945 std::vector<Instruction*> &Worklist, 946 Loop *L) { 947 if (pred_begin(BB) != pred_end(BB)) { 948 // This block isn't dead, since an edge to BB was just removed, see if there 949 // are any easy simplifications we can do now. 950 if (BasicBlock *Pred = BB->getSinglePredecessor()) { 951 // If it has one pred, fold phi nodes in BB. 952 while (isa<PHINode>(BB->begin())) 953 ReplaceUsesOfWith(BB->begin(), 954 cast<PHINode>(BB->begin())->getIncomingValue(0), 955 Worklist, L, LPM); 956 957 // If this is the header of a loop and the only pred is the latch, we now 958 // have an unreachable loop. 959 if (Loop *L = LI->getLoopFor(BB)) 960 if (loopHeader == BB && L->contains(Pred)) { 961 // Remove the branch from the latch to the header block, this makes 962 // the header dead, which will make the latch dead (because the header 963 // dominates the latch). 964 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L); 965 Pred->getTerminator()->eraseFromParent(); 966 new UnreachableInst(BB->getContext(), Pred); 967 968 // The loop is now broken, remove it from LI. 969 RemoveLoopFromHierarchy(L); 970 971 // Reprocess the header, which now IS dead. 972 RemoveBlockIfDead(BB, Worklist, L); 973 return; 974 } 975 976 // If pred ends in a uncond branch, add uncond branch to worklist so that 977 // the two blocks will get merged. 978 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) 979 if (BI->isUnconditional()) 980 Worklist.push_back(BI); 981 } 982 return; 983 } 984 985 DEBUG(dbgs() << "Nuking dead block: " << *BB); 986 987 // Remove the instructions in the basic block from the worklist. 988 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 989 RemoveFromWorklist(I, Worklist); 990 991 // Anything that uses the instructions in this basic block should have their 992 // uses replaced with undefs. 993 // If I is not void type then replaceAllUsesWith undef. 994 // This allows ValueHandlers and custom metadata to adjust itself. 995 if (!I->getType()->isVoidTy()) 996 I->replaceAllUsesWith(UndefValue::get(I->getType())); 997 } 998 999 // If this is the edge to the header block for a loop, remove the loop and 1000 // promote all subloops. 1001 if (Loop *BBLoop = LI->getLoopFor(BB)) { 1002 if (BBLoop->getLoopLatch() == BB) { 1003 RemoveLoopFromHierarchy(BBLoop); 1004 if (currentLoop == BBLoop) { 1005 currentLoop = 0; 1006 redoLoop = false; 1007 } 1008 } 1009 } 1010 1011 // Remove the block from the loop info, which removes it from any loops it 1012 // was in. 1013 LI->removeBlock(BB); 1014 1015 1016 // Remove phi node entries in successors for this block. 1017 TerminatorInst *TI = BB->getTerminator(); 1018 SmallVector<BasicBlock*, 4> Succs; 1019 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) { 1020 Succs.push_back(TI->getSuccessor(i)); 1021 TI->getSuccessor(i)->removePredecessor(BB); 1022 } 1023 1024 // Unique the successors, remove anything with multiple uses. 1025 array_pod_sort(Succs.begin(), Succs.end()); 1026 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end()); 1027 1028 // Remove the basic block, including all of the instructions contained in it. 1029 LPM->deleteSimpleAnalysisValue(BB, L); 1030 BB->eraseFromParent(); 1031 // Remove successor blocks here that are not dead, so that we know we only 1032 // have dead blocks in this list. Nondead blocks have a way of becoming dead, 1033 // then getting removed before we revisit them, which is badness. 1034 // 1035 for (unsigned i = 0; i != Succs.size(); ++i) 1036 if (pred_begin(Succs[i]) != pred_end(Succs[i])) { 1037 // One exception is loop headers. If this block was the preheader for a 1038 // loop, then we DO want to visit the loop so the loop gets deleted. 1039 // We know that if the successor is a loop header, that this loop had to 1040 // be the preheader: the case where this was the latch block was handled 1041 // above and headers can only have two predecessors. 1042 if (!LI->isLoopHeader(Succs[i])) { 1043 Succs.erase(Succs.begin()+i); 1044 --i; 1045 } 1046 } 1047 1048 for (unsigned i = 0, e = Succs.size(); i != e; ++i) 1049 RemoveBlockIfDead(Succs[i], Worklist, L); 1050 } 1051 1052 /// RemoveLoopFromHierarchy - We have discovered that the specified loop has 1053 /// become unwrapped, either because the backedge was deleted, or because the 1054 /// edge into the header was removed. If the edge into the header from the 1055 /// latch block was removed, the loop is unwrapped but subloops are still alive, 1056 /// so they just reparent loops. If the loops are actually dead, they will be 1057 /// removed later. 1058 void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) { 1059 LPM->deleteLoopFromQueue(L); 1060 RemoveLoopFromWorklist(L); 1061 } 1062 1063 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has 1064 // the value specified by Val in the specified loop, or we know it does NOT have 1065 // that value. Rewrite any uses of LIC or of properties correlated to it. 1066 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 1067 Constant *Val, 1068 bool IsEqual) { 1069 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 1070 1071 // FIXME: Support correlated properties, like: 1072 // for (...) 1073 // if (li1 < li2) 1074 // ... 1075 // if (li1 > li2) 1076 // ... 1077 1078 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 1079 // selects, switches. 1080 std::vector<Instruction*> Worklist; 1081 LLVMContext &Context = Val->getContext(); 1082 1083 1084 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC 1085 // in the loop with the appropriate one directly. 1086 if (IsEqual || (isa<ConstantInt>(Val) && 1087 Val->getType()->isIntegerTy(1))) { 1088 Value *Replacement; 1089 if (IsEqual) 1090 Replacement = Val; 1091 else 1092 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()), 1093 !cast<ConstantInt>(Val)->getZExtValue()); 1094 1095 for (Value::use_iterator UI = LIC->use_begin(), E = LIC->use_end(); 1096 UI != E; ++UI) { 1097 Instruction *U = dyn_cast<Instruction>(*UI); 1098 if (!U || !L->contains(U)) 1099 continue; 1100 Worklist.push_back(U); 1101 } 1102 1103 for (std::vector<Instruction*>::iterator UI = Worklist.begin(); 1104 UI != Worklist.end(); ++UI) 1105 (*UI)->replaceUsesOfWith(LIC, Replacement); 1106 1107 SimplifyCode(Worklist, L); 1108 return; 1109 } 1110 1111 // Otherwise, we don't know the precise value of LIC, but we do know that it 1112 // is certainly NOT "Val". As such, simplify any uses in the loop that we 1113 // can. This case occurs when we unswitch switch statements. 1114 for (Value::use_iterator UI = LIC->use_begin(), E = LIC->use_end(); 1115 UI != E; ++UI) { 1116 Instruction *U = dyn_cast<Instruction>(*UI); 1117 if (!U || !L->contains(U)) 1118 continue; 1119 1120 Worklist.push_back(U); 1121 1122 // TODO: We could do other simplifications, for example, turning 1123 // 'icmp eq LIC, Val' -> false. 1124 1125 // If we know that LIC is not Val, use this info to simplify code. 1126 SwitchInst *SI = dyn_cast<SwitchInst>(U); 1127 if (SI == 0 || !isa<ConstantInt>(Val)) continue; 1128 1129 SwitchInst::CaseIt DeadCase = SI->findCaseValue(cast<ConstantInt>(Val)); 1130 // Default case is live for multiple values. 1131 if (DeadCase == SI->case_default()) continue; 1132 1133 // Found a dead case value. Don't remove PHI nodes in the 1134 // successor if they become single-entry, those PHI nodes may 1135 // be in the Users list. 1136 1137 BasicBlock *Switch = SI->getParent(); 1138 BasicBlock *SISucc = DeadCase.getCaseSuccessor(); 1139 BasicBlock *Latch = L->getLoopLatch(); 1140 1141 BranchesInfo.setUnswitched(SI, Val); 1142 1143 if (!SI->findCaseDest(SISucc)) continue; // Edge is critical. 1144 // If the DeadCase successor dominates the loop latch, then the 1145 // transformation isn't safe since it will delete the sole predecessor edge 1146 // to the latch. 1147 if (Latch && DT->dominates(SISucc, Latch)) 1148 continue; 1149 1150 // FIXME: This is a hack. We need to keep the successor around 1151 // and hooked up so as to preserve the loop structure, because 1152 // trying to update it is complicated. So instead we preserve the 1153 // loop structure and put the block on a dead code path. 1154 SplitEdge(Switch, SISucc, this); 1155 // Compute the successors instead of relying on the return value 1156 // of SplitEdge, since it may have split the switch successor 1157 // after PHI nodes. 1158 BasicBlock *NewSISucc = DeadCase.getCaseSuccessor(); 1159 BasicBlock *OldSISucc = *succ_begin(NewSISucc); 1160 // Create an "unreachable" destination. 1161 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable", 1162 Switch->getParent(), 1163 OldSISucc); 1164 new UnreachableInst(Context, Abort); 1165 // Force the new case destination to branch to the "unreachable" 1166 // block while maintaining a (dead) CFG edge to the old block. 1167 NewSISucc->getTerminator()->eraseFromParent(); 1168 BranchInst::Create(Abort, OldSISucc, 1169 ConstantInt::getTrue(Context), NewSISucc); 1170 // Release the PHI operands for this edge. 1171 for (BasicBlock::iterator II = NewSISucc->begin(); 1172 PHINode *PN = dyn_cast<PHINode>(II); ++II) 1173 PN->setIncomingValue(PN->getBasicBlockIndex(Switch), 1174 UndefValue::get(PN->getType())); 1175 // Tell the domtree about the new block. We don't fully update the 1176 // domtree here -- instead we force it to do a full recomputation 1177 // after the pass is complete -- but we do need to inform it of 1178 // new blocks. 1179 if (DT) 1180 DT->addNewBlock(Abort, NewSISucc); 1181 } 1182 1183 SimplifyCode(Worklist, L); 1184 } 1185 1186 /// SimplifyCode - Okay, now that we have simplified some instructions in the 1187 /// loop, walk over it and constant prop, dce, and fold control flow where 1188 /// possible. Note that this is effectively a very simple loop-structure-aware 1189 /// optimizer. During processing of this loop, L could very well be deleted, so 1190 /// it must not be used. 1191 /// 1192 /// FIXME: When the loop optimizer is more mature, separate this out to a new 1193 /// pass. 1194 /// 1195 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) { 1196 while (!Worklist.empty()) { 1197 Instruction *I = Worklist.back(); 1198 Worklist.pop_back(); 1199 1200 // Simple DCE. 1201 if (isInstructionTriviallyDead(I)) { 1202 DEBUG(dbgs() << "Remove dead instruction '" << *I); 1203 1204 // Add uses to the worklist, which may be dead now. 1205 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 1206 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 1207 Worklist.push_back(Use); 1208 LPM->deleteSimpleAnalysisValue(I, L); 1209 RemoveFromWorklist(I, Worklist); 1210 I->eraseFromParent(); 1211 ++NumSimplify; 1212 continue; 1213 } 1214 1215 // See if instruction simplification can hack this up. This is common for 1216 // things like "select false, X, Y" after unswitching made the condition be 1217 // 'false'. TODO: update the domtree properly so we can pass it here. 1218 if (Value *V = SimplifyInstruction(I)) 1219 if (LI->replacementPreservesLCSSAForm(I, V)) { 1220 ReplaceUsesOfWith(I, V, Worklist, L, LPM); 1221 continue; 1222 } 1223 1224 // Special case hacks that appear commonly in unswitched code. 1225 if (BranchInst *BI = dyn_cast<BranchInst>(I)) { 1226 if (BI->isUnconditional()) { 1227 // If BI's parent is the only pred of the successor, fold the two blocks 1228 // together. 1229 BasicBlock *Pred = BI->getParent(); 1230 BasicBlock *Succ = BI->getSuccessor(0); 1231 BasicBlock *SinglePred = Succ->getSinglePredecessor(); 1232 if (!SinglePred) continue; // Nothing to do. 1233 assert(SinglePred == Pred && "CFG broken"); 1234 1235 DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- " 1236 << Succ->getName() << "\n"); 1237 1238 // Resolve any single entry PHI nodes in Succ. 1239 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) 1240 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM); 1241 1242 // If Succ has any successors with PHI nodes, update them to have 1243 // entries coming from Pred instead of Succ. 1244 Succ->replaceAllUsesWith(Pred); 1245 1246 // Move all of the successor contents from Succ to Pred. 1247 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(), 1248 Succ->end()); 1249 LPM->deleteSimpleAnalysisValue(BI, L); 1250 BI->eraseFromParent(); 1251 RemoveFromWorklist(BI, Worklist); 1252 1253 // Remove Succ from the loop tree. 1254 LI->removeBlock(Succ); 1255 LPM->deleteSimpleAnalysisValue(Succ, L); 1256 Succ->eraseFromParent(); 1257 ++NumSimplify; 1258 continue; 1259 } 1260 1261 if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){ 1262 // Conditional branch. Turn it into an unconditional branch, then 1263 // remove dead blocks. 1264 continue; // FIXME: Enable. 1265 1266 DEBUG(dbgs() << "Folded branch: " << *BI); 1267 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue()); 1268 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue()); 1269 DeadSucc->removePredecessor(BI->getParent(), true); 1270 Worklist.push_back(BranchInst::Create(LiveSucc, BI)); 1271 LPM->deleteSimpleAnalysisValue(BI, L); 1272 BI->eraseFromParent(); 1273 RemoveFromWorklist(BI, Worklist); 1274 ++NumSimplify; 1275 1276 RemoveBlockIfDead(DeadSucc, Worklist, L); 1277 } 1278 continue; 1279 } 1280 } 1281 } 1282