LoopUnswitch.cpp revision 7b550ccfc5a3346c17e0390a59e2d6d19bc52705
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/ConstantFolding.h" 36#include "llvm/Analysis/InlineCost.h" 37#include "llvm/Analysis/LoopInfo.h" 38#include "llvm/Analysis/LoopPass.h" 39#include "llvm/Analysis/Dominators.h" 40#include "llvm/Transforms/Utils/Cloning.h" 41#include "llvm/Transforms/Utils/Local.h" 42#include "llvm/Transforms/Utils/BasicBlockUtils.h" 43#include "llvm/ADT/Statistic.h" 44#include "llvm/ADT/SmallPtrSet.h" 45#include "llvm/ADT/STLExtras.h" 46#include "llvm/Support/CommandLine.h" 47#include "llvm/Support/Debug.h" 48#include "llvm/Support/raw_ostream.h" 49#include <algorithm> 50#include <set> 51using namespace llvm; 52 53STATISTIC(NumBranches, "Number of branches unswitched"); 54STATISTIC(NumSwitches, "Number of switches unswitched"); 55STATISTIC(NumSelects , "Number of selects unswitched"); 56STATISTIC(NumTrivial , "Number of unswitches that are trivial"); 57STATISTIC(NumSimplify, "Number of simplifications of unswitched code"); 58 59// The specific value of 50 here was chosen based only on intuition and a 60// few specific examples. 61static cl::opt<unsigned> 62Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), 63 cl::init(50), cl::Hidden); 64 65namespace { 66 class LoopUnswitch : public LoopPass { 67 LoopInfo *LI; // Loop information 68 LPPassManager *LPM; 69 70 // LoopProcessWorklist - Used to check if second loop needs processing 71 // after RewriteLoopBodyWithConditionConstant rewrites first loop. 72 std::vector<Loop*> LoopProcessWorklist; 73 SmallPtrSet<Value *,8> UnswitchedVals; 74 75 bool OptimizeForSize; 76 bool redoLoop; 77 78 Loop *currentLoop; 79 DominanceFrontier *DF; 80 DominatorTree *DT; 81 BasicBlock *loopHeader; 82 BasicBlock *loopPreheader; 83 84 // LoopBlocks contains all of the basic blocks of the loop, including the 85 // preheader of the loop, the body of the loop, and the exit blocks of the 86 // loop, in that order. 87 std::vector<BasicBlock*> LoopBlocks; 88 // NewBlocks contained cloned copy of basic blocks from LoopBlocks. 89 std::vector<BasicBlock*> NewBlocks; 90 91 public: 92 static char ID; // Pass ID, replacement for typeid 93 explicit LoopUnswitch(bool Os = false) : 94 LoopPass(&ID), OptimizeForSize(Os), redoLoop(false), 95 currentLoop(NULL), DF(NULL), DT(NULL), loopHeader(NULL), 96 loopPreheader(NULL) {} 97 98 bool runOnLoop(Loop *L, LPPassManager &LPM); 99 bool processCurrentLoop(); 100 101 /// This transformation requires natural loop information & requires that 102 /// loop preheaders be inserted into the CFG... 103 /// 104 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 105 AU.addRequiredID(LoopSimplifyID); 106 AU.addPreservedID(LoopSimplifyID); 107 AU.addRequired<LoopInfo>(); 108 AU.addPreserved<LoopInfo>(); 109 AU.addRequiredID(LCSSAID); 110 AU.addPreservedID(LCSSAID); 111 AU.addPreserved<DominatorTree>(); 112 AU.addPreserved<DominanceFrontier>(); 113 } 114 115 private: 116 117 virtual void releaseMemory() { 118 UnswitchedVals.clear(); 119 } 120 121 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist, 122 /// remove it. 123 void RemoveLoopFromWorklist(Loop *L) { 124 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(), 125 LoopProcessWorklist.end(), L); 126 if (I != LoopProcessWorklist.end()) 127 LoopProcessWorklist.erase(I); 128 } 129 130 void initLoopData() { 131 loopHeader = currentLoop->getHeader(); 132 loopPreheader = currentLoop->getLoopPreheader(); 133 } 134 135 /// Split all of the edges from inside the loop to their exit blocks. 136 /// Update the appropriate Phi nodes as we do so. 137 void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks); 138 139 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val); 140 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 141 BasicBlock *ExitBlock); 142 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L); 143 144 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 145 Constant *Val, bool isEqual); 146 147 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 148 BasicBlock *TrueDest, 149 BasicBlock *FalseDest, 150 Instruction *InsertPt); 151 152 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); 153 void RemoveBlockIfDead(BasicBlock *BB, 154 std::vector<Instruction*> &Worklist, Loop *l); 155 void RemoveLoopFromHierarchy(Loop *L); 156 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0, 157 BasicBlock **LoopExit = 0); 158 159 }; 160} 161char LoopUnswitch::ID = 0; 162static RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops"); 163 164Pass *llvm::createLoopUnswitchPass(bool Os) { 165 return new LoopUnswitch(Os); 166} 167 168/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is 169/// invariant in the loop, or has an invariant piece, return the invariant. 170/// Otherwise, return null. 171static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { 172 // Constants should be folded, not unswitched on! 173 if (isa<Constant>(Cond)) return 0; 174 175 // TODO: Handle: br (VARIANT|INVARIANT). 176 177 // Hoist simple values out. 178 if (L->makeLoopInvariant(Cond, Changed)) 179 return Cond; 180 181 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 182 if (BO->getOpcode() == Instruction::And || 183 BO->getOpcode() == Instruction::Or) { 184 // If either the left or right side is invariant, we can unswitch on this, 185 // which will cause the branch to go away in one loop and the condition to 186 // simplify in the other one. 187 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) 188 return LHS; 189 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) 190 return RHS; 191 } 192 193 return 0; 194} 195 196bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) { 197 LI = &getAnalysis<LoopInfo>(); 198 LPM = &LPM_Ref; 199 DF = getAnalysisIfAvailable<DominanceFrontier>(); 200 DT = getAnalysisIfAvailable<DominatorTree>(); 201 currentLoop = L; 202 Function *F = currentLoop->getHeader()->getParent(); 203 bool Changed = false; 204 do { 205 assert(currentLoop->isLCSSAForm()); 206 redoLoop = false; 207 Changed |= processCurrentLoop(); 208 } while(redoLoop); 209 210 if (Changed) { 211 // FIXME: Reconstruct dom info, because it is not preserved properly. 212 if (DT) 213 DT->runOnFunction(*F); 214 if (DF) 215 DF->runOnFunction(*F); 216 } 217 return Changed; 218} 219 220/// processCurrentLoop - Do actual work and unswitch loop if possible 221/// and profitable. 222bool LoopUnswitch::processCurrentLoop() { 223 bool Changed = false; 224 LLVMContext &Context = currentLoop->getHeader()->getContext(); 225 226 // Loop over all of the basic blocks in the loop. If we find an interior 227 // block that is branching on a loop-invariant condition, we can unswitch this 228 // loop. 229 for (Loop::block_iterator I = currentLoop->block_begin(), 230 E = currentLoop->block_end(); 231 I != E; ++I) { 232 TerminatorInst *TI = (*I)->getTerminator(); 233 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 234 // If this isn't branching on an invariant condition, we can't unswitch 235 // it. 236 if (BI->isConditional()) { 237 // See if this, or some part of it, is loop invariant. If so, we can 238 // unswitch on it if we desire. 239 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), 240 currentLoop, Changed); 241 if (LoopCond && UnswitchIfProfitable(LoopCond, 242 ConstantInt::getTrue(Context))) { 243 ++NumBranches; 244 return true; 245 } 246 } 247 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 248 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 249 currentLoop, Changed); 250 if (LoopCond && SI->getNumCases() > 1) { 251 // Find a value to unswitch on: 252 // FIXME: this should chose the most expensive case! 253 Constant *UnswitchVal = SI->getCaseValue(1); 254 // Do not process same value again and again. 255 if (!UnswitchedVals.insert(UnswitchVal)) 256 continue; 257 258 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) { 259 ++NumSwitches; 260 return true; 261 } 262 } 263 } 264 265 // Scan the instructions to check for unswitchable values. 266 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 267 BBI != E; ++BBI) 268 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 269 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 270 currentLoop, Changed); 271 if (LoopCond && UnswitchIfProfitable(LoopCond, 272 ConstantInt::getTrue(Context))) { 273 ++NumSelects; 274 return true; 275 } 276 } 277 } 278 return Changed; 279} 280 281/// isTrivialLoopExitBlock - Check to see if all paths from BB either: 282/// 1. Exit the loop with no side effects. 283/// 2. Branch to the latch block with no side-effects. 284/// 285/// If these conditions are true, we return true and set ExitBB to the block we 286/// exit through. 287/// 288static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, 289 BasicBlock *&ExitBB, 290 std::set<BasicBlock*> &Visited) { 291 if (!Visited.insert(BB).second) { 292 // Already visited and Ok, end of recursion. 293 return true; 294 } else if (!L->contains(BB)) { 295 // Otherwise, this is a loop exit, this is fine so long as this is the 296 // first exit. 297 if (ExitBB != 0) return false; 298 ExitBB = BB; 299 return true; 300 } 301 302 // Otherwise, this is an unvisited intra-loop node. Check all successors. 303 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { 304 // Check to see if the successor is a trivial loop exit. 305 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) 306 return false; 307 } 308 309 // Okay, everything after this looks good, check to make sure that this block 310 // doesn't include any side effects. 311 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 312 if (I->mayHaveSideEffects()) 313 return false; 314 315 return true; 316} 317 318/// isTrivialLoopExitBlock - Return true if the specified block unconditionally 319/// leads to an exit from the specified loop, and has no side-effects in the 320/// process. If so, return the block that is exited to, otherwise return null. 321static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { 322 std::set<BasicBlock*> Visited; 323 Visited.insert(L->getHeader()); // Branches to header are ok. 324 BasicBlock *ExitBB = 0; 325 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) 326 return ExitBB; 327 return 0; 328} 329 330/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is 331/// trivial: that is, that the condition controls whether or not the loop does 332/// anything at all. If this is a trivial condition, unswitching produces no 333/// code duplications (equivalently, it produces a simpler loop and a new empty 334/// loop, which gets deleted). 335/// 336/// If this is a trivial condition, return true, otherwise return false. When 337/// returning true, this sets Cond and Val to the condition that controls the 338/// trivial condition: when Cond dynamically equals Val, the loop is known to 339/// exit. Finally, this sets LoopExit to the BB that the loop exits to when 340/// Cond == Val. 341/// 342bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val, 343 BasicBlock **LoopExit) { 344 BasicBlock *Header = currentLoop->getHeader(); 345 TerminatorInst *HeaderTerm = Header->getTerminator(); 346 LLVMContext &Context = Header->getContext(); 347 348 BasicBlock *LoopExitBB = 0; 349 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) { 350 // If the header block doesn't end with a conditional branch on Cond, we 351 // can't handle it. 352 if (!BI->isConditional() || BI->getCondition() != Cond) 353 return false; 354 355 // Check to see if a successor of the branch is guaranteed to go to the 356 // latch block or exit through a one exit block without having any 357 // side-effects. If so, determine the value of Cond that causes it to do 358 // this. 359 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 360 BI->getSuccessor(0)))) { 361 if (Val) *Val = ConstantInt::getTrue(Context); 362 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 363 BI->getSuccessor(1)))) { 364 if (Val) *Val = ConstantInt::getFalse(Context); 365 } 366 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) { 367 // If this isn't a switch on Cond, we can't handle it. 368 if (SI->getCondition() != Cond) return false; 369 370 // Check to see if a successor of the switch is guaranteed to go to the 371 // latch block or exit through a one exit block without having any 372 // side-effects. If so, determine the value of Cond that causes it to do 373 // this. Note that we can't trivially unswitch on the default case. 374 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) 375 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 376 SI->getSuccessor(i)))) { 377 // Okay, we found a trivial case, remember the value that is trivial. 378 if (Val) *Val = SI->getCaseValue(i); 379 break; 380 } 381 } 382 383 // If we didn't find a single unique LoopExit block, or if the loop exit block 384 // contains phi nodes, this isn't trivial. 385 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) 386 return false; // Can't handle this. 387 388 if (LoopExit) *LoopExit = LoopExitBB; 389 390 // We already know that nothing uses any scalar values defined inside of this 391 // loop. As such, we just have to check to see if this loop will execute any 392 // side-effecting instructions (e.g. stores, calls, volatile loads) in the 393 // part of the loop that the code *would* execute. We already checked the 394 // tail, check the header now. 395 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 396 if (I->mayHaveSideEffects()) 397 return false; 398 return true; 399} 400 401/// UnswitchIfProfitable - We have found that we can unswitch currentLoop when 402/// LoopCond == Val to simplify the loop. If we decide that this is profitable, 403/// unswitch the loop, reprocess the pieces, then return true. 404bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val){ 405 406 initLoopData(); 407 Function *F = loopHeader->getParent(); 408 409 // If LoopSimplify was unable to form a preheader, don't do any unswitching. 410 if (!loopPreheader) 411 return false; 412 413 // If the condition is trivial, always unswitch. There is no code growth for 414 // this case. 415 if (!IsTrivialUnswitchCondition(LoopCond)) { 416 // Check to see if it would be profitable to unswitch current loop. 417 418 // Do not do non-trivial unswitch while optimizing for size. 419 if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize)) 420 return false; 421 422 // FIXME: This is overly conservative because it does not take into 423 // consideration code simplification opportunities and code that can 424 // be shared by the resultant unswitched loops. 425 CodeMetrics Metrics; 426 for (Loop::block_iterator I = currentLoop->block_begin(), 427 E = currentLoop->block_end(); 428 I != E; ++I) 429 Metrics.analyzeBasicBlock(*I); 430 431 // Limit the number of instructions to avoid causing significant code 432 // expansion, and the number of basic blocks, to avoid loops with 433 // large numbers of branches which cause loop unswitching to go crazy. 434 // This is a very ad-hoc heuristic. 435 if (Metrics.NumInsts > Threshold || 436 Metrics.NumBlocks * 5 > Threshold || 437 Metrics.NeverInline) { 438 DEBUG(errs() << "NOT unswitching loop %" 439 << currentLoop->getHeader()->getName() << ", cost too high: " 440 << currentLoop->getBlocks().size() << "\n"); 441 return false; 442 } 443 } 444 445 Constant *CondVal; 446 BasicBlock *ExitBlock; 447 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) { 448 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock); 449 } else { 450 UnswitchNontrivialCondition(LoopCond, Val, currentLoop); 451 } 452 453 return true; 454} 455 456// RemapInstruction - Convert the instruction operands from referencing the 457// current values into those specified by ValueMap. 458// 459static inline void RemapInstruction(Instruction *I, 460 DenseMap<const Value *, Value*> &ValueMap) { 461 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 462 Value *Op = I->getOperand(op); 463 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op); 464 if (It != ValueMap.end()) Op = It->second; 465 I->setOperand(op, Op); 466 } 467} 468 469/// CloneLoop - Recursively clone the specified loop and all of its children, 470/// mapping the blocks with the specified map. 471static Loop *CloneLoop(Loop *L, Loop *PL, DenseMap<const Value*, Value*> &VM, 472 LoopInfo *LI, LPPassManager *LPM) { 473 Loop *New = new Loop(); 474 475 LPM->insertLoop(New, PL); 476 477 // Add all of the blocks in L to the new loop. 478 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 479 I != E; ++I) 480 if (LI->getLoopFor(*I) == L) 481 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase()); 482 483 // Add all of the subloops to the new loop. 484 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 485 CloneLoop(*I, New, VM, LI, LPM); 486 487 return New; 488} 489 490/// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values 491/// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the 492/// code immediately before InsertPt. 493void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 494 BasicBlock *TrueDest, 495 BasicBlock *FalseDest, 496 Instruction *InsertPt) { 497 // Insert a conditional branch on LIC to the two preheaders. The original 498 // code is the true version and the new code is the false version. 499 Value *BranchVal = LIC; 500 if (!isa<ConstantInt>(Val) || 501 Val->getType() != Type::getInt1Ty(LIC->getContext())) 502 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val, "tmp"); 503 else if (Val != ConstantInt::getTrue(Val->getContext())) 504 // We want to enter the new loop when the condition is true. 505 std::swap(TrueDest, FalseDest); 506 507 // Insert the new branch. 508 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt); 509 510 // If either edge is critical, split it. This helps preserve LoopSimplify 511 // form for enclosing loops. 512 SplitCriticalEdge(BI, 0, this); 513 SplitCriticalEdge(BI, 1, this); 514} 515 516/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable 517/// condition in it (a cond branch from its header block to its latch block, 518/// where the path through the loop that doesn't execute its body has no 519/// side-effects), unswitch it. This doesn't involve any code duplication, just 520/// moving the conditional branch outside of the loop and updating loop info. 521void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, 522 Constant *Val, 523 BasicBlock *ExitBlock) { 524 DEBUG(errs() << "loop-unswitch: Trivial-Unswitch loop %" 525 << loopHeader->getName() << " [" << L->getBlocks().size() 526 << " blocks] in Function " << L->getHeader()->getParent()->getName() 527 << " on cond: " << *Val << " == " << *Cond << "\n"); 528 529 // First step, split the preheader, so that we know that there is a safe place 530 // to insert the conditional branch. We will change loopPreheader to have a 531 // conditional branch on Cond. 532 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this); 533 534 // Now that we have a place to insert the conditional branch, create a place 535 // to branch to: this is the exit block out of the loop that we should 536 // short-circuit to. 537 538 // Split this block now, so that the loop maintains its exit block, and so 539 // that the jump from the preheader can execute the contents of the exit block 540 // without actually branching to it (the exit block should be dominated by the 541 // loop header, not the preheader). 542 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 543 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this); 544 545 // Okay, now we have a position to branch from and a position to branch to, 546 // insert the new conditional branch. 547 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, 548 loopPreheader->getTerminator()); 549 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L); 550 loopPreheader->getTerminator()->eraseFromParent(); 551 552 // We need to reprocess this loop, it could be unswitched again. 553 redoLoop = true; 554 555 // Now that we know that the loop is never entered when this condition is a 556 // particular value, rewrite the loop with this info. We know that this will 557 // at least eliminate the old branch. 558 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false); 559 ++NumTrivial; 560} 561 562/// SplitExitEdges - Split all of the edges from inside the loop to their exit 563/// blocks. Update the appropriate Phi nodes as we do so. 564void LoopUnswitch::SplitExitEdges(Loop *L, 565 const SmallVector<BasicBlock *, 8> &ExitBlocks) 566{ 567 568 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 569 BasicBlock *ExitBlock = ExitBlocks[i]; 570 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock), 571 pred_end(ExitBlock)); 572 SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(), 573 ".us-lcssa", this); 574 } 575} 576 577/// UnswitchNontrivialCondition - We determined that the loop is profitable 578/// to unswitch when LIC equal Val. Split it into loop versions and test the 579/// condition outside of either loop. Return the loops created as Out1/Out2. 580void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, 581 Loop *L) { 582 Function *F = loopHeader->getParent(); 583 DEBUG(errs() << "loop-unswitch: Unswitching loop %" 584 << loopHeader->getName() << " [" << L->getBlocks().size() 585 << " blocks] in Function " << F->getName() 586 << " when '" << *Val << "' == " << *LIC << "\n"); 587 588 LoopBlocks.clear(); 589 NewBlocks.clear(); 590 591 // First step, split the preheader and exit blocks, and add these blocks to 592 // the LoopBlocks list. 593 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this); 594 LoopBlocks.push_back(NewPreheader); 595 596 // We want the loop to come after the preheader, but before the exit blocks. 597 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 598 599 SmallVector<BasicBlock*, 8> ExitBlocks; 600 L->getUniqueExitBlocks(ExitBlocks); 601 602 // Split all of the edges from inside the loop to their exit blocks. Update 603 // the appropriate Phi nodes as we do so. 604 SplitExitEdges(L, ExitBlocks); 605 606 // The exit blocks may have been changed due to edge splitting, recompute. 607 ExitBlocks.clear(); 608 L->getUniqueExitBlocks(ExitBlocks); 609 610 // Add exit blocks to the loop blocks. 611 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 612 613 // Next step, clone all of the basic blocks that make up the loop (including 614 // the loop preheader and exit blocks), keeping track of the mapping between 615 // the instructions and blocks. 616 NewBlocks.reserve(LoopBlocks.size()); 617 DenseMap<const Value*, Value*> ValueMap; 618 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 619 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); 620 NewBlocks.push_back(New); 621 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping. 622 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], New, L); 623 } 624 625 // Splice the newly inserted blocks into the function right before the 626 // original preheader. 627 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(), 628 NewBlocks[0], F->end()); 629 630 // Now we create the new Loop object for the versioned loop. 631 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM); 632 Loop *ParentLoop = L->getParentLoop(); 633 if (ParentLoop) { 634 // Make sure to add the cloned preheader and exit blocks to the parent loop 635 // as well. 636 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase()); 637 } 638 639 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 640 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]); 641 // The new exit block should be in the same loop as the old one. 642 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i])) 643 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase()); 644 645 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 646 "Exit block should have been split to have one successor!"); 647 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 648 649 // If the successor of the exit block had PHI nodes, add an entry for 650 // NewExit. 651 PHINode *PN; 652 for (BasicBlock::iterator I = ExitSucc->begin(); 653 (PN = dyn_cast<PHINode>(I)); ++I) { 654 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); 655 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V); 656 if (It != ValueMap.end()) V = It->second; 657 PN->addIncoming(V, NewExit); 658 } 659 } 660 661 // Rewrite the code to refer to itself. 662 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) 663 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 664 E = NewBlocks[i]->end(); I != E; ++I) 665 RemapInstruction(I, ValueMap); 666 667 // Rewrite the original preheader to select between versions of the loop. 668 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator()); 669 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 670 "Preheader splitting did not work correctly!"); 671 672 // Emit the new branch that selects between the two versions of this loop. 673 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); 674 LPM->deleteSimpleAnalysisValue(OldBR, L); 675 OldBR->eraseFromParent(); 676 677 LoopProcessWorklist.push_back(NewLoop); 678 redoLoop = true; 679 680 // Now we rewrite the original code to know that the condition is true and the 681 // new code to know that the condition is false. 682 RewriteLoopBodyWithConditionConstant(L , LIC, Val, false); 683 684 // It's possible that simplifying one loop could cause the other to be 685 // deleted. If so, don't simplify it. 686 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop) 687 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); 688 689} 690 691/// RemoveFromWorklist - Remove all instances of I from the worklist vector 692/// specified. 693static void RemoveFromWorklist(Instruction *I, 694 std::vector<Instruction*> &Worklist) { 695 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(), 696 Worklist.end(), I); 697 while (WI != Worklist.end()) { 698 unsigned Offset = WI-Worklist.begin(); 699 Worklist.erase(WI); 700 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I); 701 } 702} 703 704/// ReplaceUsesOfWith - When we find that I really equals V, remove I from the 705/// program, replacing all uses with V and update the worklist. 706static void ReplaceUsesOfWith(Instruction *I, Value *V, 707 std::vector<Instruction*> &Worklist, 708 Loop *L, LPPassManager *LPM) { 709 DEBUG(errs() << "Replace with '" << *V << "': " << *I); 710 711 // Add uses to the worklist, which may be dead now. 712 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 713 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 714 Worklist.push_back(Use); 715 716 // Add users to the worklist which may be simplified now. 717 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 718 UI != E; ++UI) 719 Worklist.push_back(cast<Instruction>(*UI)); 720 LPM->deleteSimpleAnalysisValue(I, L); 721 RemoveFromWorklist(I, Worklist); 722 I->replaceAllUsesWith(V); 723 I->eraseFromParent(); 724 ++NumSimplify; 725} 726 727/// RemoveBlockIfDead - If the specified block is dead, remove it, update loop 728/// information, and remove any dead successors it has. 729/// 730void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB, 731 std::vector<Instruction*> &Worklist, 732 Loop *L) { 733 if (pred_begin(BB) != pred_end(BB)) { 734 // This block isn't dead, since an edge to BB was just removed, see if there 735 // are any easy simplifications we can do now. 736 if (BasicBlock *Pred = BB->getSinglePredecessor()) { 737 // If it has one pred, fold phi nodes in BB. 738 while (isa<PHINode>(BB->begin())) 739 ReplaceUsesOfWith(BB->begin(), 740 cast<PHINode>(BB->begin())->getIncomingValue(0), 741 Worklist, L, LPM); 742 743 // If this is the header of a loop and the only pred is the latch, we now 744 // have an unreachable loop. 745 if (Loop *L = LI->getLoopFor(BB)) 746 if (loopHeader == BB && L->contains(Pred)) { 747 // Remove the branch from the latch to the header block, this makes 748 // the header dead, which will make the latch dead (because the header 749 // dominates the latch). 750 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L); 751 Pred->getTerminator()->eraseFromParent(); 752 new UnreachableInst(BB->getContext(), Pred); 753 754 // The loop is now broken, remove it from LI. 755 RemoveLoopFromHierarchy(L); 756 757 // Reprocess the header, which now IS dead. 758 RemoveBlockIfDead(BB, Worklist, L); 759 return; 760 } 761 762 // If pred ends in a uncond branch, add uncond branch to worklist so that 763 // the two blocks will get merged. 764 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) 765 if (BI->isUnconditional()) 766 Worklist.push_back(BI); 767 } 768 return; 769 } 770 771 DEBUG(errs() << "Nuking dead block: " << *BB); 772 773 // Remove the instructions in the basic block from the worklist. 774 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 775 RemoveFromWorklist(I, Worklist); 776 777 // Anything that uses the instructions in this basic block should have their 778 // uses replaced with undefs. 779 // If I is not void type then replaceAllUsesWith undef. 780 // This allows ValueHandlers and custom metadata to adjust itself. 781 if (!I->getType()->isVoidTy()) 782 I->replaceAllUsesWith(UndefValue::get(I->getType())); 783 } 784 785 // If this is the edge to the header block for a loop, remove the loop and 786 // promote all subloops. 787 if (Loop *BBLoop = LI->getLoopFor(BB)) { 788 if (BBLoop->getLoopLatch() == BB) 789 RemoveLoopFromHierarchy(BBLoop); 790 } 791 792 // Remove the block from the loop info, which removes it from any loops it 793 // was in. 794 LI->removeBlock(BB); 795 796 797 // Remove phi node entries in successors for this block. 798 TerminatorInst *TI = BB->getTerminator(); 799 SmallVector<BasicBlock*, 4> Succs; 800 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) { 801 Succs.push_back(TI->getSuccessor(i)); 802 TI->getSuccessor(i)->removePredecessor(BB); 803 } 804 805 // Unique the successors, remove anything with multiple uses. 806 array_pod_sort(Succs.begin(), Succs.end()); 807 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end()); 808 809 // Remove the basic block, including all of the instructions contained in it. 810 LPM->deleteSimpleAnalysisValue(BB, L); 811 BB->eraseFromParent(); 812 // Remove successor blocks here that are not dead, so that we know we only 813 // have dead blocks in this list. Nondead blocks have a way of becoming dead, 814 // then getting removed before we revisit them, which is badness. 815 // 816 for (unsigned i = 0; i != Succs.size(); ++i) 817 if (pred_begin(Succs[i]) != pred_end(Succs[i])) { 818 // One exception is loop headers. If this block was the preheader for a 819 // loop, then we DO want to visit the loop so the loop gets deleted. 820 // We know that if the successor is a loop header, that this loop had to 821 // be the preheader: the case where this was the latch block was handled 822 // above and headers can only have two predecessors. 823 if (!LI->isLoopHeader(Succs[i])) { 824 Succs.erase(Succs.begin()+i); 825 --i; 826 } 827 } 828 829 for (unsigned i = 0, e = Succs.size(); i != e; ++i) 830 RemoveBlockIfDead(Succs[i], Worklist, L); 831} 832 833/// RemoveLoopFromHierarchy - We have discovered that the specified loop has 834/// become unwrapped, either because the backedge was deleted, or because the 835/// edge into the header was removed. If the edge into the header from the 836/// latch block was removed, the loop is unwrapped but subloops are still alive, 837/// so they just reparent loops. If the loops are actually dead, they will be 838/// removed later. 839void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) { 840 LPM->deleteLoopFromQueue(L); 841 RemoveLoopFromWorklist(L); 842} 843 844// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has 845// the value specified by Val in the specified loop, or we know it does NOT have 846// that value. Rewrite any uses of LIC or of properties correlated to it. 847void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 848 Constant *Val, 849 bool IsEqual) { 850 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 851 852 // FIXME: Support correlated properties, like: 853 // for (...) 854 // if (li1 < li2) 855 // ... 856 // if (li1 > li2) 857 // ... 858 859 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 860 // selects, switches. 861 std::vector<User*> Users(LIC->use_begin(), LIC->use_end()); 862 std::vector<Instruction*> Worklist; 863 LLVMContext &Context = Val->getContext(); 864 865 866 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC 867 // in the loop with the appropriate one directly. 868 if (IsEqual || (isa<ConstantInt>(Val) && 869 Val->getType() == Type::getInt1Ty(Val->getContext()))) { 870 Value *Replacement; 871 if (IsEqual) 872 Replacement = Val; 873 else 874 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()), 875 !cast<ConstantInt>(Val)->getZExtValue()); 876 877 for (unsigned i = 0, e = Users.size(); i != e; ++i) 878 if (Instruction *U = cast<Instruction>(Users[i])) { 879 if (!L->contains(U->getParent())) 880 continue; 881 U->replaceUsesOfWith(LIC, Replacement); 882 Worklist.push_back(U); 883 } 884 } else { 885 // Otherwise, we don't know the precise value of LIC, but we do know that it 886 // is certainly NOT "Val". As such, simplify any uses in the loop that we 887 // can. This case occurs when we unswitch switch statements. 888 for (unsigned i = 0, e = Users.size(); i != e; ++i) 889 if (Instruction *U = cast<Instruction>(Users[i])) { 890 if (!L->contains(U->getParent())) 891 continue; 892 893 Worklist.push_back(U); 894 895 // If we know that LIC is not Val, use this info to simplify code. 896 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) { 897 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) { 898 if (SI->getCaseValue(i) == Val) { 899 // Found a dead case value. Don't remove PHI nodes in the 900 // successor if they become single-entry, those PHI nodes may 901 // be in the Users list. 902 903 // FIXME: This is a hack. We need to keep the successor around 904 // and hooked up so as to preserve the loop structure, because 905 // trying to update it is complicated. So instead we preserve the 906 // loop structure and put the block on a dead code path. 907 BasicBlock *Switch = SI->getParent(); 908 SplitEdge(Switch, SI->getSuccessor(i), this); 909 // Compute the successors instead of relying on the return value 910 // of SplitEdge, since it may have split the switch successor 911 // after PHI nodes. 912 BasicBlock *NewSISucc = SI->getSuccessor(i); 913 BasicBlock *OldSISucc = *succ_begin(NewSISucc); 914 // Create an "unreachable" destination. 915 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable", 916 Switch->getParent(), 917 OldSISucc); 918 new UnreachableInst(Context, Abort); 919 // Force the new case destination to branch to the "unreachable" 920 // block while maintaining a (dead) CFG edge to the old block. 921 NewSISucc->getTerminator()->eraseFromParent(); 922 BranchInst::Create(Abort, OldSISucc, 923 ConstantInt::getTrue(Context), NewSISucc); 924 // Release the PHI operands for this edge. 925 for (BasicBlock::iterator II = NewSISucc->begin(); 926 PHINode *PN = dyn_cast<PHINode>(II); ++II) 927 PN->setIncomingValue(PN->getBasicBlockIndex(Switch), 928 UndefValue::get(PN->getType())); 929 // Tell the domtree about the new block. We don't fully update the 930 // domtree here -- instead we force it to do a full recomputation 931 // after the pass is complete -- but we do need to inform it of 932 // new blocks. 933 if (DT) 934 DT->addNewBlock(Abort, NewSISucc); 935 break; 936 } 937 } 938 } 939 940 // TODO: We could do other simplifications, for example, turning 941 // LIC == Val -> false. 942 } 943 } 944 945 SimplifyCode(Worklist, L); 946} 947 948/// SimplifyCode - Okay, now that we have simplified some instructions in the 949/// loop, walk over it and constant prop, dce, and fold control flow where 950/// possible. Note that this is effectively a very simple loop-structure-aware 951/// optimizer. During processing of this loop, L could very well be deleted, so 952/// it must not be used. 953/// 954/// FIXME: When the loop optimizer is more mature, separate this out to a new 955/// pass. 956/// 957void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) { 958 while (!Worklist.empty()) { 959 Instruction *I = Worklist.back(); 960 Worklist.pop_back(); 961 962 // Simple constant folding. 963 if (Constant *C = ConstantFoldInstruction(I)) { 964 ReplaceUsesOfWith(I, C, Worklist, L, LPM); 965 continue; 966 } 967 968 // Simple DCE. 969 if (isInstructionTriviallyDead(I)) { 970 DEBUG(errs() << "Remove dead instruction '" << *I); 971 972 // Add uses to the worklist, which may be dead now. 973 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 974 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 975 Worklist.push_back(Use); 976 LPM->deleteSimpleAnalysisValue(I, L); 977 RemoveFromWorklist(I, Worklist); 978 I->eraseFromParent(); 979 ++NumSimplify; 980 continue; 981 } 982 983 // Special case hacks that appear commonly in unswitched code. 984 switch (I->getOpcode()) { 985 case Instruction::Select: 986 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(0))) { 987 ReplaceUsesOfWith(I, I->getOperand(!CB->getZExtValue()+1), Worklist, L, 988 LPM); 989 continue; 990 } 991 break; 992 case Instruction::And: 993 if (isa<ConstantInt>(I->getOperand(0)) && 994 // constant -> RHS 995 I->getOperand(0)->getType() == Type::getInt1Ty(I->getContext())) 996 cast<BinaryOperator>(I)->swapOperands(); 997 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1))) 998 if (CB->getType() == Type::getInt1Ty(I->getContext())) { 999 if (CB->isOne()) // X & 1 -> X 1000 ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM); 1001 else // X & 0 -> 0 1002 ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM); 1003 continue; 1004 } 1005 break; 1006 case Instruction::Or: 1007 if (isa<ConstantInt>(I->getOperand(0)) && 1008 // constant -> RHS 1009 I->getOperand(0)->getType() == Type::getInt1Ty(I->getContext())) 1010 cast<BinaryOperator>(I)->swapOperands(); 1011 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1))) 1012 if (CB->getType() == Type::getInt1Ty(I->getContext())) { 1013 if (CB->isOne()) // X | 1 -> 1 1014 ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM); 1015 else // X | 0 -> X 1016 ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM); 1017 continue; 1018 } 1019 break; 1020 case Instruction::Br: { 1021 BranchInst *BI = cast<BranchInst>(I); 1022 if (BI->isUnconditional()) { 1023 // If BI's parent is the only pred of the successor, fold the two blocks 1024 // together. 1025 BasicBlock *Pred = BI->getParent(); 1026 BasicBlock *Succ = BI->getSuccessor(0); 1027 BasicBlock *SinglePred = Succ->getSinglePredecessor(); 1028 if (!SinglePred) continue; // Nothing to do. 1029 assert(SinglePred == Pred && "CFG broken"); 1030 1031 DEBUG(errs() << "Merging blocks: " << Pred->getName() << " <- " 1032 << Succ->getName() << "\n"); 1033 1034 // Resolve any single entry PHI nodes in Succ. 1035 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) 1036 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM); 1037 1038 // Move all of the successor contents from Succ to Pred. 1039 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(), 1040 Succ->end()); 1041 LPM->deleteSimpleAnalysisValue(BI, L); 1042 BI->eraseFromParent(); 1043 RemoveFromWorklist(BI, Worklist); 1044 1045 // If Succ has any successors with PHI nodes, update them to have 1046 // entries coming from Pred instead of Succ. 1047 Succ->replaceAllUsesWith(Pred); 1048 1049 // Remove Succ from the loop tree. 1050 LI->removeBlock(Succ); 1051 LPM->deleteSimpleAnalysisValue(Succ, L); 1052 Succ->eraseFromParent(); 1053 ++NumSimplify; 1054 } else if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){ 1055 // Conditional branch. Turn it into an unconditional branch, then 1056 // remove dead blocks. 1057 break; // FIXME: Enable. 1058 1059 DEBUG(errs() << "Folded branch: " << *BI); 1060 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue()); 1061 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue()); 1062 DeadSucc->removePredecessor(BI->getParent(), true); 1063 Worklist.push_back(BranchInst::Create(LiveSucc, BI)); 1064 LPM->deleteSimpleAnalysisValue(BI, L); 1065 BI->eraseFromParent(); 1066 RemoveFromWorklist(BI, Worklist); 1067 ++NumSimplify; 1068 1069 RemoveBlockIfDead(DeadSucc, Worklist, L); 1070 } 1071 break; 1072 } 1073 } 1074 } 1075} 1076