LoopUnswitch.cpp revision 508955156a25a9abc470a29e1760aa176d341cf9
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/LLVMContext.h" 36#include "llvm/Analysis/ConstantFolding.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/Compiler.h" 48#include "llvm/Support/Debug.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 59static cl::opt<unsigned> 60Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), 61 cl::init(10), cl::Hidden); 62 63namespace { 64 class VISIBILITY_HIDDEN LoopUnswitch : public LoopPass { 65 LoopInfo *LI; // Loop information 66 LPPassManager *LPM; 67 68 // LoopProcessWorklist - Used to check if second loop needs processing 69 // after RewriteLoopBodyWithConditionConstant rewrites first loop. 70 std::vector<Loop*> LoopProcessWorklist; 71 SmallPtrSet<Value *,8> UnswitchedVals; 72 73 bool OptimizeForSize; 74 bool redoLoop; 75 76 Loop *currentLoop; 77 DominanceFrontier *DF; 78 DominatorTree *DT; 79 BasicBlock *loopHeader; 80 BasicBlock *loopPreheader; 81 82 // LoopBlocks contains all of the basic blocks of the loop, including the 83 // preheader of the loop, the body of the loop, and the exit blocks of the 84 // loop, in that order. 85 std::vector<BasicBlock*> LoopBlocks; 86 // NewBlocks contained cloned copy of basic blocks from LoopBlocks. 87 std::vector<BasicBlock*> NewBlocks; 88 89 public: 90 static char ID; // Pass ID, replacement for typeid 91 explicit LoopUnswitch(bool Os = false) : 92 LoopPass(&ID), OptimizeForSize(Os), redoLoop(false), 93 currentLoop(NULL), DF(NULL), DT(NULL), loopHeader(NULL), 94 loopPreheader(NULL) {} 95 96 bool runOnLoop(Loop *L, LPPassManager &LPM); 97 bool processCurrentLoop(); 98 99 /// This transformation requires natural loop information & requires that 100 /// loop preheaders be inserted into the CFG... 101 /// 102 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 103 AU.addRequiredID(LoopSimplifyID); 104 AU.addPreservedID(LoopSimplifyID); 105 AU.addRequired<LoopInfo>(); 106 AU.addPreserved<LoopInfo>(); 107 AU.addRequiredID(LCSSAID); 108 AU.addPreservedID(LCSSAID); 109 AU.addPreserved<DominatorTree>(); 110 AU.addPreserved<DominanceFrontier>(); 111 } 112 113 private: 114 115 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist, 116 /// remove it. 117 void RemoveLoopFromWorklist(Loop *L) { 118 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(), 119 LoopProcessWorklist.end(), L); 120 if (I != LoopProcessWorklist.end()) 121 LoopProcessWorklist.erase(I); 122 } 123 124 void initLoopData() { 125 loopHeader = currentLoop->getHeader(); 126 loopPreheader = currentLoop->getLoopPreheader(); 127 } 128 129 /// Split all of the edges from inside the loop to their exit blocks. 130 /// Update the appropriate Phi nodes as we do so. 131 void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks); 132 133 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val); 134 unsigned getLoopUnswitchCost(Value *LIC); 135 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 136 BasicBlock *ExitBlock); 137 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L); 138 139 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 140 Constant *Val, bool isEqual); 141 142 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 143 BasicBlock *TrueDest, 144 BasicBlock *FalseDest, 145 Instruction *InsertPt); 146 147 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); 148 void RemoveBlockIfDead(BasicBlock *BB, 149 std::vector<Instruction*> &Worklist, Loop *l); 150 void RemoveLoopFromHierarchy(Loop *L); 151 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0, 152 BasicBlock **LoopExit = 0); 153 154 }; 155} 156char LoopUnswitch::ID = 0; 157static RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops"); 158 159Pass *llvm::createLoopUnswitchPass(bool Os) { 160 return new LoopUnswitch(Os); 161} 162 163/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is 164/// invariant in the loop, or has an invariant piece, return the invariant. 165/// Otherwise, return null. 166static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { 167 // Constants should be folded, not unswitched on! 168 if (isa<Constant>(Cond)) return 0; 169 170 // TODO: Handle: br (VARIANT|INVARIANT). 171 // TODO: Hoist simple expressions out of loops. 172 if (L->isLoopInvariant(Cond)) return Cond; 173 174 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 175 if (BO->getOpcode() == Instruction::And || 176 BO->getOpcode() == Instruction::Or) { 177 // If either the left or right side is invariant, we can unswitch on this, 178 // which will cause the branch to go away in one loop and the condition to 179 // simplify in the other one. 180 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) 181 return LHS; 182 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) 183 return RHS; 184 } 185 186 return 0; 187} 188 189bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) { 190 LI = &getAnalysis<LoopInfo>(); 191 LPM = &LPM_Ref; 192 DF = getAnalysisIfAvailable<DominanceFrontier>(); 193 DT = getAnalysisIfAvailable<DominatorTree>(); 194 currentLoop = L; 195 Function *F = currentLoop->getHeader()->getParent(); 196 bool Changed = false; 197 do { 198 assert(currentLoop->isLCSSAForm()); 199 redoLoop = false; 200 Changed |= processCurrentLoop(); 201 } while(redoLoop); 202 203 if (Changed) { 204 // FIXME: Reconstruct dom info, because it is not preserved properly. 205 if (DT) 206 DT->runOnFunction(*F); 207 if (DF) 208 DF->runOnFunction(*F); 209 } 210 return Changed; 211} 212 213/// processCurrentLoop - Do actual work and unswitch loop if possible 214/// and profitable. 215bool LoopUnswitch::processCurrentLoop() { 216 bool Changed = false; 217 218 // Loop over all of the basic blocks in the loop. If we find an interior 219 // block that is branching on a loop-invariant condition, we can unswitch this 220 // loop. 221 for (Loop::block_iterator I = currentLoop->block_begin(), 222 E = currentLoop->block_end(); 223 I != E; ++I) { 224 TerminatorInst *TI = (*I)->getTerminator(); 225 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 226 // If this isn't branching on an invariant condition, we can't unswitch 227 // it. 228 if (BI->isConditional()) { 229 // See if this, or some part of it, is loop invariant. If so, we can 230 // unswitch on it if we desire. 231 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), 232 currentLoop, Changed); 233 if (LoopCond && UnswitchIfProfitable(LoopCond, 234 Context->getConstantIntTrue())) { 235 ++NumBranches; 236 return true; 237 } 238 } 239 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 240 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 241 currentLoop, Changed); 242 if (LoopCond && SI->getNumCases() > 1) { 243 // Find a value to unswitch on: 244 // FIXME: this should chose the most expensive case! 245 Constant *UnswitchVal = SI->getCaseValue(1); 246 // Do not process same value again and again. 247 if (!UnswitchedVals.insert(UnswitchVal)) 248 continue; 249 250 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) { 251 ++NumSwitches; 252 return true; 253 } 254 } 255 } 256 257 // Scan the instructions to check for unswitchable values. 258 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 259 BBI != E; ++BBI) 260 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 261 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 262 currentLoop, Changed); 263 if (LoopCond && UnswitchIfProfitable(LoopCond, 264 Context->getConstantIntTrue())) { 265 ++NumSelects; 266 return true; 267 } 268 } 269 } 270 return Changed; 271} 272 273/// isTrivialLoopExitBlock - Check to see if all paths from BB either: 274/// 1. Exit the loop with no side effects. 275/// 2. Branch to the latch block with no side-effects. 276/// 277/// If these conditions are true, we return true and set ExitBB to the block we 278/// exit through. 279/// 280static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, 281 BasicBlock *&ExitBB, 282 std::set<BasicBlock*> &Visited) { 283 if (!Visited.insert(BB).second) { 284 // Already visited and Ok, end of recursion. 285 return true; 286 } else if (!L->contains(BB)) { 287 // Otherwise, this is a loop exit, this is fine so long as this is the 288 // first exit. 289 if (ExitBB != 0) return false; 290 ExitBB = BB; 291 return true; 292 } 293 294 // Otherwise, this is an unvisited intra-loop node. Check all successors. 295 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { 296 // Check to see if the successor is a trivial loop exit. 297 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) 298 return false; 299 } 300 301 // Okay, everything after this looks good, check to make sure that this block 302 // doesn't include any side effects. 303 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 304 if (I->mayHaveSideEffects()) 305 return false; 306 307 return true; 308} 309 310/// isTrivialLoopExitBlock - Return true if the specified block unconditionally 311/// leads to an exit from the specified loop, and has no side-effects in the 312/// process. If so, return the block that is exited to, otherwise return null. 313static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { 314 std::set<BasicBlock*> Visited; 315 Visited.insert(L->getHeader()); // Branches to header are ok. 316 BasicBlock *ExitBB = 0; 317 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) 318 return ExitBB; 319 return 0; 320} 321 322/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is 323/// trivial: that is, that the condition controls whether or not the loop does 324/// anything at all. If this is a trivial condition, unswitching produces no 325/// code duplications (equivalently, it produces a simpler loop and a new empty 326/// loop, which gets deleted). 327/// 328/// If this is a trivial condition, return true, otherwise return false. When 329/// returning true, this sets Cond and Val to the condition that controls the 330/// trivial condition: when Cond dynamically equals Val, the loop is known to 331/// exit. Finally, this sets LoopExit to the BB that the loop exits to when 332/// Cond == Val. 333/// 334bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val, 335 BasicBlock **LoopExit) { 336 BasicBlock *Header = currentLoop->getHeader(); 337 TerminatorInst *HeaderTerm = Header->getTerminator(); 338 339 BasicBlock *LoopExitBB = 0; 340 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) { 341 // If the header block doesn't end with a conditional branch on Cond, we 342 // can't handle it. 343 if (!BI->isConditional() || BI->getCondition() != Cond) 344 return false; 345 346 // Check to see if a successor of the branch is guaranteed to go to the 347 // latch block or exit through a one exit block without having any 348 // side-effects. If so, determine the value of Cond that causes it to do 349 // this. 350 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 351 BI->getSuccessor(0)))) { 352 if (Val) *Val = Context->getConstantIntTrue(); 353 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 354 BI->getSuccessor(1)))) { 355 if (Val) *Val = Context->getConstantIntFalse(); 356 } 357 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) { 358 // If this isn't a switch on Cond, we can't handle it. 359 if (SI->getCondition() != Cond) return false; 360 361 // Check to see if a successor of the switch is guaranteed to go to the 362 // latch block or exit through a one exit block without having any 363 // side-effects. If so, determine the value of Cond that causes it to do 364 // this. Note that we can't trivially unswitch on the default case. 365 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) 366 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 367 SI->getSuccessor(i)))) { 368 // Okay, we found a trivial case, remember the value that is trivial. 369 if (Val) *Val = SI->getCaseValue(i); 370 break; 371 } 372 } 373 374 // If we didn't find a single unique LoopExit block, or if the loop exit block 375 // contains phi nodes, this isn't trivial. 376 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) 377 return false; // Can't handle this. 378 379 if (LoopExit) *LoopExit = LoopExitBB; 380 381 // We already know that nothing uses any scalar values defined inside of this 382 // loop. As such, we just have to check to see if this loop will execute any 383 // side-effecting instructions (e.g. stores, calls, volatile loads) in the 384 // part of the loop that the code *would* execute. We already checked the 385 // tail, check the header now. 386 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 387 if (I->mayHaveSideEffects()) 388 return false; 389 return true; 390} 391 392/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if 393/// we choose to unswitch current loop on the specified value. 394/// 395unsigned LoopUnswitch::getLoopUnswitchCost(Value *LIC) { 396 // If the condition is trivial, always unswitch. There is no code growth for 397 // this case. 398 if (IsTrivialUnswitchCondition(LIC)) 399 return 0; 400 401 // FIXME: This is really overly conservative. However, more liberal 402 // estimations have thus far resulted in excessive unswitching, which is bad 403 // both in compile time and in code size. This should be replaced once 404 // someone figures out how a good estimation. 405 return currentLoop->getBlocks().size(); 406 407 unsigned Cost = 0; 408 // FIXME: this is brain dead. It should take into consideration code 409 // shrinkage. 410 for (Loop::block_iterator I = currentLoop->block_begin(), 411 E = currentLoop->block_end(); 412 I != E; ++I) { 413 BasicBlock *BB = *I; 414 // Do not include empty blocks in the cost calculation. This happen due to 415 // loop canonicalization and will be removed. 416 if (BB->begin() == BasicBlock::iterator(BB->getTerminator())) 417 continue; 418 419 // Count basic blocks. 420 ++Cost; 421 } 422 423 return Cost; 424} 425 426/// UnswitchIfProfitable - We have found that we can unswitch currentLoop when 427/// LoopCond == Val to simplify the loop. If we decide that this is profitable, 428/// unswitch the loop, reprocess the pieces, then return true. 429bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val){ 430 431 initLoopData(); 432 Function *F = loopHeader->getParent(); 433 434 435 // Check to see if it would be profitable to unswitch current loop. 436 unsigned Cost = getLoopUnswitchCost(LoopCond); 437 438 // Do not do non-trivial unswitch while optimizing for size. 439 if (Cost && OptimizeForSize) 440 return false; 441 if (Cost && !F->isDeclaration() && F->hasFnAttr(Attribute::OptimizeForSize)) 442 return false; 443 444 if (Cost > Threshold) { 445 // FIXME: this should estimate growth by the amount of code shared by the 446 // resultant unswitched loops. 447 // 448 DOUT << "NOT unswitching loop %" 449 << currentLoop->getHeader()->getName() << ", cost too high: " 450 << currentLoop->getBlocks().size() << "\n"; 451 return false; 452 } 453 454 Constant *CondVal; 455 BasicBlock *ExitBlock; 456 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) { 457 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock); 458 } else { 459 UnswitchNontrivialCondition(LoopCond, Val, currentLoop); 460 } 461 462 return true; 463} 464 465// RemapInstruction - Convert the instruction operands from referencing the 466// current values into those specified by ValueMap. 467// 468static inline void RemapInstruction(Instruction *I, 469 DenseMap<const Value *, Value*> &ValueMap) { 470 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 471 Value *Op = I->getOperand(op); 472 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op); 473 if (It != ValueMap.end()) Op = It->second; 474 I->setOperand(op, Op); 475 } 476} 477 478/// CloneLoop - Recursively clone the specified loop and all of its children, 479/// mapping the blocks with the specified map. 480static Loop *CloneLoop(Loop *L, Loop *PL, DenseMap<const Value*, Value*> &VM, 481 LoopInfo *LI, LPPassManager *LPM) { 482 Loop *New = new Loop(); 483 484 LPM->insertLoop(New, PL); 485 486 // Add all of the blocks in L to the new loop. 487 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 488 I != E; ++I) 489 if (LI->getLoopFor(*I) == L) 490 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase()); 491 492 // Add all of the subloops to the new loop. 493 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 494 CloneLoop(*I, New, VM, LI, LPM); 495 496 return New; 497} 498 499/// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values 500/// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the 501/// code immediately before InsertPt. 502void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 503 BasicBlock *TrueDest, 504 BasicBlock *FalseDest, 505 Instruction *InsertPt) { 506 // Insert a conditional branch on LIC to the two preheaders. The original 507 // code is the true version and the new code is the false version. 508 Value *BranchVal = LIC; 509 if (!isa<ConstantInt>(Val) || Val->getType() != Type::Int1Ty) 510 BranchVal = new ICmpInst(ICmpInst::ICMP_EQ, LIC, Val, "tmp", InsertPt); 511 else if (Val != Context->getConstantIntTrue()) 512 // We want to enter the new loop when the condition is true. 513 std::swap(TrueDest, FalseDest); 514 515 // Insert the new branch. 516 BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt); 517} 518 519/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable 520/// condition in it (a cond branch from its header block to its latch block, 521/// where the path through the loop that doesn't execute its body has no 522/// side-effects), unswitch it. This doesn't involve any code duplication, just 523/// moving the conditional branch outside of the loop and updating loop info. 524void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, 525 Constant *Val, 526 BasicBlock *ExitBlock) { 527 DOUT << "loop-unswitch: Trivial-Unswitch loop %" 528 << loopHeader->getName() << " [" << L->getBlocks().size() 529 << " blocks] in Function " << L->getHeader()->getParent()->getName() 530 << " on cond: " << *Val << " == " << *Cond << "\n"; 531 532 // First step, split the preheader, so that we know that there is a safe place 533 // to insert the conditional branch. We will change loopPreheader to have a 534 // conditional branch on Cond. 535 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this); 536 537 // Now that we have a place to insert the conditional branch, create a place 538 // to branch to: this is the exit block out of the loop that we should 539 // short-circuit to. 540 541 // Split this block now, so that the loop maintains its exit block, and so 542 // that the jump from the preheader can execute the contents of the exit block 543 // without actually branching to it (the exit block should be dominated by the 544 // loop header, not the preheader). 545 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 546 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this); 547 548 // Okay, now we have a position to branch from and a position to branch to, 549 // insert the new conditional branch. 550 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, 551 loopPreheader->getTerminator()); 552 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L); 553 loopPreheader->getTerminator()->eraseFromParent(); 554 555 // We need to reprocess this loop, it could be unswitched again. 556 redoLoop = true; 557 558 // Now that we know that the loop is never entered when this condition is a 559 // particular value, rewrite the loop with this info. We know that this will 560 // at least eliminate the old branch. 561 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false); 562 ++NumTrivial; 563} 564 565/// SplitExitEdges - Split all of the edges from inside the loop to their exit 566/// blocks. Update the appropriate Phi nodes as we do so. 567void LoopUnswitch::SplitExitEdges(Loop *L, 568 const SmallVector<BasicBlock *, 8> &ExitBlocks) 569{ 570 571 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 572 BasicBlock *ExitBlock = ExitBlocks[i]; 573 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock)); 574 575 for (unsigned j = 0, e = Preds.size(); j != e; ++j) { 576 BasicBlock* NewExitBlock = SplitEdge(Preds[j], ExitBlock, this); 577 BasicBlock* StartBlock = Preds[j]; 578 BasicBlock* EndBlock; 579 if (NewExitBlock->getSinglePredecessor() == ExitBlock) { 580 EndBlock = NewExitBlock; 581 NewExitBlock = EndBlock->getSinglePredecessor(); 582 } else { 583 EndBlock = ExitBlock; 584 } 585 586 std::set<PHINode*> InsertedPHIs; 587 PHINode* OldLCSSA = 0; 588 for (BasicBlock::iterator I = EndBlock->begin(); 589 (OldLCSSA = dyn_cast<PHINode>(I)); ++I) { 590 Value* OldValue = OldLCSSA->getIncomingValueForBlock(NewExitBlock); 591 PHINode* NewLCSSA = PHINode::Create(OldLCSSA->getType(), 592 OldLCSSA->getName() + ".us-lcssa", 593 NewExitBlock->getTerminator()); 594 NewLCSSA->addIncoming(OldValue, StartBlock); 595 OldLCSSA->setIncomingValue(OldLCSSA->getBasicBlockIndex(NewExitBlock), 596 NewLCSSA); 597 InsertedPHIs.insert(NewLCSSA); 598 } 599 600 BasicBlock::iterator InsertPt = EndBlock->getFirstNonPHI(); 601 for (BasicBlock::iterator I = NewExitBlock->begin(); 602 (OldLCSSA = dyn_cast<PHINode>(I)) && InsertedPHIs.count(OldLCSSA) == 0; 603 ++I) { 604 PHINode *NewLCSSA = PHINode::Create(OldLCSSA->getType(), 605 OldLCSSA->getName() + ".us-lcssa", 606 InsertPt); 607 OldLCSSA->replaceAllUsesWith(NewLCSSA); 608 NewLCSSA->addIncoming(OldLCSSA, NewExitBlock); 609 } 610 611 } 612 } 613 614} 615 616/// UnswitchNontrivialCondition - We determined that the loop is profitable 617/// to unswitch when LIC equal Val. Split it into loop versions and test the 618/// condition outside of either loop. Return the loops created as Out1/Out2. 619void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, 620 Loop *L) { 621 Function *F = loopHeader->getParent(); 622 DOUT << "loop-unswitch: Unswitching loop %" 623 << loopHeader->getName() << " [" << L->getBlocks().size() 624 << " blocks] in Function " << F->getName() 625 << " when '" << *Val << "' == " << *LIC << "\n"; 626 627 LoopBlocks.clear(); 628 NewBlocks.clear(); 629 630 // First step, split the preheader and exit blocks, and add these blocks to 631 // the LoopBlocks list. 632 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this); 633 LoopBlocks.push_back(NewPreheader); 634 635 // We want the loop to come after the preheader, but before the exit blocks. 636 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 637 638 SmallVector<BasicBlock*, 8> ExitBlocks; 639 L->getUniqueExitBlocks(ExitBlocks); 640 641 // Split all of the edges from inside the loop to their exit blocks. Update 642 // the appropriate Phi nodes as we do so. 643 SplitExitEdges(L, ExitBlocks); 644 645 // The exit blocks may have been changed due to edge splitting, recompute. 646 ExitBlocks.clear(); 647 L->getUniqueExitBlocks(ExitBlocks); 648 649 // Add exit blocks to the loop blocks. 650 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 651 652 // Next step, clone all of the basic blocks that make up the loop (including 653 // the loop preheader and exit blocks), keeping track of the mapping between 654 // the instructions and blocks. 655 NewBlocks.reserve(LoopBlocks.size()); 656 DenseMap<const Value*, Value*> ValueMap; 657 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 658 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); 659 NewBlocks.push_back(New); 660 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping. 661 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], New, L); 662 } 663 664 // Splice the newly inserted blocks into the function right before the 665 // original preheader. 666 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(), 667 NewBlocks[0], F->end()); 668 669 // Now we create the new Loop object for the versioned loop. 670 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM); 671 Loop *ParentLoop = L->getParentLoop(); 672 if (ParentLoop) { 673 // Make sure to add the cloned preheader and exit blocks to the parent loop 674 // as well. 675 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase()); 676 } 677 678 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 679 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]); 680 // The new exit block should be in the same loop as the old one. 681 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i])) 682 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase()); 683 684 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 685 "Exit block should have been split to have one successor!"); 686 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 687 688 // If the successor of the exit block had PHI nodes, add an entry for 689 // NewExit. 690 PHINode *PN; 691 for (BasicBlock::iterator I = ExitSucc->begin(); 692 (PN = dyn_cast<PHINode>(I)); ++I) { 693 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); 694 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V); 695 if (It != ValueMap.end()) V = It->second; 696 PN->addIncoming(V, NewExit); 697 } 698 } 699 700 // Rewrite the code to refer to itself. 701 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) 702 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 703 E = NewBlocks[i]->end(); I != E; ++I) 704 RemapInstruction(I, ValueMap); 705 706 // Rewrite the original preheader to select between versions of the loop. 707 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator()); 708 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 709 "Preheader splitting did not work correctly!"); 710 711 // Emit the new branch that selects between the two versions of this loop. 712 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); 713 LPM->deleteSimpleAnalysisValue(OldBR, L); 714 OldBR->eraseFromParent(); 715 716 LoopProcessWorklist.push_back(NewLoop); 717 redoLoop = true; 718 719 // Now we rewrite the original code to know that the condition is true and the 720 // new code to know that the condition is false. 721 RewriteLoopBodyWithConditionConstant(L , LIC, Val, false); 722 723 // It's possible that simplifying one loop could cause the other to be 724 // deleted. If so, don't simplify it. 725 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop) 726 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); 727 728} 729 730/// RemoveFromWorklist - Remove all instances of I from the worklist vector 731/// specified. 732static void RemoveFromWorklist(Instruction *I, 733 std::vector<Instruction*> &Worklist) { 734 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(), 735 Worklist.end(), I); 736 while (WI != Worklist.end()) { 737 unsigned Offset = WI-Worklist.begin(); 738 Worklist.erase(WI); 739 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I); 740 } 741} 742 743/// ReplaceUsesOfWith - When we find that I really equals V, remove I from the 744/// program, replacing all uses with V and update the worklist. 745static void ReplaceUsesOfWith(Instruction *I, Value *V, 746 std::vector<Instruction*> &Worklist, 747 Loop *L, LPPassManager *LPM) { 748 DOUT << "Replace with '" << *V << "': " << *I; 749 750 // Add uses to the worklist, which may be dead now. 751 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 752 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 753 Worklist.push_back(Use); 754 755 // Add users to the worklist which may be simplified now. 756 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 757 UI != E; ++UI) 758 Worklist.push_back(cast<Instruction>(*UI)); 759 LPM->deleteSimpleAnalysisValue(I, L); 760 RemoveFromWorklist(I, Worklist); 761 I->replaceAllUsesWith(V); 762 I->eraseFromParent(); 763 ++NumSimplify; 764} 765 766/// RemoveBlockIfDead - If the specified block is dead, remove it, update loop 767/// information, and remove any dead successors it has. 768/// 769void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB, 770 std::vector<Instruction*> &Worklist, 771 Loop *L) { 772 if (pred_begin(BB) != pred_end(BB)) { 773 // This block isn't dead, since an edge to BB was just removed, see if there 774 // are any easy simplifications we can do now. 775 if (BasicBlock *Pred = BB->getSinglePredecessor()) { 776 // If it has one pred, fold phi nodes in BB. 777 while (isa<PHINode>(BB->begin())) 778 ReplaceUsesOfWith(BB->begin(), 779 cast<PHINode>(BB->begin())->getIncomingValue(0), 780 Worklist, L, LPM); 781 782 // If this is the header of a loop and the only pred is the latch, we now 783 // have an unreachable loop. 784 if (Loop *L = LI->getLoopFor(BB)) 785 if (loopHeader == BB && L->contains(Pred)) { 786 // Remove the branch from the latch to the header block, this makes 787 // the header dead, which will make the latch dead (because the header 788 // dominates the latch). 789 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L); 790 Pred->getTerminator()->eraseFromParent(); 791 new UnreachableInst(Pred); 792 793 // The loop is now broken, remove it from LI. 794 RemoveLoopFromHierarchy(L); 795 796 // Reprocess the header, which now IS dead. 797 RemoveBlockIfDead(BB, Worklist, L); 798 return; 799 } 800 801 // If pred ends in a uncond branch, add uncond branch to worklist so that 802 // the two blocks will get merged. 803 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) 804 if (BI->isUnconditional()) 805 Worklist.push_back(BI); 806 } 807 return; 808 } 809 810 DOUT << "Nuking dead block: " << *BB; 811 812 // Remove the instructions in the basic block from the worklist. 813 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 814 RemoveFromWorklist(I, Worklist); 815 816 // Anything that uses the instructions in this basic block should have their 817 // uses replaced with undefs. 818 if (!I->use_empty()) 819 I->replaceAllUsesWith(Context->getUndef(I->getType())); 820 } 821 822 // If this is the edge to the header block for a loop, remove the loop and 823 // promote all subloops. 824 if (Loop *BBLoop = LI->getLoopFor(BB)) { 825 if (BBLoop->getLoopLatch() == BB) 826 RemoveLoopFromHierarchy(BBLoop); 827 } 828 829 // Remove the block from the loop info, which removes it from any loops it 830 // was in. 831 LI->removeBlock(BB); 832 833 834 // Remove phi node entries in successors for this block. 835 TerminatorInst *TI = BB->getTerminator(); 836 SmallVector<BasicBlock*, 4> Succs; 837 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) { 838 Succs.push_back(TI->getSuccessor(i)); 839 TI->getSuccessor(i)->removePredecessor(BB); 840 } 841 842 // Unique the successors, remove anything with multiple uses. 843 array_pod_sort(Succs.begin(), Succs.end()); 844 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end()); 845 846 // Remove the basic block, including all of the instructions contained in it. 847 LPM->deleteSimpleAnalysisValue(BB, L); 848 BB->eraseFromParent(); 849 // Remove successor blocks here that are not dead, so that we know we only 850 // have dead blocks in this list. Nondead blocks have a way of becoming dead, 851 // then getting removed before we revisit them, which is badness. 852 // 853 for (unsigned i = 0; i != Succs.size(); ++i) 854 if (pred_begin(Succs[i]) != pred_end(Succs[i])) { 855 // One exception is loop headers. If this block was the preheader for a 856 // loop, then we DO want to visit the loop so the loop gets deleted. 857 // We know that if the successor is a loop header, that this loop had to 858 // be the preheader: the case where this was the latch block was handled 859 // above and headers can only have two predecessors. 860 if (!LI->isLoopHeader(Succs[i])) { 861 Succs.erase(Succs.begin()+i); 862 --i; 863 } 864 } 865 866 for (unsigned i = 0, e = Succs.size(); i != e; ++i) 867 RemoveBlockIfDead(Succs[i], Worklist, L); 868} 869 870/// RemoveLoopFromHierarchy - We have discovered that the specified loop has 871/// become unwrapped, either because the backedge was deleted, or because the 872/// edge into the header was removed. If the edge into the header from the 873/// latch block was removed, the loop is unwrapped but subloops are still alive, 874/// so they just reparent loops. If the loops are actually dead, they will be 875/// removed later. 876void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) { 877 LPM->deleteLoopFromQueue(L); 878 RemoveLoopFromWorklist(L); 879} 880 881// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has 882// the value specified by Val in the specified loop, or we know it does NOT have 883// that value. Rewrite any uses of LIC or of properties correlated to it. 884void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 885 Constant *Val, 886 bool IsEqual) { 887 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 888 889 // FIXME: Support correlated properties, like: 890 // for (...) 891 // if (li1 < li2) 892 // ... 893 // if (li1 > li2) 894 // ... 895 896 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 897 // selects, switches. 898 std::vector<User*> Users(LIC->use_begin(), LIC->use_end()); 899 std::vector<Instruction*> Worklist; 900 901 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC 902 // in the loop with the appropriate one directly. 903 if (IsEqual || (isa<ConstantInt>(Val) && Val->getType() == Type::Int1Ty)) { 904 Value *Replacement; 905 if (IsEqual) 906 Replacement = Val; 907 else 908 Replacement = Context->getConstantInt(Type::Int1Ty, 909 !cast<ConstantInt>(Val)->getZExtValue()); 910 911 for (unsigned i = 0, e = Users.size(); i != e; ++i) 912 if (Instruction *U = cast<Instruction>(Users[i])) { 913 if (!L->contains(U->getParent())) 914 continue; 915 U->replaceUsesOfWith(LIC, Replacement); 916 Worklist.push_back(U); 917 } 918 } else { 919 // Otherwise, we don't know the precise value of LIC, but we do know that it 920 // is certainly NOT "Val". As such, simplify any uses in the loop that we 921 // can. This case occurs when we unswitch switch statements. 922 for (unsigned i = 0, e = Users.size(); i != e; ++i) 923 if (Instruction *U = cast<Instruction>(Users[i])) { 924 if (!L->contains(U->getParent())) 925 continue; 926 927 Worklist.push_back(U); 928 929 // If we know that LIC is not Val, use this info to simplify code. 930 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) { 931 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) { 932 if (SI->getCaseValue(i) == Val) { 933 // Found a dead case value. Don't remove PHI nodes in the 934 // successor if they become single-entry, those PHI nodes may 935 // be in the Users list. 936 937 // FIXME: This is a hack. We need to keep the successor around 938 // and hooked up so as to preserve the loop structure, because 939 // trying to update it is complicated. So instead we preserve the 940 // loop structure and put the block on an dead code path. 941 942 BasicBlock *SISucc = SI->getSuccessor(i); 943 BasicBlock* Old = SI->getParent(); 944 BasicBlock* Split = SplitBlock(Old, SI, this); 945 946 Instruction* OldTerm = Old->getTerminator(); 947 BranchInst::Create(Split, SISucc, 948 Context->getConstantIntTrue(), OldTerm); 949 950 LPM->deleteSimpleAnalysisValue(Old->getTerminator(), L); 951 Old->getTerminator()->eraseFromParent(); 952 953 PHINode *PN; 954 for (BasicBlock::iterator II = SISucc->begin(); 955 (PN = dyn_cast<PHINode>(II)); ++II) { 956 Value *InVal = PN->removeIncomingValue(Split, false); 957 PN->addIncoming(InVal, Old); 958 } 959 960 SI->removeCase(i); 961 break; 962 } 963 } 964 } 965 966 // TODO: We could do other simplifications, for example, turning 967 // LIC == Val -> false. 968 } 969 } 970 971 SimplifyCode(Worklist, L); 972} 973 974/// SimplifyCode - Okay, now that we have simplified some instructions in the 975/// loop, walk over it and constant prop, dce, and fold control flow where 976/// possible. Note that this is effectively a very simple loop-structure-aware 977/// optimizer. During processing of this loop, L could very well be deleted, so 978/// it must not be used. 979/// 980/// FIXME: When the loop optimizer is more mature, separate this out to a new 981/// pass. 982/// 983void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) { 984 while (!Worklist.empty()) { 985 Instruction *I = Worklist.back(); 986 Worklist.pop_back(); 987 988 // Simple constant folding. 989 if (Constant *C = ConstantFoldInstruction(I, Context)) { 990 ReplaceUsesOfWith(I, C, Worklist, L, LPM); 991 continue; 992 } 993 994 // Simple DCE. 995 if (isInstructionTriviallyDead(I)) { 996 DOUT << "Remove dead instruction '" << *I; 997 998 // Add uses to the worklist, which may be dead now. 999 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 1000 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 1001 Worklist.push_back(Use); 1002 LPM->deleteSimpleAnalysisValue(I, L); 1003 RemoveFromWorklist(I, Worklist); 1004 I->eraseFromParent(); 1005 ++NumSimplify; 1006 continue; 1007 } 1008 1009 // Special case hacks that appear commonly in unswitched code. 1010 switch (I->getOpcode()) { 1011 case Instruction::Select: 1012 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(0))) { 1013 ReplaceUsesOfWith(I, I->getOperand(!CB->getZExtValue()+1), Worklist, L, 1014 LPM); 1015 continue; 1016 } 1017 break; 1018 case Instruction::And: 1019 if (isa<ConstantInt>(I->getOperand(0)) && 1020 I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS 1021 cast<BinaryOperator>(I)->swapOperands(); 1022 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1))) 1023 if (CB->getType() == Type::Int1Ty) { 1024 if (CB->isOne()) // X & 1 -> X 1025 ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM); 1026 else // X & 0 -> 0 1027 ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM); 1028 continue; 1029 } 1030 break; 1031 case Instruction::Or: 1032 if (isa<ConstantInt>(I->getOperand(0)) && 1033 I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS 1034 cast<BinaryOperator>(I)->swapOperands(); 1035 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1))) 1036 if (CB->getType() == Type::Int1Ty) { 1037 if (CB->isOne()) // X | 1 -> 1 1038 ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM); 1039 else // X | 0 -> X 1040 ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM); 1041 continue; 1042 } 1043 break; 1044 case Instruction::Br: { 1045 BranchInst *BI = cast<BranchInst>(I); 1046 if (BI->isUnconditional()) { 1047 // If BI's parent is the only pred of the successor, fold the two blocks 1048 // together. 1049 BasicBlock *Pred = BI->getParent(); 1050 BasicBlock *Succ = BI->getSuccessor(0); 1051 BasicBlock *SinglePred = Succ->getSinglePredecessor(); 1052 if (!SinglePred) continue; // Nothing to do. 1053 assert(SinglePred == Pred && "CFG broken"); 1054 1055 DOUT << "Merging blocks: " << Pred->getName() << " <- " 1056 << Succ->getName() << "\n"; 1057 1058 // Resolve any single entry PHI nodes in Succ. 1059 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) 1060 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM); 1061 1062 // Move all of the successor contents from Succ to Pred. 1063 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(), 1064 Succ->end()); 1065 LPM->deleteSimpleAnalysisValue(BI, L); 1066 BI->eraseFromParent(); 1067 RemoveFromWorklist(BI, Worklist); 1068 1069 // If Succ has any successors with PHI nodes, update them to have 1070 // entries coming from Pred instead of Succ. 1071 Succ->replaceAllUsesWith(Pred); 1072 1073 // Remove Succ from the loop tree. 1074 LI->removeBlock(Succ); 1075 LPM->deleteSimpleAnalysisValue(Succ, L); 1076 Succ->eraseFromParent(); 1077 ++NumSimplify; 1078 } else if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){ 1079 // Conditional branch. Turn it into an unconditional branch, then 1080 // remove dead blocks. 1081 break; // FIXME: Enable. 1082 1083 DOUT << "Folded branch: " << *BI; 1084 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue()); 1085 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue()); 1086 DeadSucc->removePredecessor(BI->getParent(), true); 1087 Worklist.push_back(BranchInst::Create(LiveSucc, BI)); 1088 LPM->deleteSimpleAnalysisValue(BI, L); 1089 BI->eraseFromParent(); 1090 RemoveFromWorklist(BI, Worklist); 1091 ++NumSimplify; 1092 1093 RemoveBlockIfDead(DeadSucc, Worklist, L); 1094 } 1095 break; 1096 } 1097 } 1098 } 1099} 1100