LoopUnswitch.cpp revision f3ba70861ccf00b1072ae1b6ade3ebe2da6cff40
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/LoopInfo.h" 37#include "llvm/Analysis/LoopPass.h" 38#include "llvm/Analysis/Dominators.h" 39#include "llvm/Transforms/Utils/Cloning.h" 40#include "llvm/Transforms/Utils/Local.h" 41#include "llvm/Transforms/Utils/BasicBlockUtils.h" 42#include "llvm/ADT/Statistic.h" 43#include "llvm/ADT/SmallPtrSet.h" 44#include "llvm/Support/CommandLine.h" 45#include "llvm/Support/Compiler.h" 46#include "llvm/Support/Debug.h" 47#include <algorithm> 48#include <set> 49using namespace llvm; 50 51STATISTIC(NumBranches, "Number of branches unswitched"); 52STATISTIC(NumSwitches, "Number of switches unswitched"); 53STATISTIC(NumSelects , "Number of selects unswitched"); 54STATISTIC(NumTrivial , "Number of unswitches that are trivial"); 55STATISTIC(NumSimplify, "Number of simplifications of unswitched code"); 56 57static cl::opt<unsigned> 58Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), 59 cl::init(10), cl::Hidden); 60 61namespace { 62 class VISIBILITY_HIDDEN LoopUnswitch : public LoopPass { 63 LoopInfo *LI; // Loop information 64 LPPassManager *LPM; 65 66 // LoopProcessWorklist - Used to check if second loop needs processing 67 // after RewriteLoopBodyWithConditionConstant rewrites first loop. 68 std::vector<Loop*> LoopProcessWorklist; 69 SmallPtrSet<Value *,8> UnswitchedVals; 70 71 bool OptimizeForSize; 72 bool redoLoop; 73 74 Loop *currentLoop; 75 DominanceFrontier *DF; 76 DominatorTree *DT; 77 BasicBlock *loopHeader; 78 BasicBlock *loopPreheader; 79 80 // LoopBlocks contains all of the basic blocks of the loop, including the 81 // preheader of the loop, the body of the loop, and the exit blocks of the 82 // loop, in that order. 83 std::vector<BasicBlock*> LoopBlocks; 84 // NewBlocks contained cloned copy of basic blocks from LoopBlocks. 85 std::vector<BasicBlock*> NewBlocks; 86 87 public: 88 static char ID; // Pass ID, replacement for typeid 89 explicit LoopUnswitch(bool Os = false) : 90 LoopPass(&ID), OptimizeForSize(Os), redoLoop(false), 91 currentLoop(NULL), DF(NULL), DT(NULL), loopHeader(NULL), 92 loopPreheader(NULL) {} 93 94 bool runOnLoop(Loop *L, LPPassManager &LPM); 95 bool processCurrentLoop(); 96 97 /// This transformation requires natural loop information & requires that 98 /// loop preheaders be inserted into the CFG... 99 /// 100 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 101 AU.addRequiredID(LoopSimplifyID); 102 AU.addPreservedID(LoopSimplifyID); 103 AU.addRequired<LoopInfo>(); 104 AU.addPreserved<LoopInfo>(); 105 AU.addRequiredID(LCSSAID); 106 AU.addPreservedID(LCSSAID); 107 AU.addPreserved<DominatorTree>(); 108 AU.addPreserved<DominanceFrontier>(); 109 } 110 111 private: 112 113 /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist, 114 /// remove it. 115 void RemoveLoopFromWorklist(Loop *L) { 116 std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(), 117 LoopProcessWorklist.end(), L); 118 if (I != LoopProcessWorklist.end()) 119 LoopProcessWorklist.erase(I); 120 } 121 122 void initLoopData() { 123 loopHeader = currentLoop->getHeader(); 124 loopPreheader = currentLoop->getLoopPreheader(); 125 } 126 127 /// Split all of the edges from inside the loop to their exit blocks. 128 /// Update the appropriate Phi nodes as we do so. 129 void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks); 130 131 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val); 132 unsigned getLoopUnswitchCost(Value *LIC); 133 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 134 BasicBlock *ExitBlock); 135 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L); 136 137 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 138 Constant *Val, bool isEqual); 139 140 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 141 BasicBlock *TrueDest, 142 BasicBlock *FalseDest, 143 Instruction *InsertPt); 144 145 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); 146 void RemoveBlockIfDead(BasicBlock *BB, 147 std::vector<Instruction*> &Worklist, Loop *l); 148 void RemoveLoopFromHierarchy(Loop *L); 149 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0, 150 BasicBlock **LoopExit = 0); 151 152 }; 153} 154char LoopUnswitch::ID = 0; 155static RegisterPass<LoopUnswitch> X("loop-unswitch", "Unswitch loops"); 156 157LoopPass *llvm::createLoopUnswitchPass(bool Os) { 158 return new LoopUnswitch(Os); 159} 160 161/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is 162/// invariant in the loop, or has an invariant piece, return the invariant. 163/// Otherwise, return null. 164static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { 165 // Constants should be folded, not unswitched on! 166 if (isa<Constant>(Cond)) return false; 167 168 // TODO: Handle: br (VARIANT|INVARIANT). 169 // TODO: Hoist simple expressions out of loops. 170 if (L->isLoopInvariant(Cond)) return Cond; 171 172 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 173 if (BO->getOpcode() == Instruction::And || 174 BO->getOpcode() == Instruction::Or) { 175 // If either the left or right side is invariant, we can unswitch on this, 176 // which will cause the branch to go away in one loop and the condition to 177 // simplify in the other one. 178 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) 179 return LHS; 180 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) 181 return RHS; 182 } 183 184 return 0; 185} 186 187bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) { 188 LI = &getAnalysis<LoopInfo>(); 189 LPM = &LPM_Ref; 190 DF = getAnalysisToUpdate<DominanceFrontier>(); 191 DT = getAnalysisToUpdate<DominatorTree>(); 192 currentLoop = L; 193 Function *F = currentLoop->getHeader()->getParent(); 194 bool Changed = false; 195 do { 196 assert(currentLoop->isLCSSAForm()); 197 redoLoop = false; 198 Changed |= processCurrentLoop(); 199 } while(redoLoop); 200 201 if (Changed) { 202 // FIXME: Reconstruct dom info, because it is not preserved properly. 203 if (DT) 204 DT->runOnFunction(*F); 205 if (DF) 206 DF->runOnFunction(*F); 207 } 208 return Changed; 209} 210 211/// processCurrentLoop - Do actual work and unswitch loop if possible 212/// and profitable. 213bool LoopUnswitch::processCurrentLoop() { 214 bool Changed = false; 215 216 // Loop over all of the basic blocks in the loop. If we find an interior 217 // block that is branching on a loop-invariant condition, we can unswitch this 218 // loop. 219 for (Loop::block_iterator I = currentLoop->block_begin(), 220 E = currentLoop->block_end(); 221 I != E; ++I) { 222 TerminatorInst *TI = (*I)->getTerminator(); 223 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 224 // If this isn't branching on an invariant condition, we can't unswitch 225 // it. 226 if (BI->isConditional()) { 227 // See if this, or some part of it, is loop invariant. If so, we can 228 // unswitch on it if we desire. 229 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), 230 currentLoop, Changed); 231 if (LoopCond && UnswitchIfProfitable(LoopCond, 232 ConstantInt::getTrue())) { 233 ++NumBranches; 234 return true; 235 } 236 } 237 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 238 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 239 currentLoop, Changed); 240 if (LoopCond && SI->getNumCases() > 1) { 241 // Find a value to unswitch on: 242 // FIXME: this should chose the most expensive case! 243 Constant *UnswitchVal = SI->getCaseValue(1); 244 // Do not process same value again and again. 245 if (!UnswitchedVals.insert(UnswitchVal)) 246 continue; 247 248 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) { 249 ++NumSwitches; 250 return true; 251 } 252 } 253 } 254 255 // Scan the instructions to check for unswitchable values. 256 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 257 BBI != E; ++BBI) 258 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 259 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), 260 currentLoop, Changed); 261 if (LoopCond && UnswitchIfProfitable(LoopCond, 262 ConstantInt::getTrue())) { 263 ++NumSelects; 264 return true; 265 } 266 } 267 } 268 return Changed; 269} 270 271/// isTrivialLoopExitBlock - Check to see if all paths from BB either: 272/// 1. Exit the loop with no side effects. 273/// 2. Branch to the latch block with no side-effects. 274/// 275/// If these conditions are true, we return true and set ExitBB to the block we 276/// exit through. 277/// 278static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, 279 BasicBlock *&ExitBB, 280 std::set<BasicBlock*> &Visited) { 281 if (!Visited.insert(BB).second) { 282 // Already visited and Ok, end of recursion. 283 return true; 284 } else if (!L->contains(BB)) { 285 // Otherwise, this is a loop exit, this is fine so long as this is the 286 // first exit. 287 if (ExitBB != 0) return false; 288 ExitBB = BB; 289 return true; 290 } 291 292 // Otherwise, this is an unvisited intra-loop node. Check all successors. 293 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { 294 // Check to see if the successor is a trivial loop exit. 295 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) 296 return false; 297 } 298 299 // Okay, everything after this looks good, check to make sure that this block 300 // doesn't include any side effects. 301 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 302 if (I->mayWriteToMemory()) 303 return false; 304 305 return true; 306} 307 308/// isTrivialLoopExitBlock - Return true if the specified block unconditionally 309/// leads to an exit from the specified loop, and has no side-effects in the 310/// process. If so, return the block that is exited to, otherwise return null. 311static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { 312 std::set<BasicBlock*> Visited; 313 Visited.insert(L->getHeader()); // Branches to header are ok. 314 BasicBlock *ExitBB = 0; 315 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) 316 return ExitBB; 317 return 0; 318} 319 320/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is 321/// trivial: that is, that the condition controls whether or not the loop does 322/// anything at all. If this is a trivial condition, unswitching produces no 323/// code duplications (equivalently, it produces a simpler loop and a new empty 324/// loop, which gets deleted). 325/// 326/// If this is a trivial condition, return true, otherwise return false. When 327/// returning true, this sets Cond and Val to the condition that controls the 328/// trivial condition: when Cond dynamically equals Val, the loop is known to 329/// exit. Finally, this sets LoopExit to the BB that the loop exits to when 330/// Cond == Val. 331/// 332bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val, 333 BasicBlock **LoopExit) { 334 BasicBlock *Header = currentLoop->getHeader(); 335 TerminatorInst *HeaderTerm = Header->getTerminator(); 336 337 BasicBlock *LoopExitBB = 0; 338 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) { 339 // If the header block doesn't end with a conditional branch on Cond, we 340 // can't handle it. 341 if (!BI->isConditional() || BI->getCondition() != Cond) 342 return false; 343 344 // Check to see if a successor of the branch is guaranteed to go to the 345 // latch block or exit through a one exit block without having any 346 // side-effects. If so, determine the value of Cond that causes it to do 347 // this. 348 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 349 BI->getSuccessor(0)))) { 350 if (Val) *Val = ConstantInt::getTrue(); 351 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 352 BI->getSuccessor(1)))) { 353 if (Val) *Val = ConstantInt::getFalse(); 354 } 355 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) { 356 // If this isn't a switch on Cond, we can't handle it. 357 if (SI->getCondition() != Cond) return false; 358 359 // Check to see if a successor of the switch is guaranteed to go to the 360 // latch block or exit through a one exit block without having any 361 // side-effects. If so, determine the value of Cond that causes it to do 362 // this. Note that we can't trivially unswitch on the default case. 363 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) 364 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 365 SI->getSuccessor(i)))) { 366 // Okay, we found a trivial case, remember the value that is trivial. 367 if (Val) *Val = SI->getCaseValue(i); 368 break; 369 } 370 } 371 372 // If we didn't find a single unique LoopExit block, or if the loop exit block 373 // contains phi nodes, this isn't trivial. 374 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) 375 return false; // Can't handle this. 376 377 if (LoopExit) *LoopExit = LoopExitBB; 378 379 // We already know that nothing uses any scalar values defined inside of this 380 // loop. As such, we just have to check to see if this loop will execute any 381 // side-effecting instructions (e.g. stores, calls, volatile loads) in the 382 // part of the loop that the code *would* execute. We already checked the 383 // tail, check the header now. 384 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 385 if (I->mayWriteToMemory()) 386 return false; 387 return true; 388} 389 390/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if 391/// we choose to unswitch current loop on the specified value. 392/// 393unsigned LoopUnswitch::getLoopUnswitchCost(Value *LIC) { 394 // If the condition is trivial, always unswitch. There is no code growth for 395 // this case. 396 if (IsTrivialUnswitchCondition(LIC)) 397 return 0; 398 399 // FIXME: This is really overly conservative. However, more liberal 400 // estimations have thus far resulted in excessive unswitching, which is bad 401 // both in compile time and in code size. This should be replaced once 402 // someone figures out how a good estimation. 403 return currentLoop->getBlocks().size(); 404 405 unsigned Cost = 0; 406 // FIXME: this is brain dead. It should take into consideration code 407 // shrinkage. 408 for (Loop::block_iterator I = currentLoop->block_begin(), 409 E = currentLoop->block_end(); 410 I != E; ++I) { 411 BasicBlock *BB = *I; 412 // Do not include empty blocks in the cost calculation. This happen due to 413 // loop canonicalization and will be removed. 414 if (BB->begin() == BasicBlock::iterator(BB->getTerminator())) 415 continue; 416 417 // Count basic blocks. 418 ++Cost; 419 } 420 421 return Cost; 422} 423 424/// UnswitchIfProfitable - We have found that we can unswitch currentLoop when 425/// LoopCond == Val to simplify the loop. If we decide that this is profitable, 426/// unswitch the loop, reprocess the pieces, then return true. 427bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val){ 428 429 initLoopData(); 430 Function *F = loopHeader->getParent(); 431 432 // Do not unswitch if the function is optimized for size. 433 if (F->hasNote(FN_NOTE_OptimizeForSize)) 434 return false; 435 436 // Check to see if it would be profitable to unswitch current loop. 437 unsigned Cost = getLoopUnswitchCost(LoopCond); 438 439 // Do not do non-trivial unswitch while optimizing for size. 440 if (Cost && OptimizeForSize) 441 return false; 442 443 if (Cost > Threshold) { 444 // FIXME: this should estimate growth by the amount of code shared by the 445 // resultant unswitched loops. 446 // 447 DOUT << "NOT unswitching loop %" 448 << currentLoop->getHeader()->getName() << ", cost too high: " 449 << currentLoop->getBlocks().size() << "\n"; 450 return false; 451 } 452 453 Constant *CondVal; 454 BasicBlock *ExitBlock; 455 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) { 456 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock); 457 } else { 458 UnswitchNontrivialCondition(LoopCond, Val, currentLoop); 459 } 460 461 return true; 462} 463 464// RemapInstruction - Convert the instruction operands from referencing the 465// current values into those specified by ValueMap. 466// 467static inline void RemapInstruction(Instruction *I, 468 DenseMap<const Value *, Value*> &ValueMap) { 469 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 470 Value *Op = I->getOperand(op); 471 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op); 472 if (It != ValueMap.end()) Op = It->second; 473 I->setOperand(op, Op); 474 } 475} 476 477/// CloneLoop - Recursively clone the specified loop and all of its children, 478/// mapping the blocks with the specified map. 479static Loop *CloneLoop(Loop *L, Loop *PL, DenseMap<const Value*, Value*> &VM, 480 LoopInfo *LI, LPPassManager *LPM) { 481 Loop *New = new Loop(); 482 483 LPM->insertLoop(New, PL); 484 485 // Add all of the blocks in L to the new loop. 486 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 487 I != E; ++I) 488 if (LI->getLoopFor(*I) == L) 489 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase()); 490 491 // Add all of the subloops to the new loop. 492 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 493 CloneLoop(*I, New, VM, LI, LPM); 494 495 return New; 496} 497 498/// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values 499/// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the 500/// code immediately before InsertPt. 501void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 502 BasicBlock *TrueDest, 503 BasicBlock *FalseDest, 504 Instruction *InsertPt) { 505 // Insert a conditional branch on LIC to the two preheaders. The original 506 // code is the true version and the new code is the false version. 507 Value *BranchVal = LIC; 508 if (!isa<ConstantInt>(Val) || Val->getType() != Type::Int1Ty) 509 BranchVal = new ICmpInst(ICmpInst::ICMP_EQ, LIC, Val, "tmp", InsertPt); 510 else if (Val != ConstantInt::getTrue()) 511 // We want to enter the new loop when the condition is true. 512 std::swap(TrueDest, FalseDest); 513 514 // Insert the new branch. 515 BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt); 516} 517 518/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable 519/// condition in it (a cond branch from its header block to its latch block, 520/// where the path through the loop that doesn't execute its body has no 521/// side-effects), unswitch it. This doesn't involve any code duplication, just 522/// moving the conditional branch outside of the loop and updating loop info. 523void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, 524 Constant *Val, 525 BasicBlock *ExitBlock) { 526 DOUT << "loop-unswitch: Trivial-Unswitch loop %" 527 << loopHeader->getName() << " [" << L->getBlocks().size() 528 << " blocks] in Function " << L->getHeader()->getParent()->getName() 529 << " on cond: " << *Val << " == " << *Cond << "\n"; 530 531 // First step, split the preheader, so that we know that there is a safe place 532 // to insert the conditional branch. We will change loopPreheader to have a 533 // conditional branch on Cond. 534 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this); 535 536 // Now that we have a place to insert the conditional branch, create a place 537 // to branch to: this is the exit block out of the loop that we should 538 // short-circuit to. 539 540 // Split this block now, so that the loop maintains its exit block, and so 541 // that the jump from the preheader can execute the contents of the exit block 542 // without actually branching to it (the exit block should be dominated by the 543 // loop header, not the preheader). 544 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 545 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this); 546 547 // Okay, now we have a position to branch from and a position to branch to, 548 // insert the new conditional branch. 549 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, 550 loopPreheader->getTerminator()); 551 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L); 552 loopPreheader->getTerminator()->eraseFromParent(); 553 554 // We need to reprocess this loop, it could be unswitched again. 555 redoLoop = true; 556 557 // Now that we know that the loop is never entered when this condition is a 558 // particular value, rewrite the loop with this info. We know that this will 559 // at least eliminate the old branch. 560 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false); 561 ++NumTrivial; 562} 563 564/// SplitExitEdges - Split all of the edges from inside the loop to their exit 565/// blocks. Update the appropriate Phi nodes as we do so. 566void LoopUnswitch::SplitExitEdges(Loop *L, 567 const SmallVector<BasicBlock *, 8> &ExitBlocks) 568{ 569 570 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 571 BasicBlock *ExitBlock = ExitBlocks[i]; 572 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock)); 573 574 for (unsigned j = 0, e = Preds.size(); j != e; ++j) { 575 BasicBlock* NewExitBlock = SplitEdge(Preds[j], ExitBlock, this); 576 BasicBlock* StartBlock = Preds[j]; 577 BasicBlock* EndBlock; 578 if (NewExitBlock->getSinglePredecessor() == ExitBlock) { 579 EndBlock = NewExitBlock; 580 NewExitBlock = EndBlock->getSinglePredecessor();; 581 } else { 582 EndBlock = ExitBlock; 583 } 584 585 std::set<PHINode*> InsertedPHIs; 586 PHINode* OldLCSSA = 0; 587 for (BasicBlock::iterator I = EndBlock->begin(); 588 (OldLCSSA = dyn_cast<PHINode>(I)); ++I) { 589 Value* OldValue = OldLCSSA->getIncomingValueForBlock(NewExitBlock); 590 PHINode* NewLCSSA = PHINode::Create(OldLCSSA->getType(), 591 OldLCSSA->getName() + ".us-lcssa", 592 NewExitBlock->getTerminator()); 593 NewLCSSA->addIncoming(OldValue, StartBlock); 594 OldLCSSA->setIncomingValue(OldLCSSA->getBasicBlockIndex(NewExitBlock), 595 NewLCSSA); 596 InsertedPHIs.insert(NewLCSSA); 597 } 598 599 BasicBlock::iterator InsertPt = EndBlock->getFirstNonPHI(); 600 for (BasicBlock::iterator I = NewExitBlock->begin(); 601 (OldLCSSA = dyn_cast<PHINode>(I)) && InsertedPHIs.count(OldLCSSA) == 0; 602 ++I) { 603 PHINode *NewLCSSA = PHINode::Create(OldLCSSA->getType(), 604 OldLCSSA->getName() + ".us-lcssa", 605 InsertPt); 606 OldLCSSA->replaceAllUsesWith(NewLCSSA); 607 NewLCSSA->addIncoming(OldLCSSA, NewExitBlock); 608 } 609 610 } 611 } 612 613} 614 615/// UnswitchNontrivialCondition - We determined that the loop is profitable 616/// to unswitch when LIC equal Val. Split it into loop versions and test the 617/// condition outside of either loop. Return the loops created as Out1/Out2. 618void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, 619 Loop *L) { 620 Function *F = loopHeader->getParent(); 621 DOUT << "loop-unswitch: Unswitching loop %" 622 << loopHeader->getName() << " [" << L->getBlocks().size() 623 << " blocks] in Function " << F->getName() 624 << " when '" << *Val << "' == " << *LIC << "\n"; 625 626 LoopBlocks.clear(); 627 NewBlocks.clear(); 628 629 // First step, split the preheader and exit blocks, and add these blocks to 630 // the LoopBlocks list. 631 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this); 632 LoopBlocks.push_back(NewPreheader); 633 634 // We want the loop to come after the preheader, but before the exit blocks. 635 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 636 637 SmallVector<BasicBlock*, 8> ExitBlocks; 638 L->getUniqueExitBlocks(ExitBlocks); 639 640 // Split all of the edges from inside the loop to their exit blocks. Update 641 // the appropriate Phi nodes as we do so. 642 SplitExitEdges(L, ExitBlocks); 643 644 // The exit blocks may have been changed due to edge splitting, recompute. 645 ExitBlocks.clear(); 646 L->getUniqueExitBlocks(ExitBlocks); 647 648 // Add exit blocks to the loop blocks. 649 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 650 651 // Next step, clone all of the basic blocks that make up the loop (including 652 // the loop preheader and exit blocks), keeping track of the mapping between 653 // the instructions and blocks. 654 NewBlocks.reserve(LoopBlocks.size()); 655 DenseMap<const Value*, Value*> ValueMap; 656 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 657 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); 658 NewBlocks.push_back(New); 659 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping. 660 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], New, L); 661 } 662 663 // Splice the newly inserted blocks into the function right before the 664 // original preheader. 665 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(), 666 NewBlocks[0], F->end()); 667 668 // Now we create the new Loop object for the versioned loop. 669 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM); 670 Loop *ParentLoop = L->getParentLoop(); 671 if (ParentLoop) { 672 // Make sure to add the cloned preheader and exit blocks to the parent loop 673 // as well. 674 ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase()); 675 } 676 677 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 678 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]); 679 // The new exit block should be in the same loop as the old one. 680 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i])) 681 ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase()); 682 683 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 684 "Exit block should have been split to have one successor!"); 685 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 686 687 // If the successor of the exit block had PHI nodes, add an entry for 688 // NewExit. 689 PHINode *PN; 690 for (BasicBlock::iterator I = ExitSucc->begin(); 691 (PN = dyn_cast<PHINode>(I)); ++I) { 692 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); 693 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V); 694 if (It != ValueMap.end()) V = It->second; 695 PN->addIncoming(V, NewExit); 696 } 697 } 698 699 // Rewrite the code to refer to itself. 700 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) 701 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 702 E = NewBlocks[i]->end(); I != E; ++I) 703 RemapInstruction(I, ValueMap); 704 705 // Rewrite the original preheader to select between versions of the loop. 706 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator()); 707 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 708 "Preheader splitting did not work correctly!"); 709 710 // Emit the new branch that selects between the two versions of this loop. 711 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); 712 LPM->deleteSimpleAnalysisValue(OldBR, L); 713 OldBR->eraseFromParent(); 714 715 LoopProcessWorklist.push_back(NewLoop); 716 redoLoop = true; 717 718 // Now we rewrite the original code to know that the condition is true and the 719 // new code to know that the condition is false. 720 RewriteLoopBodyWithConditionConstant(L , LIC, Val, false); 721 722 // It's possible that simplifying one loop could cause the other to be 723 // deleted. If so, don't simplify it. 724 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop) 725 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); 726 727} 728 729/// RemoveFromWorklist - Remove all instances of I from the worklist vector 730/// specified. 731static void RemoveFromWorklist(Instruction *I, 732 std::vector<Instruction*> &Worklist) { 733 std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(), 734 Worklist.end(), I); 735 while (WI != Worklist.end()) { 736 unsigned Offset = WI-Worklist.begin(); 737 Worklist.erase(WI); 738 WI = std::find(Worklist.begin()+Offset, Worklist.end(), I); 739 } 740} 741 742/// ReplaceUsesOfWith - When we find that I really equals V, remove I from the 743/// program, replacing all uses with V and update the worklist. 744static void ReplaceUsesOfWith(Instruction *I, Value *V, 745 std::vector<Instruction*> &Worklist, 746 Loop *L, LPPassManager *LPM) { 747 DOUT << "Replace with '" << *V << "': " << *I; 748 749 // Add uses to the worklist, which may be dead now. 750 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 751 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 752 Worklist.push_back(Use); 753 754 // Add users to the worklist which may be simplified now. 755 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); 756 UI != E; ++UI) 757 Worklist.push_back(cast<Instruction>(*UI)); 758 LPM->deleteSimpleAnalysisValue(I, L); 759 RemoveFromWorklist(I, Worklist); 760 I->replaceAllUsesWith(V); 761 I->eraseFromParent(); 762 ++NumSimplify; 763} 764 765/// RemoveBlockIfDead - If the specified block is dead, remove it, update loop 766/// information, and remove any dead successors it has. 767/// 768void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB, 769 std::vector<Instruction*> &Worklist, 770 Loop *L) { 771 if (pred_begin(BB) != pred_end(BB)) { 772 // This block isn't dead, since an edge to BB was just removed, see if there 773 // are any easy simplifications we can do now. 774 if (BasicBlock *Pred = BB->getSinglePredecessor()) { 775 // If it has one pred, fold phi nodes in BB. 776 while (isa<PHINode>(BB->begin())) 777 ReplaceUsesOfWith(BB->begin(), 778 cast<PHINode>(BB->begin())->getIncomingValue(0), 779 Worklist, L, LPM); 780 781 // If this is the header of a loop and the only pred is the latch, we now 782 // have an unreachable loop. 783 if (Loop *L = LI->getLoopFor(BB)) 784 if (loopHeader == BB && L->contains(Pred)) { 785 // Remove the branch from the latch to the header block, this makes 786 // the header dead, which will make the latch dead (because the header 787 // dominates the latch). 788 LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L); 789 Pred->getTerminator()->eraseFromParent(); 790 new UnreachableInst(Pred); 791 792 // The loop is now broken, remove it from LI. 793 RemoveLoopFromHierarchy(L); 794 795 // Reprocess the header, which now IS dead. 796 RemoveBlockIfDead(BB, Worklist, L); 797 return; 798 } 799 800 // If pred ends in a uncond branch, add uncond branch to worklist so that 801 // the two blocks will get merged. 802 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) 803 if (BI->isUnconditional()) 804 Worklist.push_back(BI); 805 } 806 return; 807 } 808 809 DOUT << "Nuking dead block: " << *BB; 810 811 // Remove the instructions in the basic block from the worklist. 812 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 813 RemoveFromWorklist(I, Worklist); 814 815 // Anything that uses the instructions in this basic block should have their 816 // uses replaced with undefs. 817 if (!I->use_empty()) 818 I->replaceAllUsesWith(UndefValue::get(I->getType())); 819 } 820 821 // If this is the edge to the header block for a loop, remove the loop and 822 // promote all subloops. 823 if (Loop *BBLoop = LI->getLoopFor(BB)) { 824 if (BBLoop->getLoopLatch() == BB) 825 RemoveLoopFromHierarchy(BBLoop); 826 } 827 828 // Remove the block from the loop info, which removes it from any loops it 829 // was in. 830 LI->removeBlock(BB); 831 832 833 // Remove phi node entries in successors for this block. 834 TerminatorInst *TI = BB->getTerminator(); 835 std::vector<BasicBlock*> Succs; 836 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) { 837 Succs.push_back(TI->getSuccessor(i)); 838 TI->getSuccessor(i)->removePredecessor(BB); 839 } 840 841 // Unique the successors, remove anything with multiple uses. 842 std::sort(Succs.begin(), Succs.end()); 843 Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end()); 844 845 // Remove the basic block, including all of the instructions contained in it. 846 LPM->deleteSimpleAnalysisValue(BB, L); 847 BB->eraseFromParent(); 848 // Remove successor blocks here that are not dead, so that we know we only 849 // have dead blocks in this list. Nondead blocks have a way of becoming dead, 850 // then getting removed before we revisit them, which is badness. 851 // 852 for (unsigned i = 0; i != Succs.size(); ++i) 853 if (pred_begin(Succs[i]) != pred_end(Succs[i])) { 854 // One exception is loop headers. If this block was the preheader for a 855 // loop, then we DO want to visit the loop so the loop gets deleted. 856 // We know that if the successor is a loop header, that this loop had to 857 // be the preheader: the case where this was the latch block was handled 858 // above and headers can only have two predecessors. 859 if (!LI->isLoopHeader(Succs[i])) { 860 Succs.erase(Succs.begin()+i); 861 --i; 862 } 863 } 864 865 for (unsigned i = 0, e = Succs.size(); i != e; ++i) 866 RemoveBlockIfDead(Succs[i], Worklist, L); 867} 868 869/// RemoveLoopFromHierarchy - We have discovered that the specified loop has 870/// become unwrapped, either because the backedge was deleted, or because the 871/// edge into the header was removed. If the edge into the header from the 872/// latch block was removed, the loop is unwrapped but subloops are still alive, 873/// so they just reparent loops. If the loops are actually dead, they will be 874/// removed later. 875void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) { 876 LPM->deleteLoopFromQueue(L); 877 RemoveLoopFromWorklist(L); 878} 879 880// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has 881// the value specified by Val in the specified loop, or we know it does NOT have 882// that value. Rewrite any uses of LIC or of properties correlated to it. 883void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 884 Constant *Val, 885 bool IsEqual) { 886 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 887 888 // FIXME: Support correlated properties, like: 889 // for (...) 890 // if (li1 < li2) 891 // ... 892 // if (li1 > li2) 893 // ... 894 895 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 896 // selects, switches. 897 std::vector<User*> Users(LIC->use_begin(), LIC->use_end()); 898 std::vector<Instruction*> Worklist; 899 900 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC 901 // in the loop with the appropriate one directly. 902 if (IsEqual || (isa<ConstantInt>(Val) && Val->getType() == Type::Int1Ty)) { 903 Value *Replacement; 904 if (IsEqual) 905 Replacement = Val; 906 else 907 Replacement = ConstantInt::get(Type::Int1Ty, 908 !cast<ConstantInt>(Val)->getZExtValue()); 909 910 for (unsigned i = 0, e = Users.size(); i != e; ++i) 911 if (Instruction *U = cast<Instruction>(Users[i])) { 912 if (!L->contains(U->getParent())) 913 continue; 914 U->replaceUsesOfWith(LIC, Replacement); 915 Worklist.push_back(U); 916 } 917 } else { 918 // Otherwise, we don't know the precise value of LIC, but we do know that it 919 // is certainly NOT "Val". As such, simplify any uses in the loop that we 920 // can. This case occurs when we unswitch switch statements. 921 for (unsigned i = 0, e = Users.size(); i != e; ++i) 922 if (Instruction *U = cast<Instruction>(Users[i])) { 923 if (!L->contains(U->getParent())) 924 continue; 925 926 Worklist.push_back(U); 927 928 // If we know that LIC is not Val, use this info to simplify code. 929 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) { 930 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) { 931 if (SI->getCaseValue(i) == Val) { 932 // Found a dead case value. Don't remove PHI nodes in the 933 // successor if they become single-entry, those PHI nodes may 934 // be in the Users list. 935 936 // FIXME: This is a hack. We need to keep the successor around 937 // and hooked up so as to preserve the loop structure, because 938 // trying to update it is complicated. So instead we preserve the 939 // loop structure and put the block on an dead code path. 940 941 BasicBlock *SISucc = SI->getSuccessor(i); 942 BasicBlock* Old = SI->getParent(); 943 BasicBlock* Split = SplitBlock(Old, SI, this); 944 945 Instruction* OldTerm = Old->getTerminator(); 946 BranchInst::Create(Split, SISucc, 947 ConstantInt::getTrue(), OldTerm); 948 949 LPM->deleteSimpleAnalysisValue(Old->getTerminator(), L); 950 Old->getTerminator()->eraseFromParent(); 951 952 PHINode *PN; 953 for (BasicBlock::iterator II = SISucc->begin(); 954 (PN = dyn_cast<PHINode>(II)); ++II) { 955 Value *InVal = PN->removeIncomingValue(Split, false); 956 PN->addIncoming(InVal, Old); 957 } 958 959 SI->removeCase(i); 960 break; 961 } 962 } 963 } 964 965 // TODO: We could do other simplifications, for example, turning 966 // LIC == Val -> false. 967 } 968 } 969 970 SimplifyCode(Worklist, L); 971} 972 973/// SimplifyCode - Okay, now that we have simplified some instructions in the 974/// loop, walk over it and constant prop, dce, and fold control flow where 975/// possible. Note that this is effectively a very simple loop-structure-aware 976/// optimizer. During processing of this loop, L could very well be deleted, so 977/// it must not be used. 978/// 979/// FIXME: When the loop optimizer is more mature, separate this out to a new 980/// pass. 981/// 982void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) { 983 while (!Worklist.empty()) { 984 Instruction *I = Worklist.back(); 985 Worklist.pop_back(); 986 987 // Simple constant folding. 988 if (Constant *C = ConstantFoldInstruction(I)) { 989 ReplaceUsesOfWith(I, C, Worklist, L, LPM); 990 continue; 991 } 992 993 // Simple DCE. 994 if (isInstructionTriviallyDead(I)) { 995 DOUT << "Remove dead instruction '" << *I; 996 997 // Add uses to the worklist, which may be dead now. 998 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 999 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) 1000 Worklist.push_back(Use); 1001 LPM->deleteSimpleAnalysisValue(I, L); 1002 RemoveFromWorklist(I, Worklist); 1003 I->eraseFromParent(); 1004 ++NumSimplify; 1005 continue; 1006 } 1007 1008 // Special case hacks that appear commonly in unswitched code. 1009 switch (I->getOpcode()) { 1010 case Instruction::Select: 1011 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(0))) { 1012 ReplaceUsesOfWith(I, I->getOperand(!CB->getZExtValue()+1), Worklist, L, 1013 LPM); 1014 continue; 1015 } 1016 break; 1017 case Instruction::And: 1018 if (isa<ConstantInt>(I->getOperand(0)) && 1019 I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS 1020 cast<BinaryOperator>(I)->swapOperands(); 1021 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1))) 1022 if (CB->getType() == Type::Int1Ty) { 1023 if (CB->isOne()) // X & 1 -> X 1024 ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM); 1025 else // X & 0 -> 0 1026 ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM); 1027 continue; 1028 } 1029 break; 1030 case Instruction::Or: 1031 if (isa<ConstantInt>(I->getOperand(0)) && 1032 I->getOperand(0)->getType() == Type::Int1Ty) // constant -> RHS 1033 cast<BinaryOperator>(I)->swapOperands(); 1034 if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1))) 1035 if (CB->getType() == Type::Int1Ty) { 1036 if (CB->isOne()) // X | 1 -> 1 1037 ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM); 1038 else // X | 0 -> X 1039 ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM); 1040 continue; 1041 } 1042 break; 1043 case Instruction::Br: { 1044 BranchInst *BI = cast<BranchInst>(I); 1045 if (BI->isUnconditional()) { 1046 // If BI's parent is the only pred of the successor, fold the two blocks 1047 // together. 1048 BasicBlock *Pred = BI->getParent(); 1049 BasicBlock *Succ = BI->getSuccessor(0); 1050 BasicBlock *SinglePred = Succ->getSinglePredecessor(); 1051 if (!SinglePred) continue; // Nothing to do. 1052 assert(SinglePred == Pred && "CFG broken"); 1053 1054 DOUT << "Merging blocks: " << Pred->getName() << " <- " 1055 << Succ->getName() << "\n"; 1056 1057 // Resolve any single entry PHI nodes in Succ. 1058 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) 1059 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM); 1060 1061 // Move all of the successor contents from Succ to Pred. 1062 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(), 1063 Succ->end()); 1064 LPM->deleteSimpleAnalysisValue(BI, L); 1065 BI->eraseFromParent(); 1066 RemoveFromWorklist(BI, Worklist); 1067 1068 // If Succ has any successors with PHI nodes, update them to have 1069 // entries coming from Pred instead of Succ. 1070 Succ->replaceAllUsesWith(Pred); 1071 1072 // Remove Succ from the loop tree. 1073 LI->removeBlock(Succ); 1074 LPM->deleteSimpleAnalysisValue(Succ, L); 1075 Succ->eraseFromParent(); 1076 ++NumSimplify; 1077 } else if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){ 1078 // Conditional branch. Turn it into an unconditional branch, then 1079 // remove dead blocks. 1080 break; // FIXME: Enable. 1081 1082 DOUT << "Folded branch: " << *BI; 1083 BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue()); 1084 BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue()); 1085 DeadSucc->removePredecessor(BI->getParent(), true); 1086 Worklist.push_back(BranchInst::Create(LiveSucc, BI)); 1087 LPM->deleteSimpleAnalysisValue(BI, L); 1088 BI->eraseFromParent(); 1089 RemoveFromWorklist(BI, Worklist); 1090 ++NumSimplify; 1091 1092 RemoveBlockIfDead(DeadSucc, Worklist, L); 1093 } 1094 break; 1095 } 1096 } 1097 } 1098} 1099