LoopUnswitch.cpp revision 4e1323969c4afa60cd6675cbbaade90afe7e6632
1//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source 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/Function.h" 33#include "llvm/Instructions.h" 34#include "llvm/Analysis/LoopInfo.h" 35#include "llvm/Transforms/Utils/Cloning.h" 36#include "llvm/Transforms/Utils/Local.h" 37#include "llvm/Transforms/Utils/BasicBlockUtils.h" 38#include "llvm/ADT/Statistic.h" 39#include "llvm/Support/Debug.h" 40#include "llvm/Support/CommandLine.h" 41#include <algorithm> 42#include <iostream> 43#include <set> 44using namespace llvm; 45 46namespace { 47 Statistic<> NumBranches("loop-unswitch", "Number of branches unswitched"); 48 Statistic<> NumSwitches("loop-unswitch", "Number of switches unswitched"); 49 Statistic<> NumSelects ("loop-unswitch", "Number of selects unswitched"); 50 Statistic<> NumTrivial ("loop-unswitch", 51 "Number of unswitches that are trivial"); 52 cl::opt<unsigned> 53 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), 54 cl::init(10), cl::Hidden); 55 56 class LoopUnswitch : public FunctionPass { 57 LoopInfo *LI; // Loop information 58 public: 59 virtual bool runOnFunction(Function &F); 60 bool visitLoop(Loop *L); 61 62 /// This transformation requires natural loop information & requires that 63 /// loop preheaders be inserted into the CFG... 64 /// 65 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 66 AU.addRequiredID(LoopSimplifyID); 67 AU.addPreservedID(LoopSimplifyID); 68 AU.addRequired<LoopInfo>(); 69 AU.addPreserved<LoopInfo>(); 70 } 71 72 private: 73 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L); 74 unsigned getLoopUnswitchCost(Loop *L, Value *LIC); 75 void VersionLoop(Value *LIC, Constant *OnVal, 76 Loop *L, Loop *&Out1, Loop *&Out2); 77 BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To); 78 BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt); 79 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val, 80 bool isEqual); 81 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 82 BasicBlock *ExitBlock); 83 }; 84 RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops"); 85} 86 87FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); } 88 89bool LoopUnswitch::runOnFunction(Function &F) { 90 bool Changed = false; 91 LI = &getAnalysis<LoopInfo>(); 92 93 // Transform all the top-level loops. Copy the loop list so that the child 94 // can update the loop tree if it needs to delete the loop. 95 std::vector<Loop*> SubLoops(LI->begin(), LI->end()); 96 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) 97 Changed |= visitLoop(SubLoops[i]); 98 99 return Changed; 100} 101 102 103/// LoopValuesUsedOutsideLoop - Return true if there are any values defined in 104/// the loop that are used by instructions outside of it. 105static bool LoopValuesUsedOutsideLoop(Loop *L) { 106 // We will be doing lots of "loop contains block" queries. Loop::contains is 107 // linear time, use a set to speed this up. 108 std::set<BasicBlock*> LoopBlocks; 109 110 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); 111 BB != E; ++BB) 112 LoopBlocks.insert(*BB); 113 114 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); 115 BB != E; ++BB) { 116 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I) 117 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 118 ++UI) { 119 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent(); 120 if (!LoopBlocks.count(UserBB)) 121 return true; 122 } 123 } 124 return false; 125} 126 127/// isTrivialLoopExitBlock - Check to see if all paths from BB either: 128/// 1. Exit the loop with no side effects. 129/// 2. Branch to the latch block with no side-effects. 130/// 131/// If these conditions are true, we return true and set ExitBB to the block we 132/// exit through. 133/// 134static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, 135 BasicBlock *&ExitBB, 136 std::set<BasicBlock*> &Visited) { 137 BasicBlock *Header = L->getHeader(); 138 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { 139 if (!Visited.insert(*SI).second) { 140 // Already visited and Ok, end of recursion. 141 } else if (L->contains(*SI)) { 142 // Check to see if the successor is a trivial loop exit. 143 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) 144 return false; 145 } else { 146 // Otherwise, this is a loop exit, this is fine so long as this is the 147 // first exit. 148 if (ExitBB != 0) return false; 149 ExitBB = *SI; 150 } 151 } 152 153 // Okay, everything after this looks good, check to make sure that this block 154 // doesn't include any side effects. 155 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 156 if (I->mayWriteToMemory()) 157 return false; 158 159 return true; 160} 161 162static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { 163 std::set<BasicBlock*> Visited; 164 Visited.insert(L->getHeader()); // Branches to header are ok. 165 Visited.insert(BB); // Don't revisit BB after we do. 166 BasicBlock *ExitBB = 0; 167 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) 168 return ExitBB; 169 return 0; 170} 171 172/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is 173/// trivial: that is, that the condition controls whether or not the loop does 174/// anything at all. If this is a trivial condition, unswitching produces no 175/// code duplications (equivalently, it produces a simpler loop and a new empty 176/// loop, which gets deleted). 177/// 178/// If this is a trivial condition, return ConstantBool::True if the loop body 179/// runs when the condition is true, False if the loop body executes when the 180/// condition is false. Otherwise, return null to indicate a complex condition. 181static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond, 182 Constant **Val = 0, 183 BasicBlock **LoopExit = 0) { 184 BasicBlock *Header = L->getHeader(); 185 BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator()); 186 187 // If the header block doesn't end with a conditional branch on Cond, we can't 188 // handle it. 189 if (!HeaderTerm || !HeaderTerm->isConditional() || 190 HeaderTerm->getCondition() != Cond) 191 return false; 192 193 // Check to see if a successor of the branch is guaranteed to go to the latch 194 // block or exit through a one exit block without having any side-effects. If 195 // so, determine the value of Cond that causes it to do this. 196 BasicBlock *LoopExitBlock = 0; 197 if ((LoopExitBlock = isTrivialLoopExitBlock(L, HeaderTerm->getSuccessor(0)))){ 198 if (Val) *Val = ConstantBool::True; 199 } else if ((LoopExitBlock = 200 isTrivialLoopExitBlock(L, HeaderTerm->getSuccessor(1)))) { 201 if (Val) *Val = ConstantBool::False; 202 } 203 204 if (!LoopExitBlock) 205 return false; // Can't handle this. 206 207 if (LoopExit) *LoopExit = LoopExitBlock; 208 209 // We already know that nothing uses any scalar values defined inside of this 210 // loop. As such, we just have to check to see if this loop will execute any 211 // side-effecting instructions (e.g. stores, calls, volatile loads) in the 212 // part of the loop that the code *would* execute. We already checked the 213 // tail, check the header now. 214 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 215 if (I->mayWriteToMemory()) 216 return false; 217 return true; 218} 219 220/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if 221/// we choose to unswitch the specified loop on the specified value. 222/// 223unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) { 224 // If the condition is trivial, always unswitch. There is no code growth for 225 // this case. 226 if (IsTrivialUnswitchCondition(L, LIC)) 227 return 0; 228 229 unsigned Cost = 0; 230 // FIXME: this is brain dead. It should take into consideration code 231 // shrinkage. 232 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 233 I != E; ++I) { 234 BasicBlock *BB = *I; 235 // Do not include empty blocks in the cost calculation. This happen due to 236 // loop canonicalization and will be removed. 237 if (BB->begin() == BasicBlock::iterator(BB->getTerminator())) 238 continue; 239 240 // Count basic blocks. 241 ++Cost; 242 } 243 244 return Cost; 245} 246 247/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is 248/// invariant in the loop, or has an invariant piece, return the invariant. 249/// Otherwise, return null. 250static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { 251 // Constants should be folded, not unswitched on! 252 if (isa<Constant>(Cond)) return false; 253 254 // TODO: Handle: br (VARIANT|INVARIANT). 255 // TODO: Hoist simple expressions out of loops. 256 if (L->isLoopInvariant(Cond)) return Cond; 257 258 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 259 if (BO->getOpcode() == Instruction::And || 260 BO->getOpcode() == Instruction::Or) { 261 // If either the left or right side is invariant, we can unswitch on this, 262 // which will cause the branch to go away in one loop and the condition to 263 // simplify in the other one. 264 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) 265 return LHS; 266 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) 267 return RHS; 268 } 269 270 return 0; 271} 272 273bool LoopUnswitch::visitLoop(Loop *L) { 274 bool Changed = false; 275 276 // Recurse through all subloops before we process this loop. Copy the loop 277 // list so that the child can update the loop tree if it needs to delete the 278 // loop. 279 std::vector<Loop*> SubLoops(L->begin(), L->end()); 280 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) 281 Changed |= visitLoop(SubLoops[i]); 282 283 // Loop over all of the basic blocks in the loop. If we find an interior 284 // block that is branching on a loop-invariant condition, we can unswitch this 285 // loop. 286 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 287 I != E; ++I) { 288 TerminatorInst *TI = (*I)->getTerminator(); 289 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 290 // If this isn't branching on an invariant condition, we can't unswitch 291 // it. 292 if (BI->isConditional()) { 293 // See if this, or some part of it, is loop invariant. If so, we can 294 // unswitch on it if we desire. 295 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed); 296 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { 297 ++NumBranches; 298 return true; 299 } 300 } 301 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 302 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); 303 if (LoopCond && SI->getNumCases() > 1) { 304 // Find a value to unswitch on: 305 // FIXME: this should chose the most expensive case! 306 Constant *UnswitchVal = SI->getCaseValue(1); 307 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) { 308 ++NumSwitches; 309 return true; 310 } 311 } 312 } 313 314 // Scan the instructions to check for unswitchable values. 315 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 316 BBI != E; ++BBI) 317 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 318 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); 319 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { 320 ++NumSelects; 321 return true; 322 } 323 } 324 } 325 326 return Changed; 327} 328 329/// UnswitchIfProfitable - We have found that we can unswitch L when 330/// LoopCond == Val to simplify the loop. If we decide that this is profitable, 331/// unswitch the loop, reprocess the pieces, then return true. 332bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){ 333 // Check to see if it would be profitable to unswitch this loop. 334 if (getLoopUnswitchCost(L, LoopCond) > Threshold) { 335 // FIXME: this should estimate growth by the amount of code shared by the 336 // resultant unswitched loops. 337 // 338 DEBUG(std::cerr << "NOT unswitching loop %" 339 << L->getHeader()->getName() << ", cost too high: " 340 << L->getBlocks().size() << "\n"); 341 return false; 342 } 343 344 // If this loop has live-out values, we can't unswitch it. We need something 345 // like loop-closed SSA form in order to know how to insert PHI nodes for 346 // these values. 347 if (LoopValuesUsedOutsideLoop(L)) { 348 DEBUG(std::cerr << "NOT unswitching loop %" << L->getHeader()->getName() 349 << ", a loop value is used outside loop!\n"); 350 return false; 351 } 352 353 //std::cerr << "BEFORE:\n"; LI->dump(); 354 Loop *NewLoop1 = 0, *NewLoop2 = 0; 355 356 // If this is a trivial condition to unswitch (which results in no code 357 // duplication), do it now. 358 Constant *CondVal; 359 BasicBlock *ExitBlock; 360 if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &ExitBlock)){ 361 UnswitchTrivialCondition(L, LoopCond, CondVal, ExitBlock); 362 NewLoop1 = L; 363 } else { 364 VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2); 365 } 366 367 //std::cerr << "AFTER:\n"; LI->dump(); 368 369 // Try to unswitch each of our new loops now! 370 if (NewLoop1) visitLoop(NewLoop1); 371 if (NewLoop2) visitLoop(NewLoop2); 372 return true; 373} 374 375/// SplitBlock - Split the specified block at the specified instruction - every 376/// thing before SplitPt stays in Old and everything starting with SplitPt moves 377/// to a new block. The two blocks are joined by an unconditional branch and 378/// the loop info is updated. 379/// 380BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *Old, Instruction *SplitPt) { 381 while (isa<PHINode>(SplitPt)) 382 ++SplitPt; 383 BasicBlock *New = Old->splitBasicBlock(SplitPt, Old->getName()+".split"); 384 385 // The new block lives in whichever loop the old one did. 386 if (Loop *L = LI->getLoopFor(Old)) 387 L->addBasicBlockToLoop(New, *LI); 388 389 return New; 390} 391 392 393BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) { 394 TerminatorInst *LatchTerm = BB->getTerminator(); 395 unsigned SuccNum = 0; 396 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) { 397 assert(i != e && "Didn't find edge?"); 398 if (LatchTerm->getSuccessor(i) == Succ) { 399 SuccNum = i; 400 break; 401 } 402 } 403 404 // If this is a critical edge, let SplitCriticalEdge do it. 405 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this)) 406 return LatchTerm->getSuccessor(SuccNum); 407 408 // If the edge isn't critical, then BB has a single successor or Succ has a 409 // single pred. Split the block. 410 BasicBlock *BlockToSplit; 411 BasicBlock::iterator SplitPoint; 412 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 413 // If the successor only has a single pred, split the top of the successor 414 // block. 415 assert(SP == BB && "CFG broken"); 416 return SplitBlock(Succ, Succ->begin()); 417 } else { 418 // Otherwise, if BB has a single successor, split it at the bottom of the 419 // block. 420 assert(BB->getTerminator()->getNumSuccessors() == 1 && 421 "Should have a single succ!"); 422 return SplitBlock(BB, BB->getTerminator()); 423 } 424} 425 426 427 428// RemapInstruction - Convert the instruction operands from referencing the 429// current values into those specified by ValueMap. 430// 431static inline void RemapInstruction(Instruction *I, 432 std::map<const Value *, Value*> &ValueMap) { 433 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 434 Value *Op = I->getOperand(op); 435 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op); 436 if (It != ValueMap.end()) Op = It->second; 437 I->setOperand(op, Op); 438 } 439} 440 441/// CloneLoop - Recursively clone the specified loop and all of its children, 442/// mapping the blocks with the specified map. 443static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM, 444 LoopInfo *LI) { 445 Loop *New = new Loop(); 446 447 if (PL) 448 PL->addChildLoop(New); 449 else 450 LI->addTopLevelLoop(New); 451 452 // Add all of the blocks in L to the new loop. 453 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 454 I != E; ++I) 455 if (LI->getLoopFor(*I) == L) 456 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI); 457 458 // Add all of the subloops to the new loop. 459 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 460 CloneLoop(*I, New, VM, LI); 461 462 return New; 463} 464 465/// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values 466/// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the 467/// code immediately before InsertPt. 468static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 469 BasicBlock *TrueDest, 470 BasicBlock *FalseDest, 471 Instruction *InsertPt) { 472 // Insert a conditional branch on LIC to the two preheaders. The original 473 // code is the true version and the new code is the false version. 474 Value *BranchVal = LIC; 475 if (!isa<ConstantBool>(Val)) { 476 BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp", InsertPt); 477 } else if (Val != ConstantBool::True) { 478 // We want to enter the new loop when the condition is true. 479 std::swap(TrueDest, FalseDest); 480 } 481 482 // Insert the new branch. 483 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt); 484} 485 486 487/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable 488/// condition in it (a cond branch from its header block to its latch block, 489/// where the path through the loop that doesn't execute its body has no 490/// side-effects), unswitch it. This doesn't involve any code duplication, just 491/// moving the conditional branch outside of the loop and updating loop info. 492void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, 493 Constant *Val, 494 BasicBlock *ExitBlock) { 495 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %" 496 << L->getHeader()->getName() << " [" << L->getBlocks().size() 497 << " blocks] in Function " << L->getHeader()->getParent()->getName() 498 << " on cond:" << *Cond << "\n"); 499 500 // First step, split the preheader, so that we know that there is a safe place 501 // to insert the conditional branch. We will change 'OrigPH' to have a 502 // conditional branch on Cond. 503 BasicBlock *OrigPH = L->getLoopPreheader(); 504 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader()); 505 506 // Now that we have a place to insert the conditional branch, create a place 507 // to branch to: this is the exit block out of the loop that we should 508 // short-circuit to. 509 510 // Split this block now, so that the loop maintains its exit block, and so 511 // that the jump from the preheader can execute the contents of the exit block 512 // without actually branching to it (the exit block should be dominated by the 513 // loop header, not the preheader). 514 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 515 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin()); 516 517 // Okay, now we have a position to branch from and a position to branch to, 518 // insert the new conditional branch. 519 EmitPreheaderBranchOnCondition(Cond, Val, NewPH, NewExit, 520 OrigPH->getTerminator()); 521 OrigPH->getTerminator()->eraseFromParent(); 522 523 // Now that we know that the loop is never entered when this condition is a 524 // particular value, rewrite the loop with this info. We know that this will 525 // at least eliminate the old branch. 526 RewriteLoopBodyWithConditionConstant(L, Cond, Val, true); 527 ++NumTrivial; 528} 529 530 531/// VersionLoop - We determined that the loop is profitable to unswitch when LIC 532/// equal Val. Split it into loop versions and test the condition outside of 533/// either loop. Return the loops created as Out1/Out2. 534void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L, 535 Loop *&Out1, Loop *&Out2) { 536 Function *F = L->getHeader()->getParent(); 537 538 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %" 539 << L->getHeader()->getName() << " [" << L->getBlocks().size() 540 << " blocks] in Function " << F->getName() 541 << " when '" << *Val << "' == " << *LIC << "\n"); 542 543 // LoopBlocks contains all of the basic blocks of the loop, including the 544 // preheader of the loop, the body of the loop, and the exit blocks of the 545 // loop, in that order. 546 std::vector<BasicBlock*> LoopBlocks; 547 548 // First step, split the preheader and exit blocks, and add these blocks to 549 // the LoopBlocks list. 550 BasicBlock *OrigPreheader = L->getLoopPreheader(); 551 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader())); 552 553 // We want the loop to come after the preheader, but before the exit blocks. 554 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 555 556 std::vector<BasicBlock*> ExitBlocks; 557 L->getExitBlocks(ExitBlocks); 558 std::sort(ExitBlocks.begin(), ExitBlocks.end()); 559 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()), 560 ExitBlocks.end()); 561 562 // Split all of the edges from inside the loop to their exit blocks. This 563 // unswitching trivial: no phi nodes to update. 564 unsigned NumBlocks = L->getBlocks().size(); 565 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 566 BasicBlock *ExitBlock = ExitBlocks[i]; 567 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock)); 568 569 for (unsigned j = 0, e = Preds.size(); j != e; ++j) { 570 assert(L->contains(Preds[j]) && 571 "All preds of loop exit blocks must be the same loop!"); 572 SplitEdge(Preds[j], ExitBlock); 573 } 574 } 575 576 // The exit blocks may have been changed due to edge splitting, recompute. 577 ExitBlocks.clear(); 578 L->getExitBlocks(ExitBlocks); 579 std::sort(ExitBlocks.begin(), ExitBlocks.end()); 580 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()), 581 ExitBlocks.end()); 582 583 // Add exit blocks to the loop blocks. 584 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 585 586 // Next step, clone all of the basic blocks that make up the loop (including 587 // the loop preheader and exit blocks), keeping track of the mapping between 588 // the instructions and blocks. 589 std::vector<BasicBlock*> NewBlocks; 590 NewBlocks.reserve(LoopBlocks.size()); 591 std::map<const Value*, Value*> ValueMap; 592 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 593 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); 594 NewBlocks.push_back(New); 595 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping. 596 } 597 598 // Splice the newly inserted blocks into the function right before the 599 // original preheader. 600 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(), 601 NewBlocks[0], F->end()); 602 603 // Now we create the new Loop object for the versioned loop. 604 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI); 605 Loop *ParentLoop = L->getParentLoop(); 606 if (ParentLoop) { 607 // Make sure to add the cloned preheader and exit blocks to the parent loop 608 // as well. 609 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI); 610 } 611 612 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 613 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]); 614 if (ParentLoop) 615 ParentLoop->addBasicBlockToLoop(cast<BasicBlock>(NewExit), *LI); 616 617 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 618 "Exit block should have been split to have one successor!"); 619 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 620 621 // If the successor of the exit block had PHI nodes, add an entry for 622 // NewExit. 623 PHINode *PN; 624 for (BasicBlock::iterator I = ExitSucc->begin(); 625 (PN = dyn_cast<PHINode>(I)); ++I) { 626 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); 627 std::map<const Value *, Value*>::iterator It = ValueMap.find(V); 628 if (It != ValueMap.end()) V = It->second; 629 PN->addIncoming(V, NewExit); 630 } 631 } 632 633 // Rewrite the code to refer to itself. 634 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) 635 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 636 E = NewBlocks[i]->end(); I != E; ++I) 637 RemapInstruction(I, ValueMap); 638 639 // Rewrite the original preheader to select between versions of the loop. 640 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator()); 641 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 642 "Preheader splitting did not work correctly!"); 643 644 // Emit the new branch that selects between the two versions of this loop. 645 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); 646 OldBR->eraseFromParent(); 647 648 // Now we rewrite the original code to know that the condition is true and the 649 // new code to know that the condition is false. 650 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false); 651 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); 652 Out1 = L; 653 Out2 = NewLoop; 654} 655 656// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has 657// the value specified by Val in the specified loop, or we know it does NOT have 658// that value. Rewrite any uses of LIC or of properties correlated to it. 659void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 660 Constant *Val, 661 bool IsEqual) { 662 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 663 664 // FIXME: Support correlated properties, like: 665 // for (...) 666 // if (li1 < li2) 667 // ... 668 // if (li1 > li2) 669 // ... 670 671 // NotVal - If Val is a bool, this contains its inverse. 672 Constant *NotVal = 0; 673 if (ConstantBool *CB = dyn_cast<ConstantBool>(Val)) 674 NotVal = ConstantBool::get(!CB->getValue()); 675 676 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 677 // selects, switches. 678 std::vector<User*> Users(LIC->use_begin(), LIC->use_end()); 679 680 // Haha, this loop could be unswitched. Get it? The unswitch pass could 681 // unswitch itself. Amazing. 682 for (unsigned i = 0, e = Users.size(); i != e; ++i) 683 if (Instruction *U = cast<Instruction>(Users[i])) 684 if (L->contains(U->getParent())) 685 if (IsEqual) { 686 U->replaceUsesOfWith(LIC, Val); 687 } else if (NotVal) { 688 U->replaceUsesOfWith(LIC, NotVal); 689 } else { 690 // If we know that LIC is not Val, use this info to simplify code. 691 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) { 692 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) { 693 if (SI->getCaseValue(i) == Val) { 694 // Found a dead case value. Don't remove PHI nodes in the 695 // successor if they become single-entry, those PHI nodes may 696 // be in the Users list. 697 SI->getSuccessor(i)->removePredecessor(SI->getParent(), true); 698 SI->removeCase(i); 699 break; 700 } 701 } 702 } 703 704 // TODO: We could simplify stuff like X == C. 705 } 706} 707