LoopUnswitch.cpp revision 6d9d13d2e493250b62fdf9812058a7a4dbcc6513
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 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val, 79 bool isEqual); 80 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, 81 BasicBlock *ExitBlock); 82 }; 83 RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops"); 84} 85 86FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); } 87 88bool LoopUnswitch::runOnFunction(Function &F) { 89 bool Changed = false; 90 LI = &getAnalysis<LoopInfo>(); 91 92 // Transform all the top-level loops. Copy the loop list so that the child 93 // can update the loop tree if it needs to delete the loop. 94 std::vector<Loop*> SubLoops(LI->begin(), LI->end()); 95 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) 96 Changed |= visitLoop(SubLoops[i]); 97 98 return Changed; 99} 100 101 102/// LoopValuesUsedOutsideLoop - Return true if there are any values defined in 103/// the loop that are used by instructions outside of it. 104static bool LoopValuesUsedOutsideLoop(Loop *L) { 105 // We will be doing lots of "loop contains block" queries. Loop::contains is 106 // linear time, use a set to speed this up. 107 std::set<BasicBlock*> LoopBlocks; 108 109 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); 110 BB != E; ++BB) 111 LoopBlocks.insert(*BB); 112 113 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); 114 BB != E; ++BB) { 115 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I) 116 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 117 ++UI) { 118 BasicBlock *UserBB = cast<Instruction>(*UI)->getParent(); 119 if (!LoopBlocks.count(UserBB)) 120 return true; 121 } 122 } 123 return false; 124} 125 126/// FindTrivialLoopExitBlock - We know that we have a branch from the loop 127/// header to the specified latch block. See if one of the successors of the 128/// latch block is an exit, and if so what block it is. 129static BasicBlock *FindTrivialLoopExitBlock(Loop *L, BasicBlock *Latch) { 130 BasicBlock *Header = L->getHeader(); 131 BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator()); 132 if (!LatchBranch || !LatchBranch->isConditional()) return 0; 133 134 // Simple case, the latch block is a conditional branch. The target that 135 // doesn't go to the loop header is our block if it is not in the loop. 136 if (LatchBranch->getSuccessor(0) == Header) { 137 if (L->contains(LatchBranch->getSuccessor(1))) return false; 138 return LatchBranch->getSuccessor(1); 139 } else { 140 assert(LatchBranch->getSuccessor(1) == Header); 141 if (L->contains(LatchBranch->getSuccessor(0))) return false; 142 return LatchBranch->getSuccessor(0); 143 } 144} 145 146 147/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is 148/// trivial: that is, that the condition controls whether or not the loop does 149/// anything at all. If this is a trivial condition, unswitching produces no 150/// code duplications (equivalently, it produces a simpler loop and a new empty 151/// loop, which gets deleted). 152/// 153/// If this is a trivial condition, return ConstantBool::True if the loop body 154/// runs when the condition is true, False if the loop body executes when the 155/// condition is false. Otherwise, return null to indicate a complex condition. 156static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond, 157 Constant **Val = 0, 158 BasicBlock **LoopExit = 0) { 159 BasicBlock *Header = L->getHeader(); 160 BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator()); 161 162 // If the header block doesn't end with a conditional branch on Cond, we can't 163 // handle it. 164 if (!HeaderTerm || !HeaderTerm->isConditional() || 165 HeaderTerm->getCondition() != Cond) 166 return false; 167 168 // Check to see if the conditional branch goes to the latch block. If not, 169 // it's not trivial. This also determines the value of Cond that will execute 170 // the loop. 171 BasicBlock *Latch = L->getLoopLatch(); 172 if (HeaderTerm->getSuccessor(1) == Latch) { 173 if (Val) *Val = ConstantBool::True; 174 } else if (HeaderTerm->getSuccessor(0) == Latch) 175 if (Val) *Val = ConstantBool::False; 176 else 177 return false; // Doesn't branch to latch block. 178 179 // The latch block must end with a conditional branch where one edge goes to 180 // the header (this much we know) and one edge goes OUT of the loop. 181 BasicBlock *LoopExitBlock = FindTrivialLoopExitBlock(L, Latch); 182 if (!LoopExitBlock) return 0; 183 if (LoopExit) *LoopExit = LoopExitBlock; 184 185 // We already know that nothing uses any scalar values defined inside of this 186 // loop. As such, we just have to check to see if this loop will execute any 187 // side-effecting instructions (e.g. stores, calls, volatile loads) in the 188 // part of the loop that the code *would* execute. 189 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 190 if (I->mayWriteToMemory()) 191 return false; 192 for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I) 193 if (I->mayWriteToMemory()) 194 return false; 195 return true; 196} 197 198/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if 199/// we choose to unswitch the specified loop on the specified value. 200/// 201unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) { 202 // If the condition is trivial, always unswitch. There is no code growth for 203 // this case. 204 if (IsTrivialUnswitchCondition(L, LIC)) 205 return 0; 206 207 unsigned Cost = 0; 208 // FIXME: this is brain dead. It should take into consideration code 209 // shrinkage. 210 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 211 I != E; ++I) { 212 BasicBlock *BB = *I; 213 // Do not include empty blocks in the cost calculation. This happen due to 214 // loop canonicalization and will be removed. 215 if (BB->begin() == BasicBlock::iterator(BB->getTerminator())) 216 continue; 217 218 // Count basic blocks. 219 ++Cost; 220 } 221 222 return Cost; 223} 224 225/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is 226/// invariant in the loop, or has an invariant piece, return the invariant. 227/// Otherwise, return null. 228static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { 229 // Constants should be folded, not unswitched on! 230 if (isa<Constant>(Cond)) return false; 231 232 // TODO: Handle: br (VARIANT|INVARIANT). 233 // TODO: Hoist simple expressions out of loops. 234 if (L->isLoopInvariant(Cond)) return Cond; 235 236 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 237 if (BO->getOpcode() == Instruction::And || 238 BO->getOpcode() == Instruction::Or) { 239 // If either the left or right side is invariant, we can unswitch on this, 240 // which will cause the branch to go away in one loop and the condition to 241 // simplify in the other one. 242 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) 243 return LHS; 244 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) 245 return RHS; 246 } 247 248 return 0; 249} 250 251bool LoopUnswitch::visitLoop(Loop *L) { 252 bool Changed = false; 253 254 // Recurse through all subloops before we process this loop. Copy the loop 255 // list so that the child can update the loop tree if it needs to delete the 256 // loop. 257 std::vector<Loop*> SubLoops(L->begin(), L->end()); 258 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) 259 Changed |= visitLoop(SubLoops[i]); 260 261 // Loop over all of the basic blocks in the loop. If we find an interior 262 // block that is branching on a loop-invariant condition, we can unswitch this 263 // loop. 264 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 265 I != E; ++I) { 266 TerminatorInst *TI = (*I)->getTerminator(); 267 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 268 // If this isn't branching on an invariant condition, we can't unswitch 269 // it. 270 if (BI->isConditional()) { 271 // See if this, or some part of it, is loop invariant. If so, we can 272 // unswitch on it if we desire. 273 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed); 274 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { 275 ++NumBranches; 276 return true; 277 } 278 } 279 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 280 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); 281 if (LoopCond && SI->getNumCases() > 1) { 282 // Find a value to unswitch on: 283 // FIXME: this should chose the most expensive case! 284 Constant *UnswitchVal = SI->getCaseValue(1); 285 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) { 286 ++NumSwitches; 287 return true; 288 } 289 } 290 } 291 292 // Scan the instructions to check for unswitchable values. 293 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 294 BBI != E; ++BBI) 295 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 296 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); 297 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { 298 ++NumSelects; 299 return true; 300 } 301 } 302 } 303 304 return Changed; 305} 306 307/// UnswitchIfProfitable - We have found that we can unswitch L when 308/// LoopCond == Val to simplify the loop. If we decide that this is profitable, 309/// unswitch the loop, reprocess the pieces, then return true. 310bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){ 311 // Check to see if it would be profitable to unswitch this loop. 312 if (getLoopUnswitchCost(L, LoopCond) > Threshold) { 313 // FIXME: this should estimate growth by the amount of code shared by the 314 // resultant unswitched loops. 315 // 316 DEBUG(std::cerr << "NOT unswitching loop %" 317 << L->getHeader()->getName() << ", cost too high: " 318 << L->getBlocks().size() << "\n"); 319 return false; 320 } 321 322 // If this loop has live-out values, we can't unswitch it. We need something 323 // like loop-closed SSA form in order to know how to insert PHI nodes for 324 // these values. 325 if (LoopValuesUsedOutsideLoop(L)) { 326 DEBUG(std::cerr << "NOT unswitching loop %" << L->getHeader()->getName() 327 << ", a loop value is used outside loop!\n"); 328 return false; 329 } 330 331 //std::cerr << "BEFORE:\n"; LI->dump(); 332 Loop *NewLoop1 = 0, *NewLoop2 = 0; 333 334 // If this is a trivial condition to unswitch (which results in no code 335 // duplication), do it now. 336 Constant *CondVal; 337 BasicBlock *ExitBlock; 338 if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &ExitBlock)){ 339 UnswitchTrivialCondition(L, LoopCond, CondVal, ExitBlock); 340 NewLoop1 = L; 341 } else { 342 VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2); 343 } 344 345 //std::cerr << "AFTER:\n"; LI->dump(); 346 347 // Try to unswitch each of our new loops now! 348 if (NewLoop1) visitLoop(NewLoop1); 349 if (NewLoop2) visitLoop(NewLoop2); 350 return true; 351} 352 353BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) { 354 TerminatorInst *LatchTerm = BB->getTerminator(); 355 unsigned SuccNum = 0; 356 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) { 357 assert(i != e && "Didn't find edge?"); 358 if (LatchTerm->getSuccessor(i) == Succ) { 359 SuccNum = i; 360 break; 361 } 362 } 363 364 // If this is a critical edge, let SplitCriticalEdge do it. 365 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this)) 366 return LatchTerm->getSuccessor(SuccNum); 367 368 // If the edge isn't critical, then BB has a single successor or Succ has a 369 // single pred. Split the block. 370 BasicBlock *BlockToSplit; 371 BasicBlock::iterator SplitPoint; 372 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 373 // If the successor only has a single pred, split the top of the successor 374 // block. 375 assert(SP == BB && "CFG broken"); 376 BlockToSplit = Succ; 377 SplitPoint = Succ->begin(); 378 } else { 379 // Otherwise, if BB has a single successor, split it at the bottom of the 380 // block. 381 assert(BB->getTerminator()->getNumSuccessors() == 1 && 382 "Should have a single succ!"); 383 BlockToSplit = BB; 384 SplitPoint = BB->getTerminator(); 385 } 386 387 BasicBlock *New = 388 BlockToSplit->splitBasicBlock(SplitPoint, 389 BlockToSplit->getName()+".tail"); 390 // New now lives in whichever loop that BB used to. 391 if (Loop *L = LI->getLoopFor(BlockToSplit)) 392 L->addBasicBlockToLoop(New, *LI); 393 return New; 394} 395 396 397 398// RemapInstruction - Convert the instruction operands from referencing the 399// current values into those specified by ValueMap. 400// 401static inline void RemapInstruction(Instruction *I, 402 std::map<const Value *, Value*> &ValueMap) { 403 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 404 Value *Op = I->getOperand(op); 405 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op); 406 if (It != ValueMap.end()) Op = It->second; 407 I->setOperand(op, Op); 408 } 409} 410 411/// CloneLoop - Recursively clone the specified loop and all of its children, 412/// mapping the blocks with the specified map. 413static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM, 414 LoopInfo *LI) { 415 Loop *New = new Loop(); 416 417 if (PL) 418 PL->addChildLoop(New); 419 else 420 LI->addTopLevelLoop(New); 421 422 // Add all of the blocks in L to the new loop. 423 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 424 I != E; ++I) 425 if (LI->getLoopFor(*I) == L) 426 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI); 427 428 // Add all of the subloops to the new loop. 429 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 430 CloneLoop(*I, New, VM, LI); 431 432 return New; 433} 434 435/// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values 436/// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the 437/// code immediately before InsertPt. 438static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 439 BasicBlock *TrueDest, 440 BasicBlock *FalseDest, 441 Instruction *InsertPt) { 442 // Insert a conditional branch on LIC to the two preheaders. The original 443 // code is the true version and the new code is the false version. 444 Value *BranchVal = LIC; 445 if (!isa<ConstantBool>(BranchVal)) { 446 BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp", InsertPt); 447 } else if (Val != ConstantBool::True) { 448 // We want to enter the new loop when the condition is true. 449 std::swap(TrueDest, FalseDest); 450 } 451 452 // Insert the new branch. 453 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt); 454} 455 456 457/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable 458/// condition in it (a cond branch from its header block to its latch block, 459/// where the path through the loop that doesn't execute its body has no 460/// side-effects), unswitch it. This doesn't involve any code duplication, just 461/// moving the conditional branch outside of the loop and updating loop info. 462void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, 463 Constant *Val, 464 BasicBlock *ExitBlock) { 465 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %" 466 << L->getHeader()->getName() << " [" << L->getBlocks().size() 467 << " blocks] in Function " << L->getHeader()->getParent()->getName() 468 << " on cond:" << *Cond << "\n"); 469 470 // First step, split the preheader, so that we know that there is a safe place 471 // to insert the conditional branch. We will change 'OrigPH' to have a 472 // conditional branch on Cond. 473 BasicBlock *OrigPH = L->getLoopPreheader(); 474 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader()); 475 476 // Now that we have a place to insert the conditional branch, create a place 477 // to branch to: this is the exit block out of the loop that we should 478 // short-circuit to. 479 480 // Split this edge now, so that the loop maintains its exit block. 481 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 482 BasicBlock *NewExit = SplitEdge(L->getLoopLatch(), ExitBlock); 483 assert(NewExit != ExitBlock && "Edge not split!"); 484 485 // Okay, now we have a position to branch from and a position to branch to, 486 // insert the new conditional branch. 487 EmitPreheaderBranchOnCondition(Cond, Val, NewPH, NewExit, 488 OrigPH->getTerminator()); 489 OrigPH->getTerminator()->eraseFromParent(); 490 491 // Now that we know that the loop is never entered when this condition is a 492 // particular value, rewrite the loop with this info. We know that this will 493 // at least eliminate the old branch. 494 RewriteLoopBodyWithConditionConstant(L, Cond, Val, true); 495 ++NumTrivial; 496} 497 498 499/// VersionLoop - We determined that the loop is profitable to unswitch when LIC 500/// equal Val. Split it into loop versions and test the condition outside of 501/// either loop. Return the loops created as Out1/Out2. 502void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L, 503 Loop *&Out1, Loop *&Out2) { 504 Function *F = L->getHeader()->getParent(); 505 506 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %" 507 << L->getHeader()->getName() << " [" << L->getBlocks().size() 508 << " blocks] in Function " << F->getName() 509 << " when '" << *Val << "' == " << *LIC << "\n"); 510 511 // LoopBlocks contains all of the basic blocks of the loop, including the 512 // preheader of the loop, the body of the loop, and the exit blocks of the 513 // loop, in that order. 514 std::vector<BasicBlock*> LoopBlocks; 515 516 // First step, split the preheader and exit blocks, and add these blocks to 517 // the LoopBlocks list. 518 BasicBlock *OrigPreheader = L->getLoopPreheader(); 519 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader())); 520 521 // We want the loop to come after the preheader, but before the exit blocks. 522 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 523 524 std::vector<BasicBlock*> ExitBlocks; 525 L->getExitBlocks(ExitBlocks); 526 std::sort(ExitBlocks.begin(), ExitBlocks.end()); 527 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()), 528 ExitBlocks.end()); 529 530 // Split all of the edges from inside the loop to their exit blocks. This 531 // unswitching trivial: no phi nodes to update. 532 unsigned NumBlocks = L->getBlocks().size(); 533 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 534 BasicBlock *ExitBlock = ExitBlocks[i]; 535 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock)); 536 537 for (unsigned j = 0, e = Preds.size(); j != e; ++j) { 538 assert(L->contains(Preds[j]) && 539 "All preds of loop exit blocks must be the same loop!"); 540 SplitEdge(Preds[j], ExitBlock); 541 } 542 } 543 544 // The exit blocks may have been changed due to edge splitting, recompute. 545 ExitBlocks.clear(); 546 L->getExitBlocks(ExitBlocks); 547 std::sort(ExitBlocks.begin(), ExitBlocks.end()); 548 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()), 549 ExitBlocks.end()); 550 551 // Add exit blocks to the loop blocks. 552 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 553 554 // Next step, clone all of the basic blocks that make up the loop (including 555 // the loop preheader and exit blocks), keeping track of the mapping between 556 // the instructions and blocks. 557 std::vector<BasicBlock*> NewBlocks; 558 NewBlocks.reserve(LoopBlocks.size()); 559 std::map<const Value*, Value*> ValueMap; 560 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 561 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); 562 NewBlocks.push_back(New); 563 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping. 564 } 565 566 // Splice the newly inserted blocks into the function right before the 567 // original preheader. 568 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(), 569 NewBlocks[0], F->end()); 570 571 // Now we create the new Loop object for the versioned loop. 572 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI); 573 Loop *ParentLoop = L->getParentLoop(); 574 if (ParentLoop) { 575 // Make sure to add the cloned preheader and exit blocks to the parent loop 576 // as well. 577 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI); 578 } 579 580 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 581 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]); 582 if (ParentLoop) 583 ParentLoop->addBasicBlockToLoop(cast<BasicBlock>(NewExit), *LI); 584 585 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 586 "Exit block should have been split to have one successor!"); 587 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 588 589 // If the successor of the exit block had PHI nodes, add an entry for 590 // NewExit. 591 PHINode *PN; 592 for (BasicBlock::iterator I = ExitSucc->begin(); 593 (PN = dyn_cast<PHINode>(I)); ++I) { 594 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); 595 std::map<const Value *, Value*>::iterator It = ValueMap.find(V); 596 if (It != ValueMap.end()) V = It->second; 597 PN->addIncoming(V, NewExit); 598 } 599 } 600 601 // Rewrite the code to refer to itself. 602 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) 603 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 604 E = NewBlocks[i]->end(); I != E; ++I) 605 RemapInstruction(I, ValueMap); 606 607 // Rewrite the original preheader to select between versions of the loop. 608 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator()); 609 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 610 "Preheader splitting did not work correctly!"); 611 612 // Emit the new branch that selects between the two versions of this loop. 613 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); 614 OldBR->eraseFromParent(); 615 616 // Now we rewrite the original code to know that the condition is true and the 617 // new code to know that the condition is false. 618 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false); 619 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); 620 Out1 = L; 621 Out2 = NewLoop; 622} 623 624// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has 625// the value specified by Val in the specified loop, or we know it does NOT have 626// that value. Rewrite any uses of LIC or of properties correlated to it. 627void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 628 Constant *Val, 629 bool IsEqual) { 630 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 631 632 // FIXME: Support correlated properties, like: 633 // for (...) 634 // if (li1 < li2) 635 // ... 636 // if (li1 > li2) 637 // ... 638 639 // NotVal - If Val is a bool, this contains its inverse. 640 Constant *NotVal = 0; 641 if (ConstantBool *CB = dyn_cast<ConstantBool>(Val)) 642 NotVal = ConstantBool::get(!CB->getValue()); 643 644 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 645 // selects, switches. 646 std::vector<User*> Users(LIC->use_begin(), LIC->use_end()); 647 648 // Haha, this loop could be unswitched. Get it? The unswitch pass could 649 // unswitch itself. Amazing. 650 for (unsigned i = 0, e = Users.size(); i != e; ++i) 651 if (Instruction *U = cast<Instruction>(Users[i])) 652 if (L->contains(U->getParent())) 653 if (IsEqual) { 654 U->replaceUsesOfWith(LIC, Val); 655 } else if (NotVal) { 656 U->replaceUsesOfWith(LIC, NotVal); 657 } else { 658 // If we know that LIC is not Val, use this info to simplify code. 659 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) { 660 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) { 661 if (SI->getCaseValue(i) == Val) { 662 // Found a dead case value. Don't remove PHI nodes in the 663 // successor if they become single-entry, those PHI nodes may 664 // be in the Users list. 665 SI->getSuccessor(i)->removePredecessor(SI->getParent(), true); 666 SI->removeCase(i); 667 break; 668 } 669 } 670 } 671 672 // TODO: We could simplify stuff like X == C. 673 } 674} 675