LoopUnswitch.cpp revision 10cd9bbde7bda0b125cb5153f330ddc1f8509bd1
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 bool EntersWhenTrue, 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 = BB->begin(), E = BB->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 bool *EntersWhenTrue = 0, 184 BasicBlock **LoopExit = 0) { 185 BasicBlock *Header = L->getHeader(); 186 TerminatorInst *HeaderTerm = Header->getTerminator(); 187 188 BasicBlock *LoopExitBB = 0; 189 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) { 190 // If the header block doesn't end with a conditional branch on Cond, we 191 // can't handle it. 192 if (!BI->isConditional() || BI->getCondition() != Cond) 193 return false; 194 195 // Check to see if a successor of the branch is guaranteed to go to the 196 // latch block or exit through a one exit block without having any 197 // side-effects. If so, determine the value of Cond that causes it to do 198 // this. 199 if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(0)))) { 200 if (Val) *Val = ConstantBool::False; 201 } else if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(1)))) { 202 if (Val) *Val = ConstantBool::True; 203 } 204 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) { 205 // If this isn't a switch on Cond, we can't handle it. 206 if (SI->getCondition() != Cond) return false; 207 208 // Check to see if a successor of the switch is guaranteed to go to the 209 // latch block or exit through a one exit block without having any 210 // side-effects. If so, determine the value of Cond that causes it to do 211 // this. Note that we can't trivially unswitch on the default case. 212 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) 213 if ((LoopExitBB = isTrivialLoopExitBlock(L, SI->getSuccessor(i)))) { 214 // Okay, we found a trivial case, remember the value that is trivial. 215 if (Val) *Val = SI->getCaseValue(i); 216 if (EntersWhenTrue) *EntersWhenTrue = false; 217 break; 218 } 219 } 220 221 if (!LoopExitBB) 222 return false; // Can't handle this. 223 224 if (LoopExit) *LoopExit = LoopExitBB; 225 226 // We already know that nothing uses any scalar values defined inside of this 227 // loop. As such, we just have to check to see if this loop will execute any 228 // side-effecting instructions (e.g. stores, calls, volatile loads) in the 229 // part of the loop that the code *would* execute. We already checked the 230 // tail, check the header now. 231 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) 232 if (I->mayWriteToMemory()) 233 return false; 234 return true; 235} 236 237/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if 238/// we choose to unswitch the specified loop on the specified value. 239/// 240unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) { 241 // If the condition is trivial, always unswitch. There is no code growth for 242 // this case. 243 if (IsTrivialUnswitchCondition(L, LIC)) 244 return 0; 245 246 unsigned Cost = 0; 247 // FIXME: this is brain dead. It should take into consideration code 248 // shrinkage. 249 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 250 I != E; ++I) { 251 BasicBlock *BB = *I; 252 // Do not include empty blocks in the cost calculation. This happen due to 253 // loop canonicalization and will be removed. 254 if (BB->begin() == BasicBlock::iterator(BB->getTerminator())) 255 continue; 256 257 // Count basic blocks. 258 ++Cost; 259 } 260 261 return Cost; 262} 263 264/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is 265/// invariant in the loop, or has an invariant piece, return the invariant. 266/// Otherwise, return null. 267static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { 268 // Constants should be folded, not unswitched on! 269 if (isa<Constant>(Cond)) return false; 270 271 // TODO: Handle: br (VARIANT|INVARIANT). 272 // TODO: Hoist simple expressions out of loops. 273 if (L->isLoopInvariant(Cond)) return Cond; 274 275 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) 276 if (BO->getOpcode() == Instruction::And || 277 BO->getOpcode() == Instruction::Or) { 278 // If either the left or right side is invariant, we can unswitch on this, 279 // which will cause the branch to go away in one loop and the condition to 280 // simplify in the other one. 281 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed)) 282 return LHS; 283 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) 284 return RHS; 285 } 286 287 return 0; 288} 289 290bool LoopUnswitch::visitLoop(Loop *L) { 291 bool Changed = false; 292 293 // Recurse through all subloops before we process this loop. Copy the loop 294 // list so that the child can update the loop tree if it needs to delete the 295 // loop. 296 std::vector<Loop*> SubLoops(L->begin(), L->end()); 297 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i) 298 Changed |= visitLoop(SubLoops[i]); 299 300 // Loop over all of the basic blocks in the loop. If we find an interior 301 // block that is branching on a loop-invariant condition, we can unswitch this 302 // loop. 303 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 304 I != E; ++I) { 305 TerminatorInst *TI = (*I)->getTerminator(); 306 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 307 // If this isn't branching on an invariant condition, we can't unswitch 308 // it. 309 if (BI->isConditional()) { 310 // See if this, or some part of it, is loop invariant. If so, we can 311 // unswitch on it if we desire. 312 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed); 313 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { 314 ++NumBranches; 315 return true; 316 } 317 } 318 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 319 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); 320 if (LoopCond && SI->getNumCases() > 1) { 321 // Find a value to unswitch on: 322 // FIXME: this should chose the most expensive case! 323 Constant *UnswitchVal = SI->getCaseValue(1); 324 if (UnswitchIfProfitable(LoopCond, UnswitchVal, L)) { 325 ++NumSwitches; 326 return true; 327 } 328 } 329 } 330 331 // Scan the instructions to check for unswitchable values. 332 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); 333 BBI != E; ++BBI) 334 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { 335 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed); 336 if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L)) { 337 ++NumSelects; 338 return true; 339 } 340 } 341 } 342 343 return Changed; 344} 345 346/// UnswitchIfProfitable - We have found that we can unswitch L when 347/// LoopCond == Val to simplify the loop. If we decide that this is profitable, 348/// unswitch the loop, reprocess the pieces, then return true. 349bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){ 350 // Check to see if it would be profitable to unswitch this loop. 351 if (getLoopUnswitchCost(L, LoopCond) > Threshold) { 352 // FIXME: this should estimate growth by the amount of code shared by the 353 // resultant unswitched loops. 354 // 355 DEBUG(std::cerr << "NOT unswitching loop %" 356 << L->getHeader()->getName() << ", cost too high: " 357 << L->getBlocks().size() << "\n"); 358 return false; 359 } 360 361 // If this loop has live-out values, we can't unswitch it. We need something 362 // like loop-closed SSA form in order to know how to insert PHI nodes for 363 // these values. 364 if (LoopValuesUsedOutsideLoop(L)) { 365 DEBUG(std::cerr << "NOT unswitching loop %" << L->getHeader()->getName() 366 << ", a loop value is used outside loop!\n"); 367 return false; 368 } 369 370 //std::cerr << "BEFORE:\n"; LI->dump(); 371 Loop *NewLoop1 = 0, *NewLoop2 = 0; 372 373 // If this is a trivial condition to unswitch (which results in no code 374 // duplication), do it now. 375 Constant *CondVal; 376 bool EntersWhenTrue = true; 377 BasicBlock *ExitBlock; 378 if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, 379 &EntersWhenTrue, &ExitBlock)) { 380 UnswitchTrivialCondition(L, LoopCond, CondVal, EntersWhenTrue, ExitBlock); 381 NewLoop1 = L; 382 } else { 383 VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2); 384 } 385 386 //std::cerr << "AFTER:\n"; LI->dump(); 387 388 // Try to unswitch each of our new loops now! 389 if (NewLoop1) visitLoop(NewLoop1); 390 if (NewLoop2) visitLoop(NewLoop2); 391 return true; 392} 393 394/// SplitBlock - Split the specified block at the specified instruction - every 395/// thing before SplitPt stays in Old and everything starting with SplitPt moves 396/// to a new block. The two blocks are joined by an unconditional branch and 397/// the loop info is updated. 398/// 399BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *Old, Instruction *SplitPt) { 400 BasicBlock::iterator SplitIt = SplitPt; 401 while (isa<PHINode>(SplitIt)) 402 ++SplitIt; 403 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split"); 404 405 // The new block lives in whichever loop the old one did. 406 if (Loop *L = LI->getLoopFor(Old)) 407 L->addBasicBlockToLoop(New, *LI); 408 409 return New; 410} 411 412 413BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) { 414 TerminatorInst *LatchTerm = BB->getTerminator(); 415 unsigned SuccNum = 0; 416 for (unsigned i = 0, e = LatchTerm->getNumSuccessors(); ; ++i) { 417 assert(i != e && "Didn't find edge?"); 418 if (LatchTerm->getSuccessor(i) == Succ) { 419 SuccNum = i; 420 break; 421 } 422 } 423 424 // If this is a critical edge, let SplitCriticalEdge do it. 425 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, this)) 426 return LatchTerm->getSuccessor(SuccNum); 427 428 // If the edge isn't critical, then BB has a single successor or Succ has a 429 // single pred. Split the block. 430 BasicBlock::iterator SplitPoint; 431 if (BasicBlock *SP = Succ->getSinglePredecessor()) { 432 // If the successor only has a single pred, split the top of the successor 433 // block. 434 assert(SP == BB && "CFG broken"); 435 return SplitBlock(Succ, Succ->begin()); 436 } else { 437 // Otherwise, if BB has a single successor, split it at the bottom of the 438 // block. 439 assert(BB->getTerminator()->getNumSuccessors() == 1 && 440 "Should have a single succ!"); 441 return SplitBlock(BB, BB->getTerminator()); 442 } 443} 444 445 446 447// RemapInstruction - Convert the instruction operands from referencing the 448// current values into those specified by ValueMap. 449// 450static inline void RemapInstruction(Instruction *I, 451 std::map<const Value *, Value*> &ValueMap) { 452 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 453 Value *Op = I->getOperand(op); 454 std::map<const Value *, Value*>::iterator It = ValueMap.find(Op); 455 if (It != ValueMap.end()) Op = It->second; 456 I->setOperand(op, Op); 457 } 458} 459 460/// CloneLoop - Recursively clone the specified loop and all of its children, 461/// mapping the blocks with the specified map. 462static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM, 463 LoopInfo *LI) { 464 Loop *New = new Loop(); 465 466 if (PL) 467 PL->addChildLoop(New); 468 else 469 LI->addTopLevelLoop(New); 470 471 // Add all of the blocks in L to the new loop. 472 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 473 I != E; ++I) 474 if (LI->getLoopFor(*I) == L) 475 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI); 476 477 // Add all of the subloops to the new loop. 478 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 479 CloneLoop(*I, New, VM, LI); 480 481 return New; 482} 483 484/// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values 485/// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the 486/// code immediately before InsertPt. 487static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, 488 BasicBlock *TrueDest, 489 BasicBlock *FalseDest, 490 Instruction *InsertPt) { 491 // Insert a conditional branch on LIC to the two preheaders. The original 492 // code is the true version and the new code is the false version. 493 Value *BranchVal = LIC; 494 if (!isa<ConstantBool>(Val)) { 495 BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp", InsertPt); 496 } else if (Val != ConstantBool::True) { 497 // We want to enter the new loop when the condition is true. 498 std::swap(TrueDest, FalseDest); 499 } 500 501 // Insert the new branch. 502 new BranchInst(TrueDest, FalseDest, BranchVal, InsertPt); 503} 504 505 506/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable 507/// condition in it (a cond branch from its header block to its latch block, 508/// where the path through the loop that doesn't execute its body has no 509/// side-effects), unswitch it. This doesn't involve any code duplication, just 510/// moving the conditional branch outside of the loop and updating loop info. 511void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, 512 Constant *Val, bool EntersWhenTrue, 513 BasicBlock *ExitBlock) { 514 DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %" 515 << L->getHeader()->getName() << " [" << L->getBlocks().size() 516 << " blocks] in Function " << L->getHeader()->getParent()->getName() 517 << " on cond: " << *Val << (EntersWhenTrue ? " == " : " != ") << 518 *Cond << "\n"); 519 520 // First step, split the preheader, so that we know that there is a safe place 521 // to insert the conditional branch. We will change 'OrigPH' to have a 522 // conditional branch on Cond. 523 BasicBlock *OrigPH = L->getLoopPreheader(); 524 BasicBlock *NewPH = SplitEdge(OrigPH, L->getHeader()); 525 526 // Now that we have a place to insert the conditional branch, create a place 527 // to branch to: this is the exit block out of the loop that we should 528 // short-circuit to. 529 530 // Split this block now, so that the loop maintains its exit block, and so 531 // that the jump from the preheader can execute the contents of the exit block 532 // without actually branching to it (the exit block should be dominated by the 533 // loop header, not the preheader). 534 assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); 535 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin()); 536 537 // Okay, now we have a position to branch from and a position to branch to, 538 // insert the new conditional branch. 539 { 540 BasicBlock *TrueDest = NewPH, *FalseDest = NewExit; 541 if (!EntersWhenTrue) std::swap(TrueDest, FalseDest); 542 EmitPreheaderBranchOnCondition(Cond, Val, TrueDest, FalseDest, 543 OrigPH->getTerminator()); 544 } 545 OrigPH->getTerminator()->eraseFromParent(); 546 547 // Now that we know that the loop is never entered when this condition is a 548 // particular value, rewrite the loop with this info. We know that this will 549 // at least eliminate the old branch. 550 RewriteLoopBodyWithConditionConstant(L, Cond, Val, EntersWhenTrue); 551 ++NumTrivial; 552} 553 554 555/// VersionLoop - We determined that the loop is profitable to unswitch when LIC 556/// equal Val. Split it into loop versions and test the condition outside of 557/// either loop. Return the loops created as Out1/Out2. 558void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L, 559 Loop *&Out1, Loop *&Out2) { 560 Function *F = L->getHeader()->getParent(); 561 562 DEBUG(std::cerr << "loop-unswitch: Unswitching loop %" 563 << L->getHeader()->getName() << " [" << L->getBlocks().size() 564 << " blocks] in Function " << F->getName() 565 << " when '" << *Val << "' == " << *LIC << "\n"); 566 567 // LoopBlocks contains all of the basic blocks of the loop, including the 568 // preheader of the loop, the body of the loop, and the exit blocks of the 569 // loop, in that order. 570 std::vector<BasicBlock*> LoopBlocks; 571 572 // First step, split the preheader and exit blocks, and add these blocks to 573 // the LoopBlocks list. 574 BasicBlock *OrigPreheader = L->getLoopPreheader(); 575 LoopBlocks.push_back(SplitEdge(OrigPreheader, L->getHeader())); 576 577 // We want the loop to come after the preheader, but before the exit blocks. 578 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); 579 580 std::vector<BasicBlock*> ExitBlocks; 581 L->getExitBlocks(ExitBlocks); 582 std::sort(ExitBlocks.begin(), ExitBlocks.end()); 583 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()), 584 ExitBlocks.end()); 585 586 // Split all of the edges from inside the loop to their exit blocks. This 587 // unswitching trivial: no phi nodes to update. 588 unsigned NumBlocks = L->getBlocks().size(); 589 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 590 BasicBlock *ExitBlock = ExitBlocks[i]; 591 std::vector<BasicBlock*> Preds(pred_begin(ExitBlock), pred_end(ExitBlock)); 592 593 for (unsigned j = 0, e = Preds.size(); j != e; ++j) { 594 assert(L->contains(Preds[j]) && 595 "All preds of loop exit blocks must be the same loop!"); 596 SplitEdge(Preds[j], ExitBlock); 597 } 598 } 599 600 // The exit blocks may have been changed due to edge splitting, recompute. 601 ExitBlocks.clear(); 602 L->getExitBlocks(ExitBlocks); 603 std::sort(ExitBlocks.begin(), ExitBlocks.end()); 604 ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()), 605 ExitBlocks.end()); 606 607 // Add exit blocks to the loop blocks. 608 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); 609 610 // Next step, clone all of the basic blocks that make up the loop (including 611 // the loop preheader and exit blocks), keeping track of the mapping between 612 // the instructions and blocks. 613 std::vector<BasicBlock*> NewBlocks; 614 NewBlocks.reserve(LoopBlocks.size()); 615 std::map<const Value*, Value*> ValueMap; 616 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { 617 BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F); 618 NewBlocks.push_back(New); 619 ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping. 620 } 621 622 // Splice the newly inserted blocks into the function right before the 623 // original preheader. 624 F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(), 625 NewBlocks[0], F->end()); 626 627 // Now we create the new Loop object for the versioned loop. 628 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI); 629 Loop *ParentLoop = L->getParentLoop(); 630 if (ParentLoop) { 631 // Make sure to add the cloned preheader and exit blocks to the parent loop 632 // as well. 633 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI); 634 } 635 636 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 637 BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]); 638 if (ParentLoop) 639 ParentLoop->addBasicBlockToLoop(cast<BasicBlock>(NewExit), *LI); 640 641 assert(NewExit->getTerminator()->getNumSuccessors() == 1 && 642 "Exit block should have been split to have one successor!"); 643 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); 644 645 // If the successor of the exit block had PHI nodes, add an entry for 646 // NewExit. 647 PHINode *PN; 648 for (BasicBlock::iterator I = ExitSucc->begin(); 649 (PN = dyn_cast<PHINode>(I)); ++I) { 650 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]); 651 std::map<const Value *, Value*>::iterator It = ValueMap.find(V); 652 if (It != ValueMap.end()) V = It->second; 653 PN->addIncoming(V, NewExit); 654 } 655 } 656 657 // Rewrite the code to refer to itself. 658 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) 659 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 660 E = NewBlocks[i]->end(); I != E; ++I) 661 RemapInstruction(I, ValueMap); 662 663 // Rewrite the original preheader to select between versions of the loop. 664 BranchInst *OldBR = cast<BranchInst>(OrigPreheader->getTerminator()); 665 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && 666 "Preheader splitting did not work correctly!"); 667 668 // Emit the new branch that selects between the two versions of this loop. 669 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR); 670 OldBR->eraseFromParent(); 671 672 // Now we rewrite the original code to know that the condition is true and the 673 // new code to know that the condition is false. 674 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false); 675 RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true); 676 Out1 = L; 677 Out2 = NewLoop; 678} 679 680// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has 681// the value specified by Val in the specified loop, or we know it does NOT have 682// that value. Rewrite any uses of LIC or of properties correlated to it. 683void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, 684 Constant *Val, 685 bool IsEqual) { 686 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); 687 688 // FIXME: Support correlated properties, like: 689 // for (...) 690 // if (li1 < li2) 691 // ... 692 // if (li1 > li2) 693 // ... 694 695 // NotVal - If Val is a bool, this contains its inverse. 696 Constant *NotVal = 0; 697 if (ConstantBool *CB = dyn_cast<ConstantBool>(Val)) 698 NotVal = ConstantBool::get(!CB->getValue()); 699 700 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, 701 // selects, switches. 702 std::vector<User*> Users(LIC->use_begin(), LIC->use_end()); 703 704 // Haha, this loop could be unswitched. Get it? The unswitch pass could 705 // unswitch itself. Amazing. 706 for (unsigned i = 0, e = Users.size(); i != e; ++i) 707 if (Instruction *U = cast<Instruction>(Users[i])) { 708 if (!L->contains(U->getParent())) 709 continue; 710 711 if (IsEqual) { 712 U->replaceUsesOfWith(LIC, Val); 713 } else if (NotVal) { 714 U->replaceUsesOfWith(LIC, NotVal); 715 } else { 716 // If we know that LIC is not Val, use this info to simplify code. 717 if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) { 718 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) { 719 if (SI->getCaseValue(i) == Val) { 720 // Found a dead case value. Don't remove PHI nodes in the 721 // successor if they become single-entry, those PHI nodes may 722 // be in the Users list. 723 SI->getSuccessor(i)->removePredecessor(SI->getParent(), true); 724 SI->removeCase(i); 725 break; 726 } 727 } 728 } 729 730 // TODO: We could simplify stuff like X == C. 731 } 732 } 733} 734