LoopSimplify.cpp revision baec98d00bda1cc904405d92716ea9d2f4c1fe9d
1//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===// 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 performs several transformations to transform natural loops into a 11// simpler form, which makes subsequent analyses and transformations simpler and 12// more effective. 13// 14// Loop pre-header insertion guarantees that there is a single, non-critical 15// entry edge from outside of the loop to the loop header. This simplifies a 16// number of analyses and transformations, such as LICM. 17// 18// Loop exit-block insertion guarantees that all exit blocks from the loop 19// (blocks which are outside of the loop that have predecessors inside of the 20// loop) only have predecessors from inside of the loop (and are thus dominated 21// by the loop header). This simplifies transformations such as store-sinking 22// that are built into LICM. 23// 24// This pass also guarantees that loops will have exactly one backedge. 25// 26// Note that the simplifycfg pass will clean up blocks which are split out but 27// end up being unnecessary, so usage of this pass should not pessimize 28// generated code. 29// 30// This pass obviously modifies the CFG, but updates loop information and 31// dominator information. 32// 33//===----------------------------------------------------------------------===// 34 35#include "llvm/Transforms/Scalar.h" 36#include "llvm/Constant.h" 37#include "llvm/Instructions.h" 38#include "llvm/Function.h" 39#include "llvm/Type.h" 40#include "llvm/Analysis/AliasAnalysis.h" 41#include "llvm/Analysis/Dominators.h" 42#include "llvm/Analysis/LoopInfo.h" 43#include "llvm/Support/CFG.h" 44#include "llvm/ADT/SetOperations.h" 45#include "llvm/ADT/SetVector.h" 46#include "llvm/ADT/Statistic.h" 47#include "llvm/ADT/DepthFirstIterator.h" 48using namespace llvm; 49 50namespace { 51 Statistic<> 52 NumInserted("loopsimplify", "Number of pre-header or exit blocks inserted"); 53 Statistic<> 54 NumNested("loopsimplify", "Number of nested loops split out"); 55 56 struct LoopSimplify : public FunctionPass { 57 // AA - If we have an alias analysis object to update, this is it, otherwise 58 // this is null. 59 AliasAnalysis *AA; 60 61 virtual bool runOnFunction(Function &F); 62 63 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 64 // We need loop information to identify the loops... 65 AU.addRequired<LoopInfo>(); 66 AU.addRequired<DominatorSet>(); 67 AU.addRequired<DominatorTree>(); 68 69 AU.addPreserved<LoopInfo>(); 70 AU.addPreserved<DominatorSet>(); 71 AU.addPreserved<ImmediateDominators>(); 72 AU.addPreserved<ETForest>(); 73 AU.addPreserved<DominatorTree>(); 74 AU.addPreserved<DominanceFrontier>(); 75 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added. 76 } 77 private: 78 bool ProcessLoop(Loop *L); 79 BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix, 80 const std::vector<BasicBlock*> &Preds); 81 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit); 82 void InsertPreheaderForLoop(Loop *L); 83 Loop *SeparateNestedLoop(Loop *L); 84 void InsertUniqueBackedgeBlock(Loop *L); 85 86 void UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB, 87 std::vector<BasicBlock*> &PredBlocks); 88 }; 89 90 RegisterOpt<LoopSimplify> 91 X("loopsimplify", "Canonicalize natural loops", true); 92} 93 94// Publically exposed interface to pass... 95const PassInfo *llvm::LoopSimplifyID = X.getPassInfo(); 96FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); } 97 98/// runOnFunction - Run down all loops in the CFG (recursively, but we could do 99/// it in any convenient order) inserting preheaders... 100/// 101bool LoopSimplify::runOnFunction(Function &F) { 102 bool Changed = false; 103 LoopInfo &LI = getAnalysis<LoopInfo>(); 104 AA = getAnalysisToUpdate<AliasAnalysis>(); 105 106 for (LoopInfo::iterator I = LI.begin(), E = LI.end(); I != E; ++I) 107 Changed |= ProcessLoop(*I); 108 109 return Changed; 110} 111 112 113/// ProcessLoop - Walk the loop structure in depth first order, ensuring that 114/// all loops have preheaders. 115/// 116bool LoopSimplify::ProcessLoop(Loop *L) { 117 bool Changed = false; 118 119 // Check to see that no blocks (other than the header) in the loop have 120 // predecessors that are not in the loop. This is not valid for natural 121 // loops, but can occur if the blocks are unreachable. Since they are 122 // unreachable we can just shamelessly destroy their terminators to make them 123 // not branch into the loop! 124 assert(L->getBlocks()[0] == L->getHeader() && 125 "Header isn't first block in loop?"); 126 for (unsigned i = 1, e = L->getBlocks().size(); i != e; ++i) { 127 BasicBlock *LoopBB = L->getBlocks()[i]; 128 Retry: 129 for (pred_iterator PI = pred_begin(LoopBB), E = pred_end(LoopBB); 130 PI != E; ++PI) 131 if (!L->contains(*PI)) { 132 // This predecessor is not in the loop. Kill its terminator! 133 BasicBlock *DeadBlock = *PI; 134 for (succ_iterator SI = succ_begin(DeadBlock), E = succ_end(DeadBlock); 135 SI != E; ++SI) 136 (*SI)->removePredecessor(DeadBlock); // Remove PHI node entries 137 138 // Delete the dead terminator. 139 if (AA) AA->deleteValue(&DeadBlock->back()); 140 DeadBlock->getInstList().pop_back(); 141 142 Value *RetVal = 0; 143 if (LoopBB->getParent()->getReturnType() != Type::VoidTy) 144 RetVal = Constant::getNullValue(LoopBB->getParent()->getReturnType()); 145 new ReturnInst(RetVal, DeadBlock); 146 goto Retry; // We just invalidated the pred_iterator. Retry. 147 } 148 } 149 150 // Does the loop already have a preheader? If so, don't modify the loop... 151 if (L->getLoopPreheader() == 0) { 152 InsertPreheaderForLoop(L); 153 NumInserted++; 154 Changed = true; 155 } 156 157 // Next, check to make sure that all exit nodes of the loop only have 158 // predecessors that are inside of the loop. This check guarantees that the 159 // loop preheader/header will dominate the exit blocks. If the exit block has 160 // predecessors from outside of the loop, split the edge now. 161 std::vector<BasicBlock*> ExitBlocks; 162 L->getExitBlocks(ExitBlocks); 163 164 SetVector<BasicBlock*> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end()); 165 for (SetVector<BasicBlock*>::iterator I = ExitBlockSet.begin(), 166 E = ExitBlockSet.end(); I != E; ++I) { 167 BasicBlock *ExitBlock = *I; 168 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock); 169 PI != PE; ++PI) 170 if (!L->contains(*PI)) { 171 RewriteLoopExitBlock(L, ExitBlock); 172 NumInserted++; 173 Changed = true; 174 break; 175 } 176 } 177 178 // If the header has more than two predecessors at this point (from the 179 // preheader and from multiple backedges), we must adjust the loop. 180 if (L->getNumBackEdges() != 1) { 181 // If this is really a nested loop, rip it out into a child loop. 182 if (Loop *NL = SeparateNestedLoop(L)) { 183 ++NumNested; 184 // This is a big restructuring change, reprocess the whole loop. 185 ProcessLoop(NL); 186 return true; 187 } 188 189 InsertUniqueBackedgeBlock(L); 190 NumInserted++; 191 Changed = true; 192 } 193 194 // Scan over the PHI nodes in the loop header. Since they now have only two 195 // incoming values (the loop is canonicalized), we may have simplified the PHI 196 // down to 'X = phi [X, Y]', which should be replaced with 'Y'. 197 PHINode *PN; 198 DominatorSet &DS = getAnalysis<DominatorSet>(); 199 for (BasicBlock::iterator I = L->getHeader()->begin(); 200 (PN = dyn_cast<PHINode>(I++)); ) 201 if (Value *V = PN->hasConstantValue()) { 202 PN->replaceAllUsesWith(V); 203 PN->eraseFromParent(); 204 } 205 206 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 207 Changed |= ProcessLoop(*I); 208 209 return Changed; 210} 211 212/// SplitBlockPredecessors - Split the specified block into two blocks. We want 213/// to move the predecessors specified in the Preds list to point to the new 214/// block, leaving the remaining predecessors pointing to BB. This method 215/// updates the SSA PHINode's, but no other analyses. 216/// 217BasicBlock *LoopSimplify::SplitBlockPredecessors(BasicBlock *BB, 218 const char *Suffix, 219 const std::vector<BasicBlock*> &Preds) { 220 221 // Create new basic block, insert right before the original block... 222 BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB->getParent(), BB); 223 224 // The preheader first gets an unconditional branch to the loop header... 225 BranchInst *BI = new BranchInst(BB, NewBB); 226 227 // For every PHI node in the block, insert a PHI node into NewBB where the 228 // incoming values from the out of loop edges are moved to NewBB. We have two 229 // possible cases here. If the loop is dead, we just insert dummy entries 230 // into the PHI nodes for the new edge. If the loop is not dead, we move the 231 // incoming edges in BB into new PHI nodes in NewBB. 232 // 233 if (!Preds.empty()) { // Is the loop not obviously dead? 234 // Check to see if the values being merged into the new block need PHI 235 // nodes. If so, insert them. 236 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) { 237 PHINode *PN = cast<PHINode>(I); 238 ++I; 239 240 // Check to see if all of the values coming in are the same. If so, we 241 // don't need to create a new PHI node. 242 Value *InVal = PN->getIncomingValueForBlock(Preds[0]); 243 for (unsigned i = 1, e = Preds.size(); i != e; ++i) 244 if (InVal != PN->getIncomingValueForBlock(Preds[i])) { 245 InVal = 0; 246 break; 247 } 248 249 // If the values coming into the block are not the same, we need a PHI. 250 if (InVal == 0) { 251 // Create the new PHI node, insert it into NewBB at the end of the block 252 PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI); 253 if (AA) AA->copyValue(PN, NewPHI); 254 255 // Move all of the edges from blocks outside the loop to the new PHI 256 for (unsigned i = 0, e = Preds.size(); i != e; ++i) { 257 Value *V = PN->removeIncomingValue(Preds[i], false); 258 NewPHI->addIncoming(V, Preds[i]); 259 } 260 InVal = NewPHI; 261 } else { 262 // Remove all of the edges coming into the PHI nodes from outside of the 263 // block. 264 for (unsigned i = 0, e = Preds.size(); i != e; ++i) 265 PN->removeIncomingValue(Preds[i], false); 266 } 267 268 // Add an incoming value to the PHI node in the loop for the preheader 269 // edge. 270 PN->addIncoming(InVal, NewBB); 271 272 // Can we eliminate this phi node now? 273 if (Value *V = PN->hasConstantValue(true)) { 274 if (!isa<Instruction>(V) || 275 getAnalysis<DominatorSet>().dominates(cast<Instruction>(V), PN)) { 276 PN->replaceAllUsesWith(V); 277 if (AA) AA->deleteValue(PN); 278 BB->getInstList().erase(PN); 279 } 280 } 281 } 282 283 // Now that the PHI nodes are updated, actually move the edges from 284 // Preds to point to NewBB instead of BB. 285 // 286 for (unsigned i = 0, e = Preds.size(); i != e; ++i) { 287 TerminatorInst *TI = Preds[i]->getTerminator(); 288 for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s) 289 if (TI->getSuccessor(s) == BB) 290 TI->setSuccessor(s, NewBB); 291 } 292 293 } else { // Otherwise the loop is dead... 294 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) { 295 PHINode *PN = cast<PHINode>(I); 296 // Insert dummy values as the incoming value... 297 PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB); 298 } 299 } 300 return NewBB; 301} 302 303/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a 304/// preheader, this method is called to insert one. This method has two phases: 305/// preheader insertion and analysis updating. 306/// 307void LoopSimplify::InsertPreheaderForLoop(Loop *L) { 308 BasicBlock *Header = L->getHeader(); 309 310 // Compute the set of predecessors of the loop that are not in the loop. 311 std::vector<BasicBlock*> OutsideBlocks; 312 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header); 313 PI != PE; ++PI) 314 if (!L->contains(*PI)) // Coming in from outside the loop? 315 OutsideBlocks.push_back(*PI); // Keep track of it... 316 317 // Split out the loop pre-header 318 BasicBlock *NewBB = 319 SplitBlockPredecessors(Header, ".preheader", OutsideBlocks); 320 321 //===--------------------------------------------------------------------===// 322 // Update analysis results now that we have performed the transformation 323 // 324 325 // We know that we have loop information to update... update it now. 326 if (Loop *Parent = L->getParentLoop()) 327 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>()); 328 329 DominatorSet &DS = getAnalysis<DominatorSet>(); // Update dominator info 330 DominatorTree &DT = getAnalysis<DominatorTree>(); 331 332 333 // Update the dominator tree information. 334 // The immediate dominator of the preheader is the immediate dominator of 335 // the old header. 336 DominatorTree::Node *PHDomTreeNode = 337 DT.createNewNode(NewBB, DT.getNode(Header)->getIDom()); 338 BasicBlock *oldHeaderIDom = DT.getNode(Header)->getIDom()->getBlock(); 339 340 // Change the header node so that PNHode is the new immediate dominator 341 DT.changeImmediateDominator(DT.getNode(Header), PHDomTreeNode); 342 343 { 344 // The blocks that dominate NewBB are the blocks that dominate Header, 345 // minus Header, plus NewBB. 346 DominatorSet::DomSetType DomSet = DS.getDominators(Header); 347 DomSet.erase(Header); // Header does not dominate us... 348 DS.addBasicBlock(NewBB, DomSet); 349 350 // The newly created basic block dominates all nodes dominated by Header. 351 for (df_iterator<DominatorTree::Node*> DFI = df_begin(PHDomTreeNode), 352 E = df_end(PHDomTreeNode); DFI != E; ++DFI) 353 DS.addDominator((*DFI)->getBlock(), NewBB); 354 } 355 356 // Update immediate dominator information if we have it... 357 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) { 358 // Whatever i-dominated the header node now immediately dominates NewBB 359 ID->addNewBlock(NewBB, ID->get(Header)); 360 361 // The preheader now is the immediate dominator for the header node... 362 ID->setImmediateDominator(Header, NewBB); 363 } 364 365 // Update ET Forest information if we have it... 366 if (ETForest *EF = getAnalysisToUpdate<ETForest>()) { 367 // Whatever i-dominated the header node now immediately dominates NewBB 368 EF->addNewBlock(NewBB, oldHeaderIDom); 369 370 // The preheader now is the immediate dominator for the header node... 371 EF->setImmediateDominator(Header, NewBB); 372 } 373 374 // Update dominance frontier information... 375 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) { 376 // The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates 377 // everything that Header does, and it strictly dominates Header in 378 // addition. 379 assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?"); 380 DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second; 381 NewDFSet.erase(Header); 382 DF->addBasicBlock(NewBB, NewDFSet); 383 384 // Now we must loop over all of the dominance frontiers in the function, 385 // replacing occurrences of Header with NewBB in some cases. If a block 386 // dominates a (now) predecessor of NewBB, but did not strictly dominate 387 // Header, it will have Header in it's DF set, but should now have NewBB in 388 // its set. 389 for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) { 390 // Get all of the dominators of the predecessor... 391 const DominatorSet::DomSetType &PredDoms = 392 DS.getDominators(OutsideBlocks[i]); 393 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(), 394 PDE = PredDoms.end(); PDI != PDE; ++PDI) { 395 BasicBlock *PredDom = *PDI; 396 // If the loop header is in DF(PredDom), then PredDom didn't dominate 397 // the header but did dominate a predecessor outside of the loop. Now 398 // we change this entry to include the preheader in the DF instead of 399 // the header. 400 DominanceFrontier::iterator DFI = DF->find(PredDom); 401 assert(DFI != DF->end() && "No dominance frontier for node?"); 402 if (DFI->second.count(Header)) { 403 DF->removeFromFrontier(DFI, Header); 404 DF->addToFrontier(DFI, NewBB); 405 } 406 } 407 } 408 } 409} 410 411/// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit 412/// blocks. This method is used to split exit blocks that have predecessors 413/// outside of the loop. 414BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) { 415 DominatorSet &DS = getAnalysis<DominatorSet>(); 416 417 std::vector<BasicBlock*> LoopBlocks; 418 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) 419 if (L->contains(*I)) 420 LoopBlocks.push_back(*I); 421 422 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?"); 423 BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks); 424 425 // Update Loop Information - we know that the new block will be in the parent 426 // loop of L. 427 if (Loop *Parent = L->getParentLoop()) 428 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>()); 429 430 // Update dominator information (set, immdom, domtree, and domfrontier) 431 UpdateDomInfoForRevectoredPreds(NewBB, LoopBlocks); 432 return NewBB; 433} 434 435/// AddBlockAndPredsToSet - Add the specified block, and all of its 436/// predecessors, to the specified set, if it's not already in there. Stop 437/// predecessor traversal when we reach StopBlock. 438static void AddBlockAndPredsToSet(BasicBlock *BB, BasicBlock *StopBlock, 439 std::set<BasicBlock*> &Blocks) { 440 if (!Blocks.insert(BB).second) return; // already processed. 441 if (BB == StopBlock) return; // Stop here! 442 443 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) 444 AddBlockAndPredsToSet(*I, StopBlock, Blocks); 445} 446 447/// FindPHIToPartitionLoops - The first part of loop-nestification is to find a 448/// PHI node that tells us how to partition the loops. 449static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorSet &DS, 450 AliasAnalysis *AA) { 451 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) { 452 PHINode *PN = cast<PHINode>(I); 453 ++I; 454 if (Value *V = PN->hasConstantValue()) 455 if (!isa<Instruction>(V) || DS.dominates(cast<Instruction>(V), PN)) { 456 // This is a degenerate PHI already, don't modify it! 457 PN->replaceAllUsesWith(V); 458 if (AA) AA->deleteValue(PN); 459 PN->eraseFromParent(); 460 continue; 461 } 462 463 // Scan this PHI node looking for a use of the PHI node by itself. 464 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 465 if (PN->getIncomingValue(i) == PN && 466 L->contains(PN->getIncomingBlock(i))) 467 // We found something tasty to remove. 468 return PN; 469 } 470 return 0; 471} 472 473/// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of 474/// them out into a nested loop. This is important for code that looks like 475/// this: 476/// 477/// Loop: 478/// ... 479/// br cond, Loop, Next 480/// ... 481/// br cond2, Loop, Out 482/// 483/// To identify this common case, we look at the PHI nodes in the header of the 484/// loop. PHI nodes with unchanging values on one backedge correspond to values 485/// that change in the "outer" loop, but not in the "inner" loop. 486/// 487/// If we are able to separate out a loop, return the new outer loop that was 488/// created. 489/// 490Loop *LoopSimplify::SeparateNestedLoop(Loop *L) { 491 PHINode *PN = FindPHIToPartitionLoops(L, getAnalysis<DominatorSet>(), AA); 492 if (PN == 0) return 0; // No known way to partition. 493 494 // Pull out all predecessors that have varying values in the loop. This 495 // handles the case when a PHI node has multiple instances of itself as 496 // arguments. 497 std::vector<BasicBlock*> OuterLoopPreds; 498 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 499 if (PN->getIncomingValue(i) != PN || 500 !L->contains(PN->getIncomingBlock(i))) 501 OuterLoopPreds.push_back(PN->getIncomingBlock(i)); 502 503 BasicBlock *Header = L->getHeader(); 504 BasicBlock *NewBB = SplitBlockPredecessors(Header, ".outer", OuterLoopPreds); 505 506 // Update dominator information (set, immdom, domtree, and domfrontier) 507 UpdateDomInfoForRevectoredPreds(NewBB, OuterLoopPreds); 508 509 // Create the new outer loop. 510 Loop *NewOuter = new Loop(); 511 512 LoopInfo &LI = getAnalysis<LoopInfo>(); 513 514 // Change the parent loop to use the outer loop as its child now. 515 if (Loop *Parent = L->getParentLoop()) 516 Parent->replaceChildLoopWith(L, NewOuter); 517 else 518 LI.changeTopLevelLoop(L, NewOuter); 519 520 // This block is going to be our new header block: add it to this loop and all 521 // parent loops. 522 NewOuter->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>()); 523 524 // L is now a subloop of our outer loop. 525 NewOuter->addChildLoop(L); 526 527 for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i) 528 NewOuter->addBlockEntry(L->getBlocks()[i]); 529 530 // Determine which blocks should stay in L and which should be moved out to 531 // the Outer loop now. 532 DominatorSet &DS = getAnalysis<DominatorSet>(); 533 std::set<BasicBlock*> BlocksInL; 534 for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) 535 if (DS.dominates(Header, *PI)) 536 AddBlockAndPredsToSet(*PI, Header, BlocksInL); 537 538 539 // Scan all of the loop children of L, moving them to OuterLoop if they are 540 // not part of the inner loop. 541 for (Loop::iterator I = L->begin(); I != L->end(); ) 542 if (BlocksInL.count((*I)->getHeader())) 543 ++I; // Loop remains in L 544 else 545 NewOuter->addChildLoop(L->removeChildLoop(I)); 546 547 // Now that we know which blocks are in L and which need to be moved to 548 // OuterLoop, move any blocks that need it. 549 for (unsigned i = 0; i != L->getBlocks().size(); ++i) { 550 BasicBlock *BB = L->getBlocks()[i]; 551 if (!BlocksInL.count(BB)) { 552 // Move this block to the parent, updating the exit blocks sets 553 L->removeBlockFromLoop(BB); 554 if (LI[BB] == L) 555 LI.changeLoopFor(BB, NewOuter); 556 --i; 557 } 558 } 559 560 return NewOuter; 561} 562 563 564 565/// InsertUniqueBackedgeBlock - This method is called when the specified loop 566/// has more than one backedge in it. If this occurs, revector all of these 567/// backedges to target a new basic block and have that block branch to the loop 568/// header. This ensures that loops have exactly one backedge. 569/// 570void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) { 571 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!"); 572 573 // Get information about the loop 574 BasicBlock *Preheader = L->getLoopPreheader(); 575 BasicBlock *Header = L->getHeader(); 576 Function *F = Header->getParent(); 577 578 // Figure out which basic blocks contain back-edges to the loop header. 579 std::vector<BasicBlock*> BackedgeBlocks; 580 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I) 581 if (*I != Preheader) BackedgeBlocks.push_back(*I); 582 583 // Create and insert the new backedge block... 584 BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F); 585 BranchInst *BETerminator = new BranchInst(Header, BEBlock); 586 587 // Move the new backedge block to right after the last backedge block. 588 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos; 589 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock); 590 591 // Now that the block has been inserted into the function, create PHI nodes in 592 // the backedge block which correspond to any PHI nodes in the header block. 593 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 594 PHINode *PN = cast<PHINode>(I); 595 PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be", 596 BETerminator); 597 NewPN->reserveOperandSpace(BackedgeBlocks.size()); 598 if (AA) AA->copyValue(PN, NewPN); 599 600 // Loop over the PHI node, moving all entries except the one for the 601 // preheader over to the new PHI node. 602 unsigned PreheaderIdx = ~0U; 603 bool HasUniqueIncomingValue = true; 604 Value *UniqueValue = 0; 605 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 606 BasicBlock *IBB = PN->getIncomingBlock(i); 607 Value *IV = PN->getIncomingValue(i); 608 if (IBB == Preheader) { 609 PreheaderIdx = i; 610 } else { 611 NewPN->addIncoming(IV, IBB); 612 if (HasUniqueIncomingValue) { 613 if (UniqueValue == 0) 614 UniqueValue = IV; 615 else if (UniqueValue != IV) 616 HasUniqueIncomingValue = false; 617 } 618 } 619 } 620 621 // Delete all of the incoming values from the old PN except the preheader's 622 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??"); 623 if (PreheaderIdx != 0) { 624 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx)); 625 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx)); 626 } 627 // Nuke all entries except the zero'th. 628 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i) 629 PN->removeIncomingValue(e-i, false); 630 631 // Finally, add the newly constructed PHI node as the entry for the BEBlock. 632 PN->addIncoming(NewPN, BEBlock); 633 634 // As an optimization, if all incoming values in the new PhiNode (which is a 635 // subset of the incoming values of the old PHI node) have the same value, 636 // eliminate the PHI Node. 637 if (HasUniqueIncomingValue) { 638 NewPN->replaceAllUsesWith(UniqueValue); 639 if (AA) AA->deleteValue(NewPN); 640 BEBlock->getInstList().erase(NewPN); 641 } 642 } 643 644 // Now that all of the PHI nodes have been inserted and adjusted, modify the 645 // backedge blocks to just to the BEBlock instead of the header. 646 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) { 647 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator(); 648 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op) 649 if (TI->getSuccessor(Op) == Header) 650 TI->setSuccessor(Op, BEBlock); 651 } 652 653 //===--- Update all analyses which we must preserve now -----------------===// 654 655 // Update Loop Information - we know that this block is now in the current 656 // loop and all parent loops. 657 L->addBasicBlockToLoop(BEBlock, getAnalysis<LoopInfo>()); 658 659 // Update dominator information (set, immdom, domtree, and domfrontier) 660 UpdateDomInfoForRevectoredPreds(BEBlock, BackedgeBlocks); 661} 662 663/// UpdateDomInfoForRevectoredPreds - This method is used to update the four 664/// different kinds of dominator information (dominator sets, immediate 665/// dominators, dominator trees, and dominance frontiers) after a new block has 666/// been added to the CFG. 667/// 668/// This only supports the case when an existing block (known as "NewBBSucc"), 669/// had some of its predecessors factored into a new basic block. This 670/// transformation inserts a new basic block ("NewBB"), with a single 671/// unconditional branch to NewBBSucc, and moves some predecessors of 672/// "NewBBSucc" to now branch to NewBB. These predecessors are listed in 673/// PredBlocks, even though they are the same as 674/// pred_begin(NewBB)/pred_end(NewBB). 675/// 676void LoopSimplify::UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB, 677 std::vector<BasicBlock*> &PredBlocks) { 678 assert(!PredBlocks.empty() && "No predblocks??"); 679 assert(succ_begin(NewBB) != succ_end(NewBB) && 680 ++succ_begin(NewBB) == succ_end(NewBB) && 681 "NewBB should have a single successor!"); 682 BasicBlock *NewBBSucc = *succ_begin(NewBB); 683 DominatorSet &DS = getAnalysis<DominatorSet>(); 684 685 // Update dominator information... The blocks that dominate NewBB are the 686 // intersection of the dominators of predecessors, plus the block itself. 687 // 688 DominatorSet::DomSetType NewBBDomSet = DS.getDominators(PredBlocks[0]); 689 for (unsigned i = 1, e = PredBlocks.size(); i != e; ++i) 690 set_intersect(NewBBDomSet, DS.getDominators(PredBlocks[i])); 691 NewBBDomSet.insert(NewBB); // All blocks dominate themselves... 692 DS.addBasicBlock(NewBB, NewBBDomSet); 693 694 // The newly inserted basic block will dominate existing basic blocks iff the 695 // PredBlocks dominate all of the non-pred blocks. If all predblocks dominate 696 // the non-pred blocks, then they all must be the same block! 697 // 698 bool NewBBDominatesNewBBSucc = true; 699 { 700 BasicBlock *OnePred = PredBlocks[0]; 701 for (unsigned i = 1, e = PredBlocks.size(); i != e; ++i) 702 if (PredBlocks[i] != OnePred) { 703 NewBBDominatesNewBBSucc = false; 704 break; 705 } 706 707 if (NewBBDominatesNewBBSucc) 708 for (pred_iterator PI = pred_begin(NewBBSucc), E = pred_end(NewBBSucc); 709 PI != E; ++PI) 710 if (*PI != NewBB && !DS.dominates(NewBBSucc, *PI)) { 711 NewBBDominatesNewBBSucc = false; 712 break; 713 } 714 } 715 716 // The other scenario where the new block can dominate its successors are when 717 // all predecessors of NewBBSucc that are not NewBB are dominated by NewBBSucc 718 // already. 719 if (!NewBBDominatesNewBBSucc) { 720 NewBBDominatesNewBBSucc = true; 721 for (pred_iterator PI = pred_begin(NewBBSucc), E = pred_end(NewBBSucc); 722 PI != E; ++PI) 723 if (*PI != NewBB && !DS.dominates(NewBBSucc, *PI)) { 724 NewBBDominatesNewBBSucc = false; 725 break; 726 } 727 } 728 729 // If NewBB dominates some blocks, then it will dominate all blocks that 730 // NewBBSucc does. 731 if (NewBBDominatesNewBBSucc) { 732 BasicBlock *PredBlock = PredBlocks[0]; 733 Function *F = NewBB->getParent(); 734 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) 735 if (DS.dominates(NewBBSucc, I)) 736 DS.addDominator(I, NewBB); 737 } 738 739 // Update immediate dominator information if we have it... 740 BasicBlock *NewBBIDom = 0; 741 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) { 742 // To find the immediate dominator of the new exit node, we trace up the 743 // immediate dominators of a predecessor until we find a basic block that 744 // dominates the exit block. 745 // 746 BasicBlock *Dom = PredBlocks[0]; // Some random predecessor... 747 while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator... 748 assert(Dom != 0 && "No shared dominator found???"); 749 Dom = ID->get(Dom); 750 } 751 752 // Set the immediate dominator now... 753 ID->addNewBlock(NewBB, Dom); 754 NewBBIDom = Dom; // Reuse this if calculating DominatorTree info... 755 756 // If NewBB strictly dominates other blocks, we need to update their idom's 757 // now. The only block that need adjustment is the NewBBSucc block, whose 758 // idom should currently be set to PredBlocks[0]. 759 if (NewBBDominatesNewBBSucc) 760 ID->setImmediateDominator(NewBBSucc, NewBB); 761 } 762 763 // Update DominatorTree information if it is active. 764 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) { 765 // If we don't have ImmediateDominator info around, calculate the idom as 766 // above. 767 DominatorTree::Node *NewBBIDomNode; 768 if (NewBBIDom) { 769 NewBBIDomNode = DT->getNode(NewBBIDom); 770 } else { 771 NewBBIDomNode = DT->getNode(PredBlocks[0]); // Random pred 772 while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) { 773 NewBBIDomNode = NewBBIDomNode->getIDom(); 774 assert(NewBBIDomNode && "No shared dominator found??"); 775 } 776 NewBBIDom = NewBBIDomNode->getBlock(); 777 } 778 779 // Create the new dominator tree node... and set the idom of NewBB. 780 DominatorTree::Node *NewBBNode = DT->createNewNode(NewBB, NewBBIDomNode); 781 782 // If NewBB strictly dominates other blocks, then it is now the immediate 783 // dominator of NewBBSucc. Update the dominator tree as appropriate. 784 if (NewBBDominatesNewBBSucc) { 785 DominatorTree::Node *NewBBSuccNode = DT->getNode(NewBBSucc); 786 DT->changeImmediateDominator(NewBBSuccNode, NewBBNode); 787 } 788 } 789 790 // Update ET-Forest information if it is active. 791 if (ETForest *EF = getAnalysisToUpdate<ETForest>()) { 792 EF->addNewBlock(NewBB, NewBBIDom); 793 if (NewBBDominatesNewBBSucc) 794 EF->setImmediateDominator(NewBBSucc, NewBB); 795 } 796 797 // Update dominance frontier information... 798 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) { 799 // If NewBB dominates NewBBSucc, then DF(NewBB) is now going to be the 800 // DF(PredBlocks[0]) without the stuff that the new block does not dominate 801 // a predecessor of. 802 if (NewBBDominatesNewBBSucc) { 803 DominanceFrontier::iterator DFI = DF->find(PredBlocks[0]); 804 if (DFI != DF->end()) { 805 DominanceFrontier::DomSetType Set = DFI->second; 806 // Filter out stuff in Set that we do not dominate a predecessor of. 807 for (DominanceFrontier::DomSetType::iterator SetI = Set.begin(), 808 E = Set.end(); SetI != E;) { 809 bool DominatesPred = false; 810 for (pred_iterator PI = pred_begin(*SetI), E = pred_end(*SetI); 811 PI != E; ++PI) 812 if (DS.dominates(NewBB, *PI)) 813 DominatesPred = true; 814 if (!DominatesPred) 815 Set.erase(SetI++); 816 else 817 ++SetI; 818 } 819 820 DF->addBasicBlock(NewBB, Set); 821 } 822 823 } else { 824 // DF(NewBB) is {NewBBSucc} because NewBB does not strictly dominate 825 // NewBBSucc, but it does dominate itself (and there is an edge (NewBB -> 826 // NewBBSucc)). NewBBSucc is the single successor of NewBB. 827 DominanceFrontier::DomSetType NewDFSet; 828 NewDFSet.insert(NewBBSucc); 829 DF->addBasicBlock(NewBB, NewDFSet); 830 } 831 832 // Now we must loop over all of the dominance frontiers in the function, 833 // replacing occurrences of NewBBSucc with NewBB in some cases. All 834 // blocks that dominate a block in PredBlocks and contained NewBBSucc in 835 // their dominance frontier must be updated to contain NewBB instead. 836 // 837 for (unsigned i = 0, e = PredBlocks.size(); i != e; ++i) { 838 BasicBlock *Pred = PredBlocks[i]; 839 // Get all of the dominators of the predecessor... 840 const DominatorSet::DomSetType &PredDoms = DS.getDominators(Pred); 841 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(), 842 PDE = PredDoms.end(); PDI != PDE; ++PDI) { 843 BasicBlock *PredDom = *PDI; 844 845 // If the NewBBSucc node is in DF(PredDom), then PredDom didn't 846 // dominate NewBBSucc but did dominate a predecessor of it. Now we 847 // change this entry to include NewBB in the DF instead of NewBBSucc. 848 DominanceFrontier::iterator DFI = DF->find(PredDom); 849 assert(DFI != DF->end() && "No dominance frontier for node?"); 850 if (DFI->second.count(NewBBSucc)) { 851 // If NewBBSucc should not stay in our dominator frontier, remove it. 852 // We remove it unless there is a predecessor of NewBBSucc that we 853 // dominate, but we don't strictly dominate NewBBSucc. 854 bool ShouldRemove = true; 855 if (PredDom == NewBBSucc || !DS.dominates(PredDom, NewBBSucc)) { 856 // Okay, we know that PredDom does not strictly dominate NewBBSucc. 857 // Check to see if it dominates any predecessors of NewBBSucc. 858 for (pred_iterator PI = pred_begin(NewBBSucc), 859 E = pred_end(NewBBSucc); PI != E; ++PI) 860 if (DS.dominates(PredDom, *PI)) { 861 ShouldRemove = false; 862 break; 863 } 864 } 865 866 if (ShouldRemove) 867 DF->removeFromFrontier(DFI, NewBBSucc); 868 DF->addToFrontier(DFI, NewBB); 869 } 870 } 871 } 872 } 873} 874 875