LoopSimplify.cpp revision ffa75cdcf82ef2034249a313b9276eaa1bee6c43
1//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This pass 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// Indirectbr instructions introduce several complications. If the loop 27// contains or is entered by an indirectbr instruction, it may not be possible 28// to transform the loop and make these guarantees. Client code should check 29// that these conditions are true before relying on them. 30// 31// Note that the simplifycfg pass will clean up blocks which are split out but 32// end up being unnecessary, so usage of this pass should not pessimize 33// generated code. 34// 35// This pass obviously modifies the CFG, but updates loop information and 36// dominator information. 37// 38//===----------------------------------------------------------------------===// 39 40#define DEBUG_TYPE "loopsimplify" 41#include "llvm/Transforms/Scalar.h" 42#include "llvm/Constants.h" 43#include "llvm/Instructions.h" 44#include "llvm/IntrinsicInst.h" 45#include "llvm/Function.h" 46#include "llvm/LLVMContext.h" 47#include "llvm/Type.h" 48#include "llvm/Analysis/AliasAnalysis.h" 49#include "llvm/Analysis/ScalarEvolution.h" 50#include "llvm/Analysis/Dominators.h" 51#include "llvm/Analysis/LoopPass.h" 52#include "llvm/Transforms/Utils/BasicBlockUtils.h" 53#include "llvm/Transforms/Utils/Local.h" 54#include "llvm/Support/CFG.h" 55#include "llvm/Support/Debug.h" 56#include "llvm/ADT/SetOperations.h" 57#include "llvm/ADT/SetVector.h" 58#include "llvm/ADT/Statistic.h" 59#include "llvm/ADT/DepthFirstIterator.h" 60using namespace llvm; 61 62STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted"); 63STATISTIC(NumNested , "Number of nested loops split out"); 64 65namespace { 66 struct LoopSimplify : public LoopPass { 67 static char ID; // Pass identification, replacement for typeid 68 LoopSimplify() : LoopPass(ID) {} 69 70 // AA - If we have an alias analysis object to update, this is it, otherwise 71 // this is null. 72 AliasAnalysis *AA; 73 LoopInfo *LI; 74 DominatorTree *DT; 75 ScalarEvolution *SE; 76 Loop *L; 77 virtual bool runOnLoop(Loop *L, LPPassManager &LPM); 78 79 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 80 // We need loop information to identify the loops... 81 AU.addRequired<DominatorTree>(); 82 AU.addPreserved<DominatorTree>(); 83 84 AU.addRequired<LoopInfo>(); 85 AU.addPreserved<LoopInfo>(); 86 87 AU.addPreserved<AliasAnalysis>(); 88 AU.addPreserved<ScalarEvolution>(); 89 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added. 90 AU.addPreserved<DominanceFrontier>(); 91 AU.addPreservedID(LCSSAID); 92 } 93 94 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees. 95 void verifyAnalysis() const; 96 97 private: 98 bool ProcessLoop(Loop *L, LPPassManager &LPM); 99 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit); 100 BasicBlock *InsertPreheaderForLoop(Loop *L); 101 Loop *SeparateNestedLoop(Loop *L, LPPassManager &LPM); 102 BasicBlock *InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader); 103 void PlaceSplitBlockCarefully(BasicBlock *NewBB, 104 SmallVectorImpl<BasicBlock*> &SplitPreds, 105 Loop *L); 106 }; 107} 108 109char LoopSimplify::ID = 0; 110INITIALIZE_PASS(LoopSimplify, "loopsimplify", 111 "Canonicalize natural loops", true, false); 112 113// Publically exposed interface to pass... 114char &llvm::LoopSimplifyID = LoopSimplify::ID; 115Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); } 116 117/// runOnLoop - Run down all loops in the CFG (recursively, but we could do 118/// it in any convenient order) inserting preheaders... 119/// 120bool LoopSimplify::runOnLoop(Loop *l, LPPassManager &LPM) { 121 L = l; 122 bool Changed = false; 123 LI = &getAnalysis<LoopInfo>(); 124 AA = getAnalysisIfAvailable<AliasAnalysis>(); 125 DT = &getAnalysis<DominatorTree>(); 126 SE = getAnalysisIfAvailable<ScalarEvolution>(); 127 128 Changed |= ProcessLoop(L, LPM); 129 130 return Changed; 131} 132 133/// ProcessLoop - Walk the loop structure in depth first order, ensuring that 134/// all loops have preheaders. 135/// 136bool LoopSimplify::ProcessLoop(Loop *L, LPPassManager &LPM) { 137 bool Changed = false; 138ReprocessLoop: 139 140 // Check to see that no blocks (other than the header) in this loop have 141 // predecessors that are not in the loop. This is not valid for natural 142 // loops, but can occur if the blocks are unreachable. Since they are 143 // unreachable we can just shamelessly delete those CFG edges! 144 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); 145 BB != E; ++BB) { 146 if (*BB == L->getHeader()) continue; 147 148 SmallPtrSet<BasicBlock*, 4> BadPreds; 149 for (pred_iterator PI = pred_begin(*BB), 150 PE = pred_end(*BB); PI != PE; ++PI) { 151 BasicBlock *P = *PI; 152 if (!L->contains(P)) 153 BadPreds.insert(P); 154 } 155 156 // Delete each unique out-of-loop (and thus dead) predecessor. 157 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(), 158 E = BadPreds.end(); I != E; ++I) { 159 160 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "; 161 WriteAsOperand(dbgs(), *I, false); 162 dbgs() << "\n"); 163 164 // Inform each successor of each dead pred. 165 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI) 166 (*SI)->removePredecessor(*I); 167 // Zap the dead pred's terminator and replace it with unreachable. 168 TerminatorInst *TI = (*I)->getTerminator(); 169 TI->replaceAllUsesWith(UndefValue::get(TI->getType())); 170 (*I)->getTerminator()->eraseFromParent(); 171 new UnreachableInst((*I)->getContext(), *I); 172 Changed = true; 173 } 174 } 175 176 // If there are exiting blocks with branches on undef, resolve the undef in 177 // the direction which will exit the loop. This will help simplify loop 178 // trip count computations. 179 SmallVector<BasicBlock*, 8> ExitingBlocks; 180 L->getExitingBlocks(ExitingBlocks); 181 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(), 182 E = ExitingBlocks.end(); I != E; ++I) 183 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator())) 184 if (BI->isConditional()) { 185 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) { 186 187 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "; 188 WriteAsOperand(dbgs(), *I, false); 189 dbgs() << "\n"); 190 191 BI->setCondition(ConstantInt::get(Cond->getType(), 192 !L->contains(BI->getSuccessor(0)))); 193 Changed = true; 194 } 195 } 196 197 // Does the loop already have a preheader? If so, don't insert one. 198 BasicBlock *Preheader = L->getLoopPreheader(); 199 if (!Preheader) { 200 Preheader = InsertPreheaderForLoop(L); 201 if (Preheader) { 202 ++NumInserted; 203 Changed = true; 204 } 205 } 206 207 // Next, check to make sure that all exit nodes of the loop only have 208 // predecessors that are inside of the loop. This check guarantees that the 209 // loop preheader/header will dominate the exit blocks. If the exit block has 210 // predecessors from outside of the loop, split the edge now. 211 SmallVector<BasicBlock*, 8> ExitBlocks; 212 L->getExitBlocks(ExitBlocks); 213 214 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(), 215 ExitBlocks.end()); 216 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(), 217 E = ExitBlockSet.end(); I != E; ++I) { 218 BasicBlock *ExitBlock = *I; 219 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock); 220 PI != PE; ++PI) 221 // Must be exactly this loop: no subloops, parent loops, or non-loop preds 222 // allowed. 223 if (!L->contains(*PI)) { 224 if (RewriteLoopExitBlock(L, ExitBlock)) { 225 ++NumInserted; 226 Changed = true; 227 } 228 break; 229 } 230 } 231 232 // If the header has more than two predecessors at this point (from the 233 // preheader and from multiple backedges), we must adjust the loop. 234 BasicBlock *LoopLatch = L->getLoopLatch(); 235 if (!LoopLatch) { 236 // If this is really a nested loop, rip it out into a child loop. Don't do 237 // this for loops with a giant number of backedges, just factor them into a 238 // common backedge instead. 239 if (L->getNumBackEdges() < 8) { 240 if (SeparateNestedLoop(L, LPM)) { 241 ++NumNested; 242 // This is a big restructuring change, reprocess the whole loop. 243 Changed = true; 244 // GCC doesn't tail recursion eliminate this. 245 goto ReprocessLoop; 246 } 247 } 248 249 // If we either couldn't, or didn't want to, identify nesting of the loops, 250 // insert a new block that all backedges target, then make it jump to the 251 // loop header. 252 LoopLatch = InsertUniqueBackedgeBlock(L, Preheader); 253 if (LoopLatch) { 254 ++NumInserted; 255 Changed = true; 256 } 257 } 258 259 // Scan over the PHI nodes in the loop header. Since they now have only two 260 // incoming values (the loop is canonicalized), we may have simplified the PHI 261 // down to 'X = phi [X, Y]', which should be replaced with 'Y'. 262 PHINode *PN; 263 for (BasicBlock::iterator I = L->getHeader()->begin(); 264 (PN = dyn_cast<PHINode>(I++)); ) 265 if (Value *V = PN->hasConstantValue(DT)) { 266 if (AA) AA->deleteValue(PN); 267 PN->replaceAllUsesWith(V); 268 PN->eraseFromParent(); 269 } 270 271 // If this loop has multiple exits and the exits all go to the same 272 // block, attempt to merge the exits. This helps several passes, such 273 // as LoopRotation, which do not support loops with multiple exits. 274 // SimplifyCFG also does this (and this code uses the same utility 275 // function), however this code is loop-aware, where SimplifyCFG is 276 // not. That gives it the advantage of being able to hoist 277 // loop-invariant instructions out of the way to open up more 278 // opportunities, and the disadvantage of having the responsibility 279 // to preserve dominator information. 280 bool UniqueExit = true; 281 if (!ExitBlocks.empty()) 282 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i) 283 if (ExitBlocks[i] != ExitBlocks[0]) { 284 UniqueExit = false; 285 break; 286 } 287 if (UniqueExit) { 288 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { 289 BasicBlock *ExitingBlock = ExitingBlocks[i]; 290 if (!ExitingBlock->getSinglePredecessor()) continue; 291 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()); 292 if (!BI || !BI->isConditional()) continue; 293 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition()); 294 if (!CI || CI->getParent() != ExitingBlock) continue; 295 296 // Attempt to hoist out all instructions except for the 297 // comparison and the branch. 298 bool AllInvariant = true; 299 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) { 300 Instruction *Inst = I++; 301 // Skip debug info intrinsics. 302 if (isa<DbgInfoIntrinsic>(Inst)) 303 continue; 304 if (Inst == CI) 305 continue; 306 if (!L->makeLoopInvariant(Inst, Changed, 307 Preheader ? Preheader->getTerminator() : 0)) { 308 AllInvariant = false; 309 break; 310 } 311 } 312 if (!AllInvariant) continue; 313 314 // The block has now been cleared of all instructions except for 315 // a comparison and a conditional branch. SimplifyCFG may be able 316 // to fold it now. 317 if (!FoldBranchToCommonDest(BI)) continue; 318 319 // Success. The block is now dead, so remove it from the loop, 320 // update the dominator tree and dominance frontier, and delete it. 321 322 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "; 323 WriteAsOperand(dbgs(), ExitingBlock, false); 324 dbgs() << "\n"); 325 326 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock)); 327 Changed = true; 328 LI->removeBlock(ExitingBlock); 329 330 DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>(); 331 DomTreeNode *Node = DT->getNode(ExitingBlock); 332 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children = 333 Node->getChildren(); 334 while (!Children.empty()) { 335 DomTreeNode *Child = Children.front(); 336 DT->changeImmediateDominator(Child, Node->getIDom()); 337 if (DF) DF->changeImmediateDominator(Child->getBlock(), 338 Node->getIDom()->getBlock(), 339 DT); 340 } 341 DT->eraseNode(ExitingBlock); 342 if (DF) DF->removeBlock(ExitingBlock); 343 344 BI->getSuccessor(0)->removePredecessor(ExitingBlock); 345 BI->getSuccessor(1)->removePredecessor(ExitingBlock); 346 ExitingBlock->eraseFromParent(); 347 } 348 } 349 350 return Changed; 351} 352 353/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a 354/// preheader, this method is called to insert one. This method has two phases: 355/// preheader insertion and analysis updating. 356/// 357BasicBlock *LoopSimplify::InsertPreheaderForLoop(Loop *L) { 358 BasicBlock *Header = L->getHeader(); 359 360 // Compute the set of predecessors of the loop that are not in the loop. 361 SmallVector<BasicBlock*, 8> OutsideBlocks; 362 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header); 363 PI != PE; ++PI) { 364 BasicBlock *P = *PI; 365 if (!L->contains(P)) { // Coming in from outside the loop? 366 // If the loop is branched to from an indirect branch, we won't 367 // be able to fully transform the loop, because it prohibits 368 // edge splitting. 369 if (isa<IndirectBrInst>(P->getTerminator())) return 0; 370 371 // Keep track of it. 372 OutsideBlocks.push_back(P); 373 } 374 } 375 376 // Split out the loop pre-header. 377 BasicBlock *NewBB = 378 SplitBlockPredecessors(Header, &OutsideBlocks[0], OutsideBlocks.size(), 379 ".preheader", this); 380 381 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "; 382 WriteAsOperand(dbgs(), NewBB, false); 383 dbgs() << "\n"); 384 385 // Make sure that NewBB is put someplace intelligent, which doesn't mess up 386 // code layout too horribly. 387 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L); 388 389 return NewBB; 390} 391 392/// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit 393/// blocks. This method is used to split exit blocks that have predecessors 394/// outside of the loop. 395BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) { 396 SmallVector<BasicBlock*, 8> LoopBlocks; 397 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) { 398 BasicBlock *P = *I; 399 if (L->contains(P)) { 400 // Don't do this if the loop is exited via an indirect branch. 401 if (isa<IndirectBrInst>(P->getTerminator())) return 0; 402 403 LoopBlocks.push_back(P); 404 } 405 } 406 407 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?"); 408 BasicBlock *NewBB = SplitBlockPredecessors(Exit, &LoopBlocks[0], 409 LoopBlocks.size(), ".loopexit", 410 this); 411 412 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "; 413 WriteAsOperand(dbgs(), NewBB, false); 414 dbgs() << "\n"); 415 416 return NewBB; 417} 418 419/// AddBlockAndPredsToSet - Add the specified block, and all of its 420/// predecessors, to the specified set, if it's not already in there. Stop 421/// predecessor traversal when we reach StopBlock. 422static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock, 423 std::set<BasicBlock*> &Blocks) { 424 std::vector<BasicBlock *> WorkList; 425 WorkList.push_back(InputBB); 426 do { 427 BasicBlock *BB = WorkList.back(); WorkList.pop_back(); 428 if (Blocks.insert(BB).second && BB != StopBlock) 429 // If BB is not already processed and it is not a stop block then 430 // insert its predecessor in the work list 431 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) { 432 BasicBlock *WBB = *I; 433 WorkList.push_back(WBB); 434 } 435 } while(!WorkList.empty()); 436} 437 438/// FindPHIToPartitionLoops - The first part of loop-nestification is to find a 439/// PHI node that tells us how to partition the loops. 440static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT, 441 AliasAnalysis *AA) { 442 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) { 443 PHINode *PN = cast<PHINode>(I); 444 ++I; 445 if (Value *V = PN->hasConstantValue(DT)) { 446 // This is a degenerate PHI already, don't modify it! 447 PN->replaceAllUsesWith(V); 448 if (AA) AA->deleteValue(PN); 449 PN->eraseFromParent(); 450 continue; 451 } 452 453 // Scan this PHI node looking for a use of the PHI node by itself. 454 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 455 if (PN->getIncomingValue(i) == PN && 456 L->contains(PN->getIncomingBlock(i))) 457 // We found something tasty to remove. 458 return PN; 459 } 460 return 0; 461} 462 463// PlaceSplitBlockCarefully - If the block isn't already, move the new block to 464// right after some 'outside block' block. This prevents the preheader from 465// being placed inside the loop body, e.g. when the loop hasn't been rotated. 466void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB, 467 SmallVectorImpl<BasicBlock*> &SplitPreds, 468 Loop *L) { 469 // Check to see if NewBB is already well placed. 470 Function::iterator BBI = NewBB; --BBI; 471 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { 472 if (&*BBI == SplitPreds[i]) 473 return; 474 } 475 476 // If it isn't already after an outside block, move it after one. This is 477 // always good as it makes the uncond branch from the outside block into a 478 // fall-through. 479 480 // Figure out *which* outside block to put this after. Prefer an outside 481 // block that neighbors a BB actually in the loop. 482 BasicBlock *FoundBB = 0; 483 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { 484 Function::iterator BBI = SplitPreds[i]; 485 if (++BBI != NewBB->getParent()->end() && 486 L->contains(BBI)) { 487 FoundBB = SplitPreds[i]; 488 break; 489 } 490 } 491 492 // If our heuristic for a *good* bb to place this after doesn't find 493 // anything, just pick something. It's likely better than leaving it within 494 // the loop. 495 if (!FoundBB) 496 FoundBB = SplitPreds[0]; 497 NewBB->moveAfter(FoundBB); 498} 499 500 501/// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of 502/// them out into a nested loop. This is important for code that looks like 503/// this: 504/// 505/// Loop: 506/// ... 507/// br cond, Loop, Next 508/// ... 509/// br cond2, Loop, Out 510/// 511/// To identify this common case, we look at the PHI nodes in the header of the 512/// loop. PHI nodes with unchanging values on one backedge correspond to values 513/// that change in the "outer" loop, but not in the "inner" loop. 514/// 515/// If we are able to separate out a loop, return the new outer loop that was 516/// created. 517/// 518Loop *LoopSimplify::SeparateNestedLoop(Loop *L, LPPassManager &LPM) { 519 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA); 520 if (PN == 0) return 0; // No known way to partition. 521 522 // Pull out all predecessors that have varying values in the loop. This 523 // handles the case when a PHI node has multiple instances of itself as 524 // arguments. 525 SmallVector<BasicBlock*, 8> OuterLoopPreds; 526 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 527 if (PN->getIncomingValue(i) != PN || 528 !L->contains(PN->getIncomingBlock(i))) { 529 // We can't split indirectbr edges. 530 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator())) 531 return 0; 532 533 OuterLoopPreds.push_back(PN->getIncomingBlock(i)); 534 } 535 536 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n"); 537 538 // If ScalarEvolution is around and knows anything about values in 539 // this loop, tell it to forget them, because we're about to 540 // substantially change it. 541 if (SE) 542 SE->forgetLoop(L); 543 544 BasicBlock *Header = L->getHeader(); 545 BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0], 546 OuterLoopPreds.size(), 547 ".outer", this); 548 549 // Make sure that NewBB is put someplace intelligent, which doesn't mess up 550 // code layout too horribly. 551 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L); 552 553 // Create the new outer loop. 554 Loop *NewOuter = new Loop(); 555 556 // Change the parent loop to use the outer loop as its child now. 557 if (Loop *Parent = L->getParentLoop()) 558 Parent->replaceChildLoopWith(L, NewOuter); 559 else 560 LI->changeTopLevelLoop(L, NewOuter); 561 562 // L is now a subloop of our outer loop. 563 NewOuter->addChildLoop(L); 564 565 // Add the new loop to the pass manager queue. 566 LPM.insertLoopIntoQueue(NewOuter); 567 568 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 569 I != E; ++I) 570 NewOuter->addBlockEntry(*I); 571 572 // Now reset the header in L, which had been moved by 573 // SplitBlockPredecessors for the outer loop. 574 L->moveToHeader(Header); 575 576 // Determine which blocks should stay in L and which should be moved out to 577 // the Outer loop now. 578 std::set<BasicBlock*> BlocksInL; 579 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) { 580 BasicBlock *P = *PI; 581 if (DT->dominates(Header, P)) 582 AddBlockAndPredsToSet(P, Header, BlocksInL); 583 } 584 585 // Scan all of the loop children of L, moving them to OuterLoop if they are 586 // not part of the inner loop. 587 const std::vector<Loop*> &SubLoops = L->getSubLoops(); 588 for (size_t I = 0; I != SubLoops.size(); ) 589 if (BlocksInL.count(SubLoops[I]->getHeader())) 590 ++I; // Loop remains in L 591 else 592 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I)); 593 594 // Now that we know which blocks are in L and which need to be moved to 595 // OuterLoop, move any blocks that need it. 596 for (unsigned i = 0; i != L->getBlocks().size(); ++i) { 597 BasicBlock *BB = L->getBlocks()[i]; 598 if (!BlocksInL.count(BB)) { 599 // Move this block to the parent, updating the exit blocks sets 600 L->removeBlockFromLoop(BB); 601 if ((*LI)[BB] == L) 602 LI->changeLoopFor(BB, NewOuter); 603 --i; 604 } 605 } 606 607 return NewOuter; 608} 609 610 611 612/// InsertUniqueBackedgeBlock - This method is called when the specified loop 613/// has more than one backedge in it. If this occurs, revector all of these 614/// backedges to target a new basic block and have that block branch to the loop 615/// header. This ensures that loops have exactly one backedge. 616/// 617BasicBlock * 618LoopSimplify::InsertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader) { 619 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!"); 620 621 // Get information about the loop 622 BasicBlock *Header = L->getHeader(); 623 Function *F = Header->getParent(); 624 625 // Unique backedge insertion currently depends on having a preheader. 626 if (!Preheader) 627 return 0; 628 629 // Figure out which basic blocks contain back-edges to the loop header. 630 std::vector<BasicBlock*> BackedgeBlocks; 631 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){ 632 BasicBlock *P = *I; 633 634 // Indirectbr edges cannot be split, so we must fail if we find one. 635 if (isa<IndirectBrInst>(P->getTerminator())) 636 return 0; 637 638 if (P != Preheader) BackedgeBlocks.push_back(P); 639 } 640 641 // Create and insert the new backedge block... 642 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(), 643 Header->getName()+".backedge", F); 644 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock); 645 646 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "; 647 WriteAsOperand(dbgs(), BEBlock, false); 648 dbgs() << "\n"); 649 650 // Move the new backedge block to right after the last backedge block. 651 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos; 652 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock); 653 654 // Now that the block has been inserted into the function, create PHI nodes in 655 // the backedge block which correspond to any PHI nodes in the header block. 656 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 657 PHINode *PN = cast<PHINode>(I); 658 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be", 659 BETerminator); 660 NewPN->reserveOperandSpace(BackedgeBlocks.size()); 661 if (AA) AA->copyValue(PN, NewPN); 662 663 // Loop over the PHI node, moving all entries except the one for the 664 // preheader over to the new PHI node. 665 unsigned PreheaderIdx = ~0U; 666 bool HasUniqueIncomingValue = true; 667 Value *UniqueValue = 0; 668 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 669 BasicBlock *IBB = PN->getIncomingBlock(i); 670 Value *IV = PN->getIncomingValue(i); 671 if (IBB == Preheader) { 672 PreheaderIdx = i; 673 } else { 674 NewPN->addIncoming(IV, IBB); 675 if (HasUniqueIncomingValue) { 676 if (UniqueValue == 0) 677 UniqueValue = IV; 678 else if (UniqueValue != IV) 679 HasUniqueIncomingValue = false; 680 } 681 } 682 } 683 684 // Delete all of the incoming values from the old PN except the preheader's 685 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??"); 686 if (PreheaderIdx != 0) { 687 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx)); 688 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx)); 689 } 690 // Nuke all entries except the zero'th. 691 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i) 692 PN->removeIncomingValue(e-i, false); 693 694 // Finally, add the newly constructed PHI node as the entry for the BEBlock. 695 PN->addIncoming(NewPN, BEBlock); 696 697 // As an optimization, if all incoming values in the new PhiNode (which is a 698 // subset of the incoming values of the old PHI node) have the same value, 699 // eliminate the PHI Node. 700 if (HasUniqueIncomingValue) { 701 NewPN->replaceAllUsesWith(UniqueValue); 702 if (AA) AA->deleteValue(NewPN); 703 BEBlock->getInstList().erase(NewPN); 704 } 705 } 706 707 // Now that all of the PHI nodes have been inserted and adjusted, modify the 708 // backedge blocks to just to the BEBlock instead of the header. 709 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) { 710 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator(); 711 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op) 712 if (TI->getSuccessor(Op) == Header) 713 TI->setSuccessor(Op, BEBlock); 714 } 715 716 //===--- Update all analyses which we must preserve now -----------------===// 717 718 // Update Loop Information - we know that this block is now in the current 719 // loop and all parent loops. 720 L->addBasicBlockToLoop(BEBlock, LI->getBase()); 721 722 // Update dominator information 723 DT->splitBlock(BEBlock); 724 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>()) 725 DF->splitBlock(BEBlock); 726 727 return BEBlock; 728} 729 730void LoopSimplify::verifyAnalysis() const { 731 // It used to be possible to just assert L->isLoopSimplifyForm(), however 732 // with the introduction of indirectbr, there are now cases where it's 733 // not possible to transform a loop as necessary. We can at least check 734 // that there is an indirectbr near any time there's trouble. 735 736 // Indirectbr can interfere with preheader and unique backedge insertion. 737 if (!L->getLoopPreheader() || !L->getLoopLatch()) { 738 bool HasIndBrPred = false; 739 for (pred_iterator PI = pred_begin(L->getHeader()), 740 PE = pred_end(L->getHeader()); PI != PE; ++PI) 741 if (isa<IndirectBrInst>((*PI)->getTerminator())) { 742 HasIndBrPred = true; 743 break; 744 } 745 assert(HasIndBrPred && 746 "LoopSimplify has no excuse for missing loop header info!"); 747 } 748 749 // Indirectbr can interfere with exit block canonicalization. 750 if (!L->hasDedicatedExits()) { 751 bool HasIndBrExiting = false; 752 SmallVector<BasicBlock*, 8> ExitingBlocks; 753 L->getExitingBlocks(ExitingBlocks); 754 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) 755 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) { 756 HasIndBrExiting = true; 757 break; 758 } 759 assert(HasIndBrExiting && 760 "LoopSimplify has no excuse for missing exit block info!"); 761 } 762} 763