LoopSimplify.cpp revision dce4a407a24b04eebc6a376f8e62b41aaa7b071f
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#include "llvm/Transforms/Scalar.h" 41#include "llvm/ADT/DepthFirstIterator.h" 42#include "llvm/ADT/SetOperations.h" 43#include "llvm/ADT/SetVector.h" 44#include "llvm/ADT/SmallVector.h" 45#include "llvm/ADT/Statistic.h" 46#include "llvm/Analysis/AliasAnalysis.h" 47#include "llvm/Analysis/DependenceAnalysis.h" 48#include "llvm/Analysis/InstructionSimplify.h" 49#include "llvm/Analysis/LoopInfo.h" 50#include "llvm/Analysis/ScalarEvolution.h" 51#include "llvm/IR/CFG.h" 52#include "llvm/IR/Constants.h" 53#include "llvm/IR/Dominators.h" 54#include "llvm/IR/Function.h" 55#include "llvm/IR/Instructions.h" 56#include "llvm/IR/IntrinsicInst.h" 57#include "llvm/IR/LLVMContext.h" 58#include "llvm/IR/Type.h" 59#include "llvm/Support/Debug.h" 60#include "llvm/Transforms/Utils/BasicBlockUtils.h" 61#include "llvm/Transforms/Utils/Local.h" 62#include "llvm/Transforms/Utils/LoopUtils.h" 63using namespace llvm; 64 65#define DEBUG_TYPE "loop-simplify" 66 67STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted"); 68STATISTIC(NumNested , "Number of nested loops split out"); 69 70// If the block isn't already, move the new block to right after some 'outside 71// block' block. This prevents the preheader from being placed inside the loop 72// body, e.g. when the loop hasn't been rotated. 73static void placeSplitBlockCarefully(BasicBlock *NewBB, 74 SmallVectorImpl<BasicBlock *> &SplitPreds, 75 Loop *L) { 76 // Check to see if NewBB is already well placed. 77 Function::iterator BBI = NewBB; --BBI; 78 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { 79 if (&*BBI == SplitPreds[i]) 80 return; 81 } 82 83 // If it isn't already after an outside block, move it after one. This is 84 // always good as it makes the uncond branch from the outside block into a 85 // fall-through. 86 87 // Figure out *which* outside block to put this after. Prefer an outside 88 // block that neighbors a BB actually in the loop. 89 BasicBlock *FoundBB = nullptr; 90 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { 91 Function::iterator BBI = SplitPreds[i]; 92 if (++BBI != NewBB->getParent()->end() && 93 L->contains(BBI)) { 94 FoundBB = SplitPreds[i]; 95 break; 96 } 97 } 98 99 // If our heuristic for a *good* bb to place this after doesn't find 100 // anything, just pick something. It's likely better than leaving it within 101 // the loop. 102 if (!FoundBB) 103 FoundBB = SplitPreds[0]; 104 NewBB->moveAfter(FoundBB); 105} 106 107/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a 108/// preheader, this method is called to insert one. This method has two phases: 109/// preheader insertion and analysis updating. 110/// 111BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, Pass *PP) { 112 BasicBlock *Header = L->getHeader(); 113 114 // Compute the set of predecessors of the loop that are not in the loop. 115 SmallVector<BasicBlock*, 8> OutsideBlocks; 116 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header); 117 PI != PE; ++PI) { 118 BasicBlock *P = *PI; 119 if (!L->contains(P)) { // Coming in from outside the loop? 120 // If the loop is branched to from an indirect branch, we won't 121 // be able to fully transform the loop, because it prohibits 122 // edge splitting. 123 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr; 124 125 // Keep track of it. 126 OutsideBlocks.push_back(P); 127 } 128 } 129 130 // Split out the loop pre-header. 131 BasicBlock *PreheaderBB; 132 if (!Header->isLandingPad()) { 133 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", 134 PP); 135 } else { 136 SmallVector<BasicBlock*, 2> NewBBs; 137 SplitLandingPadPredecessors(Header, OutsideBlocks, ".preheader", 138 ".split-lp", PP, NewBBs); 139 PreheaderBB = NewBBs[0]; 140 } 141 142 PreheaderBB->getTerminator()->setDebugLoc( 143 Header->getFirstNonPHI()->getDebugLoc()); 144 DEBUG(dbgs() << "LoopSimplify: Creating pre-header " 145 << PreheaderBB->getName() << "\n"); 146 147 // Make sure that NewBB is put someplace intelligent, which doesn't mess up 148 // code layout too horribly. 149 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L); 150 151 return PreheaderBB; 152} 153 154/// \brief Ensure that the loop preheader dominates all exit blocks. 155/// 156/// This method is used to split exit blocks that have predecessors outside of 157/// the loop. 158static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit, Pass *PP) { 159 SmallVector<BasicBlock*, 8> LoopBlocks; 160 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) { 161 BasicBlock *P = *I; 162 if (L->contains(P)) { 163 // Don't do this if the loop is exited via an indirect branch. 164 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr; 165 166 LoopBlocks.push_back(P); 167 } 168 } 169 170 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?"); 171 BasicBlock *NewExitBB = nullptr; 172 173 if (Exit->isLandingPad()) { 174 SmallVector<BasicBlock*, 2> NewBBs; 175 SplitLandingPadPredecessors(Exit, ArrayRef<BasicBlock*>(&LoopBlocks[0], 176 LoopBlocks.size()), 177 ".loopexit", ".nonloopexit", 178 PP, NewBBs); 179 NewExitBB = NewBBs[0]; 180 } else { 181 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", PP); 182 } 183 184 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block " 185 << NewExitBB->getName() << "\n"); 186 return NewExitBB; 187} 188 189/// Add the specified block, and all of its predecessors, to the specified set, 190/// if it's not already in there. Stop predecessor traversal when we reach 191/// StopBlock. 192static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock, 193 std::set<BasicBlock*> &Blocks) { 194 SmallVector<BasicBlock *, 8> Worklist; 195 Worklist.push_back(InputBB); 196 do { 197 BasicBlock *BB = Worklist.pop_back_val(); 198 if (Blocks.insert(BB).second && BB != StopBlock) 199 // If BB is not already processed and it is not a stop block then 200 // insert its predecessor in the work list 201 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) { 202 BasicBlock *WBB = *I; 203 Worklist.push_back(WBB); 204 } 205 } while (!Worklist.empty()); 206} 207 208/// \brief The first part of loop-nestification is to find a PHI node that tells 209/// us how to partition the loops. 210static PHINode *findPHIToPartitionLoops(Loop *L, AliasAnalysis *AA, 211 DominatorTree *DT) { 212 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) { 213 PHINode *PN = cast<PHINode>(I); 214 ++I; 215 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, DT)) { 216 // This is a degenerate PHI already, don't modify it! 217 PN->replaceAllUsesWith(V); 218 if (AA) AA->deleteValue(PN); 219 PN->eraseFromParent(); 220 continue; 221 } 222 223 // Scan this PHI node looking for a use of the PHI node by itself. 224 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 225 if (PN->getIncomingValue(i) == PN && 226 L->contains(PN->getIncomingBlock(i))) 227 // We found something tasty to remove. 228 return PN; 229 } 230 return nullptr; 231} 232 233/// \brief If this loop has multiple backedges, try to pull one of them out into 234/// a nested loop. 235/// 236/// This is important for code that looks like 237/// this: 238/// 239/// Loop: 240/// ... 241/// br cond, Loop, Next 242/// ... 243/// br cond2, Loop, Out 244/// 245/// To identify this common case, we look at the PHI nodes in the header of the 246/// loop. PHI nodes with unchanging values on one backedge correspond to values 247/// that change in the "outer" loop, but not in the "inner" loop. 248/// 249/// If we are able to separate out a loop, return the new outer loop that was 250/// created. 251/// 252static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader, 253 AliasAnalysis *AA, DominatorTree *DT, 254 LoopInfo *LI, ScalarEvolution *SE, Pass *PP) { 255 // Don't try to separate loops without a preheader. 256 if (!Preheader) 257 return nullptr; 258 259 // The header is not a landing pad; preheader insertion should ensure this. 260 assert(!L->getHeader()->isLandingPad() && 261 "Can't insert backedge to landing pad"); 262 263 PHINode *PN = findPHIToPartitionLoops(L, AA, DT); 264 if (!PN) return nullptr; // No known way to partition. 265 266 // Pull out all predecessors that have varying values in the loop. This 267 // handles the case when a PHI node has multiple instances of itself as 268 // arguments. 269 SmallVector<BasicBlock*, 8> OuterLoopPreds; 270 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 271 if (PN->getIncomingValue(i) != PN || 272 !L->contains(PN->getIncomingBlock(i))) { 273 // We can't split indirectbr edges. 274 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator())) 275 return nullptr; 276 OuterLoopPreds.push_back(PN->getIncomingBlock(i)); 277 } 278 } 279 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n"); 280 281 // If ScalarEvolution is around and knows anything about values in 282 // this loop, tell it to forget them, because we're about to 283 // substantially change it. 284 if (SE) 285 SE->forgetLoop(L); 286 287 BasicBlock *Header = L->getHeader(); 288 BasicBlock *NewBB = 289 SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", PP); 290 291 // Make sure that NewBB is put someplace intelligent, which doesn't mess up 292 // code layout too horribly. 293 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L); 294 295 // Create the new outer loop. 296 Loop *NewOuter = new Loop(); 297 298 // Change the parent loop to use the outer loop as its child now. 299 if (Loop *Parent = L->getParentLoop()) 300 Parent->replaceChildLoopWith(L, NewOuter); 301 else 302 LI->changeTopLevelLoop(L, NewOuter); 303 304 // L is now a subloop of our outer loop. 305 NewOuter->addChildLoop(L); 306 307 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 308 I != E; ++I) 309 NewOuter->addBlockEntry(*I); 310 311 // Now reset the header in L, which had been moved by 312 // SplitBlockPredecessors for the outer loop. 313 L->moveToHeader(Header); 314 315 // Determine which blocks should stay in L and which should be moved out to 316 // the Outer loop now. 317 std::set<BasicBlock*> BlocksInL; 318 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) { 319 BasicBlock *P = *PI; 320 if (DT->dominates(Header, P)) 321 addBlockAndPredsToSet(P, Header, BlocksInL); 322 } 323 324 // Scan all of the loop children of L, moving them to OuterLoop if they are 325 // not part of the inner loop. 326 const std::vector<Loop*> &SubLoops = L->getSubLoops(); 327 for (size_t I = 0; I != SubLoops.size(); ) 328 if (BlocksInL.count(SubLoops[I]->getHeader())) 329 ++I; // Loop remains in L 330 else 331 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I)); 332 333 // Now that we know which blocks are in L and which need to be moved to 334 // OuterLoop, move any blocks that need it. 335 for (unsigned i = 0; i != L->getBlocks().size(); ++i) { 336 BasicBlock *BB = L->getBlocks()[i]; 337 if (!BlocksInL.count(BB)) { 338 // Move this block to the parent, updating the exit blocks sets 339 L->removeBlockFromLoop(BB); 340 if ((*LI)[BB] == L) 341 LI->changeLoopFor(BB, NewOuter); 342 --i; 343 } 344 } 345 346 return NewOuter; 347} 348 349/// \brief This method is called when the specified loop has more than one 350/// backedge in it. 351/// 352/// If this occurs, revector all of these backedges to target a new basic block 353/// and have that block branch to the loop header. This ensures that loops 354/// have exactly one backedge. 355static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader, 356 AliasAnalysis *AA, 357 DominatorTree *DT, LoopInfo *LI) { 358 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!"); 359 360 // Get information about the loop 361 BasicBlock *Header = L->getHeader(); 362 Function *F = Header->getParent(); 363 364 // Unique backedge insertion currently depends on having a preheader. 365 if (!Preheader) 366 return nullptr; 367 368 // The header is not a landing pad; preheader insertion should ensure this. 369 assert(!Header->isLandingPad() && "Can't insert backedge to landing pad"); 370 371 // Figure out which basic blocks contain back-edges to the loop header. 372 std::vector<BasicBlock*> BackedgeBlocks; 373 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){ 374 BasicBlock *P = *I; 375 376 // Indirectbr edges cannot be split, so we must fail if we find one. 377 if (isa<IndirectBrInst>(P->getTerminator())) 378 return nullptr; 379 380 if (P != Preheader) BackedgeBlocks.push_back(P); 381 } 382 383 // Create and insert the new backedge block... 384 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(), 385 Header->getName()+".backedge", F); 386 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock); 387 388 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block " 389 << BEBlock->getName() << "\n"); 390 391 // Move the new backedge block to right after the last backedge block. 392 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos; 393 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock); 394 395 // Now that the block has been inserted into the function, create PHI nodes in 396 // the backedge block which correspond to any PHI nodes in the header block. 397 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 398 PHINode *PN = cast<PHINode>(I); 399 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(), 400 PN->getName()+".be", BETerminator); 401 if (AA) AA->copyValue(PN, NewPN); 402 403 // Loop over the PHI node, moving all entries except the one for the 404 // preheader over to the new PHI node. 405 unsigned PreheaderIdx = ~0U; 406 bool HasUniqueIncomingValue = true; 407 Value *UniqueValue = nullptr; 408 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 409 BasicBlock *IBB = PN->getIncomingBlock(i); 410 Value *IV = PN->getIncomingValue(i); 411 if (IBB == Preheader) { 412 PreheaderIdx = i; 413 } else { 414 NewPN->addIncoming(IV, IBB); 415 if (HasUniqueIncomingValue) { 416 if (!UniqueValue) 417 UniqueValue = IV; 418 else if (UniqueValue != IV) 419 HasUniqueIncomingValue = false; 420 } 421 } 422 } 423 424 // Delete all of the incoming values from the old PN except the preheader's 425 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??"); 426 if (PreheaderIdx != 0) { 427 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx)); 428 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx)); 429 } 430 // Nuke all entries except the zero'th. 431 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i) 432 PN->removeIncomingValue(e-i, false); 433 434 // Finally, add the newly constructed PHI node as the entry for the BEBlock. 435 PN->addIncoming(NewPN, BEBlock); 436 437 // As an optimization, if all incoming values in the new PhiNode (which is a 438 // subset of the incoming values of the old PHI node) have the same value, 439 // eliminate the PHI Node. 440 if (HasUniqueIncomingValue) { 441 NewPN->replaceAllUsesWith(UniqueValue); 442 if (AA) AA->deleteValue(NewPN); 443 BEBlock->getInstList().erase(NewPN); 444 } 445 } 446 447 // Now that all of the PHI nodes have been inserted and adjusted, modify the 448 // backedge blocks to just to the BEBlock instead of the header. 449 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) { 450 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator(); 451 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op) 452 if (TI->getSuccessor(Op) == Header) 453 TI->setSuccessor(Op, BEBlock); 454 } 455 456 //===--- Update all analyses which we must preserve now -----------------===// 457 458 // Update Loop Information - we know that this block is now in the current 459 // loop and all parent loops. 460 L->addBasicBlockToLoop(BEBlock, LI->getBase()); 461 462 // Update dominator information 463 DT->splitBlock(BEBlock); 464 465 return BEBlock; 466} 467 468/// \brief Simplify one loop and queue further loops for simplification. 469/// 470/// FIXME: Currently this accepts both lots of analyses that it uses and a raw 471/// Pass pointer. The Pass pointer is used by numerous utilities to update 472/// specific analyses. Rather than a pass it would be much cleaner and more 473/// explicit if they accepted the analysis directly and then updated it. 474static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist, 475 AliasAnalysis *AA, DominatorTree *DT, LoopInfo *LI, 476 ScalarEvolution *SE, Pass *PP) { 477 bool Changed = false; 478ReprocessLoop: 479 480 // Check to see that no blocks (other than the header) in this loop have 481 // predecessors that are not in the loop. This is not valid for natural 482 // loops, but can occur if the blocks are unreachable. Since they are 483 // unreachable we can just shamelessly delete those CFG edges! 484 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); 485 BB != E; ++BB) { 486 if (*BB == L->getHeader()) continue; 487 488 SmallPtrSet<BasicBlock*, 4> BadPreds; 489 for (pred_iterator PI = pred_begin(*BB), 490 PE = pred_end(*BB); PI != PE; ++PI) { 491 BasicBlock *P = *PI; 492 if (!L->contains(P)) 493 BadPreds.insert(P); 494 } 495 496 // Delete each unique out-of-loop (and thus dead) predecessor. 497 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(), 498 E = BadPreds.end(); I != E; ++I) { 499 500 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor " 501 << (*I)->getName() << "\n"); 502 503 // Inform each successor of each dead pred. 504 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI) 505 (*SI)->removePredecessor(*I); 506 // Zap the dead pred's terminator and replace it with unreachable. 507 TerminatorInst *TI = (*I)->getTerminator(); 508 TI->replaceAllUsesWith(UndefValue::get(TI->getType())); 509 (*I)->getTerminator()->eraseFromParent(); 510 new UnreachableInst((*I)->getContext(), *I); 511 Changed = true; 512 } 513 } 514 515 // If there are exiting blocks with branches on undef, resolve the undef in 516 // the direction which will exit the loop. This will help simplify loop 517 // trip count computations. 518 SmallVector<BasicBlock*, 8> ExitingBlocks; 519 L->getExitingBlocks(ExitingBlocks); 520 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(), 521 E = ExitingBlocks.end(); I != E; ++I) 522 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator())) 523 if (BI->isConditional()) { 524 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) { 525 526 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in " 527 << (*I)->getName() << "\n"); 528 529 BI->setCondition(ConstantInt::get(Cond->getType(), 530 !L->contains(BI->getSuccessor(0)))); 531 532 // This may make the loop analyzable, force SCEV recomputation. 533 if (SE) 534 SE->forgetLoop(L); 535 536 Changed = true; 537 } 538 } 539 540 // Does the loop already have a preheader? If so, don't insert one. 541 BasicBlock *Preheader = L->getLoopPreheader(); 542 if (!Preheader) { 543 Preheader = InsertPreheaderForLoop(L, PP); 544 if (Preheader) { 545 ++NumInserted; 546 Changed = true; 547 } 548 } 549 550 // Next, check to make sure that all exit nodes of the loop only have 551 // predecessors that are inside of the loop. This check guarantees that the 552 // loop preheader/header will dominate the exit blocks. If the exit block has 553 // predecessors from outside of the loop, split the edge now. 554 SmallVector<BasicBlock*, 8> ExitBlocks; 555 L->getExitBlocks(ExitBlocks); 556 557 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(), 558 ExitBlocks.end()); 559 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(), 560 E = ExitBlockSet.end(); I != E; ++I) { 561 BasicBlock *ExitBlock = *I; 562 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock); 563 PI != PE; ++PI) 564 // Must be exactly this loop: no subloops, parent loops, or non-loop preds 565 // allowed. 566 if (!L->contains(*PI)) { 567 if (rewriteLoopExitBlock(L, ExitBlock, PP)) { 568 ++NumInserted; 569 Changed = true; 570 } 571 break; 572 } 573 } 574 575 // If the header has more than two predecessors at this point (from the 576 // preheader and from multiple backedges), we must adjust the loop. 577 BasicBlock *LoopLatch = L->getLoopLatch(); 578 if (!LoopLatch) { 579 // If this is really a nested loop, rip it out into a child loop. Don't do 580 // this for loops with a giant number of backedges, just factor them into a 581 // common backedge instead. 582 if (L->getNumBackEdges() < 8) { 583 if (Loop *OuterL = separateNestedLoop(L, Preheader, AA, DT, LI, SE, PP)) { 584 ++NumNested; 585 // Enqueue the outer loop as it should be processed next in our 586 // depth-first nest walk. 587 Worklist.push_back(OuterL); 588 589 // This is a big restructuring change, reprocess the whole loop. 590 Changed = true; 591 // GCC doesn't tail recursion eliminate this. 592 // FIXME: It isn't clear we can't rely on LLVM to TRE this. 593 goto ReprocessLoop; 594 } 595 } 596 597 // If we either couldn't, or didn't want to, identify nesting of the loops, 598 // insert a new block that all backedges target, then make it jump to the 599 // loop header. 600 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, AA, DT, LI); 601 if (LoopLatch) { 602 ++NumInserted; 603 Changed = true; 604 } 605 } 606 607 // Scan over the PHI nodes in the loop header. Since they now have only two 608 // incoming values (the loop is canonicalized), we may have simplified the PHI 609 // down to 'X = phi [X, Y]', which should be replaced with 'Y'. 610 PHINode *PN; 611 for (BasicBlock::iterator I = L->getHeader()->begin(); 612 (PN = dyn_cast<PHINode>(I++)); ) 613 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, DT)) { 614 if (AA) AA->deleteValue(PN); 615 if (SE) SE->forgetValue(PN); 616 PN->replaceAllUsesWith(V); 617 PN->eraseFromParent(); 618 } 619 620 // If this loop has multiple exits and the exits all go to the same 621 // block, attempt to merge the exits. This helps several passes, such 622 // as LoopRotation, which do not support loops with multiple exits. 623 // SimplifyCFG also does this (and this code uses the same utility 624 // function), however this code is loop-aware, where SimplifyCFG is 625 // not. That gives it the advantage of being able to hoist 626 // loop-invariant instructions out of the way to open up more 627 // opportunities, and the disadvantage of having the responsibility 628 // to preserve dominator information. 629 bool UniqueExit = true; 630 if (!ExitBlocks.empty()) 631 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i) 632 if (ExitBlocks[i] != ExitBlocks[0]) { 633 UniqueExit = false; 634 break; 635 } 636 if (UniqueExit) { 637 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { 638 BasicBlock *ExitingBlock = ExitingBlocks[i]; 639 if (!ExitingBlock->getSinglePredecessor()) continue; 640 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()); 641 if (!BI || !BI->isConditional()) continue; 642 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition()); 643 if (!CI || CI->getParent() != ExitingBlock) continue; 644 645 // Attempt to hoist out all instructions except for the 646 // comparison and the branch. 647 bool AllInvariant = true; 648 bool AnyInvariant = false; 649 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) { 650 Instruction *Inst = I++; 651 // Skip debug info intrinsics. 652 if (isa<DbgInfoIntrinsic>(Inst)) 653 continue; 654 if (Inst == CI) 655 continue; 656 if (!L->makeLoopInvariant(Inst, AnyInvariant, 657 Preheader ? Preheader->getTerminator() 658 : nullptr)) { 659 AllInvariant = false; 660 break; 661 } 662 } 663 if (AnyInvariant) { 664 Changed = true; 665 // The loop disposition of all SCEV expressions that depend on any 666 // hoisted values have also changed. 667 if (SE) 668 SE->forgetLoopDispositions(L); 669 } 670 if (!AllInvariant) continue; 671 672 // The block has now been cleared of all instructions except for 673 // a comparison and a conditional branch. SimplifyCFG may be able 674 // to fold it now. 675 if (!FoldBranchToCommonDest(BI)) continue; 676 677 // Success. The block is now dead, so remove it from the loop, 678 // update the dominator tree and delete it. 679 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block " 680 << ExitingBlock->getName() << "\n"); 681 682 // Notify ScalarEvolution before deleting this block. Currently assume the 683 // parent loop doesn't change (spliting edges doesn't count). If blocks, 684 // CFG edges, or other values in the parent loop change, then we need call 685 // to forgetLoop() for the parent instead. 686 if (SE) 687 SE->forgetLoop(L); 688 689 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock)); 690 Changed = true; 691 LI->removeBlock(ExitingBlock); 692 693 DomTreeNode *Node = DT->getNode(ExitingBlock); 694 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children = 695 Node->getChildren(); 696 while (!Children.empty()) { 697 DomTreeNode *Child = Children.front(); 698 DT->changeImmediateDominator(Child, Node->getIDom()); 699 } 700 DT->eraseNode(ExitingBlock); 701 702 BI->getSuccessor(0)->removePredecessor(ExitingBlock); 703 BI->getSuccessor(1)->removePredecessor(ExitingBlock); 704 ExitingBlock->eraseFromParent(); 705 } 706 } 707 708 return Changed; 709} 710 711bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP, 712 AliasAnalysis *AA, ScalarEvolution *SE) { 713 bool Changed = false; 714 715 // Worklist maintains our depth-first queue of loops in this nest to process. 716 SmallVector<Loop *, 4> Worklist; 717 Worklist.push_back(L); 718 719 // Walk the worklist from front to back, pushing newly found sub loops onto 720 // the back. This will let us process loops from back to front in depth-first 721 // order. We can use this simple process because loops form a tree. 722 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) { 723 Loop *L2 = Worklist[Idx]; 724 for (Loop::iterator I = L2->begin(), E = L2->end(); I != E; ++I) 725 Worklist.push_back(*I); 726 } 727 728 while (!Worklist.empty()) 729 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, AA, DT, LI, SE, PP); 730 731 return Changed; 732} 733 734namespace { 735 struct LoopSimplify : public FunctionPass { 736 static char ID; // Pass identification, replacement for typeid 737 LoopSimplify() : FunctionPass(ID) { 738 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry()); 739 } 740 741 // AA - If we have an alias analysis object to update, this is it, otherwise 742 // this is null. 743 AliasAnalysis *AA; 744 DominatorTree *DT; 745 LoopInfo *LI; 746 ScalarEvolution *SE; 747 748 bool runOnFunction(Function &F) override; 749 750 void getAnalysisUsage(AnalysisUsage &AU) const override { 751 // We need loop information to identify the loops... 752 AU.addRequired<DominatorTreeWrapperPass>(); 753 AU.addPreserved<DominatorTreeWrapperPass>(); 754 755 AU.addRequired<LoopInfo>(); 756 AU.addPreserved<LoopInfo>(); 757 758 AU.addPreserved<AliasAnalysis>(); 759 AU.addPreserved<ScalarEvolution>(); 760 AU.addPreserved<DependenceAnalysis>(); 761 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added. 762 } 763 764 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees. 765 void verifyAnalysis() const override; 766 }; 767} 768 769char LoopSimplify::ID = 0; 770INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify", 771 "Canonicalize natural loops", true, false) 772INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 773INITIALIZE_PASS_DEPENDENCY(LoopInfo) 774INITIALIZE_PASS_END(LoopSimplify, "loop-simplify", 775 "Canonicalize natural loops", true, false) 776 777// Publicly exposed interface to pass... 778char &llvm::LoopSimplifyID = LoopSimplify::ID; 779Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); } 780 781/// runOnFunction - Run down all loops in the CFG (recursively, but we could do 782/// it in any convenient order) inserting preheaders... 783/// 784bool LoopSimplify::runOnFunction(Function &F) { 785 bool Changed = false; 786 AA = getAnalysisIfAvailable<AliasAnalysis>(); 787 LI = &getAnalysis<LoopInfo>(); 788 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 789 SE = getAnalysisIfAvailable<ScalarEvolution>(); 790 791 // Simplify each loop nest in the function. 792 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 793 Changed |= simplifyLoop(*I, DT, LI, this, AA, SE); 794 795 return Changed; 796} 797 798// FIXME: Restore this code when we re-enable verification in verifyAnalysis 799// below. 800#if 0 801static void verifyLoop(Loop *L) { 802 // Verify subloops. 803 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 804 verifyLoop(*I); 805 806 // It used to be possible to just assert L->isLoopSimplifyForm(), however 807 // with the introduction of indirectbr, there are now cases where it's 808 // not possible to transform a loop as necessary. We can at least check 809 // that there is an indirectbr near any time there's trouble. 810 811 // Indirectbr can interfere with preheader and unique backedge insertion. 812 if (!L->getLoopPreheader() || !L->getLoopLatch()) { 813 bool HasIndBrPred = false; 814 for (pred_iterator PI = pred_begin(L->getHeader()), 815 PE = pred_end(L->getHeader()); PI != PE; ++PI) 816 if (isa<IndirectBrInst>((*PI)->getTerminator())) { 817 HasIndBrPred = true; 818 break; 819 } 820 assert(HasIndBrPred && 821 "LoopSimplify has no excuse for missing loop header info!"); 822 (void)HasIndBrPred; 823 } 824 825 // Indirectbr can interfere with exit block canonicalization. 826 if (!L->hasDedicatedExits()) { 827 bool HasIndBrExiting = false; 828 SmallVector<BasicBlock*, 8> ExitingBlocks; 829 L->getExitingBlocks(ExitingBlocks); 830 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { 831 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) { 832 HasIndBrExiting = true; 833 break; 834 } 835 } 836 837 assert(HasIndBrExiting && 838 "LoopSimplify has no excuse for missing exit block info!"); 839 (void)HasIndBrExiting; 840 } 841} 842#endif 843 844void LoopSimplify::verifyAnalysis() const { 845 // FIXME: This routine is being called mid-way through the loop pass manager 846 // as loop passes destroy this analysis. That's actually fine, but we have no 847 // way of expressing that here. Once all of the passes that destroy this are 848 // hoisted out of the loop pass manager we can add back verification here. 849#if 0 850 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 851 verifyLoop(*I); 852#endif 853} 854