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/AssumptionCache.h" 48 #include "llvm/Analysis/DependenceAnalysis.h" 49 #include "llvm/Analysis/InstructionSimplify.h" 50 #include "llvm/Analysis/LoopInfo.h" 51 #include "llvm/Analysis/ScalarEvolution.h" 52 #include "llvm/IR/CFG.h" 53 #include "llvm/IR/Constants.h" 54 #include "llvm/IR/DataLayout.h" 55 #include "llvm/IR/Dominators.h" 56 #include "llvm/IR/Function.h" 57 #include "llvm/IR/Instructions.h" 58 #include "llvm/IR/IntrinsicInst.h" 59 #include "llvm/IR/LLVMContext.h" 60 #include "llvm/IR/Module.h" 61 #include "llvm/IR/Type.h" 62 #include "llvm/Support/Debug.h" 63 #include "llvm/Support/raw_ostream.h" 64 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 65 #include "llvm/Transforms/Utils/Local.h" 66 #include "llvm/Transforms/Utils/LoopUtils.h" 67 using namespace llvm; 68 69 #define DEBUG_TYPE "loop-simplify" 70 71 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted"); 72 STATISTIC(NumNested , "Number of nested loops split out"); 73 74 // If the block isn't already, move the new block to right after some 'outside 75 // block' block. This prevents the preheader from being placed inside the loop 76 // body, e.g. when the loop hasn't been rotated. 77 static void placeSplitBlockCarefully(BasicBlock *NewBB, 78 SmallVectorImpl<BasicBlock *> &SplitPreds, 79 Loop *L) { 80 // Check to see if NewBB is already well placed. 81 Function::iterator BBI = NewBB; --BBI; 82 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { 83 if (&*BBI == SplitPreds[i]) 84 return; 85 } 86 87 // If it isn't already after an outside block, move it after one. This is 88 // always good as it makes the uncond branch from the outside block into a 89 // fall-through. 90 91 // Figure out *which* outside block to put this after. Prefer an outside 92 // block that neighbors a BB actually in the loop. 93 BasicBlock *FoundBB = nullptr; 94 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) { 95 Function::iterator BBI = SplitPreds[i]; 96 if (++BBI != NewBB->getParent()->end() && 97 L->contains(BBI)) { 98 FoundBB = SplitPreds[i]; 99 break; 100 } 101 } 102 103 // If our heuristic for a *good* bb to place this after doesn't find 104 // anything, just pick something. It's likely better than leaving it within 105 // the loop. 106 if (!FoundBB) 107 FoundBB = SplitPreds[0]; 108 NewBB->moveAfter(FoundBB); 109 } 110 111 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a 112 /// preheader, this method is called to insert one. This method has two phases: 113 /// preheader insertion and analysis updating. 114 /// 115 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, Pass *PP) { 116 BasicBlock *Header = L->getHeader(); 117 118 // Get analyses that we try to update. 119 auto *AA = PP->getAnalysisIfAvailable<AliasAnalysis>(); 120 auto *DTWP = PP->getAnalysisIfAvailable<DominatorTreeWrapperPass>(); 121 auto *DT = DTWP ? &DTWP->getDomTree() : nullptr; 122 auto *LIWP = PP->getAnalysisIfAvailable<LoopInfoWrapperPass>(); 123 auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr; 124 bool PreserveLCSSA = PP->mustPreserveAnalysisID(LCSSAID); 125 126 // Compute the set of predecessors of the loop that are not in the loop. 127 SmallVector<BasicBlock*, 8> OutsideBlocks; 128 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header); 129 PI != PE; ++PI) { 130 BasicBlock *P = *PI; 131 if (!L->contains(P)) { // Coming in from outside the loop? 132 // If the loop is branched to from an indirect branch, we won't 133 // be able to fully transform the loop, because it prohibits 134 // edge splitting. 135 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr; 136 137 // Keep track of it. 138 OutsideBlocks.push_back(P); 139 } 140 } 141 142 // Split out the loop pre-header. 143 BasicBlock *PreheaderBB; 144 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", 145 AA, DT, LI, PreserveLCSSA); 146 147 PreheaderBB->getTerminator()->setDebugLoc( 148 Header->getFirstNonPHI()->getDebugLoc()); 149 DEBUG(dbgs() << "LoopSimplify: Creating pre-header " 150 << PreheaderBB->getName() << "\n"); 151 152 // Make sure that NewBB is put someplace intelligent, which doesn't mess up 153 // code layout too horribly. 154 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L); 155 156 return PreheaderBB; 157 } 158 159 /// \brief Ensure that the loop preheader dominates all exit blocks. 160 /// 161 /// This method is used to split exit blocks that have predecessors outside of 162 /// the loop. 163 static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit, 164 AliasAnalysis *AA, DominatorTree *DT, 165 LoopInfo *LI, Pass *PP) { 166 SmallVector<BasicBlock*, 8> LoopBlocks; 167 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) { 168 BasicBlock *P = *I; 169 if (L->contains(P)) { 170 // Don't do this if the loop is exited via an indirect branch. 171 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr; 172 173 LoopBlocks.push_back(P); 174 } 175 } 176 177 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?"); 178 BasicBlock *NewExitBB = nullptr; 179 180 bool PreserveLCSSA = PP->mustPreserveAnalysisID(LCSSAID); 181 182 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", AA, DT, 183 LI, PreserveLCSSA); 184 185 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block " 186 << NewExitBB->getName() << "\n"); 187 return NewExitBB; 188 } 189 190 /// Add the specified block, and all of its predecessors, to the specified set, 191 /// if it's not already in there. Stop predecessor traversal when we reach 192 /// StopBlock. 193 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock, 194 std::set<BasicBlock*> &Blocks) { 195 SmallVector<BasicBlock *, 8> Worklist; 196 Worklist.push_back(InputBB); 197 do { 198 BasicBlock *BB = Worklist.pop_back_val(); 199 if (Blocks.insert(BB).second && BB != StopBlock) 200 // If BB is not already processed and it is not a stop block then 201 // insert its predecessor in the work list 202 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) { 203 BasicBlock *WBB = *I; 204 Worklist.push_back(WBB); 205 } 206 } while (!Worklist.empty()); 207 } 208 209 /// \brief The first part of loop-nestification is to find a PHI node that tells 210 /// us how to partition the loops. 211 static PHINode *findPHIToPartitionLoops(Loop *L, AliasAnalysis *AA, 212 DominatorTree *DT, 213 AssumptionCache *AC) { 214 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout(); 215 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) { 216 PHINode *PN = cast<PHINode>(I); 217 ++I; 218 if (Value *V = SimplifyInstruction(PN, DL, nullptr, DT, AC)) { 219 // This is a degenerate PHI already, don't modify it! 220 PN->replaceAllUsesWith(V); 221 if (AA) AA->deleteValue(PN); 222 PN->eraseFromParent(); 223 continue; 224 } 225 226 // Scan this PHI node looking for a use of the PHI node by itself. 227 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 228 if (PN->getIncomingValue(i) == PN && 229 L->contains(PN->getIncomingBlock(i))) 230 // We found something tasty to remove. 231 return PN; 232 } 233 return nullptr; 234 } 235 236 /// \brief If this loop has multiple backedges, try to pull one of them out into 237 /// a nested loop. 238 /// 239 /// This is important for code that looks like 240 /// this: 241 /// 242 /// Loop: 243 /// ... 244 /// br cond, Loop, Next 245 /// ... 246 /// br cond2, Loop, Out 247 /// 248 /// To identify this common case, we look at the PHI nodes in the header of the 249 /// loop. PHI nodes with unchanging values on one backedge correspond to values 250 /// that change in the "outer" loop, but not in the "inner" loop. 251 /// 252 /// If we are able to separate out a loop, return the new outer loop that was 253 /// created. 254 /// 255 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader, 256 AliasAnalysis *AA, DominatorTree *DT, 257 LoopInfo *LI, ScalarEvolution *SE, Pass *PP, 258 AssumptionCache *AC) { 259 // Don't try to separate loops without a preheader. 260 if (!Preheader) 261 return nullptr; 262 263 // The header is not a landing pad; preheader insertion should ensure this. 264 assert(!L->getHeader()->isLandingPad() && 265 "Can't insert backedge to landing pad"); 266 267 PHINode *PN = findPHIToPartitionLoops(L, AA, DT, AC); 268 if (!PN) return nullptr; // No known way to partition. 269 270 // Pull out all predecessors that have varying values in the loop. This 271 // handles the case when a PHI node has multiple instances of itself as 272 // arguments. 273 SmallVector<BasicBlock*, 8> OuterLoopPreds; 274 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 275 if (PN->getIncomingValue(i) != PN || 276 !L->contains(PN->getIncomingBlock(i))) { 277 // We can't split indirectbr edges. 278 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator())) 279 return nullptr; 280 OuterLoopPreds.push_back(PN->getIncomingBlock(i)); 281 } 282 } 283 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n"); 284 285 // If ScalarEvolution is around and knows anything about values in 286 // this loop, tell it to forget them, because we're about to 287 // substantially change it. 288 if (SE) 289 SE->forgetLoop(L); 290 291 bool PreserveLCSSA = PP->mustPreserveAnalysisID(LCSSAID); 292 293 BasicBlock *Header = L->getHeader(); 294 BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", 295 AA, DT, LI, PreserveLCSSA); 296 297 // Make sure that NewBB is put someplace intelligent, which doesn't mess up 298 // code layout too horribly. 299 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L); 300 301 // Create the new outer loop. 302 Loop *NewOuter = new Loop(); 303 304 // Change the parent loop to use the outer loop as its child now. 305 if (Loop *Parent = L->getParentLoop()) 306 Parent->replaceChildLoopWith(L, NewOuter); 307 else 308 LI->changeTopLevelLoop(L, NewOuter); 309 310 // L is now a subloop of our outer loop. 311 NewOuter->addChildLoop(L); 312 313 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 314 I != E; ++I) 315 NewOuter->addBlockEntry(*I); 316 317 // Now reset the header in L, which had been moved by 318 // SplitBlockPredecessors for the outer loop. 319 L->moveToHeader(Header); 320 321 // Determine which blocks should stay in L and which should be moved out to 322 // the Outer loop now. 323 std::set<BasicBlock*> BlocksInL; 324 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) { 325 BasicBlock *P = *PI; 326 if (DT->dominates(Header, P)) 327 addBlockAndPredsToSet(P, Header, BlocksInL); 328 } 329 330 // Scan all of the loop children of L, moving them to OuterLoop if they are 331 // not part of the inner loop. 332 const std::vector<Loop*> &SubLoops = L->getSubLoops(); 333 for (size_t I = 0; I != SubLoops.size(); ) 334 if (BlocksInL.count(SubLoops[I]->getHeader())) 335 ++I; // Loop remains in L 336 else 337 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I)); 338 339 // Now that we know which blocks are in L and which need to be moved to 340 // OuterLoop, move any blocks that need it. 341 for (unsigned i = 0; i != L->getBlocks().size(); ++i) { 342 BasicBlock *BB = L->getBlocks()[i]; 343 if (!BlocksInL.count(BB)) { 344 // Move this block to the parent, updating the exit blocks sets 345 L->removeBlockFromLoop(BB); 346 if ((*LI)[BB] == L) 347 LI->changeLoopFor(BB, NewOuter); 348 --i; 349 } 350 } 351 352 return NewOuter; 353 } 354 355 /// \brief This method is called when the specified loop has more than one 356 /// backedge in it. 357 /// 358 /// If this occurs, revector all of these backedges to target a new basic block 359 /// and have that block branch to the loop header. This ensures that loops 360 /// have exactly one backedge. 361 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader, 362 AliasAnalysis *AA, 363 DominatorTree *DT, LoopInfo *LI) { 364 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!"); 365 366 // Get information about the loop 367 BasicBlock *Header = L->getHeader(); 368 Function *F = Header->getParent(); 369 370 // Unique backedge insertion currently depends on having a preheader. 371 if (!Preheader) 372 return nullptr; 373 374 // The header is not a landing pad; preheader insertion should ensure this. 375 assert(!Header->isLandingPad() && "Can't insert backedge to landing pad"); 376 377 // Figure out which basic blocks contain back-edges to the loop header. 378 std::vector<BasicBlock*> BackedgeBlocks; 379 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){ 380 BasicBlock *P = *I; 381 382 // Indirectbr edges cannot be split, so we must fail if we find one. 383 if (isa<IndirectBrInst>(P->getTerminator())) 384 return nullptr; 385 386 if (P != Preheader) BackedgeBlocks.push_back(P); 387 } 388 389 // Create and insert the new backedge block... 390 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(), 391 Header->getName()+".backedge", F); 392 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock); 393 394 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block " 395 << BEBlock->getName() << "\n"); 396 397 // Move the new backedge block to right after the last backedge block. 398 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos; 399 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock); 400 401 // Now that the block has been inserted into the function, create PHI nodes in 402 // the backedge block which correspond to any PHI nodes in the header block. 403 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 404 PHINode *PN = cast<PHINode>(I); 405 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(), 406 PN->getName()+".be", BETerminator); 407 if (AA) AA->copyValue(PN, NewPN); 408 409 // Loop over the PHI node, moving all entries except the one for the 410 // preheader over to the new PHI node. 411 unsigned PreheaderIdx = ~0U; 412 bool HasUniqueIncomingValue = true; 413 Value *UniqueValue = nullptr; 414 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 415 BasicBlock *IBB = PN->getIncomingBlock(i); 416 Value *IV = PN->getIncomingValue(i); 417 if (IBB == Preheader) { 418 PreheaderIdx = i; 419 } else { 420 NewPN->addIncoming(IV, IBB); 421 if (HasUniqueIncomingValue) { 422 if (!UniqueValue) 423 UniqueValue = IV; 424 else if (UniqueValue != IV) 425 HasUniqueIncomingValue = false; 426 } 427 } 428 } 429 430 // Delete all of the incoming values from the old PN except the preheader's 431 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??"); 432 if (PreheaderIdx != 0) { 433 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx)); 434 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx)); 435 } 436 // Nuke all entries except the zero'th. 437 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i) 438 PN->removeIncomingValue(e-i, false); 439 440 // Finally, add the newly constructed PHI node as the entry for the BEBlock. 441 PN->addIncoming(NewPN, BEBlock); 442 443 // As an optimization, if all incoming values in the new PhiNode (which is a 444 // subset of the incoming values of the old PHI node) have the same value, 445 // eliminate the PHI Node. 446 if (HasUniqueIncomingValue) { 447 NewPN->replaceAllUsesWith(UniqueValue); 448 if (AA) AA->deleteValue(NewPN); 449 BEBlock->getInstList().erase(NewPN); 450 } 451 } 452 453 // Now that all of the PHI nodes have been inserted and adjusted, modify the 454 // backedge blocks to just to the BEBlock instead of the header. 455 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) { 456 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator(); 457 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op) 458 if (TI->getSuccessor(Op) == Header) 459 TI->setSuccessor(Op, BEBlock); 460 } 461 462 //===--- Update all analyses which we must preserve now -----------------===// 463 464 // Update Loop Information - we know that this block is now in the current 465 // loop and all parent loops. 466 L->addBasicBlockToLoop(BEBlock, *LI); 467 468 // Update dominator information 469 DT->splitBlock(BEBlock); 470 471 return BEBlock; 472 } 473 474 /// \brief Simplify one loop and queue further loops for simplification. 475 /// 476 /// FIXME: Currently this accepts both lots of analyses that it uses and a raw 477 /// Pass pointer. The Pass pointer is used by numerous utilities to update 478 /// specific analyses. Rather than a pass it would be much cleaner and more 479 /// explicit if they accepted the analysis directly and then updated it. 480 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist, 481 AliasAnalysis *AA, DominatorTree *DT, LoopInfo *LI, 482 ScalarEvolution *SE, Pass *PP, 483 AssumptionCache *AC) { 484 bool Changed = false; 485 ReprocessLoop: 486 487 // Check to see that no blocks (other than the header) in this loop have 488 // predecessors that are not in the loop. This is not valid for natural 489 // loops, but can occur if the blocks are unreachable. Since they are 490 // unreachable we can just shamelessly delete those CFG edges! 491 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end(); 492 BB != E; ++BB) { 493 if (*BB == L->getHeader()) continue; 494 495 SmallPtrSet<BasicBlock*, 4> BadPreds; 496 for (pred_iterator PI = pred_begin(*BB), 497 PE = pred_end(*BB); PI != PE; ++PI) { 498 BasicBlock *P = *PI; 499 if (!L->contains(P)) 500 BadPreds.insert(P); 501 } 502 503 // Delete each unique out-of-loop (and thus dead) predecessor. 504 for (BasicBlock *P : BadPreds) { 505 506 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor " 507 << P->getName() << "\n"); 508 509 // Inform each successor of each dead pred. 510 for (succ_iterator SI = succ_begin(P), SE = succ_end(P); SI != SE; ++SI) 511 (*SI)->removePredecessor(P); 512 // Zap the dead pred's terminator and replace it with unreachable. 513 TerminatorInst *TI = P->getTerminator(); 514 TI->replaceAllUsesWith(UndefValue::get(TI->getType())); 515 P->getTerminator()->eraseFromParent(); 516 new UnreachableInst(P->getContext(), P); 517 Changed = true; 518 } 519 } 520 521 // If there are exiting blocks with branches on undef, resolve the undef in 522 // the direction which will exit the loop. This will help simplify loop 523 // trip count computations. 524 SmallVector<BasicBlock*, 8> ExitingBlocks; 525 L->getExitingBlocks(ExitingBlocks); 526 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(), 527 E = ExitingBlocks.end(); I != E; ++I) 528 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator())) 529 if (BI->isConditional()) { 530 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) { 531 532 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in " 533 << (*I)->getName() << "\n"); 534 535 BI->setCondition(ConstantInt::get(Cond->getType(), 536 !L->contains(BI->getSuccessor(0)))); 537 538 // This may make the loop analyzable, force SCEV recomputation. 539 if (SE) 540 SE->forgetLoop(L); 541 542 Changed = true; 543 } 544 } 545 546 // Does the loop already have a preheader? If so, don't insert one. 547 BasicBlock *Preheader = L->getLoopPreheader(); 548 if (!Preheader) { 549 Preheader = InsertPreheaderForLoop(L, PP); 550 if (Preheader) { 551 ++NumInserted; 552 Changed = true; 553 } 554 } 555 556 // Next, check to make sure that all exit nodes of the loop only have 557 // predecessors that are inside of the loop. This check guarantees that the 558 // loop preheader/header will dominate the exit blocks. If the exit block has 559 // predecessors from outside of the loop, split the edge now. 560 SmallVector<BasicBlock*, 8> ExitBlocks; 561 L->getExitBlocks(ExitBlocks); 562 563 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(), 564 ExitBlocks.end()); 565 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(), 566 E = ExitBlockSet.end(); I != E; ++I) { 567 BasicBlock *ExitBlock = *I; 568 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock); 569 PI != PE; ++PI) 570 // Must be exactly this loop: no subloops, parent loops, or non-loop preds 571 // allowed. 572 if (!L->contains(*PI)) { 573 if (rewriteLoopExitBlock(L, ExitBlock, AA, DT, LI, PP)) { 574 ++NumInserted; 575 Changed = true; 576 } 577 break; 578 } 579 } 580 581 // If the header has more than two predecessors at this point (from the 582 // preheader and from multiple backedges), we must adjust the loop. 583 BasicBlock *LoopLatch = L->getLoopLatch(); 584 if (!LoopLatch) { 585 // If this is really a nested loop, rip it out into a child loop. Don't do 586 // this for loops with a giant number of backedges, just factor them into a 587 // common backedge instead. 588 if (L->getNumBackEdges() < 8) { 589 if (Loop *OuterL = 590 separateNestedLoop(L, Preheader, AA, DT, LI, SE, PP, AC)) { 591 ++NumNested; 592 // Enqueue the outer loop as it should be processed next in our 593 // depth-first nest walk. 594 Worklist.push_back(OuterL); 595 596 // This is a big restructuring change, reprocess the whole loop. 597 Changed = true; 598 // GCC doesn't tail recursion eliminate this. 599 // FIXME: It isn't clear we can't rely on LLVM to TRE this. 600 goto ReprocessLoop; 601 } 602 } 603 604 // If we either couldn't, or didn't want to, identify nesting of the loops, 605 // insert a new block that all backedges target, then make it jump to the 606 // loop header. 607 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, AA, DT, LI); 608 if (LoopLatch) { 609 ++NumInserted; 610 Changed = true; 611 } 612 } 613 614 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout(); 615 616 // Scan over the PHI nodes in the loop header. Since they now have only two 617 // incoming values (the loop is canonicalized), we may have simplified the PHI 618 // down to 'X = phi [X, Y]', which should be replaced with 'Y'. 619 PHINode *PN; 620 for (BasicBlock::iterator I = L->getHeader()->begin(); 621 (PN = dyn_cast<PHINode>(I++)); ) 622 if (Value *V = SimplifyInstruction(PN, DL, nullptr, DT, AC)) { 623 if (AA) AA->deleteValue(PN); 624 if (SE) SE->forgetValue(PN); 625 PN->replaceAllUsesWith(V); 626 PN->eraseFromParent(); 627 } 628 629 // If this loop has multiple exits and the exits all go to the same 630 // block, attempt to merge the exits. This helps several passes, such 631 // as LoopRotation, which do not support loops with multiple exits. 632 // SimplifyCFG also does this (and this code uses the same utility 633 // function), however this code is loop-aware, where SimplifyCFG is 634 // not. That gives it the advantage of being able to hoist 635 // loop-invariant instructions out of the way to open up more 636 // opportunities, and the disadvantage of having the responsibility 637 // to preserve dominator information. 638 bool UniqueExit = true; 639 if (!ExitBlocks.empty()) 640 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i) 641 if (ExitBlocks[i] != ExitBlocks[0]) { 642 UniqueExit = false; 643 break; 644 } 645 if (UniqueExit) { 646 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { 647 BasicBlock *ExitingBlock = ExitingBlocks[i]; 648 if (!ExitingBlock->getSinglePredecessor()) continue; 649 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()); 650 if (!BI || !BI->isConditional()) continue; 651 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition()); 652 if (!CI || CI->getParent() != ExitingBlock) continue; 653 654 // Attempt to hoist out all instructions except for the 655 // comparison and the branch. 656 bool AllInvariant = true; 657 bool AnyInvariant = false; 658 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) { 659 Instruction *Inst = I++; 660 // Skip debug info intrinsics. 661 if (isa<DbgInfoIntrinsic>(Inst)) 662 continue; 663 if (Inst == CI) 664 continue; 665 if (!L->makeLoopInvariant(Inst, AnyInvariant, 666 Preheader ? Preheader->getTerminator() 667 : nullptr)) { 668 AllInvariant = false; 669 break; 670 } 671 } 672 if (AnyInvariant) { 673 Changed = true; 674 // The loop disposition of all SCEV expressions that depend on any 675 // hoisted values have also changed. 676 if (SE) 677 SE->forgetLoopDispositions(L); 678 } 679 if (!AllInvariant) continue; 680 681 // The block has now been cleared of all instructions except for 682 // a comparison and a conditional branch. SimplifyCFG may be able 683 // to fold it now. 684 if (!FoldBranchToCommonDest(BI)) 685 continue; 686 687 // Success. The block is now dead, so remove it from the loop, 688 // update the dominator tree and delete it. 689 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block " 690 << ExitingBlock->getName() << "\n"); 691 692 // Notify ScalarEvolution before deleting this block. Currently assume the 693 // parent loop doesn't change (spliting edges doesn't count). If blocks, 694 // CFG edges, or other values in the parent loop change, then we need call 695 // to forgetLoop() for the parent instead. 696 if (SE) 697 SE->forgetLoop(L); 698 699 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock)); 700 Changed = true; 701 LI->removeBlock(ExitingBlock); 702 703 DomTreeNode *Node = DT->getNode(ExitingBlock); 704 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children = 705 Node->getChildren(); 706 while (!Children.empty()) { 707 DomTreeNode *Child = Children.front(); 708 DT->changeImmediateDominator(Child, Node->getIDom()); 709 } 710 DT->eraseNode(ExitingBlock); 711 712 BI->getSuccessor(0)->removePredecessor(ExitingBlock); 713 BI->getSuccessor(1)->removePredecessor(ExitingBlock); 714 ExitingBlock->eraseFromParent(); 715 } 716 } 717 718 return Changed; 719 } 720 721 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP, 722 AliasAnalysis *AA, ScalarEvolution *SE, 723 AssumptionCache *AC) { 724 bool Changed = false; 725 726 // Worklist maintains our depth-first queue of loops in this nest to process. 727 SmallVector<Loop *, 4> Worklist; 728 Worklist.push_back(L); 729 730 // Walk the worklist from front to back, pushing newly found sub loops onto 731 // the back. This will let us process loops from back to front in depth-first 732 // order. We can use this simple process because loops form a tree. 733 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) { 734 Loop *L2 = Worklist[Idx]; 735 Worklist.append(L2->begin(), L2->end()); 736 } 737 738 while (!Worklist.empty()) 739 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, AA, DT, LI, 740 SE, PP, AC); 741 742 return Changed; 743 } 744 745 namespace { 746 struct LoopSimplify : public FunctionPass { 747 static char ID; // Pass identification, replacement for typeid 748 LoopSimplify() : FunctionPass(ID) { 749 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry()); 750 } 751 752 // AA - If we have an alias analysis object to update, this is it, otherwise 753 // this is null. 754 AliasAnalysis *AA; 755 DominatorTree *DT; 756 LoopInfo *LI; 757 ScalarEvolution *SE; 758 AssumptionCache *AC; 759 760 bool runOnFunction(Function &F) override; 761 762 void getAnalysisUsage(AnalysisUsage &AU) const override { 763 AU.addRequired<AssumptionCacheTracker>(); 764 765 // We need loop information to identify the loops... 766 AU.addRequired<DominatorTreeWrapperPass>(); 767 AU.addPreserved<DominatorTreeWrapperPass>(); 768 769 AU.addRequired<LoopInfoWrapperPass>(); 770 AU.addPreserved<LoopInfoWrapperPass>(); 771 772 AU.addPreserved<AliasAnalysis>(); 773 AU.addPreserved<ScalarEvolution>(); 774 AU.addPreserved<DependenceAnalysis>(); 775 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added. 776 } 777 778 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees. 779 void verifyAnalysis() const override; 780 }; 781 } 782 783 char LoopSimplify::ID = 0; 784 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify", 785 "Canonicalize natural loops", false, false) 786 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 787 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 788 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) 789 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify", 790 "Canonicalize natural loops", false, false) 791 792 // Publicly exposed interface to pass... 793 char &llvm::LoopSimplifyID = LoopSimplify::ID; 794 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); } 795 796 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do 797 /// it in any convenient order) inserting preheaders... 798 /// 799 bool LoopSimplify::runOnFunction(Function &F) { 800 bool Changed = false; 801 AA = getAnalysisIfAvailable<AliasAnalysis>(); 802 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); 803 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 804 SE = getAnalysisIfAvailable<ScalarEvolution>(); 805 AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); 806 807 // Simplify each loop nest in the function. 808 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 809 Changed |= simplifyLoop(*I, DT, LI, this, AA, SE, AC); 810 811 return Changed; 812 } 813 814 // FIXME: Restore this code when we re-enable verification in verifyAnalysis 815 // below. 816 #if 0 817 static void verifyLoop(Loop *L) { 818 // Verify subloops. 819 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) 820 verifyLoop(*I); 821 822 // It used to be possible to just assert L->isLoopSimplifyForm(), however 823 // with the introduction of indirectbr, there are now cases where it's 824 // not possible to transform a loop as necessary. We can at least check 825 // that there is an indirectbr near any time there's trouble. 826 827 // Indirectbr can interfere with preheader and unique backedge insertion. 828 if (!L->getLoopPreheader() || !L->getLoopLatch()) { 829 bool HasIndBrPred = false; 830 for (pred_iterator PI = pred_begin(L->getHeader()), 831 PE = pred_end(L->getHeader()); PI != PE; ++PI) 832 if (isa<IndirectBrInst>((*PI)->getTerminator())) { 833 HasIndBrPred = true; 834 break; 835 } 836 assert(HasIndBrPred && 837 "LoopSimplify has no excuse for missing loop header info!"); 838 (void)HasIndBrPred; 839 } 840 841 // Indirectbr can interfere with exit block canonicalization. 842 if (!L->hasDedicatedExits()) { 843 bool HasIndBrExiting = false; 844 SmallVector<BasicBlock*, 8> ExitingBlocks; 845 L->getExitingBlocks(ExitingBlocks); 846 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) { 847 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) { 848 HasIndBrExiting = true; 849 break; 850 } 851 } 852 853 assert(HasIndBrExiting && 854 "LoopSimplify has no excuse for missing exit block info!"); 855 (void)HasIndBrExiting; 856 } 857 } 858 #endif 859 860 void LoopSimplify::verifyAnalysis() const { 861 // FIXME: This routine is being called mid-way through the loop pass manager 862 // as loop passes destroy this analysis. That's actually fine, but we have no 863 // way of expressing that here. Once all of the passes that destroy this are 864 // hoisted out of the loop pass manager we can add back verification here. 865 #if 0 866 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) 867 verifyLoop(*I); 868 #endif 869 } 870