1 //===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===// 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 file implements some loop unrolling utilities. It does not define any 11 // actual pass or policy, but provides a single function to perform loop 12 // unrolling. 13 // 14 // The process of unrolling can produce extraneous basic blocks linked with 15 // unconditional branches. This will be corrected in the future. 16 // 17 //===----------------------------------------------------------------------===// 18 19 #include "llvm/Transforms/Utils/UnrollLoop.h" 20 #include "llvm/ADT/Statistic.h" 21 #include "llvm/Analysis/InstructionSimplify.h" 22 #include "llvm/Analysis/LoopIterator.h" 23 #include "llvm/Analysis/LoopPass.h" 24 #include "llvm/Analysis/ScalarEvolution.h" 25 #include "llvm/IR/BasicBlock.h" 26 #include "llvm/IR/DataLayout.h" 27 #include "llvm/IR/Dominators.h" 28 #include "llvm/IR/DiagnosticInfo.h" 29 #include "llvm/IR/LLVMContext.h" 30 #include "llvm/Support/Debug.h" 31 #include "llvm/Support/raw_ostream.h" 32 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 33 #include "llvm/Transforms/Utils/Cloning.h" 34 #include "llvm/Transforms/Utils/Local.h" 35 #include "llvm/Transforms/Utils/LoopUtils.h" 36 #include "llvm/Transforms/Utils/SimplifyIndVar.h" 37 using namespace llvm; 38 39 #define DEBUG_TYPE "loop-unroll" 40 41 // TODO: Should these be here or in LoopUnroll? 42 STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled"); 43 STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)"); 44 45 /// RemapInstruction - Convert the instruction operands from referencing the 46 /// current values into those specified by VMap. 47 static inline void RemapInstruction(Instruction *I, 48 ValueToValueMapTy &VMap) { 49 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 50 Value *Op = I->getOperand(op); 51 ValueToValueMapTy::iterator It = VMap.find(Op); 52 if (It != VMap.end()) 53 I->setOperand(op, It->second); 54 } 55 56 if (PHINode *PN = dyn_cast<PHINode>(I)) { 57 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 58 ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i)); 59 if (It != VMap.end()) 60 PN->setIncomingBlock(i, cast<BasicBlock>(It->second)); 61 } 62 } 63 } 64 65 /// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it 66 /// only has one predecessor, and that predecessor only has one successor. 67 /// The LoopInfo Analysis that is passed will be kept consistent. 68 /// Returns the new combined block. 69 static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI, 70 LPPassManager *LPM) { 71 // Merge basic blocks into their predecessor if there is only one distinct 72 // pred, and if there is only one distinct successor of the predecessor, and 73 // if there are no PHI nodes. 74 BasicBlock *OnlyPred = BB->getSinglePredecessor(); 75 if (!OnlyPred) return nullptr; 76 77 if (OnlyPred->getTerminator()->getNumSuccessors() != 1) 78 return nullptr; 79 80 DEBUG(dbgs() << "Merging: " << *BB << "into: " << *OnlyPred); 81 82 // Resolve any PHI nodes at the start of the block. They are all 83 // guaranteed to have exactly one entry if they exist, unless there are 84 // multiple duplicate (but guaranteed to be equal) entries for the 85 // incoming edges. This occurs when there are multiple edges from 86 // OnlyPred to OnlySucc. 87 FoldSingleEntryPHINodes(BB); 88 89 // Delete the unconditional branch from the predecessor... 90 OnlyPred->getInstList().pop_back(); 91 92 // Make all PHI nodes that referred to BB now refer to Pred as their 93 // source... 94 BB->replaceAllUsesWith(OnlyPred); 95 96 // Move all definitions in the successor to the predecessor... 97 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); 98 99 // OldName will be valid until erased. 100 StringRef OldName = BB->getName(); 101 102 // Erase basic block from the function... 103 104 // ScalarEvolution holds references to loop exit blocks. 105 if (LPM) { 106 if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) { 107 if (Loop *L = LI->getLoopFor(BB)) 108 SE->forgetLoop(L); 109 } 110 } 111 LI->removeBlock(BB); 112 113 // Inherit predecessor's name if it exists... 114 if (!OldName.empty() && !OnlyPred->hasName()) 115 OnlyPred->setName(OldName); 116 117 BB->eraseFromParent(); 118 119 return OnlyPred; 120 } 121 122 /// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true 123 /// if unrolling was successful, or false if the loop was unmodified. Unrolling 124 /// can only fail when the loop's latch block is not terminated by a conditional 125 /// branch instruction. However, if the trip count (and multiple) are not known, 126 /// loop unrolling will mostly produce more code that is no faster. 127 /// 128 /// TripCount is generally defined as the number of times the loop header 129 /// executes. UnrollLoop relaxes the definition to permit early exits: here 130 /// TripCount is the iteration on which control exits LatchBlock if no early 131 /// exits were taken. Note that UnrollLoop assumes that the loop counter test 132 /// terminates LatchBlock in order to remove unnecesssary instances of the 133 /// test. In other words, control may exit the loop prior to TripCount 134 /// iterations via an early branch, but control may not exit the loop from the 135 /// LatchBlock's terminator prior to TripCount iterations. 136 /// 137 /// Similarly, TripMultiple divides the number of times that the LatchBlock may 138 /// execute without exiting the loop. 139 /// 140 /// The LoopInfo Analysis that is passed will be kept consistent. 141 /// 142 /// If a LoopPassManager is passed in, and the loop is fully removed, it will be 143 /// removed from the LoopPassManager as well. LPM can also be NULL. 144 /// 145 /// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are 146 /// available from the Pass it must also preserve those analyses. 147 bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, 148 bool AllowRuntime, unsigned TripMultiple, 149 LoopInfo *LI, Pass *PP, LPPassManager *LPM) { 150 BasicBlock *Preheader = L->getLoopPreheader(); 151 if (!Preheader) { 152 DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n"); 153 return false; 154 } 155 156 BasicBlock *LatchBlock = L->getLoopLatch(); 157 if (!LatchBlock) { 158 DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n"); 159 return false; 160 } 161 162 // Loops with indirectbr cannot be cloned. 163 if (!L->isSafeToClone()) { 164 DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n"); 165 return false; 166 } 167 168 BasicBlock *Header = L->getHeader(); 169 BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); 170 171 if (!BI || BI->isUnconditional()) { 172 // The loop-rotate pass can be helpful to avoid this in many cases. 173 DEBUG(dbgs() << 174 " Can't unroll; loop not terminated by a conditional branch.\n"); 175 return false; 176 } 177 178 if (Header->hasAddressTaken()) { 179 // The loop-rotate pass can be helpful to avoid this in many cases. 180 DEBUG(dbgs() << 181 " Won't unroll loop: address of header block is taken.\n"); 182 return false; 183 } 184 185 if (TripCount != 0) 186 DEBUG(dbgs() << " Trip Count = " << TripCount << "\n"); 187 if (TripMultiple != 1) 188 DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n"); 189 190 // Effectively "DCE" unrolled iterations that are beyond the tripcount 191 // and will never be executed. 192 if (TripCount != 0 && Count > TripCount) 193 Count = TripCount; 194 195 // Don't enter the unroll code if there is nothing to do. This way we don't 196 // need to support "partial unrolling by 1". 197 if (TripCount == 0 && Count < 2) 198 return false; 199 200 assert(Count > 0); 201 assert(TripMultiple > 0); 202 assert(TripCount == 0 || TripCount % TripMultiple == 0); 203 204 // Are we eliminating the loop control altogether? 205 bool CompletelyUnroll = Count == TripCount; 206 207 // We assume a run-time trip count if the compiler cannot 208 // figure out the loop trip count and the unroll-runtime 209 // flag is specified. 210 bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime); 211 212 if (RuntimeTripCount && !UnrollRuntimeLoopProlog(L, Count, LI, LPM)) 213 return false; 214 215 // Notify ScalarEvolution that the loop will be substantially changed, 216 // if not outright eliminated. 217 if (PP) { 218 ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>(); 219 if (SE) 220 SE->forgetLoop(L); 221 } 222 223 // If we know the trip count, we know the multiple... 224 unsigned BreakoutTrip = 0; 225 if (TripCount != 0) { 226 BreakoutTrip = TripCount % Count; 227 TripMultiple = 0; 228 } else { 229 // Figure out what multiple to use. 230 BreakoutTrip = TripMultiple = 231 (unsigned)GreatestCommonDivisor64(Count, TripMultiple); 232 } 233 234 // Report the unrolling decision. 235 DebugLoc LoopLoc = L->getStartLoc(); 236 Function *F = Header->getParent(); 237 LLVMContext &Ctx = F->getContext(); 238 239 if (CompletelyUnroll) { 240 DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName() 241 << " with trip count " << TripCount << "!\n"); 242 emitOptimizationRemark(Ctx, DEBUG_TYPE, *F, LoopLoc, 243 Twine("completely unrolled loop with ") + 244 Twine(TripCount) + " iterations"); 245 } else { 246 auto EmitDiag = [&](const Twine &T) { 247 emitOptimizationRemark(Ctx, DEBUG_TYPE, *F, LoopLoc, 248 "unrolled loop by a factor of " + Twine(Count) + 249 T); 250 }; 251 252 DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() 253 << " by " << Count); 254 if (TripMultiple == 0 || BreakoutTrip != TripMultiple) { 255 DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip); 256 EmitDiag(" with a breakout at trip " + Twine(BreakoutTrip)); 257 } else if (TripMultiple != 1) { 258 DEBUG(dbgs() << " with " << TripMultiple << " trips per branch"); 259 EmitDiag(" with " + Twine(TripMultiple) + " trips per branch"); 260 } else if (RuntimeTripCount) { 261 DEBUG(dbgs() << " with run-time trip count"); 262 EmitDiag(" with run-time trip count"); 263 } 264 DEBUG(dbgs() << "!\n"); 265 } 266 267 bool ContinueOnTrue = L->contains(BI->getSuccessor(0)); 268 BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue); 269 270 // For the first iteration of the loop, we should use the precloned values for 271 // PHI nodes. Insert associations now. 272 ValueToValueMapTy LastValueMap; 273 std::vector<PHINode*> OrigPHINode; 274 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 275 OrigPHINode.push_back(cast<PHINode>(I)); 276 } 277 278 std::vector<BasicBlock*> Headers; 279 std::vector<BasicBlock*> Latches; 280 Headers.push_back(Header); 281 Latches.push_back(LatchBlock); 282 283 // The current on-the-fly SSA update requires blocks to be processed in 284 // reverse postorder so that LastValueMap contains the correct value at each 285 // exit. 286 LoopBlocksDFS DFS(L); 287 DFS.perform(LI); 288 289 // Stash the DFS iterators before adding blocks to the loop. 290 LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO(); 291 LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO(); 292 293 for (unsigned It = 1; It != Count; ++It) { 294 std::vector<BasicBlock*> NewBlocks; 295 296 for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) { 297 ValueToValueMapTy VMap; 298 BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It)); 299 Header->getParent()->getBasicBlockList().push_back(New); 300 301 // Loop over all of the PHI nodes in the block, changing them to use the 302 // incoming values from the previous block. 303 if (*BB == Header) 304 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 305 PHINode *NewPHI = cast<PHINode>(VMap[OrigPHINode[i]]); 306 Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock); 307 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) 308 if (It > 1 && L->contains(InValI)) 309 InVal = LastValueMap[InValI]; 310 VMap[OrigPHINode[i]] = InVal; 311 New->getInstList().erase(NewPHI); 312 } 313 314 // Update our running map of newest clones 315 LastValueMap[*BB] = New; 316 for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end(); 317 VI != VE; ++VI) 318 LastValueMap[VI->first] = VI->second; 319 320 L->addBasicBlockToLoop(New, LI->getBase()); 321 322 // Add phi entries for newly created values to all exit blocks. 323 for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB); 324 SI != SE; ++SI) { 325 if (L->contains(*SI)) 326 continue; 327 for (BasicBlock::iterator BBI = (*SI)->begin(); 328 PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) { 329 Value *Incoming = phi->getIncomingValueForBlock(*BB); 330 ValueToValueMapTy::iterator It = LastValueMap.find(Incoming); 331 if (It != LastValueMap.end()) 332 Incoming = It->second; 333 phi->addIncoming(Incoming, New); 334 } 335 } 336 // Keep track of new headers and latches as we create them, so that 337 // we can insert the proper branches later. 338 if (*BB == Header) 339 Headers.push_back(New); 340 if (*BB == LatchBlock) 341 Latches.push_back(New); 342 343 NewBlocks.push_back(New); 344 } 345 346 // Remap all instructions in the most recent iteration 347 for (unsigned i = 0; i < NewBlocks.size(); ++i) 348 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 349 E = NewBlocks[i]->end(); I != E; ++I) 350 ::RemapInstruction(I, LastValueMap); 351 } 352 353 // Loop over the PHI nodes in the original block, setting incoming values. 354 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 355 PHINode *PN = OrigPHINode[i]; 356 if (CompletelyUnroll) { 357 PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); 358 Header->getInstList().erase(PN); 359 } 360 else if (Count > 1) { 361 Value *InVal = PN->removeIncomingValue(LatchBlock, false); 362 // If this value was defined in the loop, take the value defined by the 363 // last iteration of the loop. 364 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) { 365 if (L->contains(InValI)) 366 InVal = LastValueMap[InVal]; 367 } 368 assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch"); 369 PN->addIncoming(InVal, Latches.back()); 370 } 371 } 372 373 // Now that all the basic blocks for the unrolled iterations are in place, 374 // set up the branches to connect them. 375 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 376 // The original branch was replicated in each unrolled iteration. 377 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 378 379 // The branch destination. 380 unsigned j = (i + 1) % e; 381 BasicBlock *Dest = Headers[j]; 382 bool NeedConditional = true; 383 384 if (RuntimeTripCount && j != 0) { 385 NeedConditional = false; 386 } 387 388 // For a complete unroll, make the last iteration end with a branch 389 // to the exit block. 390 if (CompletelyUnroll && j == 0) { 391 Dest = LoopExit; 392 NeedConditional = false; 393 } 394 395 // If we know the trip count or a multiple of it, we can safely use an 396 // unconditional branch for some iterations. 397 if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) { 398 NeedConditional = false; 399 } 400 401 if (NeedConditional) { 402 // Update the conditional branch's successor for the following 403 // iteration. 404 Term->setSuccessor(!ContinueOnTrue, Dest); 405 } else { 406 // Remove phi operands at this loop exit 407 if (Dest != LoopExit) { 408 BasicBlock *BB = Latches[i]; 409 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); 410 SI != SE; ++SI) { 411 if (*SI == Headers[i]) 412 continue; 413 for (BasicBlock::iterator BBI = (*SI)->begin(); 414 PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) { 415 Phi->removeIncomingValue(BB, false); 416 } 417 } 418 } 419 // Replace the conditional branch with an unconditional one. 420 BranchInst::Create(Dest, Term); 421 Term->eraseFromParent(); 422 } 423 } 424 425 // Merge adjacent basic blocks, if possible. 426 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 427 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 428 if (Term->isUnconditional()) { 429 BasicBlock *Dest = Term->getSuccessor(0); 430 if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM)) 431 std::replace(Latches.begin(), Latches.end(), Dest, Fold); 432 } 433 } 434 435 DominatorTree *DT = nullptr; 436 if (PP) { 437 // FIXME: Reconstruct dom info, because it is not preserved properly. 438 // Incrementally updating domtree after loop unrolling would be easy. 439 if (DominatorTreeWrapperPass *DTWP = 440 PP->getAnalysisIfAvailable<DominatorTreeWrapperPass>()) { 441 DT = &DTWP->getDomTree(); 442 DT->recalculate(*L->getHeader()->getParent()); 443 } 444 445 // Simplify any new induction variables in the partially unrolled loop. 446 ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>(); 447 if (SE && !CompletelyUnroll) { 448 SmallVector<WeakVH, 16> DeadInsts; 449 simplifyLoopIVs(L, SE, LPM, DeadInsts); 450 451 // Aggressively clean up dead instructions that simplifyLoopIVs already 452 // identified. Any remaining should be cleaned up below. 453 while (!DeadInsts.empty()) 454 if (Instruction *Inst = 455 dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val())) 456 RecursivelyDeleteTriviallyDeadInstructions(Inst); 457 } 458 } 459 // At this point, the code is well formed. We now do a quick sweep over the 460 // inserted code, doing constant propagation and dead code elimination as we 461 // go. 462 const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks(); 463 for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(), 464 BBE = NewLoopBlocks.end(); BB != BBE; ++BB) 465 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) { 466 Instruction *Inst = I++; 467 468 if (isInstructionTriviallyDead(Inst)) 469 (*BB)->getInstList().erase(Inst); 470 else if (Value *V = SimplifyInstruction(Inst)) 471 if (LI->replacementPreservesLCSSAForm(Inst, V)) { 472 Inst->replaceAllUsesWith(V); 473 (*BB)->getInstList().erase(Inst); 474 } 475 } 476 477 NumCompletelyUnrolled += CompletelyUnroll; 478 ++NumUnrolled; 479 480 Loop *OuterL = L->getParentLoop(); 481 // Remove the loop from the LoopPassManager if it's completely removed. 482 if (CompletelyUnroll && LPM != nullptr) 483 LPM->deleteLoopFromQueue(L); 484 485 // If we have a pass and a DominatorTree we should re-simplify impacted loops 486 // to ensure subsequent analyses can rely on this form. We want to simplify 487 // at least one layer outside of the loop that was unrolled so that any 488 // changes to the parent loop exposed by the unrolling are considered. 489 if (PP && DT) { 490 if (!OuterL && !CompletelyUnroll) 491 OuterL = L; 492 if (OuterL) { 493 DataLayoutPass *DLP = PP->getAnalysisIfAvailable<DataLayoutPass>(); 494 const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr; 495 ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>(); 496 simplifyLoop(OuterL, DT, LI, PP, /*AliasAnalysis*/ nullptr, SE, DL); 497 498 // LCSSA must be performed on the outermost affected loop. The unrolled 499 // loop's last loop latch is guaranteed to be in the outermost loop after 500 // deleteLoopFromQueue updates LoopInfo. 501 Loop *LatchLoop = LI->getLoopFor(Latches.back()); 502 if (!OuterL->contains(LatchLoop)) 503 while (OuterL->getParentLoop() != LatchLoop) 504 OuterL = OuterL->getParentLoop(); 505 506 formLCSSARecursively(*OuterL, *DT, SE); 507 } 508 } 509 510 return true; 511 } 512