LoopUnroll.cpp revision 36b56886974eae4f9c5ebc96befd3e7bfe5de338
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#define DEBUG_TYPE "loop-unroll" 20#include "llvm/Transforms/Utils/UnrollLoop.h" 21#include "llvm/ADT/Statistic.h" 22#include "llvm/Analysis/InstructionSimplify.h" 23#include "llvm/Analysis/LoopIterator.h" 24#include "llvm/Analysis/LoopPass.h" 25#include "llvm/Analysis/ScalarEvolution.h" 26#include "llvm/IR/BasicBlock.h" 27#include "llvm/IR/Dominators.h" 28#include "llvm/Support/Debug.h" 29#include "llvm/Support/raw_ostream.h" 30#include "llvm/Transforms/Utils/BasicBlockUtils.h" 31#include "llvm/Transforms/Utils/Cloning.h" 32#include "llvm/Transforms/Utils/Local.h" 33#include "llvm/Transforms/Utils/LoopUtils.h" 34#include "llvm/Transforms/Utils/SimplifyIndVar.h" 35using namespace llvm; 36 37// TODO: Should these be here or in LoopUnroll? 38STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled"); 39STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)"); 40 41/// RemapInstruction - Convert the instruction operands from referencing the 42/// current values into those specified by VMap. 43static inline void RemapInstruction(Instruction *I, 44 ValueToValueMapTy &VMap) { 45 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 46 Value *Op = I->getOperand(op); 47 ValueToValueMapTy::iterator It = VMap.find(Op); 48 if (It != VMap.end()) 49 I->setOperand(op, It->second); 50 } 51 52 if (PHINode *PN = dyn_cast<PHINode>(I)) { 53 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 54 ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i)); 55 if (It != VMap.end()) 56 PN->setIncomingBlock(i, cast<BasicBlock>(It->second)); 57 } 58 } 59} 60 61/// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it 62/// only has one predecessor, and that predecessor only has one successor. 63/// The LoopInfo Analysis that is passed will be kept consistent. 64/// Returns the new combined block. 65static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI, 66 LPPassManager *LPM) { 67 // Merge basic blocks into their predecessor if there is only one distinct 68 // pred, and if there is only one distinct successor of the predecessor, and 69 // if there are no PHI nodes. 70 BasicBlock *OnlyPred = BB->getSinglePredecessor(); 71 if (!OnlyPred) return 0; 72 73 if (OnlyPred->getTerminator()->getNumSuccessors() != 1) 74 return 0; 75 76 DEBUG(dbgs() << "Merging: " << *BB << "into: " << *OnlyPred); 77 78 // Resolve any PHI nodes at the start of the block. They are all 79 // guaranteed to have exactly one entry if they exist, unless there are 80 // multiple duplicate (but guaranteed to be equal) entries for the 81 // incoming edges. This occurs when there are multiple edges from 82 // OnlyPred to OnlySucc. 83 FoldSingleEntryPHINodes(BB); 84 85 // Delete the unconditional branch from the predecessor... 86 OnlyPred->getInstList().pop_back(); 87 88 // Make all PHI nodes that referred to BB now refer to Pred as their 89 // source... 90 BB->replaceAllUsesWith(OnlyPred); 91 92 // Move all definitions in the successor to the predecessor... 93 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); 94 95 // OldName will be valid until erased. 96 StringRef OldName = BB->getName(); 97 98 // Erase basic block from the function... 99 100 // ScalarEvolution holds references to loop exit blocks. 101 if (LPM) { 102 if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) { 103 if (Loop *L = LI->getLoopFor(BB)) 104 SE->forgetLoop(L); 105 } 106 } 107 LI->removeBlock(BB); 108 109 // Inherit predecessor's name if it exists... 110 if (!OldName.empty() && !OnlyPred->hasName()) 111 OnlyPred->setName(OldName); 112 113 BB->eraseFromParent(); 114 115 return OnlyPred; 116} 117 118/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true 119/// if unrolling was successful, or false if the loop was unmodified. Unrolling 120/// can only fail when the loop's latch block is not terminated by a conditional 121/// branch instruction. However, if the trip count (and multiple) are not known, 122/// loop unrolling will mostly produce more code that is no faster. 123/// 124/// TripCount is generally defined as the number of times the loop header 125/// executes. UnrollLoop relaxes the definition to permit early exits: here 126/// TripCount is the iteration on which control exits LatchBlock if no early 127/// exits were taken. Note that UnrollLoop assumes that the loop counter test 128/// terminates LatchBlock in order to remove unnecesssary instances of the 129/// test. In other words, control may exit the loop prior to TripCount 130/// iterations via an early branch, but control may not exit the loop from the 131/// LatchBlock's terminator prior to TripCount iterations. 132/// 133/// Similarly, TripMultiple divides the number of times that the LatchBlock may 134/// execute without exiting the loop. 135/// 136/// The LoopInfo Analysis that is passed will be kept consistent. 137/// 138/// If a LoopPassManager is passed in, and the loop is fully removed, it will be 139/// removed from the LoopPassManager as well. LPM can also be NULL. 140/// 141/// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are 142/// available from the Pass it must also preserve those analyses. 143bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, 144 bool AllowRuntime, unsigned TripMultiple, 145 LoopInfo *LI, Pass *PP, LPPassManager *LPM) { 146 BasicBlock *Preheader = L->getLoopPreheader(); 147 if (!Preheader) { 148 DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n"); 149 return false; 150 } 151 152 BasicBlock *LatchBlock = L->getLoopLatch(); 153 if (!LatchBlock) { 154 DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n"); 155 return false; 156 } 157 158 // Loops with indirectbr cannot be cloned. 159 if (!L->isSafeToClone()) { 160 DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n"); 161 return false; 162 } 163 164 BasicBlock *Header = L->getHeader(); 165 BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); 166 167 if (!BI || BI->isUnconditional()) { 168 // The loop-rotate pass can be helpful to avoid this in many cases. 169 DEBUG(dbgs() << 170 " Can't unroll; loop not terminated by a conditional branch.\n"); 171 return false; 172 } 173 174 if (Header->hasAddressTaken()) { 175 // The loop-rotate pass can be helpful to avoid this in many cases. 176 DEBUG(dbgs() << 177 " Won't unroll loop: address of header block is taken.\n"); 178 return false; 179 } 180 181 if (TripCount != 0) 182 DEBUG(dbgs() << " Trip Count = " << TripCount << "\n"); 183 if (TripMultiple != 1) 184 DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n"); 185 186 // Effectively "DCE" unrolled iterations that are beyond the tripcount 187 // and will never be executed. 188 if (TripCount != 0 && Count > TripCount) 189 Count = TripCount; 190 191 // Don't enter the unroll code if there is nothing to do. This way we don't 192 // need to support "partial unrolling by 1". 193 if (TripCount == 0 && Count < 2) 194 return false; 195 196 assert(Count > 0); 197 assert(TripMultiple > 0); 198 assert(TripCount == 0 || TripCount % TripMultiple == 0); 199 200 // Are we eliminating the loop control altogether? 201 bool CompletelyUnroll = Count == TripCount; 202 203 // We assume a run-time trip count if the compiler cannot 204 // figure out the loop trip count and the unroll-runtime 205 // flag is specified. 206 bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime); 207 208 if (RuntimeTripCount && !UnrollRuntimeLoopProlog(L, Count, LI, LPM)) 209 return false; 210 211 // Notify ScalarEvolution that the loop will be substantially changed, 212 // if not outright eliminated. 213 if (PP) { 214 ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>(); 215 if (SE) 216 SE->forgetLoop(L); 217 } 218 219 // If we know the trip count, we know the multiple... 220 unsigned BreakoutTrip = 0; 221 if (TripCount != 0) { 222 BreakoutTrip = TripCount % Count; 223 TripMultiple = 0; 224 } else { 225 // Figure out what multiple to use. 226 BreakoutTrip = TripMultiple = 227 (unsigned)GreatestCommonDivisor64(Count, TripMultiple); 228 } 229 230 if (CompletelyUnroll) { 231 DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName() 232 << " with trip count " << TripCount << "!\n"); 233 } else { 234 DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() 235 << " by " << Count); 236 if (TripMultiple == 0 || BreakoutTrip != TripMultiple) { 237 DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip); 238 } else if (TripMultiple != 1) { 239 DEBUG(dbgs() << " with " << TripMultiple << " trips per branch"); 240 } else if (RuntimeTripCount) { 241 DEBUG(dbgs() << " with run-time trip count"); 242 } 243 DEBUG(dbgs() << "!\n"); 244 } 245 246 bool ContinueOnTrue = L->contains(BI->getSuccessor(0)); 247 BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue); 248 249 // For the first iteration of the loop, we should use the precloned values for 250 // PHI nodes. Insert associations now. 251 ValueToValueMapTy LastValueMap; 252 std::vector<PHINode*> OrigPHINode; 253 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 254 OrigPHINode.push_back(cast<PHINode>(I)); 255 } 256 257 std::vector<BasicBlock*> Headers; 258 std::vector<BasicBlock*> Latches; 259 Headers.push_back(Header); 260 Latches.push_back(LatchBlock); 261 262 // The current on-the-fly SSA update requires blocks to be processed in 263 // reverse postorder so that LastValueMap contains the correct value at each 264 // exit. 265 LoopBlocksDFS DFS(L); 266 DFS.perform(LI); 267 268 // Stash the DFS iterators before adding blocks to the loop. 269 LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO(); 270 LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO(); 271 272 for (unsigned It = 1; It != Count; ++It) { 273 std::vector<BasicBlock*> NewBlocks; 274 275 for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) { 276 ValueToValueMapTy VMap; 277 BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It)); 278 Header->getParent()->getBasicBlockList().push_back(New); 279 280 // Loop over all of the PHI nodes in the block, changing them to use the 281 // incoming values from the previous block. 282 if (*BB == Header) 283 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 284 PHINode *NewPHI = cast<PHINode>(VMap[OrigPHINode[i]]); 285 Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock); 286 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) 287 if (It > 1 && L->contains(InValI)) 288 InVal = LastValueMap[InValI]; 289 VMap[OrigPHINode[i]] = InVal; 290 New->getInstList().erase(NewPHI); 291 } 292 293 // Update our running map of newest clones 294 LastValueMap[*BB] = New; 295 for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end(); 296 VI != VE; ++VI) 297 LastValueMap[VI->first] = VI->second; 298 299 L->addBasicBlockToLoop(New, LI->getBase()); 300 301 // Add phi entries for newly created values to all exit blocks. 302 for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB); 303 SI != SE; ++SI) { 304 if (L->contains(*SI)) 305 continue; 306 for (BasicBlock::iterator BBI = (*SI)->begin(); 307 PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) { 308 Value *Incoming = phi->getIncomingValueForBlock(*BB); 309 ValueToValueMapTy::iterator It = LastValueMap.find(Incoming); 310 if (It != LastValueMap.end()) 311 Incoming = It->second; 312 phi->addIncoming(Incoming, New); 313 } 314 } 315 // Keep track of new headers and latches as we create them, so that 316 // we can insert the proper branches later. 317 if (*BB == Header) 318 Headers.push_back(New); 319 if (*BB == LatchBlock) 320 Latches.push_back(New); 321 322 NewBlocks.push_back(New); 323 } 324 325 // Remap all instructions in the most recent iteration 326 for (unsigned i = 0; i < NewBlocks.size(); ++i) 327 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 328 E = NewBlocks[i]->end(); I != E; ++I) 329 ::RemapInstruction(I, LastValueMap); 330 } 331 332 // Loop over the PHI nodes in the original block, setting incoming values. 333 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 334 PHINode *PN = OrigPHINode[i]; 335 if (CompletelyUnroll) { 336 PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); 337 Header->getInstList().erase(PN); 338 } 339 else if (Count > 1) { 340 Value *InVal = PN->removeIncomingValue(LatchBlock, false); 341 // If this value was defined in the loop, take the value defined by the 342 // last iteration of the loop. 343 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) { 344 if (L->contains(InValI)) 345 InVal = LastValueMap[InVal]; 346 } 347 assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch"); 348 PN->addIncoming(InVal, Latches.back()); 349 } 350 } 351 352 // Now that all the basic blocks for the unrolled iterations are in place, 353 // set up the branches to connect them. 354 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 355 // The original branch was replicated in each unrolled iteration. 356 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 357 358 // The branch destination. 359 unsigned j = (i + 1) % e; 360 BasicBlock *Dest = Headers[j]; 361 bool NeedConditional = true; 362 363 if (RuntimeTripCount && j != 0) { 364 NeedConditional = false; 365 } 366 367 // For a complete unroll, make the last iteration end with a branch 368 // to the exit block. 369 if (CompletelyUnroll && j == 0) { 370 Dest = LoopExit; 371 NeedConditional = false; 372 } 373 374 // If we know the trip count or a multiple of it, we can safely use an 375 // unconditional branch for some iterations. 376 if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) { 377 NeedConditional = false; 378 } 379 380 if (NeedConditional) { 381 // Update the conditional branch's successor for the following 382 // iteration. 383 Term->setSuccessor(!ContinueOnTrue, Dest); 384 } else { 385 // Remove phi operands at this loop exit 386 if (Dest != LoopExit) { 387 BasicBlock *BB = Latches[i]; 388 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); 389 SI != SE; ++SI) { 390 if (*SI == Headers[i]) 391 continue; 392 for (BasicBlock::iterator BBI = (*SI)->begin(); 393 PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) { 394 Phi->removeIncomingValue(BB, false); 395 } 396 } 397 } 398 // Replace the conditional branch with an unconditional one. 399 BranchInst::Create(Dest, Term); 400 Term->eraseFromParent(); 401 } 402 } 403 404 // Merge adjacent basic blocks, if possible. 405 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 406 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 407 if (Term->isUnconditional()) { 408 BasicBlock *Dest = Term->getSuccessor(0); 409 if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM)) 410 std::replace(Latches.begin(), Latches.end(), Dest, Fold); 411 } 412 } 413 414 DominatorTree *DT = 0; 415 if (PP) { 416 // FIXME: Reconstruct dom info, because it is not preserved properly. 417 // Incrementally updating domtree after loop unrolling would be easy. 418 if (DominatorTreeWrapperPass *DTWP = 419 PP->getAnalysisIfAvailable<DominatorTreeWrapperPass>()) { 420 DT = &DTWP->getDomTree(); 421 DT->recalculate(*L->getHeader()->getParent()); 422 } 423 424 // Simplify any new induction variables in the partially unrolled loop. 425 ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>(); 426 if (SE && !CompletelyUnroll) { 427 SmallVector<WeakVH, 16> DeadInsts; 428 simplifyLoopIVs(L, SE, LPM, DeadInsts); 429 430 // Aggressively clean up dead instructions that simplifyLoopIVs already 431 // identified. Any remaining should be cleaned up below. 432 while (!DeadInsts.empty()) 433 if (Instruction *Inst = 434 dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val())) 435 RecursivelyDeleteTriviallyDeadInstructions(Inst); 436 } 437 } 438 // At this point, the code is well formed. We now do a quick sweep over the 439 // inserted code, doing constant propagation and dead code elimination as we 440 // go. 441 const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks(); 442 for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(), 443 BBE = NewLoopBlocks.end(); BB != BBE; ++BB) 444 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) { 445 Instruction *Inst = I++; 446 447 if (isInstructionTriviallyDead(Inst)) 448 (*BB)->getInstList().erase(Inst); 449 else if (Value *V = SimplifyInstruction(Inst)) 450 if (LI->replacementPreservesLCSSAForm(Inst, V)) { 451 Inst->replaceAllUsesWith(V); 452 (*BB)->getInstList().erase(Inst); 453 } 454 } 455 456 NumCompletelyUnrolled += CompletelyUnroll; 457 ++NumUnrolled; 458 459 Loop *OuterL = L->getParentLoop(); 460 // Remove the loop from the LoopPassManager if it's completely removed. 461 if (CompletelyUnroll && LPM != NULL) 462 LPM->deleteLoopFromQueue(L); 463 464 // If we have a pass and a DominatorTree we should re-simplify impacted loops 465 // to ensure subsequent analyses can rely on this form. We want to simplify 466 // at least one layer outside of the loop that was unrolled so that any 467 // changes to the parent loop exposed by the unrolling are considered. 468 if (PP && DT) { 469 if (!OuterL && !CompletelyUnroll) 470 OuterL = L; 471 if (OuterL) { 472 ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>(); 473 simplifyLoop(OuterL, DT, LI, PP, /*AliasAnalysis*/ 0, SE); 474 formLCSSARecursively(*OuterL, *DT, SE); 475 } 476 } 477 478 return true; 479} 480