LoopUnroll.cpp revision 478849e98ca4661d77c1d6f3f96e8b28e1183fbd
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/BasicBlock.h" 22#include "llvm/ADT/Statistic.h" 23#include "llvm/Analysis/InstructionSimplify.h" 24#include "llvm/Analysis/LoopPass.h" 25#include "llvm/Analysis/ScalarEvolution.h" 26#include "llvm/Support/Debug.h" 27#include "llvm/Support/raw_ostream.h" 28#include "llvm/Transforms/Utils/BasicBlockUtils.h" 29#include "llvm/Transforms/Utils/Cloning.h" 30#include "llvm/Transforms/Utils/Local.h" 31using namespace llvm; 32 33// TODO: Should these be here or in LoopUnroll? 34STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled"); 35STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)"); 36 37/// RemapInstruction - Convert the instruction operands from referencing the 38/// current values into those specified by VMap. 39static inline void RemapInstruction(Instruction *I, 40 ValueToValueMapTy &VMap) { 41 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 42 Value *Op = I->getOperand(op); 43 ValueToValueMapTy::iterator It = VMap.find(Op); 44 if (It != VMap.end()) 45 I->setOperand(op, It->second); 46 } 47 48 if (PHINode *PN = dyn_cast<PHINode>(I)) { 49 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 50 ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i)); 51 if (It != VMap.end()) 52 PN->setIncomingBlock(i, cast<BasicBlock>(It->second)); 53 } 54 } 55} 56 57/// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it 58/// only has one predecessor, and that predecessor only has one successor. 59/// The LoopInfo Analysis that is passed will be kept consistent. 60/// Returns the new combined block. 61static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI) { 62 // Merge basic blocks into their predecessor if there is only one distinct 63 // pred, and if there is only one distinct successor of the predecessor, and 64 // if there are no PHI nodes. 65 BasicBlock *OnlyPred = BB->getSinglePredecessor(); 66 if (!OnlyPred) return 0; 67 68 if (OnlyPred->getTerminator()->getNumSuccessors() != 1) 69 return 0; 70 71 DEBUG(dbgs() << "Merging: " << *BB << "into: " << *OnlyPred); 72 73 // Resolve any PHI nodes at the start of the block. They are all 74 // guaranteed to have exactly one entry if they exist, unless there are 75 // multiple duplicate (but guaranteed to be equal) entries for the 76 // incoming edges. This occurs when there are multiple edges from 77 // OnlyPred to OnlySucc. 78 FoldSingleEntryPHINodes(BB); 79 80 // Delete the unconditional branch from the predecessor... 81 OnlyPred->getInstList().pop_back(); 82 83 // Make all PHI nodes that referred to BB now refer to Pred as their 84 // source... 85 BB->replaceAllUsesWith(OnlyPred); 86 87 // Move all definitions in the successor to the predecessor... 88 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); 89 90 std::string OldName = BB->getName(); 91 92 // Erase basic block from the function... 93 LI->removeBlock(BB); 94 BB->eraseFromParent(); 95 96 // Inherit predecessor's name if it exists... 97 if (!OldName.empty() && !OnlyPred->hasName()) 98 OnlyPred->setName(OldName); 99 100 return OnlyPred; 101} 102 103/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true 104/// if unrolling was successful, or false if the loop was unmodified. Unrolling 105/// can only fail when the loop's latch block is not terminated by a conditional 106/// branch instruction. However, if the trip count (and multiple) are not known, 107/// loop unrolling will mostly produce more code that is no faster. 108/// 109/// TripCount is generally defined as the number of times the loop header 110/// executes. UnrollLoop relaxes the definition to permit early exits: here 111/// TripCount is the iteration on which control exits LatchBlock if no early 112/// exits were taken. Note that UnrollLoop assumes that the loop counter test 113/// terminates LatchBlock in order to remove unnecesssary instances of the 114/// test. In other words, control may exit the loop prior to TripCount 115/// iterations via an early branch, but control may not exit the loop from the 116/// LatchBlock's terminator prior to TripCount iterations. 117/// 118/// Similarly, TripMultiple divides the number of times that the LatchBlock may 119/// execute without exiting the loop. 120/// 121/// The LoopInfo Analysis that is passed will be kept consistent. 122/// 123/// If a LoopPassManager is passed in, and the loop is fully removed, it will be 124/// removed from the LoopPassManager as well. LPM can also be NULL. 125bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, 126 unsigned TripMultiple, LoopInfo *LI, LPPassManager *LPM) { 127 BasicBlock *Preheader = L->getLoopPreheader(); 128 if (!Preheader) { 129 DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n"); 130 return false; 131 } 132 133 BasicBlock *LatchBlock = L->getLoopLatch(); 134 if (!LatchBlock) { 135 DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n"); 136 return false; 137 } 138 139 BasicBlock *Header = L->getHeader(); 140 BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); 141 142 if (!BI || BI->isUnconditional()) { 143 // The loop-rotate pass can be helpful to avoid this in many cases. 144 DEBUG(dbgs() << 145 " Can't unroll; loop not terminated by a conditional branch.\n"); 146 return false; 147 } 148 149 if (Header->hasAddressTaken()) { 150 // The loop-rotate pass can be helpful to avoid this in many cases. 151 DEBUG(dbgs() << 152 " Won't unroll loop: address of header block is taken.\n"); 153 return false; 154 } 155 156 // Notify ScalarEvolution that the loop will be substantially changed, 157 // if not outright eliminated. 158 if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) 159 SE->forgetLoop(L); 160 161 if (TripCount != 0) 162 DEBUG(dbgs() << " Trip Count = " << TripCount << "\n"); 163 if (TripMultiple != 1) 164 DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n"); 165 166 // Effectively "DCE" unrolled iterations that are beyond the tripcount 167 // and will never be executed. 168 if (TripCount != 0 && Count > TripCount) 169 Count = TripCount; 170 171 assert(Count > 0); 172 assert(TripMultiple > 0); 173 assert(TripCount == 0 || TripCount % TripMultiple == 0); 174 175 // Are we eliminating the loop control altogether? 176 bool CompletelyUnroll = Count == TripCount; 177 178 // If we know the trip count, we know the multiple... 179 unsigned BreakoutTrip = 0; 180 if (TripCount != 0) { 181 BreakoutTrip = TripCount % Count; 182 TripMultiple = 0; 183 } else { 184 // Figure out what multiple to use. 185 BreakoutTrip = TripMultiple = 186 (unsigned)GreatestCommonDivisor64(Count, TripMultiple); 187 } 188 189 if (CompletelyUnroll) { 190 DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName() 191 << " with trip count " << TripCount << "!\n"); 192 } else { 193 DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() 194 << " by " << Count); 195 if (TripMultiple == 0 || BreakoutTrip != TripMultiple) { 196 DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip); 197 } else if (TripMultiple != 1) { 198 DEBUG(dbgs() << " with " << TripMultiple << " trips per branch"); 199 } 200 DEBUG(dbgs() << "!\n"); 201 } 202 203 std::vector<BasicBlock*> LoopBlocks = L->getBlocks(); 204 205 bool ContinueOnTrue = L->contains(BI->getSuccessor(0)); 206 BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue); 207 208 // For the first iteration of the loop, we should use the precloned values for 209 // PHI nodes. Insert associations now. 210 ValueToValueMapTy LastValueMap; 211 std::vector<PHINode*> OrigPHINode; 212 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 213 PHINode *PN = cast<PHINode>(I); 214 OrigPHINode.push_back(PN); 215 if (Instruction *I = 216 dyn_cast<Instruction>(PN->getIncomingValueForBlock(LatchBlock))) 217 if (L->contains(I)) 218 LastValueMap[I] = I; 219 } 220 221 std::vector<BasicBlock*> Headers; 222 std::vector<BasicBlock*> Latches; 223 Headers.push_back(Header); 224 Latches.push_back(LatchBlock); 225 226 for (unsigned It = 1; It != Count; ++It) { 227 std::vector<BasicBlock*> NewBlocks; 228 229 for (std::vector<BasicBlock*>::iterator BB = LoopBlocks.begin(), 230 E = LoopBlocks.end(); BB != E; ++BB) { 231 ValueToValueMapTy VMap; 232 BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It)); 233 Header->getParent()->getBasicBlockList().push_back(New); 234 235 // Loop over all of the PHI nodes in the block, changing them to use the 236 // incoming values from the previous block. 237 if (*BB == Header) 238 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 239 PHINode *NewPHI = cast<PHINode>(VMap[OrigPHINode[i]]); 240 Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock); 241 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) 242 if (It > 1 && L->contains(InValI)) 243 InVal = LastValueMap[InValI]; 244 VMap[OrigPHINode[i]] = InVal; 245 New->getInstList().erase(NewPHI); 246 } 247 248 // Update our running map of newest clones 249 LastValueMap[*BB] = New; 250 for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end(); 251 VI != VE; ++VI) 252 LastValueMap[VI->first] = VI->second; 253 254 L->addBasicBlockToLoop(New, LI->getBase()); 255 256 // Add phi entries for newly created values to all exit blocks except 257 // the successor of the latch block. The successor of the exit block will 258 // be updated specially after unrolling all the way. 259 if (*BB != LatchBlock) 260 for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB); SI != SE; 261 ++SI) 262 if (!L->contains(*SI)) 263 for (BasicBlock::iterator BBI = (*SI)->begin(); 264 PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) { 265 Value *Incoming = phi->getIncomingValueForBlock(*BB); 266 phi->addIncoming(Incoming, New); 267 } 268 269 // Keep track of new headers and latches as we create them, so that 270 // we can insert the proper branches later. 271 if (*BB == Header) 272 Headers.push_back(New); 273 if (*BB == LatchBlock) { 274 Latches.push_back(New); 275 276 // Also, clear out the new latch's back edge so that it doesn't look 277 // like a new loop, so that it's amenable to being merged with adjacent 278 // blocks later on. 279 TerminatorInst *Term = New->getTerminator(); 280 assert(L->contains(Term->getSuccessor(!ContinueOnTrue))); 281 assert(Term->getSuccessor(ContinueOnTrue) == LoopExit); 282 Term->setSuccessor(!ContinueOnTrue, NULL); 283 } 284 285 NewBlocks.push_back(New); 286 } 287 288 // Remap all instructions in the most recent iteration 289 for (unsigned i = 0; i < NewBlocks.size(); ++i) 290 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 291 E = NewBlocks[i]->end(); I != E; ++I) 292 ::RemapInstruction(I, LastValueMap); 293 } 294 295 // The latch block exits the loop. If there are any PHI nodes in the 296 // successor blocks, update them to use the appropriate values computed as the 297 // last iteration of the loop. 298 if (Count != 1) { 299 BasicBlock *LastIterationBB = cast<BasicBlock>(LastValueMap[LatchBlock]); 300 for (succ_iterator SI = succ_begin(LatchBlock), SE = succ_end(LatchBlock); 301 SI != SE; ++SI) { 302 for (BasicBlock::iterator BBI = (*SI)->begin(); 303 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) { 304 Value *InVal = PN->removeIncomingValue(LatchBlock, false); 305 // If this value was defined in the loop, take the value defined by the 306 // last iteration of the loop. 307 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) { 308 if (L->contains(InValI)) 309 InVal = LastValueMap[InVal]; 310 } 311 PN->addIncoming(InVal, LastIterationBB); 312 } 313 } 314 } 315 316 // Now, if we're doing complete unrolling, loop over the PHI nodes in the 317 // original block, setting them to their incoming values. 318 if (CompletelyUnroll) { 319 BasicBlock *Preheader = L->getLoopPreheader(); 320 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 321 PHINode *PN = OrigPHINode[i]; 322 PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); 323 Header->getInstList().erase(PN); 324 } 325 } 326 327 // Now that all the basic blocks for the unrolled iterations are in place, 328 // set up the branches to connect them. 329 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 330 // The original branch was replicated in each unrolled iteration. 331 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 332 333 // The branch destination. 334 unsigned j = (i + 1) % e; 335 BasicBlock *Dest = Headers[j]; 336 bool NeedConditional = true; 337 338 // For a complete unroll, make the last iteration end with a branch 339 // to the exit block. 340 if (CompletelyUnroll && j == 0) { 341 Dest = LoopExit; 342 NeedConditional = false; 343 } 344 345 // If we know the trip count or a multiple of it, we can safely use an 346 // unconditional branch for some iterations. 347 if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) { 348 NeedConditional = false; 349 } 350 351 if (NeedConditional) { 352 // Update the conditional branch's successor for the following 353 // iteration. 354 Term->setSuccessor(!ContinueOnTrue, Dest); 355 } else { 356 // Replace the conditional branch with an unconditional one. 357 BranchInst::Create(Dest, Term); 358 Term->eraseFromParent(); 359 } 360 } 361 362 // Merge adjacent basic blocks, if possible. 363 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 364 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 365 if (Term->isUnconditional()) { 366 BasicBlock *Dest = Term->getSuccessor(0); 367 if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI)) 368 std::replace(Latches.begin(), Latches.end(), Dest, Fold); 369 } 370 } 371 372 // At this point, the code is well formed. We now do a quick sweep over the 373 // inserted code, doing constant propagation and dead code elimination as we 374 // go. 375 const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks(); 376 for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(), 377 BBE = NewLoopBlocks.end(); BB != BBE; ++BB) 378 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) { 379 Instruction *Inst = I++; 380 381 if (isInstructionTriviallyDead(Inst)) 382 (*BB)->getInstList().erase(Inst); 383 else if (Value *V = SimplifyInstruction(Inst)) 384 if (LI->replacementPreservesLCSSAForm(Inst, V)) { 385 Inst->replaceAllUsesWith(V); 386 (*BB)->getInstList().erase(Inst); 387 } 388 } 389 390 NumCompletelyUnrolled += CompletelyUnroll; 391 ++NumUnrolled; 392 // Remove the loop from the LoopPassManager if it's completely removed. 393 if (CompletelyUnroll && LPM != NULL) 394 LPM->deleteLoopFromQueue(L); 395 396 return true; 397} 398