LoopUnroll.cpp revision 1009c3299be8c147ecd3fbd2d75ba1bafb2c84b1
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 LPPassManager *LPM) { 63 // Merge basic blocks into their predecessor if there is only one distinct 64 // pred, and if there is only one distinct successor of the predecessor, and 65 // if there are no PHI nodes. 66 BasicBlock *OnlyPred = BB->getSinglePredecessor(); 67 if (!OnlyPred) return 0; 68 69 if (OnlyPred->getTerminator()->getNumSuccessors() != 1) 70 return 0; 71 72 DEBUG(dbgs() << "Merging: " << *BB << "into: " << *OnlyPred); 73 74 // Resolve any PHI nodes at the start of the block. They are all 75 // guaranteed to have exactly one entry if they exist, unless there are 76 // multiple duplicate (but guaranteed to be equal) entries for the 77 // incoming edges. This occurs when there are multiple edges from 78 // OnlyPred to OnlySucc. 79 FoldSingleEntryPHINodes(BB); 80 81 // Delete the unconditional branch from the predecessor... 82 OnlyPred->getInstList().pop_back(); 83 84 // Make all PHI nodes that referred to BB now refer to Pred as their 85 // source... 86 BB->replaceAllUsesWith(OnlyPred); 87 88 // Move all definitions in the successor to the predecessor... 89 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); 90 91 std::string OldName = BB->getName(); 92 93 // Erase basic block from the function... 94 95 // ScalarEvolution holds references to loop exit blocks. 96 if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) { 97 if (Loop *L = LI->getLoopFor(BB)) 98 SE->forgetLoop(L); 99 } 100 LI->removeBlock(BB); 101 BB->eraseFromParent(); 102 103 // Inherit predecessor's name if it exists... 104 if (!OldName.empty() && !OnlyPred->hasName()) 105 OnlyPred->setName(OldName); 106 107 return OnlyPred; 108} 109 110/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true 111/// if unrolling was successful, or false if the loop was unmodified. Unrolling 112/// can only fail when the loop's latch block is not terminated by a conditional 113/// branch instruction. However, if the trip count (and multiple) are not known, 114/// loop unrolling will mostly produce more code that is no faster. 115/// 116/// TripCount is generally defined as the number of times the loop header 117/// executes. UnrollLoop relaxes the definition to permit early exits: here 118/// TripCount is the iteration on which control exits LatchBlock if no early 119/// exits were taken. Note that UnrollLoop assumes that the loop counter test 120/// terminates LatchBlock in order to remove unnecesssary instances of the 121/// test. In other words, control may exit the loop prior to TripCount 122/// iterations via an early branch, but control may not exit the loop from the 123/// LatchBlock's terminator prior to TripCount iterations. 124/// 125/// Similarly, TripMultiple divides the number of times that the LatchBlock may 126/// execute without exiting the loop. 127/// 128/// The LoopInfo Analysis that is passed will be kept consistent. 129/// 130/// If a LoopPassManager is passed in, and the loop is fully removed, it will be 131/// removed from the LoopPassManager as well. LPM can also be NULL. 132bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, 133 unsigned TripMultiple, LoopInfo *LI, LPPassManager *LPM) { 134 BasicBlock *Preheader = L->getLoopPreheader(); 135 if (!Preheader) { 136 DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n"); 137 return false; 138 } 139 140 BasicBlock *LatchBlock = L->getLoopLatch(); 141 if (!LatchBlock) { 142 DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n"); 143 return false; 144 } 145 146 BasicBlock *Header = L->getHeader(); 147 BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); 148 149 if (!BI || BI->isUnconditional()) { 150 // The loop-rotate pass can be helpful to avoid this in many cases. 151 DEBUG(dbgs() << 152 " Can't unroll; loop not terminated by a conditional branch.\n"); 153 return false; 154 } 155 156 if (Header->hasAddressTaken()) { 157 // The loop-rotate pass can be helpful to avoid this in many cases. 158 DEBUG(dbgs() << 159 " Won't unroll loop: address of header block is taken.\n"); 160 return false; 161 } 162 163 // Notify ScalarEvolution that the loop will be substantially changed, 164 // if not outright eliminated. 165 if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) 166 SE->forgetLoop(L); 167 168 if (TripCount != 0) 169 DEBUG(dbgs() << " Trip Count = " << TripCount << "\n"); 170 if (TripMultiple != 1) 171 DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n"); 172 173 // Effectively "DCE" unrolled iterations that are beyond the tripcount 174 // and will never be executed. 175 if (TripCount != 0 && Count > TripCount) 176 Count = TripCount; 177 178 assert(Count > 0); 179 assert(TripMultiple > 0); 180 assert(TripCount == 0 || TripCount % TripMultiple == 0); 181 182 // Are we eliminating the loop control altogether? 183 bool CompletelyUnroll = Count == TripCount; 184 185 // If we know the trip count, we know the multiple... 186 unsigned BreakoutTrip = 0; 187 if (TripCount != 0) { 188 BreakoutTrip = TripCount % Count; 189 TripMultiple = 0; 190 } else { 191 // Figure out what multiple to use. 192 BreakoutTrip = TripMultiple = 193 (unsigned)GreatestCommonDivisor64(Count, TripMultiple); 194 } 195 196 if (CompletelyUnroll) { 197 DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName() 198 << " with trip count " << TripCount << "!\n"); 199 } else { 200 DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() 201 << " by " << Count); 202 if (TripMultiple == 0 || BreakoutTrip != TripMultiple) { 203 DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip); 204 } else if (TripMultiple != 1) { 205 DEBUG(dbgs() << " with " << TripMultiple << " trips per branch"); 206 } 207 DEBUG(dbgs() << "!\n"); 208 } 209 210 std::vector<BasicBlock*> LoopBlocks = L->getBlocks(); 211 212 bool ContinueOnTrue = L->contains(BI->getSuccessor(0)); 213 BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue); 214 215 // For the first iteration of the loop, we should use the precloned values for 216 // PHI nodes. Insert associations now. 217 ValueToValueMapTy LastValueMap; 218 std::vector<PHINode*> OrigPHINode; 219 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 220 PHINode *PN = cast<PHINode>(I); 221 OrigPHINode.push_back(PN); 222 if (Instruction *I = 223 dyn_cast<Instruction>(PN->getIncomingValueForBlock(LatchBlock))) 224 if (L->contains(I)) 225 LastValueMap[I] = I; 226 } 227 228 std::vector<BasicBlock*> Headers; 229 std::vector<BasicBlock*> Latches; 230 Headers.push_back(Header); 231 Latches.push_back(LatchBlock); 232 233 for (unsigned It = 1; It != Count; ++It) { 234 std::vector<BasicBlock*> NewBlocks; 235 236 for (std::vector<BasicBlock*>::iterator BB = LoopBlocks.begin(), 237 E = LoopBlocks.end(); BB != E; ++BB) { 238 ValueToValueMapTy VMap; 239 BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It)); 240 Header->getParent()->getBasicBlockList().push_back(New); 241 242 // Loop over all of the PHI nodes in the block, changing them to use the 243 // incoming values from the previous block. 244 if (*BB == Header) 245 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 246 PHINode *NewPHI = cast<PHINode>(VMap[OrigPHINode[i]]); 247 Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock); 248 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) 249 if (It > 1 && L->contains(InValI)) 250 InVal = LastValueMap[InValI]; 251 VMap[OrigPHINode[i]] = InVal; 252 New->getInstList().erase(NewPHI); 253 } 254 255 // Update our running map of newest clones 256 LastValueMap[*BB] = New; 257 for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end(); 258 VI != VE; ++VI) 259 LastValueMap[VI->first] = VI->second; 260 261 L->addBasicBlockToLoop(New, LI->getBase()); 262 263 // Add phi entries for newly created values to all exit blocks except 264 // the successor of the latch block. The successor of the exit block will 265 // be updated specially after unrolling all the way. 266 if (*BB != LatchBlock) 267 for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB); SI != SE; 268 ++SI) 269 if (!L->contains(*SI)) 270 for (BasicBlock::iterator BBI = (*SI)->begin(); 271 PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) { 272 Value *Incoming = phi->getIncomingValueForBlock(*BB); 273 phi->addIncoming(Incoming, New); 274 } 275 276 // Keep track of new headers and latches as we create them, so that 277 // we can insert the proper branches later. 278 if (*BB == Header) 279 Headers.push_back(New); 280 if (*BB == LatchBlock) { 281 Latches.push_back(New); 282 283 // Also, clear out the new latch's back edge so that it doesn't look 284 // like a new loop, so that it's amenable to being merged with adjacent 285 // blocks later on. 286 TerminatorInst *Term = New->getTerminator(); 287 assert(L->contains(Term->getSuccessor(!ContinueOnTrue))); 288 assert(Term->getSuccessor(ContinueOnTrue) == LoopExit); 289 Term->setSuccessor(!ContinueOnTrue, NULL); 290 } 291 292 NewBlocks.push_back(New); 293 } 294 295 // Remap all instructions in the most recent iteration 296 for (unsigned i = 0; i < NewBlocks.size(); ++i) 297 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 298 E = NewBlocks[i]->end(); I != E; ++I) 299 ::RemapInstruction(I, LastValueMap); 300 } 301 302 // The latch block exits the loop. If there are any PHI nodes in the 303 // successor blocks, update them to use the appropriate values computed as the 304 // last iteration of the loop. 305 if (Count != 1) { 306 BasicBlock *LastIterationBB = cast<BasicBlock>(LastValueMap[LatchBlock]); 307 for (succ_iterator SI = succ_begin(LatchBlock), SE = succ_end(LatchBlock); 308 SI != SE; ++SI) { 309 for (BasicBlock::iterator BBI = (*SI)->begin(); 310 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) { 311 Value *InVal = PN->removeIncomingValue(LatchBlock, false); 312 // If this value was defined in the loop, take the value defined by the 313 // last iteration of the loop. 314 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) { 315 if (L->contains(InValI)) 316 InVal = LastValueMap[InVal]; 317 } 318 PN->addIncoming(InVal, LastIterationBB); 319 } 320 } 321 } 322 323 // Now, if we're doing complete unrolling, loop over the PHI nodes in the 324 // original block, setting them to their incoming values. 325 if (CompletelyUnroll) { 326 BasicBlock *Preheader = L->getLoopPreheader(); 327 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 328 PHINode *PN = OrigPHINode[i]; 329 PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); 330 Header->getInstList().erase(PN); 331 } 332 } 333 334 // Now that all the basic blocks for the unrolled iterations are in place, 335 // set up the branches to connect them. 336 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 337 // The original branch was replicated in each unrolled iteration. 338 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 339 340 // The branch destination. 341 unsigned j = (i + 1) % e; 342 BasicBlock *Dest = Headers[j]; 343 bool NeedConditional = true; 344 345 // For a complete unroll, make the last iteration end with a branch 346 // to the exit block. 347 if (CompletelyUnroll && j == 0) { 348 Dest = LoopExit; 349 NeedConditional = false; 350 } 351 352 // If we know the trip count or a multiple of it, we can safely use an 353 // unconditional branch for some iterations. 354 if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) { 355 NeedConditional = false; 356 } 357 358 if (NeedConditional) { 359 // Update the conditional branch's successor for the following 360 // iteration. 361 Term->setSuccessor(!ContinueOnTrue, Dest); 362 } else { 363 // Replace the conditional branch with an unconditional one. 364 BranchInst::Create(Dest, Term); 365 Term->eraseFromParent(); 366 } 367 } 368 369 // Merge adjacent basic blocks, if possible. 370 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 371 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 372 if (Term->isUnconditional()) { 373 BasicBlock *Dest = Term->getSuccessor(0); 374 if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM)) 375 std::replace(Latches.begin(), Latches.end(), Dest, Fold); 376 } 377 } 378 379 // At this point, the code is well formed. We now do a quick sweep over the 380 // inserted code, doing constant propagation and dead code elimination as we 381 // go. 382 const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks(); 383 for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(), 384 BBE = NewLoopBlocks.end(); BB != BBE; ++BB) 385 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) { 386 Instruction *Inst = I++; 387 388 if (isInstructionTriviallyDead(Inst)) 389 (*BB)->getInstList().erase(Inst); 390 else if (Value *V = SimplifyInstruction(Inst)) 391 if (LI->replacementPreservesLCSSAForm(Inst, V)) { 392 Inst->replaceAllUsesWith(V); 393 (*BB)->getInstList().erase(Inst); 394 } 395 } 396 397 NumCompletelyUnrolled += CompletelyUnroll; 398 ++NumUnrolled; 399 // Remove the loop from the LoopPassManager if it's completely removed. 400 if (CompletelyUnroll && LPM != NULL) 401 LPM->deleteLoopFromQueue(L); 402 403 return true; 404} 405