LoopUnroll.cpp revision 199481
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// It works best when loops have been canonicalized by the -indvars pass, 15// allowing it to determine the trip counts of loops easily. 16// 17// The process of unrolling can produce extraneous basic blocks linked with 18// unconditional branches. This will be corrected in the future. 19//===----------------------------------------------------------------------===// 20 21#define DEBUG_TYPE "loop-unroll" 22#include "llvm/Transforms/Utils/UnrollLoop.h" 23#include "llvm/BasicBlock.h" 24#include "llvm/ADT/Statistic.h" 25#include "llvm/Analysis/ConstantFolding.h" 26#include "llvm/Analysis/LoopPass.h" 27#include "llvm/Support/Debug.h" 28#include "llvm/Support/raw_ostream.h" 29#include "llvm/Transforms/Utils/BasicBlockUtils.h" 30#include "llvm/Transforms/Utils/Cloning.h" 31#include "llvm/Transforms/Utils/Local.h" 32#include <cstdio> 33 34using namespace llvm; 35 36// TODO: Should these be here or in LoopUnroll? 37STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled"); 38STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)"); 39 40/// RemapInstruction - Convert the instruction operands from referencing the 41/// current values into those specified by ValueMap. 42static inline void RemapInstruction(Instruction *I, 43 DenseMap<const Value *, Value*> &ValueMap) { 44 for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { 45 Value *Op = I->getOperand(op); 46 DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op); 47 if (It != ValueMap.end()) 48 I->setOperand(op, It->second); 49 } 50} 51 52/// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it 53/// only has one predecessor, and that predecessor only has one successor. 54/// The LoopInfo Analysis that is passed will be kept consistent. 55/// Returns the new combined block. 56static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI) { 57 // Merge basic blocks into their predecessor if there is only one distinct 58 // pred, and if there is only one distinct successor of the predecessor, and 59 // if there are no PHI nodes. 60 BasicBlock *OnlyPred = BB->getSinglePredecessor(); 61 if (!OnlyPred) return 0; 62 63 if (OnlyPred->getTerminator()->getNumSuccessors() != 1) 64 return 0; 65 66 DEBUG(errs() << "Merging: " << *BB << "into: " << *OnlyPred); 67 68 // Resolve any PHI nodes at the start of the block. They are all 69 // guaranteed to have exactly one entry if they exist, unless there are 70 // multiple duplicate (but guaranteed to be equal) entries for the 71 // incoming edges. This occurs when there are multiple edges from 72 // OnlyPred to OnlySucc. 73 FoldSingleEntryPHINodes(BB); 74 75 // Delete the unconditional branch from the predecessor... 76 OnlyPred->getInstList().pop_back(); 77 78 // Move all definitions in the successor to the predecessor... 79 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); 80 81 // Make all PHI nodes that referred to BB now refer to Pred as their 82 // source... 83 BB->replaceAllUsesWith(OnlyPred); 84 85 std::string OldName = BB->getName(); 86 87 // Erase basic block from the function... 88 LI->removeBlock(BB); 89 BB->eraseFromParent(); 90 91 // Inherit predecessor's name if it exists... 92 if (!OldName.empty() && !OnlyPred->hasName()) 93 OnlyPred->setName(OldName); 94 95 return OnlyPred; 96} 97 98/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true 99/// if unrolling was succesful, or false if the loop was unmodified. Unrolling 100/// can only fail when the loop's latch block is not terminated by a conditional 101/// branch instruction. However, if the trip count (and multiple) are not known, 102/// loop unrolling will mostly produce more code that is no faster. 103/// 104/// The LoopInfo Analysis that is passed will be kept consistent. 105/// 106/// If a LoopPassManager is passed in, and the loop is fully removed, it will be 107/// removed from the LoopPassManager as well. LPM can also be NULL. 108bool llvm::UnrollLoop(Loop *L, unsigned Count, LoopInfo* LI, LPPassManager* LPM) { 109 assert(L->isLCSSAForm()); 110 111 BasicBlock *Preheader = L->getLoopPreheader(); 112 if (!Preheader) { 113 DEBUG(errs() << " Can't unroll; loop preheader-insertion failed.\n"); 114 return false; 115 } 116 117 BasicBlock *LatchBlock = L->getLoopLatch(); 118 if (!LatchBlock) { 119 DEBUG(errs() << " Can't unroll; loop exit-block-insertion failed.\n"); 120 return false; 121 } 122 123 BasicBlock *Header = L->getHeader(); 124 BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); 125 126 if (!BI || BI->isUnconditional()) { 127 // The loop-rotate pass can be helpful to avoid this in many cases. 128 DEBUG(errs() << 129 " Can't unroll; loop not terminated by a conditional branch.\n"); 130 return false; 131 } 132 133 // Find trip count 134 unsigned TripCount = L->getSmallConstantTripCount(); 135 // Find trip multiple if count is not available 136 unsigned TripMultiple = 1; 137 if (TripCount == 0) 138 TripMultiple = L->getSmallConstantTripMultiple(); 139 140 if (TripCount != 0) 141 DEBUG(errs() << " Trip Count = " << TripCount << "\n"); 142 if (TripMultiple != 1) 143 DEBUG(errs() << " Trip Multiple = " << TripMultiple << "\n"); 144 145 // Effectively "DCE" unrolled iterations that are beyond the tripcount 146 // and will never be executed. 147 if (TripCount != 0 && Count > TripCount) 148 Count = TripCount; 149 150 assert(Count > 0); 151 assert(TripMultiple > 0); 152 assert(TripCount == 0 || TripCount % TripMultiple == 0); 153 154 // Are we eliminating the loop control altogether? 155 bool CompletelyUnroll = Count == TripCount; 156 157 // If we know the trip count, we know the multiple... 158 unsigned BreakoutTrip = 0; 159 if (TripCount != 0) { 160 BreakoutTrip = TripCount % Count; 161 TripMultiple = 0; 162 } else { 163 // Figure out what multiple to use. 164 BreakoutTrip = TripMultiple = 165 (unsigned)GreatestCommonDivisor64(Count, TripMultiple); 166 } 167 168 if (CompletelyUnroll) { 169 DEBUG(errs() << "COMPLETELY UNROLLING loop %" << Header->getName() 170 << " with trip count " << TripCount << "!\n"); 171 } else { 172 DEBUG(errs() << "UNROLLING loop %" << Header->getName() 173 << " by " << Count); 174 if (TripMultiple == 0 || BreakoutTrip != TripMultiple) { 175 DEBUG(errs() << " with a breakout at trip " << BreakoutTrip); 176 } else if (TripMultiple != 1) { 177 DEBUG(errs() << " with " << TripMultiple << " trips per branch"); 178 } 179 DEBUG(errs() << "!\n"); 180 } 181 182 std::vector<BasicBlock*> LoopBlocks = L->getBlocks(); 183 184 bool ContinueOnTrue = L->contains(BI->getSuccessor(0)); 185 BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue); 186 187 // For the first iteration of the loop, we should use the precloned values for 188 // PHI nodes. Insert associations now. 189 typedef DenseMap<const Value*, Value*> ValueMapTy; 190 ValueMapTy LastValueMap; 191 std::vector<PHINode*> OrigPHINode; 192 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) { 193 PHINode *PN = cast<PHINode>(I); 194 OrigPHINode.push_back(PN); 195 if (Instruction *I = 196 dyn_cast<Instruction>(PN->getIncomingValueForBlock(LatchBlock))) 197 if (L->contains(I->getParent())) 198 LastValueMap[I] = I; 199 } 200 201 std::vector<BasicBlock*> Headers; 202 std::vector<BasicBlock*> Latches; 203 Headers.push_back(Header); 204 Latches.push_back(LatchBlock); 205 206 for (unsigned It = 1; It != Count; ++It) { 207 char SuffixBuffer[100]; 208 sprintf(SuffixBuffer, ".%d", It); 209 210 std::vector<BasicBlock*> NewBlocks; 211 212 for (std::vector<BasicBlock*>::iterator BB = LoopBlocks.begin(), 213 E = LoopBlocks.end(); BB != E; ++BB) { 214 ValueMapTy ValueMap; 215 BasicBlock *New = CloneBasicBlock(*BB, ValueMap, SuffixBuffer); 216 Header->getParent()->getBasicBlockList().push_back(New); 217 218 // Loop over all of the PHI nodes in the block, changing them to use the 219 // incoming values from the previous block. 220 if (*BB == Header) 221 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 222 PHINode *NewPHI = cast<PHINode>(ValueMap[OrigPHINode[i]]); 223 Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock); 224 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) 225 if (It > 1 && L->contains(InValI->getParent())) 226 InVal = LastValueMap[InValI]; 227 ValueMap[OrigPHINode[i]] = InVal; 228 New->getInstList().erase(NewPHI); 229 } 230 231 // Update our running map of newest clones 232 LastValueMap[*BB] = New; 233 for (ValueMapTy::iterator VI = ValueMap.begin(), VE = ValueMap.end(); 234 VI != VE; ++VI) 235 LastValueMap[VI->first] = VI->second; 236 237 L->addBasicBlockToLoop(New, LI->getBase()); 238 239 // Add phi entries for newly created values to all exit blocks except 240 // the successor of the latch block. The successor of the exit block will 241 // be updated specially after unrolling all the way. 242 if (*BB != LatchBlock) 243 for (Value::use_iterator UI = (*BB)->use_begin(), UE = (*BB)->use_end(); 244 UI != UE;) { 245 Instruction *UseInst = cast<Instruction>(*UI); 246 ++UI; 247 if (isa<PHINode>(UseInst) && !L->contains(UseInst->getParent())) { 248 PHINode *phi = cast<PHINode>(UseInst); 249 Value *Incoming = phi->getIncomingValueForBlock(*BB); 250 phi->addIncoming(Incoming, New); 251 } 252 } 253 254 // Keep track of new headers and latches as we create them, so that 255 // we can insert the proper branches later. 256 if (*BB == Header) 257 Headers.push_back(New); 258 if (*BB == LatchBlock) { 259 Latches.push_back(New); 260 261 // Also, clear out the new latch's back edge so that it doesn't look 262 // like a new loop, so that it's amenable to being merged with adjacent 263 // blocks later on. 264 TerminatorInst *Term = New->getTerminator(); 265 assert(L->contains(Term->getSuccessor(!ContinueOnTrue))); 266 assert(Term->getSuccessor(ContinueOnTrue) == LoopExit); 267 Term->setSuccessor(!ContinueOnTrue, NULL); 268 } 269 270 NewBlocks.push_back(New); 271 } 272 273 // Remap all instructions in the most recent iteration 274 for (unsigned i = 0; i < NewBlocks.size(); ++i) 275 for (BasicBlock::iterator I = NewBlocks[i]->begin(), 276 E = NewBlocks[i]->end(); I != E; ++I) 277 RemapInstruction(I, LastValueMap); 278 } 279 280 // The latch block exits the loop. If there are any PHI nodes in the 281 // successor blocks, update them to use the appropriate values computed as the 282 // last iteration of the loop. 283 if (Count != 1) { 284 SmallPtrSet<PHINode*, 8> Users; 285 for (Value::use_iterator UI = LatchBlock->use_begin(), 286 UE = LatchBlock->use_end(); UI != UE; ++UI) 287 if (PHINode *phi = dyn_cast<PHINode>(*UI)) 288 Users.insert(phi); 289 290 BasicBlock *LastIterationBB = cast<BasicBlock>(LastValueMap[LatchBlock]); 291 for (SmallPtrSet<PHINode*,8>::iterator SI = Users.begin(), SE = Users.end(); 292 SI != SE; ++SI) { 293 PHINode *PN = *SI; 294 Value *InVal = PN->removeIncomingValue(LatchBlock, false); 295 // If this value was defined in the loop, take the value defined by the 296 // last iteration of the loop. 297 if (Instruction *InValI = dyn_cast<Instruction>(InVal)) { 298 if (L->contains(InValI->getParent())) 299 InVal = LastValueMap[InVal]; 300 } 301 PN->addIncoming(InVal, LastIterationBB); 302 } 303 } 304 305 // Now, if we're doing complete unrolling, loop over the PHI nodes in the 306 // original block, setting them to their incoming values. 307 if (CompletelyUnroll) { 308 BasicBlock *Preheader = L->getLoopPreheader(); 309 for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) { 310 PHINode *PN = OrigPHINode[i]; 311 PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); 312 Header->getInstList().erase(PN); 313 } 314 } 315 316 // Now that all the basic blocks for the unrolled iterations are in place, 317 // set up the branches to connect them. 318 for (unsigned i = 0, e = Latches.size(); i != e; ++i) { 319 // The original branch was replicated in each unrolled iteration. 320 BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator()); 321 322 // The branch destination. 323 unsigned j = (i + 1) % e; 324 BasicBlock *Dest = Headers[j]; 325 bool NeedConditional = true; 326 327 // For a complete unroll, make the last iteration end with a branch 328 // to the exit block. 329 if (CompletelyUnroll && j == 0) { 330 Dest = LoopExit; 331 NeedConditional = false; 332 } 333 334 // If we know the trip count or a multiple of it, we can safely use an 335 // unconditional branch for some iterations. 336 if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) { 337 NeedConditional = false; 338 } 339 340 if (NeedConditional) { 341 // Update the conditional branch's successor for the following 342 // iteration. 343 Term->setSuccessor(!ContinueOnTrue, Dest); 344 } else { 345 Term->setUnconditionalDest(Dest); 346 // Merge adjacent basic blocks, if possible. 347 if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI)) { 348 std::replace(Latches.begin(), Latches.end(), Dest, Fold); 349 std::replace(Headers.begin(), Headers.end(), Dest, Fold); 350 } 351 } 352 } 353 354 // At this point, the code is well formed. We now do a quick sweep over the 355 // inserted code, doing constant propagation and dead code elimination as we 356 // go. 357 const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks(); 358 for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(), 359 BBE = NewLoopBlocks.end(); BB != BBE; ++BB) 360 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) { 361 Instruction *Inst = I++; 362 363 if (isInstructionTriviallyDead(Inst)) 364 (*BB)->getInstList().erase(Inst); 365 else if (Constant *C = ConstantFoldInstruction(Inst)) { 366 Inst->replaceAllUsesWith(C); 367 (*BB)->getInstList().erase(Inst); 368 } 369 } 370 371 NumCompletelyUnrolled += CompletelyUnroll; 372 ++NumUnrolled; 373 // Remove the loop from the LoopPassManager if it's completely removed. 374 if (CompletelyUnroll && LPM != NULL) 375 LPM->deleteLoopFromQueue(L); 376 377 // If we didn't completely unroll the loop, it should still be in LCSSA form. 378 if (!CompletelyUnroll) 379 assert(L->isLCSSAForm()); 380 381 return true; 382} 383