1 //===-- SimplifyIndVar.cpp - Induction variable simplification ------------===// 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 induction variable simplification. It does 11 // not define any actual pass or policy, but provides a single function to 12 // simplify a loop's induction variables based on ScalarEvolution. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include "llvm/Transforms/Utils/SimplifyIndVar.h" 17 #include "llvm/ADT/STLExtras.h" 18 #include "llvm/ADT/SmallVector.h" 19 #include "llvm/ADT/Statistic.h" 20 #include "llvm/Analysis/IVUsers.h" 21 #include "llvm/Analysis/LoopInfo.h" 22 #include "llvm/Analysis/LoopPass.h" 23 #include "llvm/Analysis/ScalarEvolutionExpressions.h" 24 #include "llvm/IR/DataLayout.h" 25 #include "llvm/IR/Dominators.h" 26 #include "llvm/IR/IRBuilder.h" 27 #include "llvm/IR/Instructions.h" 28 #include "llvm/IR/IntrinsicInst.h" 29 #include "llvm/Support/CommandLine.h" 30 #include "llvm/Support/Debug.h" 31 #include "llvm/Support/raw_ostream.h" 32 33 using namespace llvm; 34 35 #define DEBUG_TYPE "indvars" 36 37 STATISTIC(NumElimIdentity, "Number of IV identities eliminated"); 38 STATISTIC(NumElimOperand, "Number of IV operands folded into a use"); 39 STATISTIC(NumElimRem , "Number of IV remainder operations eliminated"); 40 STATISTIC(NumElimCmp , "Number of IV comparisons eliminated"); 41 42 namespace { 43 /// SimplifyIndvar - This is a utility for simplifying induction variables 44 /// based on ScalarEvolution. It is the primary instrument of the 45 /// IndvarSimplify pass, but it may also be directly invoked to cleanup after 46 /// other loop passes that preserve SCEV. 47 class SimplifyIndvar { 48 Loop *L; 49 LoopInfo *LI; 50 ScalarEvolution *SE; 51 const DataLayout *DL; // May be NULL 52 53 SmallVectorImpl<WeakVH> &DeadInsts; 54 55 bool Changed; 56 57 public: 58 SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, LPPassManager *LPM, 59 SmallVectorImpl<WeakVH> &Dead, IVUsers *IVU = nullptr) : 60 L(Loop), 61 LI(LPM->getAnalysisIfAvailable<LoopInfo>()), 62 SE(SE), 63 DeadInsts(Dead), 64 Changed(false) { 65 DataLayoutPass *DLP = LPM->getAnalysisIfAvailable<DataLayoutPass>(); 66 DL = DLP ? &DLP->getDataLayout() : nullptr; 67 assert(LI && "IV simplification requires LoopInfo"); 68 } 69 70 bool hasChanged() const { return Changed; } 71 72 /// Iteratively perform simplification on a worklist of users of the 73 /// specified induction variable. This is the top-level driver that applies 74 /// all simplicitions to users of an IV. 75 void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr); 76 77 Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand); 78 79 bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand); 80 void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand); 81 void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand, 82 bool IsSigned); 83 84 Instruction *splitOverflowIntrinsic(Instruction *IVUser, 85 const DominatorTree *DT); 86 }; 87 } 88 89 /// foldIVUser - Fold an IV operand into its use. This removes increments of an 90 /// aligned IV when used by a instruction that ignores the low bits. 91 /// 92 /// IVOperand is guaranteed SCEVable, but UseInst may not be. 93 /// 94 /// Return the operand of IVOperand for this induction variable if IVOperand can 95 /// be folded (in case more folding opportunities have been exposed). 96 /// Otherwise return null. 97 Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) { 98 Value *IVSrc = nullptr; 99 unsigned OperIdx = 0; 100 const SCEV *FoldedExpr = nullptr; 101 switch (UseInst->getOpcode()) { 102 default: 103 return nullptr; 104 case Instruction::UDiv: 105 case Instruction::LShr: 106 // We're only interested in the case where we know something about 107 // the numerator and have a constant denominator. 108 if (IVOperand != UseInst->getOperand(OperIdx) || 109 !isa<ConstantInt>(UseInst->getOperand(1))) 110 return nullptr; 111 112 // Attempt to fold a binary operator with constant operand. 113 // e.g. ((I + 1) >> 2) => I >> 2 114 if (!isa<BinaryOperator>(IVOperand) 115 || !isa<ConstantInt>(IVOperand->getOperand(1))) 116 return nullptr; 117 118 IVSrc = IVOperand->getOperand(0); 119 // IVSrc must be the (SCEVable) IV, since the other operand is const. 120 assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand"); 121 122 ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1)); 123 if (UseInst->getOpcode() == Instruction::LShr) { 124 // Get a constant for the divisor. See createSCEV. 125 uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth(); 126 if (D->getValue().uge(BitWidth)) 127 return nullptr; 128 129 D = ConstantInt::get(UseInst->getContext(), 130 APInt::getOneBitSet(BitWidth, D->getZExtValue())); 131 } 132 FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D)); 133 } 134 // We have something that might fold it's operand. Compare SCEVs. 135 if (!SE->isSCEVable(UseInst->getType())) 136 return nullptr; 137 138 // Bypass the operand if SCEV can prove it has no effect. 139 if (SE->getSCEV(UseInst) != FoldedExpr) 140 return nullptr; 141 142 DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand 143 << " -> " << *UseInst << '\n'); 144 145 UseInst->setOperand(OperIdx, IVSrc); 146 assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper"); 147 148 ++NumElimOperand; 149 Changed = true; 150 if (IVOperand->use_empty()) 151 DeadInsts.push_back(IVOperand); 152 return IVSrc; 153 } 154 155 /// eliminateIVComparison - SimplifyIVUsers helper for eliminating useless 156 /// comparisons against an induction variable. 157 void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) { 158 unsigned IVOperIdx = 0; 159 ICmpInst::Predicate Pred = ICmp->getPredicate(); 160 if (IVOperand != ICmp->getOperand(0)) { 161 // Swapped 162 assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand"); 163 IVOperIdx = 1; 164 Pred = ICmpInst::getSwappedPredicate(Pred); 165 } 166 167 // Get the SCEVs for the ICmp operands. 168 const SCEV *S = SE->getSCEV(ICmp->getOperand(IVOperIdx)); 169 const SCEV *X = SE->getSCEV(ICmp->getOperand(1 - IVOperIdx)); 170 171 // Simplify unnecessary loops away. 172 const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent()); 173 S = SE->getSCEVAtScope(S, ICmpLoop); 174 X = SE->getSCEVAtScope(X, ICmpLoop); 175 176 // If the condition is always true or always false, replace it with 177 // a constant value. 178 if (SE->isKnownPredicate(Pred, S, X)) 179 ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext())); 180 else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X)) 181 ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext())); 182 else 183 return; 184 185 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n'); 186 ++NumElimCmp; 187 Changed = true; 188 DeadInsts.push_back(ICmp); 189 } 190 191 /// eliminateIVRemainder - SimplifyIVUsers helper for eliminating useless 192 /// remainder operations operating on an induction variable. 193 void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem, 194 Value *IVOperand, 195 bool IsSigned) { 196 // We're only interested in the case where we know something about 197 // the numerator. 198 if (IVOperand != Rem->getOperand(0)) 199 return; 200 201 // Get the SCEVs for the ICmp operands. 202 const SCEV *S = SE->getSCEV(Rem->getOperand(0)); 203 const SCEV *X = SE->getSCEV(Rem->getOperand(1)); 204 205 // Simplify unnecessary loops away. 206 const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent()); 207 S = SE->getSCEVAtScope(S, ICmpLoop); 208 X = SE->getSCEVAtScope(X, ICmpLoop); 209 210 // i % n --> i if i is in [0,n). 211 if ((!IsSigned || SE->isKnownNonNegative(S)) && 212 SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT, 213 S, X)) 214 Rem->replaceAllUsesWith(Rem->getOperand(0)); 215 else { 216 // (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n). 217 const SCEV *LessOne = 218 SE->getMinusSCEV(S, SE->getConstant(S->getType(), 1)); 219 if (IsSigned && !SE->isKnownNonNegative(LessOne)) 220 return; 221 222 if (!SE->isKnownPredicate(IsSigned ? 223 ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT, 224 LessOne, X)) 225 return; 226 227 ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ, 228 Rem->getOperand(0), Rem->getOperand(1)); 229 SelectInst *Sel = 230 SelectInst::Create(ICmp, 231 ConstantInt::get(Rem->getType(), 0), 232 Rem->getOperand(0), "tmp", Rem); 233 Rem->replaceAllUsesWith(Sel); 234 } 235 236 DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n'); 237 ++NumElimRem; 238 Changed = true; 239 DeadInsts.push_back(Rem); 240 } 241 242 /// eliminateIVUser - Eliminate an operation that consumes a simple IV and has 243 /// no observable side-effect given the range of IV values. 244 /// IVOperand is guaranteed SCEVable, but UseInst may not be. 245 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst, 246 Instruction *IVOperand) { 247 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) { 248 eliminateIVComparison(ICmp, IVOperand); 249 return true; 250 } 251 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) { 252 bool IsSigned = Rem->getOpcode() == Instruction::SRem; 253 if (IsSigned || Rem->getOpcode() == Instruction::URem) { 254 eliminateIVRemainder(Rem, IVOperand, IsSigned); 255 return true; 256 } 257 } 258 259 // Eliminate any operation that SCEV can prove is an identity function. 260 if (!SE->isSCEVable(UseInst->getType()) || 261 (UseInst->getType() != IVOperand->getType()) || 262 (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand))) 263 return false; 264 265 DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n'); 266 267 UseInst->replaceAllUsesWith(IVOperand); 268 ++NumElimIdentity; 269 Changed = true; 270 DeadInsts.push_back(UseInst); 271 return true; 272 } 273 274 /// \brief Split sadd.with.overflow into add + sadd.with.overflow to allow 275 /// analysis and optimization. 276 /// 277 /// \return A new value representing the non-overflowing add if possible, 278 /// otherwise return the original value. 279 Instruction *SimplifyIndvar::splitOverflowIntrinsic(Instruction *IVUser, 280 const DominatorTree *DT) { 281 IntrinsicInst *II = dyn_cast<IntrinsicInst>(IVUser); 282 if (!II || II->getIntrinsicID() != Intrinsic::sadd_with_overflow) 283 return IVUser; 284 285 // Find a branch guarded by the overflow check. 286 BranchInst *Branch = nullptr; 287 Instruction *AddVal = nullptr; 288 for (User *U : II->users()) { 289 if (ExtractValueInst *ExtractInst = dyn_cast<ExtractValueInst>(U)) { 290 if (ExtractInst->getNumIndices() != 1) 291 continue; 292 if (ExtractInst->getIndices()[0] == 0) 293 AddVal = ExtractInst; 294 else if (ExtractInst->getIndices()[0] == 1 && ExtractInst->hasOneUse()) 295 Branch = dyn_cast<BranchInst>(ExtractInst->user_back()); 296 } 297 } 298 if (!AddVal || !Branch) 299 return IVUser; 300 301 BasicBlock *ContinueBB = Branch->getSuccessor(1); 302 if (std::next(pred_begin(ContinueBB)) != pred_end(ContinueBB)) 303 return IVUser; 304 305 // Check if all users of the add are provably NSW. 306 bool AllNSW = true; 307 for (Use &U : AddVal->uses()) { 308 if (Instruction *UseInst = dyn_cast<Instruction>(U.getUser())) { 309 BasicBlock *UseBB = UseInst->getParent(); 310 if (PHINode *PHI = dyn_cast<PHINode>(UseInst)) 311 UseBB = PHI->getIncomingBlock(U); 312 if (!DT->dominates(ContinueBB, UseBB)) { 313 AllNSW = false; 314 break; 315 } 316 } 317 } 318 if (!AllNSW) 319 return IVUser; 320 321 // Go for it... 322 IRBuilder<> Builder(IVUser); 323 Instruction *AddInst = dyn_cast<Instruction>( 324 Builder.CreateNSWAdd(II->getOperand(0), II->getOperand(1))); 325 326 // The caller expects the new add to have the same form as the intrinsic. The 327 // IV operand position must be the same. 328 assert((AddInst->getOpcode() == Instruction::Add && 329 AddInst->getOperand(0) == II->getOperand(0)) && 330 "Bad add instruction created from overflow intrinsic."); 331 332 AddVal->replaceAllUsesWith(AddInst); 333 DeadInsts.push_back(AddVal); 334 return AddInst; 335 } 336 337 /// pushIVUsers - Add all uses of Def to the current IV's worklist. 338 /// 339 static void pushIVUsers( 340 Instruction *Def, 341 SmallPtrSet<Instruction*,16> &Simplified, 342 SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) { 343 344 for (User *U : Def->users()) { 345 Instruction *UI = cast<Instruction>(U); 346 347 // Avoid infinite or exponential worklist processing. 348 // Also ensure unique worklist users. 349 // If Def is a LoopPhi, it may not be in the Simplified set, so check for 350 // self edges first. 351 if (UI != Def && Simplified.insert(UI)) 352 SimpleIVUsers.push_back(std::make_pair(UI, Def)); 353 } 354 } 355 356 /// isSimpleIVUser - Return true if this instruction generates a simple SCEV 357 /// expression in terms of that IV. 358 /// 359 /// This is similar to IVUsers' isInteresting() but processes each instruction 360 /// non-recursively when the operand is already known to be a simpleIVUser. 361 /// 362 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) { 363 if (!SE->isSCEVable(I->getType())) 364 return false; 365 366 // Get the symbolic expression for this instruction. 367 const SCEV *S = SE->getSCEV(I); 368 369 // Only consider affine recurrences. 370 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S); 371 if (AR && AR->getLoop() == L) 372 return true; 373 374 return false; 375 } 376 377 /// simplifyUsers - Iteratively perform simplification on a worklist of users 378 /// of the specified induction variable. Each successive simplification may push 379 /// more users which may themselves be candidates for simplification. 380 /// 381 /// This algorithm does not require IVUsers analysis. Instead, it simplifies 382 /// instructions in-place during analysis. Rather than rewriting induction 383 /// variables bottom-up from their users, it transforms a chain of IVUsers 384 /// top-down, updating the IR only when it encouters a clear optimization 385 /// opportunitiy. 386 /// 387 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers. 388 /// 389 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) { 390 if (!SE->isSCEVable(CurrIV->getType())) 391 return; 392 393 // Instructions processed by SimplifyIndvar for CurrIV. 394 SmallPtrSet<Instruction*,16> Simplified; 395 396 // Use-def pairs if IV users waiting to be processed for CurrIV. 397 SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers; 398 399 // Push users of the current LoopPhi. In rare cases, pushIVUsers may be 400 // called multiple times for the same LoopPhi. This is the proper thing to 401 // do for loop header phis that use each other. 402 pushIVUsers(CurrIV, Simplified, SimpleIVUsers); 403 404 while (!SimpleIVUsers.empty()) { 405 std::pair<Instruction*, Instruction*> UseOper = 406 SimpleIVUsers.pop_back_val(); 407 Instruction *UseInst = UseOper.first; 408 409 // Bypass back edges to avoid extra work. 410 if (UseInst == CurrIV) continue; 411 412 if (V && V->shouldSplitOverflowInstrinsics()) { 413 UseInst = splitOverflowIntrinsic(UseInst, V->getDomTree()); 414 if (!UseInst) 415 continue; 416 } 417 418 Instruction *IVOperand = UseOper.second; 419 for (unsigned N = 0; IVOperand; ++N) { 420 assert(N <= Simplified.size() && "runaway iteration"); 421 422 Value *NewOper = foldIVUser(UseOper.first, IVOperand); 423 if (!NewOper) 424 break; // done folding 425 IVOperand = dyn_cast<Instruction>(NewOper); 426 } 427 if (!IVOperand) 428 continue; 429 430 if (eliminateIVUser(UseOper.first, IVOperand)) { 431 pushIVUsers(IVOperand, Simplified, SimpleIVUsers); 432 continue; 433 } 434 CastInst *Cast = dyn_cast<CastInst>(UseOper.first); 435 if (V && Cast) { 436 V->visitCast(Cast); 437 continue; 438 } 439 if (isSimpleIVUser(UseOper.first, L, SE)) { 440 pushIVUsers(UseOper.first, Simplified, SimpleIVUsers); 441 } 442 } 443 } 444 445 namespace llvm { 446 447 void IVVisitor::anchor() { } 448 449 /// simplifyUsersOfIV - Simplify instructions that use this induction variable 450 /// by using ScalarEvolution to analyze the IV's recurrence. 451 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, LPPassManager *LPM, 452 SmallVectorImpl<WeakVH> &Dead, IVVisitor *V) 453 { 454 LoopInfo *LI = &LPM->getAnalysis<LoopInfo>(); 455 SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, LPM, Dead); 456 SIV.simplifyUsers(CurrIV, V); 457 return SIV.hasChanged(); 458 } 459 460 /// simplifyLoopIVs - Simplify users of induction variables within this 461 /// loop. This does not actually change or add IVs. 462 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, LPPassManager *LPM, 463 SmallVectorImpl<WeakVH> &Dead) { 464 bool Changed = false; 465 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) { 466 Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, LPM, Dead); 467 } 468 return Changed; 469 } 470 471 } // namespace llvm 472