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