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