lib/Transforms/Utils/SimplifyIndVar.cpp

   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/LoopInfo.h"
  21 #include "llvm/Analysis/ScalarEvolutionExpander.h"
  22 #include "llvm/IR/DataLayout.h"
  23 #include "llvm/IR/Dominators.h"
  24 #include "llvm/IR/IRBuilder.h"
  25 #include "llvm/IR/Instructions.h"
  26 #include "llvm/IR/PatternMatch.h"
  27 #include "llvm/Support/Debug.h"
  28 #include "llvm/Support/raw_ostream.h"
  29 #include "llvm/Transforms/Utils/Local.h"
  30
  31 using namespace llvm;
  32
  33 #define DEBUG_TYPE "indvars"
  34
  35 STATISTIC(NumElimIdentity, "Number of IV identities eliminated");
  36 STATISTIC(NumElimOperand,  "Number of IV operands folded into a use");
  37 STATISTIC(NumFoldedUser, "Number of IV users folded into a constant");
  38 STATISTIC(NumElimRem     , "Number of IV remainder operations eliminated");
  39 STATISTIC(
  40     NumSimplifiedSDiv,
  41     "Number of IV signed division operations converted to unsigned division");
  42 STATISTIC(
  43     NumSimplifiedSRem,
  44     "Number of IV signed remainder operations converted to unsigned remainder");
  45 STATISTIC(NumElimCmp     , "Number of IV comparisons eliminated");
  46
  47 namespace {
  48   /// This is a utility for simplifying induction variables
  49   /// based on ScalarEvolution. It is the primary instrument of the
  50   /// IndvarSimplify pass, but it may also be directly invoked to cleanup after
  51   /// other loop passes that preserve SCEV.
  52   class SimplifyIndvar {
  53     Loop             *L;
  54     LoopInfo         *LI;
  55     ScalarEvolution  *SE;
  56     DominatorTree    *DT;
  57     SCEVExpander     &Rewriter;
  58     SmallVectorImpl<WeakTrackingVH> &DeadInsts;
  59
  60     bool Changed;
  61
  62   public:
  63     SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, DominatorTree *DT,
  64                    LoopInfo *LI, SCEVExpander &Rewriter,
  65                    SmallVectorImpl<WeakTrackingVH> &Dead)
  66         : L(Loop), LI(LI), SE(SE), DT(DT), Rewriter(Rewriter), DeadInsts(Dead),
  67           Changed(false) {
  68       assert(LI && "IV simplification requires LoopInfo");
  69     }
  70
  71     bool hasChanged() const { return Changed; }
  72
  73     /// Iteratively perform simplification on a worklist of users of the
  74     /// specified induction variable. This is the top-level driver that applies
  75     /// all simplifications to users of an IV.
  76     void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr);
  77
  78     Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
  79
  80     bool eliminateIdentitySCEV(Instruction *UseInst, Instruction *IVOperand);
  81     bool replaceIVUserWithLoopInvariant(Instruction *UseInst);
  82
  83     bool eliminateOverflowIntrinsic(CallInst *CI);
  84     bool eliminateTrunc(TruncInst *TI);
  85     bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
  86     bool makeIVComparisonInvariant(ICmpInst *ICmp, Value *IVOperand);
  87     void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
  88     void simplifyIVRemainder(BinaryOperator *Rem, Value *IVOperand,
  89                              bool IsSigned);
  90     void replaceRemWithNumerator(BinaryOperator *Rem);
  91     void replaceRemWithNumeratorOrZero(BinaryOperator *Rem);
  92     void replaceSRemWithURem(BinaryOperator *Rem);
  93     bool eliminateSDiv(BinaryOperator *SDiv);
  94     bool strengthenOverflowingOperation(BinaryOperator *OBO, Value *IVOperand);
  95     bool strengthenRightShift(BinaryOperator *BO, Value *IVOperand);
  96   };
  97 }
  98
  99 /// Fold an IV operand into its use.  This removes increments of an
 100 /// aligned IV when used by a instruction that ignores the low bits.
 101 ///
 102 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
 103 ///
 104 /// Return the operand of IVOperand for this induction variable if IVOperand can
 105 /// be folded (in case more folding opportunities have been exposed).
 106 /// Otherwise return null.
 107 Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) {
 108   Value *IVSrc = nullptr;
 109   unsigned OperIdx = 0;
 110   const SCEV *FoldedExpr = nullptr;
 111   switch (UseInst->getOpcode()) {
 112   default:
 113     return nullptr;
 114   case Instruction::UDiv:
 115   case Instruction::LShr:
 116     // We're only interested in the case where we know something about
 117     // the numerator and have a constant denominator.
 118     if (IVOperand != UseInst->getOperand(OperIdx) ||
 119         !isa<ConstantInt>(UseInst->getOperand(1)))
 120       return nullptr;
 121
 122     // Attempt to fold a binary operator with constant operand.
 123     // e.g. ((I + 1) >> 2) => I >> 2
 124     if (!isa<BinaryOperator>(IVOperand)
 125         || !isa<ConstantInt>(IVOperand->getOperand(1)))
 126       return nullptr;
 127
 128     IVSrc = IVOperand->getOperand(0);
 129     // IVSrc must be the (SCEVable) IV, since the other operand is const.
 130     assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand");
 131
 132     ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1));
 133     if (UseInst->getOpcode() == Instruction::LShr) {
 134       // Get a constant for the divisor. See createSCEV.
 135       uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth();
 136       if (D->getValue().uge(BitWidth))
 137         return nullptr;
 138
 139       D = ConstantInt::get(UseInst->getContext(),
 140                            APInt::getOneBitSet(BitWidth, D->getZExtValue()));
 141     }
 142     FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
 143   }
 144   // We have something that might fold it's operand. Compare SCEVs.
 145   if (!SE->isSCEVable(UseInst->getType()))
 146     return nullptr;
 147
 148   // Bypass the operand if SCEV can prove it has no effect.
 149   if (SE->getSCEV(UseInst) != FoldedExpr)
 150     return nullptr;
 151
 152   LLVM_DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand
 153                     << " -> " << *UseInst << '\n');
 154
 155   UseInst->setOperand(OperIdx, IVSrc);
 156   assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
 157
 158   ++NumElimOperand;
 159   Changed = true;
 160   if (IVOperand->use_empty())
 161     DeadInsts.emplace_back(IVOperand);
 162   return IVSrc;
 163 }
 164
 165 bool SimplifyIndvar::makeIVComparisonInvariant(ICmpInst *ICmp,
 166                                                Value *IVOperand) {
 167   unsigned IVOperIdx = 0;
 168   ICmpInst::Predicate Pred = ICmp->getPredicate();
 169   if (IVOperand != ICmp->getOperand(0)) {
 170     // Swapped
 171     assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
 172     IVOperIdx = 1;
 173     Pred = ICmpInst::getSwappedPredicate(Pred);
 174   }
 175
 176   // Get the SCEVs for the ICmp operands (in the specific context of the
 177   // current loop)
 178   const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
 179   const SCEV *S = SE->getSCEVAtScope(ICmp->getOperand(IVOperIdx), ICmpLoop);
 180   const SCEV *X = SE->getSCEVAtScope(ICmp->getOperand(1 - IVOperIdx), ICmpLoop);
 181
 182   ICmpInst::Predicate InvariantPredicate;
 183   const SCEV *InvariantLHS, *InvariantRHS;
 184
 185   auto *PN = dyn_cast<PHINode>(IVOperand);
 186   if (!PN)
 187     return false;
 188   if (!SE->isLoopInvariantPredicate(Pred, S, X, L, InvariantPredicate,
 189                                     InvariantLHS, InvariantRHS))
 190     return false;
 191
 192   // Rewrite the comparison to a loop invariant comparison if it can be done
 193   // cheaply, where cheaply means "we don't need to emit any new
 194   // instructions".
 195
 196   SmallDenseMap<const SCEV*, Value*> CheapExpansions;
 197   CheapExpansions[S] = ICmp->getOperand(IVOperIdx);
 198   CheapExpansions[X] = ICmp->getOperand(1 - IVOperIdx);
 199
 200   // TODO: Support multiple entry loops?  (We currently bail out of these in
 201   // the IndVarSimplify pass)
 202   if (auto *BB = L->getLoopPredecessor()) {
 203     const int Idx = PN->getBasicBlockIndex(BB);
 204     if (Idx >= 0) {
 205       Value *Incoming = PN->getIncomingValue(Idx);
 206       const SCEV *IncomingS = SE->getSCEV(Incoming);
 207       CheapExpansions[IncomingS] = Incoming;
 208     }
 209   }
 210   Value *NewLHS = CheapExpansions[InvariantLHS];
 211   Value *NewRHS = CheapExpansions[InvariantRHS];
 212
 213   if (!NewLHS)
 214     if (auto *ConstLHS = dyn_cast<SCEVConstant>(InvariantLHS))
 215       NewLHS = ConstLHS->getValue();
 216   if (!NewRHS)
 217     if (auto *ConstRHS = dyn_cast<SCEVConstant>(InvariantRHS))
 218       NewRHS = ConstRHS->getValue();
 219
 220   if (!NewLHS || !NewRHS)
 221     // We could not find an existing value to replace either LHS or RHS.
 222     // Generating new instructions has subtler tradeoffs, so avoid doing that
 223     // for now.
 224     return false;
 225
 226   LLVM_DEBUG(dbgs() << "INDVARS: Simplified comparison: " << *ICmp << '\n');
 227   ICmp->setPredicate(InvariantPredicate);
 228   ICmp->setOperand(0, NewLHS);
 229   ICmp->setOperand(1, NewRHS);
 230   return true;
 231 }
 232
 233 /// SimplifyIVUsers helper for eliminating useless
 234 /// comparisons against an induction variable.
 235 void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
 236   unsigned IVOperIdx = 0;
 237   ICmpInst::Predicate Pred = ICmp->getPredicate();
 238   ICmpInst::Predicate OriginalPred = Pred;
 239   if (IVOperand != ICmp->getOperand(0)) {
 240     // Swapped
 241     assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
 242     IVOperIdx = 1;
 243     Pred = ICmpInst::getSwappedPredicate(Pred);
 244   }
 245
 246   // Get the SCEVs for the ICmp operands (in the specific context of the
 247   // current loop)
 248   const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
 249   const SCEV *S = SE->getSCEVAtScope(ICmp->getOperand(IVOperIdx), ICmpLoop);
 250   const SCEV *X = SE->getSCEVAtScope(ICmp->getOperand(1 - IVOperIdx), ICmpLoop);
 251
 252   // If the condition is always true or always false, replace it with
 253   // a constant value.
 254   if (SE->isKnownPredicate(Pred, S, X)) {
 255     ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
 256     DeadInsts.emplace_back(ICmp);
 257     LLVM_DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
 258   } else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X)) {
 259     ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
 260     DeadInsts.emplace_back(ICmp);
 261     LLVM_DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
 262   } else if (makeIVComparisonInvariant(ICmp, IVOperand)) {
 263     // fallthrough to end of function
 264   } else if (ICmpInst::isSigned(OriginalPred) &&
 265              SE->isKnownNonNegative(S) && SE->isKnownNonNegative(X)) {
 266     // If we were unable to make anything above, all we can is to canonicalize
 267     // the comparison hoping that it will open the doors for other
 268     // optimizations. If we find out that we compare two non-negative values,
 269     // we turn the instruction's predicate to its unsigned version. Note that
 270     // we cannot rely on Pred here unless we check if we have swapped it.
 271     assert(ICmp->getPredicate() == OriginalPred && "Predicate changed?");
 272     LLVM_DEBUG(dbgs() << "INDVARS: Turn to unsigned comparison: " << *ICmp
 273                       << '\n');
 274     ICmp->setPredicate(ICmpInst::getUnsignedPredicate(OriginalPred));
 275   } else
 276     return;
 277
 278   ++NumElimCmp;
 279   Changed = true;
 280 }
 281
 282 bool SimplifyIndvar::eliminateSDiv(BinaryOperator *SDiv) {
 283   // Get the SCEVs for the ICmp operands.
 284   auto *N = SE->getSCEV(SDiv->getOperand(0));
 285   auto *D = SE->getSCEV(SDiv->getOperand(1));
 286
 287   // Simplify unnecessary loops away.
 288   const Loop *L = LI->getLoopFor(SDiv->getParent());
 289   N = SE->getSCEVAtScope(N, L);
 290   D = SE->getSCEVAtScope(D, L);
 291
 292   // Replace sdiv by udiv if both of the operands are non-negative
 293   if (SE->isKnownNonNegative(N) && SE->isKnownNonNegative(D)) {
 294     auto *UDiv = BinaryOperator::Create(
 295         BinaryOperator::UDiv, SDiv->getOperand(0), SDiv->getOperand(1),
 296         SDiv->getName() + ".udiv", SDiv);
 297     UDiv->setIsExact(SDiv->isExact());
 298     SDiv->replaceAllUsesWith(UDiv);
 299     LLVM_DEBUG(dbgs() << "INDVARS: Simplified sdiv: " << *SDiv << '\n');
 300     ++NumSimplifiedSDiv;
 301     Changed = true;
 302     DeadInsts.push_back(SDiv);
 303     return true;
 304   }
 305
 306   return false;
 307 }
 308
 309 // i %s n -> i %u n if i >= 0 and n >= 0
 310 void SimplifyIndvar::replaceSRemWithURem(BinaryOperator *Rem) {
 311   auto *N = Rem->getOperand(0), *D = Rem->getOperand(1);
 312   auto *URem = BinaryOperator::Create(BinaryOperator::URem, N, D,
 313                                       Rem->getName() + ".urem", Rem);
 314   Rem->replaceAllUsesWith(URem);
 315   LLVM_DEBUG(dbgs() << "INDVARS: Simplified srem: " << *Rem << '\n');
 316   ++NumSimplifiedSRem;
 317   Changed = true;
 318   DeadInsts.emplace_back(Rem);
 319 }
 320
 321 // i % n  -->  i  if i is in [0,n).
 322 void SimplifyIndvar::replaceRemWithNumerator(BinaryOperator *Rem) {
 323   Rem->replaceAllUsesWith(Rem->getOperand(0));
 324   LLVM_DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
 325   ++NumElimRem;
 326   Changed = true;
 327   DeadInsts.emplace_back(Rem);
 328 }
 329
 330 // (i+1) % n  -->  (i+1)==n?0:(i+1)  if i is in [0,n).
 331 void SimplifyIndvar::replaceRemWithNumeratorOrZero(BinaryOperator *Rem) {
 332   auto *T = Rem->getType();
 333   auto *N = Rem->getOperand(0), *D = Rem->getOperand(1);
 334   ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ, N, D);
 335   SelectInst *Sel =
 336       SelectInst::Create(ICmp, ConstantInt::get(T, 0), N, "iv.rem", Rem);
 337   Rem->replaceAllUsesWith(Sel);
 338   LLVM_DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
 339   ++NumElimRem;
 340   Changed = true;
 341   DeadInsts.emplace_back(Rem);
 342 }
 343
 344 /// SimplifyIVUsers helper for eliminating useless remainder operations
 345 /// operating on an induction variable or replacing srem by urem.
 346 void SimplifyIndvar::simplifyIVRemainder(BinaryOperator *Rem, Value *IVOperand,
 347                                          bool IsSigned) {
 348   auto *NValue = Rem->getOperand(0);
 349   auto *DValue = Rem->getOperand(1);
 350   // We're only interested in the case where we know something about
 351   // the numerator, unless it is a srem, because we want to replace srem by urem
 352   // in general.
 353   bool UsedAsNumerator = IVOperand == NValue;
 354   if (!UsedAsNumerator && !IsSigned)
 355     return;
 356
 357   const SCEV *N = SE->getSCEV(NValue);
 358
 359   // Simplify unnecessary loops away.
 360   const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
 361   N = SE->getSCEVAtScope(N, ICmpLoop);
 362
 363   bool IsNumeratorNonNegative = !IsSigned || SE->isKnownNonNegative(N);
 364
 365   // Do not proceed if the Numerator may be negative
 366   if (!IsNumeratorNonNegative)
 367     return;
 368
 369   const SCEV *D = SE->getSCEV(DValue);
 370   D = SE->getSCEVAtScope(D, ICmpLoop);
 371
 372   if (UsedAsNumerator) {
 373     auto LT = IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
 374     if (SE->isKnownPredicate(LT, N, D)) {
 375       replaceRemWithNumerator(Rem);
 376       return;
 377     }
 378
 379     auto *T = Rem->getType();
 380     const auto *NLessOne = SE->getMinusSCEV(N, SE->getOne(T));
 381     if (SE->isKnownPredicate(LT, NLessOne, D)) {
 382       replaceRemWithNumeratorOrZero(Rem);
 383       return;
 384     }
 385   }
 386
 387   // Try to replace SRem with URem, if both N and D are known non-negative.
 388   // Since we had already check N, we only need to check D now
 389   if (!IsSigned || !SE->isKnownNonNegative(D))
 390     return;
 391
 392   replaceSRemWithURem(Rem);
 393 }
 394
 395 bool SimplifyIndvar::eliminateOverflowIntrinsic(CallInst *CI) {
 396   auto *F = CI->getCalledFunction();
 397   if (!F)
 398     return false;
 399
 400   typedef const SCEV *(ScalarEvolution::*OperationFunctionTy)(
 401       const SCEV *, const SCEV *, SCEV::NoWrapFlags, unsigned);
 402   typedef const SCEV *(ScalarEvolution::*ExtensionFunctionTy)(
 403       const SCEV *, Type *, unsigned);
 404
 405   OperationFunctionTy Operation;
 406   ExtensionFunctionTy Extension;
 407
 408   Instruction::BinaryOps RawOp;
 409
 410   // We always have exactly one of nsw or nuw.  If NoSignedOverflow is false, we
 411   // have nuw.
 412   bool NoSignedOverflow;
 413
 414   switch (F->getIntrinsicID()) {
 415   default:
 416     return false;
 417
 418   case Intrinsic::sadd_with_overflow:
 419     Operation = &ScalarEvolution::getAddExpr;
 420     Extension = &ScalarEvolution::getSignExtendExpr;
 421     RawOp = Instruction::Add;
 422     NoSignedOverflow = true;
 423     break;
 424
 425   case Intrinsic::uadd_with_overflow:
 426     Operation = &ScalarEvolution::getAddExpr;
 427     Extension = &ScalarEvolution::getZeroExtendExpr;
 428     RawOp = Instruction::Add;
 429     NoSignedOverflow = false;
 430     break;
 431
 432   case Intrinsic::ssub_with_overflow:
 433     Operation = &ScalarEvolution::getMinusSCEV;
 434     Extension = &ScalarEvolution::getSignExtendExpr;
 435     RawOp = Instruction::Sub;
 436     NoSignedOverflow = true;
 437     break;
 438
 439   case Intrinsic::usub_with_overflow:
 440     Operation = &ScalarEvolution::getMinusSCEV;
 441     Extension = &ScalarEvolution::getZeroExtendExpr;
 442     RawOp = Instruction::Sub;
 443     NoSignedOverflow = false;
 444     break;
 445   }
 446
 447   const SCEV *LHS = SE->getSCEV(CI->getArgOperand(0));
 448   const SCEV *RHS = SE->getSCEV(CI->getArgOperand(1));
 449
 450   auto *NarrowTy = cast<IntegerType>(LHS->getType());
 451   auto *WideTy =
 452     IntegerType::get(NarrowTy->getContext(), NarrowTy->getBitWidth() * 2);
 453
 454   const SCEV *A =
 455       (SE->*Extension)((SE->*Operation)(LHS, RHS, SCEV::FlagAnyWrap, 0),
 456                        WideTy, 0);
 457   const SCEV *B =
 458       (SE->*Operation)((SE->*Extension)(LHS, WideTy, 0),
 459                        (SE->*Extension)(RHS, WideTy, 0), SCEV::FlagAnyWrap, 0);
 460
 461   if (A != B)
 462     return false;
 463
 464   // Proved no overflow, nuke the overflow check and, if possible, the overflow
 465   // intrinsic as well.
 466
 467   BinaryOperator *NewResult = BinaryOperator::Create(
 468       RawOp, CI->getArgOperand(0), CI->getArgOperand(1), "", CI);
 469
 470   if (NoSignedOverflow)
 471     NewResult->setHasNoSignedWrap(true);
 472   else
 473     NewResult->setHasNoUnsignedWrap(true);
 474
 475   SmallVector<ExtractValueInst *, 4> ToDelete;
 476
 477   for (auto *U : CI->users()) {
 478     if (auto *EVI = dyn_cast<ExtractValueInst>(U)) {
 479       if (EVI->getIndices()[0] == 1)
 480         EVI->replaceAllUsesWith(ConstantInt::getFalse(CI->getContext()));
 481       else {
 482         assert(EVI->getIndices()[0] == 0 && "Only two possibilities!");
 483         EVI->replaceAllUsesWith(NewResult);
 484       }
 485       ToDelete.push_back(EVI);
 486     }
 487   }
 488
 489   for (auto *EVI : ToDelete)
 490     EVI->eraseFromParent();
 491
 492   if (CI->use_empty())
 493     CI->eraseFromParent();
 494
 495   return true;
 496 }
 497
 498 bool SimplifyIndvar::eliminateTrunc(TruncInst *TI) {
 499   // It is always legal to replace
 500   //   icmp <pred> i32 trunc(iv), n
 501   // with
 502   //   icmp <pred> i64 sext(trunc(iv)), sext(n), if pred is signed predicate.
 503   // Or with
 504   //   icmp <pred> i64 zext(trunc(iv)), zext(n), if pred is unsigned predicate.
 505   // Or with either of these if pred is an equality predicate.
 506   //
 507   // If we can prove that iv == sext(trunc(iv)) or iv == zext(trunc(iv)) for
 508   // every comparison which uses trunc, it means that we can replace each of
 509   // them with comparison of iv against sext/zext(n). We no longer need trunc
 510   // after that.
 511   //
 512   // TODO: Should we do this if we can widen *some* comparisons, but not all
 513   // of them? Sometimes it is enough to enable other optimizations, but the
 514   // trunc instruction will stay in the loop.
 515   Value *IV = TI->getOperand(0);
 516   Type *IVTy = IV->getType();
 517   const SCEV *IVSCEV = SE->getSCEV(IV);
 518   const SCEV *TISCEV = SE->getSCEV(TI);
 519
 520   // Check if iv == zext(trunc(iv)) and if iv == sext(trunc(iv)). If so, we can
 521   // get rid of trunc
 522   bool DoesSExtCollapse = false;
 523   bool DoesZExtCollapse = false;
 524   if (IVSCEV == SE->getSignExtendExpr(TISCEV, IVTy))
 525     DoesSExtCollapse = true;
 526   if (IVSCEV == SE->getZeroExtendExpr(TISCEV, IVTy))
 527     DoesZExtCollapse = true;
 528
 529   // If neither sext nor zext does collapse, it is not profitable to do any
 530   // transform. Bail.
 531   if (!DoesSExtCollapse && !DoesZExtCollapse)
 532     return false;
 533
 534   // Collect users of the trunc that look like comparisons against invariants.
 535   // Bail if we find something different.
 536   SmallVector<ICmpInst *, 4> ICmpUsers;
 537   for (auto *U : TI->users()) {
 538     // We don't care about users in unreachable blocks.
 539     if (isa<Instruction>(U) &&
 540         !DT->isReachableFromEntry(cast<Instruction>(U)->getParent()))
 541       continue;
 542     if (ICmpInst *ICI = dyn_cast<ICmpInst>(U)) {
 543       if (ICI->getOperand(0) == TI && L->isLoopInvariant(ICI->getOperand(1))) {
 544         assert(L->contains(ICI->getParent()) && "LCSSA form broken?");
 545         // If we cannot get rid of trunc, bail.
 546         if (ICI->isSigned() && !DoesSExtCollapse)
 547           return false;
 548         if (ICI->isUnsigned() && !DoesZExtCollapse)
 549           return false;
 550         // For equality, either signed or unsigned works.
 551         ICmpUsers.push_back(ICI);
 552       } else
 553         return false;
 554     } else
 555       return false;
 556   }
 557
 558   // Replace all comparisons against trunc with comparisons against IV.
 559   for (auto *ICI : ICmpUsers) {
 560     auto *Op1 = ICI->getOperand(1);
 561     Instruction *Ext = nullptr;
 562     // For signed/unsigned predicate, replace the old comparison with comparison
 563     // of immediate IV against sext/zext of the invariant argument. If we can
 564     // use either sext or zext (i.e. we are dealing with equality predicate),
 565     // then prefer zext as a more canonical form.
 566     // TODO: If we see a signed comparison which can be turned into unsigned,
 567     // we can do it here for canonicalization purposes.
 568     if (ICI->isUnsigned() || (ICI->isEquality() && DoesZExtCollapse)) {
 569       assert(DoesZExtCollapse && "Unprofitable zext?");
 570       Ext = new ZExtInst(Op1, IVTy, "zext", ICI);
 571     } else {
 572       assert(DoesSExtCollapse && "Unprofitable sext?");
 573       Ext = new SExtInst(Op1, IVTy, "sext", ICI);
 574     }
 575     bool Changed;
 576     L->makeLoopInvariant(Ext, Changed);
 577     (void)Changed;
 578     ICmpInst *NewICI = new ICmpInst(ICI, ICI->getPredicate(), IV, Ext);
 579     ICI->replaceAllUsesWith(NewICI);
 580     DeadInsts.emplace_back(ICI);
 581   }
 582
 583   // Trunc no longer needed.
 584   TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
 585   DeadInsts.emplace_back(TI);
 586   return true;
 587 }
 588
 589 /// Eliminate an operation that consumes a simple IV and has no observable
 590 /// side-effect given the range of IV values.  IVOperand is guaranteed SCEVable,
 591 /// but UseInst may not be.
 592 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
 593                                      Instruction *IVOperand) {
 594   if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
 595     eliminateIVComparison(ICmp, IVOperand);
 596     return true;
 597   }
 598   if (BinaryOperator *Bin = dyn_cast<BinaryOperator>(UseInst)) {
 599     bool IsSRem = Bin->getOpcode() == Instruction::SRem;
 600     if (IsSRem || Bin->getOpcode() == Instruction::URem) {
 601       simplifyIVRemainder(Bin, IVOperand, IsSRem);
 602       return true;
 603     }
 604
 605     if (Bin->getOpcode() == Instruction::SDiv)
 606       return eliminateSDiv(Bin);
 607   }
 608
 609   if (auto *CI = dyn_cast<CallInst>(UseInst))
 610     if (eliminateOverflowIntrinsic(CI))
 611       return true;
 612
 613   if (auto *TI = dyn_cast<TruncInst>(UseInst))
 614     if (eliminateTrunc(TI))
 615       return true;
 616
 617   if (eliminateIdentitySCEV(UseInst, IVOperand))
 618     return true;
 619
 620   return false;
 621 }
 622
 623 static Instruction *GetLoopInvariantInsertPosition(Loop *L, Instruction *Hint) {
 624   if (auto *BB = L->getLoopPreheader())
 625     return BB->getTerminator();
 626
 627   return Hint;
 628 }
 629
 630 /// Replace the UseInst with a constant if possible.
 631 bool SimplifyIndvar::replaceIVUserWithLoopInvariant(Instruction *I) {
 632   if (!SE->isSCEVable(I->getType()))
 633     return false;
 634
 635   // Get the symbolic expression for this instruction.
 636   const SCEV *S = SE->getSCEV(I);
 637
 638   if (!SE->isLoopInvariant(S, L))
 639     return false;
 640
 641   // Do not generate something ridiculous even if S is loop invariant.
 642   if (Rewriter.isHighCostExpansion(S, L, I))
 643     return false;
 644
 645   auto *IP = GetLoopInvariantInsertPosition(L, I);
 646   auto *Invariant = Rewriter.expandCodeFor(S, I->getType(), IP);
 647
 648   I->replaceAllUsesWith(Invariant);
 649   LLVM_DEBUG(dbgs() << "INDVARS: Replace IV user: " << *I
 650                     << " with loop invariant: " << *S << '\n');
 651   ++NumFoldedUser;
 652   Changed = true;
 653   DeadInsts.emplace_back(I);
 654   return true;
 655 }
 656
 657 /// Eliminate any operation that SCEV can prove is an identity function.
 658 bool SimplifyIndvar::eliminateIdentitySCEV(Instruction *UseInst,
 659                                            Instruction *IVOperand) {
 660   if (!SE->isSCEVable(UseInst->getType()) ||
 661       (UseInst->getType() != IVOperand->getType()) ||
 662       (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
 663     return false;
 664
 665   // getSCEV(X) == getSCEV(Y) does not guarantee that X and Y are related in the
 666   // dominator tree, even if X is an operand to Y.  For instance, in
 667   //
 668   //     %iv = phi i32 {0,+,1}
 669   //     br %cond, label %left, label %merge
 670   //
 671   //   left:
 672   //     %X = add i32 %iv, 0
 673   //     br label %merge
 674   //
 675   //   merge:
 676   //     %M = phi (%X, %iv)
 677   //
 678   // getSCEV(%M) == getSCEV(%X) == {0,+,1}, but %X does not dominate %M, and
 679   // %M.replaceAllUsesWith(%X) would be incorrect.
 680
 681   if (isa<PHINode>(UseInst))
 682     // If UseInst is not a PHI node then we know that IVOperand dominates
 683     // UseInst directly from the legality of SSA.
 684     if (!DT || !DT->dominates(IVOperand, UseInst))
 685       return false;
 686
 687   if (!LI->replacementPreservesLCSSAForm(UseInst, IVOperand))
 688     return false;
 689
 690   LLVM_DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
 691
 692   UseInst->replaceAllUsesWith(IVOperand);
 693   ++NumElimIdentity;
 694   Changed = true;
 695   DeadInsts.emplace_back(UseInst);
 696   return true;
 697 }
 698
 699 /// Annotate BO with nsw / nuw if it provably does not signed-overflow /
 700 /// unsigned-overflow.  Returns true if anything changed, false otherwise.
 701 bool SimplifyIndvar::strengthenOverflowingOperation(BinaryOperator *BO,
 702                                                     Value *IVOperand) {
 703
 704   // Fastpath: we don't have any work to do if `BO` is `nuw` and `nsw`.
 705   if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
 706     return false;
 707
 708   const SCEV *(ScalarEvolution::*GetExprForBO)(const SCEV *, const SCEV *,
 709                                                SCEV::NoWrapFlags, unsigned);
 710   switch (BO->getOpcode()) {
 711   default:
 712     return false;
 713
 714   case Instruction::Add:
 715     GetExprForBO = &ScalarEvolution::getAddExpr;
 716     break;
 717
 718   case Instruction::Sub:
 719     GetExprForBO = &ScalarEvolution::getMinusSCEV;
 720     break;
 721
 722   case Instruction::Mul:
 723     GetExprForBO = &ScalarEvolution::getMulExpr;
 724     break;
 725   }
 726
 727   unsigned BitWidth = cast<IntegerType>(BO->getType())->getBitWidth();
 728   Type *WideTy = IntegerType::get(BO->getContext(), BitWidth * 2);
 729   const SCEV *LHS = SE->getSCEV(BO->getOperand(0));
 730   const SCEV *RHS = SE->getSCEV(BO->getOperand(1));
 731
 732   bool Changed = false;
 733
 734   if (!BO->hasNoUnsignedWrap()) {
 735     const SCEV *ExtendAfterOp = SE->getZeroExtendExpr(SE->getSCEV(BO), WideTy);
 736     const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
 737       SE->getZeroExtendExpr(LHS, WideTy), SE->getZeroExtendExpr(RHS, WideTy),
 738       SCEV::FlagAnyWrap, 0u);
 739     if (ExtendAfterOp == OpAfterExtend) {
 740       BO->setHasNoUnsignedWrap();
 741       SE->forgetValue(BO);
 742       Changed = true;
 743     }
 744   }
 745
 746   if (!BO->hasNoSignedWrap()) {
 747     const SCEV *ExtendAfterOp = SE->getSignExtendExpr(SE->getSCEV(BO), WideTy);
 748     const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
 749       SE->getSignExtendExpr(LHS, WideTy), SE->getSignExtendExpr(RHS, WideTy),
 750       SCEV::FlagAnyWrap, 0u);
 751     if (ExtendAfterOp == OpAfterExtend) {
 752       BO->setHasNoSignedWrap();
 753       SE->forgetValue(BO);
 754       Changed = true;
 755     }
 756   }
 757
 758   return Changed;
 759 }
 760
 761 /// Annotate the Shr in (X << IVOperand) >> C as exact using the
 762 /// information from the IV's range. Returns true if anything changed, false
 763 /// otherwise.
 764 bool SimplifyIndvar::strengthenRightShift(BinaryOperator *BO,
 765                                           Value *IVOperand) {
 766   using namespace llvm::PatternMatch;
 767
 768   if (BO->getOpcode() == Instruction::Shl) {
 769     bool Changed = false;
 770     ConstantRange IVRange = SE->getUnsignedRange(SE->getSCEV(IVOperand));
 771     for (auto *U : BO->users()) {
 772       const APInt *C;
 773       if (match(U,
 774                 m_AShr(m_Shl(m_Value(), m_Specific(IVOperand)), m_APInt(C))) ||
 775           match(U,
 776                 m_LShr(m_Shl(m_Value(), m_Specific(IVOperand)), m_APInt(C)))) {
 777         BinaryOperator *Shr = cast<BinaryOperator>(U);
 778         if (!Shr->isExact() && IVRange.getUnsignedMin().uge(*C)) {
 779           Shr->setIsExact(true);
 780           Changed = true;
 781         }
 782       }
 783     }
 784     return Changed;
 785   }
 786
 787   return false;
 788 }
 789
 790 /// Add all uses of Def to the current IV's worklist.
 791 static void pushIVUsers(
 792   Instruction *Def, Loop *L,
 793   SmallPtrSet<Instruction*,16> &Simplified,
 794   SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
 795
 796   for (User *U : Def->users()) {
 797     Instruction *UI = cast<Instruction>(U);
 798
 799     // Avoid infinite or exponential worklist processing.
 800     // Also ensure unique worklist users.
 801     // If Def is a LoopPhi, it may not be in the Simplified set, so check for
 802     // self edges first.
 803     if (UI == Def)
 804       continue;
 805
 806     // Only change the current Loop, do not change the other parts (e.g. other
 807     // Loops).
 808     if (!L->contains(UI))
 809       continue;
 810
 811     // Do not push the same instruction more than once.
 812     if (!Simplified.insert(UI).second)
 813       continue;
 814
 815     SimpleIVUsers.push_back(std::make_pair(UI, Def));
 816   }
 817 }
 818
 819 /// Return true if this instruction generates a simple SCEV
 820 /// expression in terms of that IV.
 821 ///
 822 /// This is similar to IVUsers' isInteresting() but processes each instruction
 823 /// non-recursively when the operand is already known to be a simpleIVUser.
 824 ///
 825 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
 826   if (!SE->isSCEVable(I->getType()))
 827     return false;
 828
 829   // Get the symbolic expression for this instruction.
 830   const SCEV *S = SE->getSCEV(I);
 831
 832   // Only consider affine recurrences.
 833   const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
 834   if (AR && AR->getLoop() == L)
 835     return true;
 836
 837   return false;
 838 }
 839
 840 /// Iteratively perform simplification on a worklist of users
 841 /// of the specified induction variable. Each successive simplification may push
 842 /// more users which may themselves be candidates for simplification.
 843 ///
 844 /// This algorithm does not require IVUsers analysis. Instead, it simplifies
 845 /// instructions in-place during analysis. Rather than rewriting induction
 846 /// variables bottom-up from their users, it transforms a chain of IVUsers
 847 /// top-down, updating the IR only when it encounters a clear optimization
 848 /// opportunity.
 849 ///
 850 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
 851 ///
 852 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
 853   if (!SE->isSCEVable(CurrIV->getType()))
 854     return;
 855
 856   // Instructions processed by SimplifyIndvar for CurrIV.
 857   SmallPtrSet<Instruction*,16> Simplified;
 858
 859   // Use-def pairs if IV users waiting to be processed for CurrIV.
 860   SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
 861
 862   // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
 863   // called multiple times for the same LoopPhi. This is the proper thing to
 864   // do for loop header phis that use each other.
 865   pushIVUsers(CurrIV, L, Simplified, SimpleIVUsers);
 866
 867   while (!SimpleIVUsers.empty()) {
 868     std::pair<Instruction*, Instruction*> UseOper =
 869       SimpleIVUsers.pop_back_val();
 870     Instruction *UseInst = UseOper.first;
 871
 872     // If a user of the IndVar is trivially dead, we prefer just to mark it dead
 873     // rather than try to do some complex analysis or transformation (such as
 874     // widening) basing on it.
 875     // TODO: Propagate TLI and pass it here to handle more cases.
 876     if (isInstructionTriviallyDead(UseInst, /* TLI */ nullptr)) {
 877       DeadInsts.emplace_back(UseInst);
 878       continue;
 879     }
 880
 881     // Bypass back edges to avoid extra work.
 882     if (UseInst == CurrIV) continue;
 883
 884     // Try to replace UseInst with a loop invariant before any other
 885     // simplifications.
 886     if (replaceIVUserWithLoopInvariant(UseInst))
 887       continue;
 888
 889     Instruction *IVOperand = UseOper.second;
 890     for (unsigned N = 0; IVOperand; ++N) {
 891       assert(N <= Simplified.size() && "runaway iteration");
 892
 893       Value *NewOper = foldIVUser(UseInst, IVOperand);
 894       if (!NewOper)
 895         break; // done folding
 896       IVOperand = dyn_cast<Instruction>(NewOper);
 897     }
 898     if (!IVOperand)
 899       continue;
 900
 901     if (eliminateIVUser(UseInst, IVOperand)) {
 902       pushIVUsers(IVOperand, L, Simplified, SimpleIVUsers);
 903       continue;
 904     }
 905
 906     if (BinaryOperator *BO = dyn_cast<BinaryOperator>(UseInst)) {
 907       if ((isa<OverflowingBinaryOperator>(BO) &&
 908            strengthenOverflowingOperation(BO, IVOperand)) ||
 909           (isa<ShlOperator>(BO) && strengthenRightShift(BO, IVOperand))) {
 910         // re-queue uses of the now modified binary operator and fall
 911         // through to the checks that remain.
 912         pushIVUsers(IVOperand, L, Simplified, SimpleIVUsers);
 913       }
 914     }
 915
 916     CastInst *Cast = dyn_cast<CastInst>(UseInst);
 917     if (V && Cast) {
 918       V->visitCast(Cast);
 919       continue;
 920     }
 921     if (isSimpleIVUser(UseInst, L, SE)) {
 922       pushIVUsers(UseInst, L, Simplified, SimpleIVUsers);
 923     }
 924   }
 925 }
 926
 927 namespace llvm {
 928
 929 void IVVisitor::anchor() { }
 930
 931 /// Simplify instructions that use this induction variable
 932 /// by using ScalarEvolution to analyze the IV's recurrence.
 933 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, DominatorTree *DT,
 934                        LoopInfo *LI, SmallVectorImpl<WeakTrackingVH> &Dead,
 935                        SCEVExpander &Rewriter, IVVisitor *V) {
 936   SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, DT, LI, Rewriter,
 937                      Dead);
 938   SIV.simplifyUsers(CurrIV, V);
 939   return SIV.hasChanged();
 940 }
 941
 942 /// Simplify users of induction variables within this
 943 /// loop. This does not actually change or add IVs.
 944 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, DominatorTree *DT,
 945                      LoopInfo *LI, SmallVectorImpl<WeakTrackingVH> &Dead) {
 946   SCEVExpander Rewriter(*SE, SE->getDataLayout(), "indvars");
 947 #ifndef NDEBUG
 948   Rewriter.setDebugType(DEBUG_TYPE);
 949 #endif
 950   bool Changed = false;
 951   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
 952     Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, DT, LI, Dead, Rewriter);
 953   }
 954   return Changed;
 955 }
 956
 957 } // namespace llvm