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      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