Home | History | Annotate | Download | only in Analysis 
      1 //===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
      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 bookkeeping for "interesting" users of expressions
     11 // computed from induction variables.
     12 //
     13 //===----------------------------------------------------------------------===//
     14 
     15 #define DEBUG_TYPE "iv-users"
     16 #include "llvm/Analysis/IVUsers.h"
     17 #include "llvm/Constants.h"
     18 #include "llvm/Instructions.h"
     19 #include "llvm/Type.h"
     20 #include "llvm/DerivedTypes.h"
     21 #include "llvm/Analysis/Dominators.h"
     22 #include "llvm/Analysis/LoopPass.h"
     23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
     24 #include "llvm/Target/TargetData.h"
     25 #include "llvm/Assembly/Writer.h"
     26 #include "llvm/ADT/STLExtras.h"
     27 #include "llvm/Support/Debug.h"
     28 #include "llvm/Support/raw_ostream.h"
     29 #include <algorithm>
     30 using namespace llvm;
     31 
     32 char IVUsers::ID = 0;
     33 INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
     34                       "Induction Variable Users", false, true)
     35 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
     36 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
     37 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
     38 INITIALIZE_PASS_END(IVUsers, "iv-users",
     39                       "Induction Variable Users", false, true)
     40 
     41 Pass *llvm::createIVUsersPass() {
     42   return new IVUsers();
     43 }
     44 
     45 /// isInteresting - Test whether the given expression is "interesting" when
     46 /// used by the given expression, within the context of analyzing the
     47 /// given loop.
     48 static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
     49                           ScalarEvolution *SE, LoopInfo *LI) {
     50   // An addrec is interesting if it's affine or if it has an interesting start.
     51   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
     52     // Keep things simple. Don't touch loop-variant strides unless they're
     53     // only used outside the loop and we can simplify them.
     54     if (AR->getLoop() == L)
     55       return AR->isAffine() ||
     56              (!L->contains(I) &&
     57               SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
     58     // Otherwise recurse to see if the start value is interesting, and that
     59     // the step value is not interesting, since we don't yet know how to
     60     // do effective SCEV expansions for addrecs with interesting steps.
     61     return isInteresting(AR->getStart(), I, L, SE, LI) &&
     62           !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
     63   }
     64 
     65   // An add is interesting if exactly one of its operands is interesting.
     66   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
     67     bool AnyInterestingYet = false;
     68     for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end();
     69          OI != OE; ++OI)
     70       if (isInteresting(*OI, I, L, SE, LI)) {
     71         if (AnyInterestingYet)
     72           return false;
     73         AnyInterestingYet = true;
     74       }
     75     return AnyInterestingYet;
     76   }
     77 
     78   // Nothing else is interesting here.
     79   return false;
     80 }
     81 
     82 /// AddUsersIfInteresting - Inspect the specified instruction.  If it is a
     83 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
     84 /// return true.  Otherwise, return false.
     85 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
     86   if (!SE->isSCEVable(I->getType()))
     87     return false;   // Void and FP expressions cannot be reduced.
     88 
     89   // LSR is not APInt clean, do not touch integers bigger than 64-bits.
     90   // Also avoid creating IVs of non-native types. For example, we don't want a
     91   // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
     92   uint64_t Width = SE->getTypeSizeInBits(I->getType());
     93   if (Width > 64 || (TD && !TD->isLegalInteger(Width)))
     94     return false;
     95 
     96   if (!Processed.insert(I))
     97     return true;    // Instruction already handled.
     98 
     99   // Get the symbolic expression for this instruction.
    100   const SCEV *ISE = SE->getSCEV(I);
    101 
    102   // If we've come to an uninteresting expression, stop the traversal and
    103   // call this a user.
    104   if (!isInteresting(ISE, I, L, SE, LI))
    105     return false;
    106 
    107   SmallPtrSet<Instruction *, 4> UniqueUsers;
    108   for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
    109        UI != E; ++UI) {
    110     Instruction *User = cast<Instruction>(*UI);
    111     if (!UniqueUsers.insert(User))
    112       continue;
    113 
    114     // Do not infinitely recurse on PHI nodes.
    115     if (isa<PHINode>(User) && Processed.count(User))
    116       continue;
    117 
    118     // Descend recursively, but not into PHI nodes outside the current loop.
    119     // It's important to see the entire expression outside the loop to get
    120     // choices that depend on addressing mode use right, although we won't
    121     // consider references outside the loop in all cases.
    122     // If User is already in Processed, we don't want to recurse into it again,
    123     // but do want to record a second reference in the same instruction.
    124     bool AddUserToIVUsers = false;
    125     if (LI->getLoopFor(User->getParent()) != L) {
    126       if (isa<PHINode>(User) || Processed.count(User) ||
    127           !AddUsersIfInteresting(User)) {
    128         DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
    129                      << "   OF SCEV: " << *ISE << '\n');
    130         AddUserToIVUsers = true;
    131       }
    132     } else if (Processed.count(User) || !AddUsersIfInteresting(User)) {
    133       DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
    134                    << "   OF SCEV: " << *ISE << '\n');
    135       AddUserToIVUsers = true;
    136     }
    137 
    138     if (AddUserToIVUsers) {
    139       // Okay, we found a user that we cannot reduce.
    140       IVUses.push_back(new IVStrideUse(this, User, I));
    141       IVStrideUse &NewUse = IVUses.back();
    142       // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
    143       // The regular return value here is discarded; instead of recording
    144       // it, we just recompute it when we need it.
    145       ISE = TransformForPostIncUse(NormalizeAutodetect,
    146                                    ISE, User, I,
    147                                    NewUse.PostIncLoops,
    148                                    *SE, *DT);
    149       DEBUG(dbgs() << "   NORMALIZED TO: " << *ISE << '\n');
    150     }
    151   }
    152   return true;
    153 }
    154 
    155 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
    156   IVUses.push_back(new IVStrideUse(this, User, Operand));
    157   return IVUses.back();
    158 }
    159 
    160 IVUsers::IVUsers()
    161     : LoopPass(ID) {
    162   initializeIVUsersPass(*PassRegistry::getPassRegistry());
    163 }
    164 
    165 void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
    166   AU.addRequired<LoopInfo>();
    167   AU.addRequired<DominatorTree>();
    168   AU.addRequired<ScalarEvolution>();
    169   AU.setPreservesAll();
    170 }
    171 
    172 bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
    173 
    174   L = l;
    175   LI = &getAnalysis<LoopInfo>();
    176   DT = &getAnalysis<DominatorTree>();
    177   SE = &getAnalysis<ScalarEvolution>();
    178   TD = getAnalysisIfAvailable<TargetData>();
    179 
    180   // Find all uses of induction variables in this loop, and categorize
    181   // them by stride.  Start by finding all of the PHI nodes in the header for
    182   // this loop.  If they are induction variables, inspect their uses.
    183   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
    184     (void)AddUsersIfInteresting(I);
    185 
    186   return false;
    187 }
    188 
    189 void IVUsers::print(raw_ostream &OS, const Module *M) const {
    190   OS << "IV Users for loop ";
    191   WriteAsOperand(OS, L->getHeader(), false);
    192   if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
    193     OS << " with backedge-taken count "
    194        << *SE->getBackedgeTakenCount(L);
    195   }
    196   OS << ":\n";
    197 
    198   for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
    199        E = IVUses.end(); UI != E; ++UI) {
    200     OS << "  ";
    201     WriteAsOperand(OS, UI->getOperandValToReplace(), false);
    202     OS << " = " << *getReplacementExpr(*UI);
    203     for (PostIncLoopSet::const_iterator
    204          I = UI->PostIncLoops.begin(),
    205          E = UI->PostIncLoops.end(); I != E; ++I) {
    206       OS << " (post-inc with loop ";
    207       WriteAsOperand(OS, (*I)->getHeader(), false);
    208       OS << ")";
    209     }
    210     OS << " in  ";
    211     UI->getUser()->print(OS);
    212     OS << '\n';
    213   }
    214 }
    215 
    216 void IVUsers::dump() const {
    217   print(dbgs());
    218 }
    219 
    220 void IVUsers::releaseMemory() {
    221   Processed.clear();
    222   IVUses.clear();
    223 }
    224 
    225 /// getReplacementExpr - Return a SCEV expression which computes the
    226 /// value of the OperandValToReplace.
    227 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
    228   return SE->getSCEV(IU.getOperandValToReplace());
    229 }
    230 
    231 /// getExpr - Return the expression for the use.
    232 const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
    233   return
    234     TransformForPostIncUse(Normalize, getReplacementExpr(IU),
    235                            IU.getUser(), IU.getOperandValToReplace(),
    236                            const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
    237                            *SE, *DT);
    238 }
    239 
    240 static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
    241   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
    242     if (AR->getLoop() == L)
    243       return AR;
    244     return findAddRecForLoop(AR->getStart(), L);
    245   }
    246 
    247   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
    248     for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
    249          I != E; ++I)
    250       if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
    251         return AR;
    252     return 0;
    253   }
    254 
    255   return 0;
    256 }
    257 
    258 const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
    259   if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
    260     return AR->getStepRecurrence(*SE);
    261   return 0;
    262 }
    263 
    264 void IVStrideUse::transformToPostInc(const Loop *L) {
    265   PostIncLoops.insert(L);
    266 }
    267 
    268 void IVStrideUse::deleted() {
    269   // Remove this user from the list.
    270   Parent->IVUses.erase(this);
    271   // this now dangles!
    272 }
    273