// For all selected reductions, remove all parts except those in the first
// iteration (and the PHI). Replace outside uses of the reduced value with uses
// of the first-iteration reduced value (in other words, reroll the selected
// reductions).
void LoopReroll::ReductionTracker::replaceSelected() {
  // Fixup reductions to refer to the last instruction associated with the
  // first iteration (not the last).
  for (DenseSet<int>::iterator RI = Reds.begin(), RIE = Reds.end();
       RI != RIE; ++RI) {
    int i = *RI;
    int j = 0;
    for (int e = PossibleReds[i].size(); j != e; ++j)
      if (PossibleRedIter[PossibleReds[i][j]] != 0) {
        --j;
        break;
      }

    // Replace users with the new end-of-chain value.
    SmallInstructionVector Users;
    for (Value::use_iterator UI =
           PossibleReds[i].getReducedValue()->use_begin(),
         UIE = PossibleReds[i].getReducedValue()->use_end(); UI != UIE; ++UI)
      Users.push_back(cast<Instruction>(*UI));

    for (SmallInstructionVector::iterator J = Users.begin(),
         JE = Users.end(); J != JE; ++J)
      (*J)->replaceUsesOfWith(PossibleReds[i].getReducedValue(),
                              PossibleReds[i][j]);
  }
}
// Collect all of the users of all of the provided root instructions (combined
// into a single set).
void LoopReroll::DAGRootTracker::collectInLoopUserSet(
  const SmallInstructionVector &Roots,
  const SmallInstructionSet &Exclude,
  const SmallInstructionSet &Final,
  DenseSet<Instruction *> &Users) {
  for (SmallInstructionVector::const_iterator I = Roots.begin(),
       IE = Roots.end(); I != IE; ++I)
    collectInLoopUserSet(*I, Exclude, Final, Users);
}
Example #3
0
bool LoopReroll::runOnLoop(Loop *L, LPPassManager &LPM) {
  if (skipOptnoneFunction(L))
    return false;

  AA = &getAnalysis<AliasAnalysis>();
  LI = &getAnalysis<LoopInfo>();
  SE = &getAnalysis<ScalarEvolution>();
  TLI = &getAnalysis<TargetLibraryInfo>();
  DL = getAnalysisIfAvailable<DataLayout>();
  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();

  BasicBlock *Header = L->getHeader();
  DEBUG(dbgs() << "LRR: F[" << Header->getParent()->getName() <<
        "] Loop %" << Header->getName() << " (" <<
        L->getNumBlocks() << " block(s))\n");

  bool Changed = false;

  // For now, we'll handle only single BB loops.
  if (L->getNumBlocks() > 1)
    return Changed;

  if (!SE->hasLoopInvariantBackedgeTakenCount(L))
    return Changed;

  const SCEV *LIBETC = SE->getBackedgeTakenCount(L);
  const SCEV *IterCount =
    SE->getAddExpr(LIBETC, SE->getConstant(LIBETC->getType(), 1));
  DEBUG(dbgs() << "LRR: iteration count = " << *IterCount << "\n");

  // First, we need to find the induction variable with respect to which we can
  // reroll (there may be several possible options).
  SmallInstructionVector PossibleIVs;
  collectPossibleIVs(L, PossibleIVs);

  if (PossibleIVs.empty()) {
    DEBUG(dbgs() << "LRR: No possible IVs found\n");
    return Changed;
  }

  ReductionTracker Reductions;
  collectPossibleReductions(L, Reductions);

  // For each possible IV, collect the associated possible set of 'root' nodes
  // (i+1, i+2, etc.).
  for (SmallInstructionVector::iterator I = PossibleIVs.begin(),
       IE = PossibleIVs.end(); I != IE; ++I)
    if (reroll(*I, L, Header, IterCount, Reductions)) {
      Changed = true;
      break;
    }

  return Changed;
}
// Reroll the provided loop with respect to the provided induction variable.
// Generally, we're looking for a loop like this:
//
// %iv = phi [ (preheader, ...), (body, %iv.next) ]
// f(%iv)
// %iv.1 = add %iv, 1                <-- a root increment
// f(%iv.1)
// %iv.2 = add %iv, 2                <-- a root increment
// f(%iv.2)
// %iv.scale_m_1 = add %iv, scale-1  <-- a root increment
// f(%iv.scale_m_1)
// ...
// %iv.next = add %iv, scale
// %cmp = icmp(%iv, ...)
// br %cmp, header, exit
//
// Notably, we do not require that f(%iv), f(%iv.1), etc. be isolated groups of
// instructions. In other words, the instructions in f(%iv), f(%iv.1), etc. can
// be intermixed with eachother. The restriction imposed by this algorithm is
// that the relative order of the isomorphic instructions in f(%iv), f(%iv.1),
// etc. be the same.
//
// First, we collect the use set of %iv, excluding the other increment roots.
// This gives us f(%iv). Then we iterate over the loop instructions (scale-1)
// times, having collected the use set of f(%iv.(i+1)), during which we:
//   - Ensure that the next unmatched instruction in f(%iv) is isomorphic to
//     the next unmatched instruction in f(%iv.(i+1)).
//   - Ensure that both matched instructions don't have any external users
//     (with the exception of last-in-chain reduction instructions).
//   - Track the (aliasing) write set, and other side effects, of all
//     instructions that belong to future iterations that come before the matched
//     instructions. If the matched instructions read from that write set, then
//     f(%iv) or f(%iv.(i+1)) has some dependency on instructions in
//     f(%iv.(j+1)) for some j > i, and we cannot reroll the loop. Similarly,
//     if any of these future instructions had side effects (could not be
//     speculatively executed), and so do the matched instructions, when we
//     cannot reorder those side-effect-producing instructions, and rerolling
//     fails.
//
// Finally, we make sure that all loop instructions are either loop increment
// roots, belong to simple latch code, parts of validated reductions, part of
// f(%iv) or part of some f(%iv.i). If all of that is true (and all reductions
// have been validated), then we reroll the loop.
bool LoopReroll::reroll(Instruction *IV, Loop *L, BasicBlock *Header,
                        const SCEV *IterCount,
                        ReductionTracker &Reductions) {
  const SCEVAddRecExpr *RealIVSCEV = cast<SCEVAddRecExpr>(SE->getSCEV(IV));
  uint64_t Inc = cast<SCEVConstant>(RealIVSCEV->getOperand(1))->
                   getValue()->getZExtValue();
  // The collection of loop increment instructions.
  SmallInstructionVector LoopIncs;
  uint64_t Scale = Inc;

  // The effective induction variable, IV, is normally also the real induction
  // variable. When we're dealing with a loop like:
  //   for (int i = 0; i < 500; ++i)
  //     x[3*i] = ...;
  //     x[3*i+1] = ...;
  //     x[3*i+2] = ...;
  // then the real IV is still i, but the effective IV is (3*i).
  Instruction *RealIV = IV;
  if (Inc == 1 && !findScaleFromMul(RealIV, Scale, IV, LoopIncs))
    return false;

  assert(Scale <= MaxInc && "Scale is too large");
  assert(Scale > 1 && "Scale must be at least 2");

  // The set of increment instructions for each increment value.
  SmallVector<SmallInstructionVector, 32> Roots(Scale-1);
  SmallInstructionSet AllRoots;
  if (!collectAllRoots(L, Inc, Scale, IV, Roots, AllRoots, LoopIncs))
    return false;

  DEBUG(dbgs() << "LRR: Found all root induction increments for: " <<
                  *RealIV << "\n");

  // An array of just the possible reductions for this scale factor. When we
  // collect the set of all users of some root instructions, these reduction
  // instructions are treated as 'final' (their uses are not considered).
  // This is important because we don't want the root use set to search down
  // the reduction chain.
  SmallInstructionSet PossibleRedSet;
  SmallInstructionSet PossibleRedLastSet, PossibleRedPHISet;
  Reductions.restrictToScale(Scale, PossibleRedSet, PossibleRedPHISet,
                             PossibleRedLastSet);

  // We now need to check for equivalence of the use graph of each root with
  // that of the primary induction variable (excluding the roots). Our goal
  // here is not to solve the full graph isomorphism problem, but rather to
  // catch common cases without a lot of work. As a result, we will assume
  // that the relative order of the instructions in each unrolled iteration
  // is the same (although we will not make an assumption about how the
  // different iterations are intermixed). Note that while the order must be
  // the same, the instructions may not be in the same basic block.
  SmallInstructionSet Exclude(AllRoots);
  Exclude.insert(LoopIncs.begin(), LoopIncs.end());

  DenseSet<Instruction *> BaseUseSet;
  collectInLoopUserSet(L, IV, Exclude, PossibleRedSet, BaseUseSet);

  DenseSet<Instruction *> AllRootUses;
  std::vector<DenseSet<Instruction *> > RootUseSets(Scale-1);

  bool MatchFailed = false;
  for (unsigned i = 0; i < Scale-1 && !MatchFailed; ++i) {
    DenseSet<Instruction *> &RootUseSet = RootUseSets[i];
    collectInLoopUserSet(L, Roots[i], SmallInstructionSet(),
                         PossibleRedSet, RootUseSet);

    DEBUG(dbgs() << "LRR: base use set size: " << BaseUseSet.size() <<
                    " vs. iteration increment " << (i+1) <<
                    " use set size: " << RootUseSet.size() << "\n");

    if (BaseUseSet.size() != RootUseSet.size()) {
      MatchFailed = true;
      break;
    }

    // In addition to regular aliasing information, we need to look for
    // instructions from later (future) iterations that have side effects
    // preventing us from reordering them past other instructions with side
    // effects.
    bool FutureSideEffects = false;
    AliasSetTracker AST(*AA);

    // The map between instructions in f(%iv.(i+1)) and f(%iv).
    DenseMap<Value *, Value *> BaseMap;

    assert(L->getNumBlocks() == 1 && "Cannot handle multi-block loops");
    for (BasicBlock::iterator J1 = Header->begin(), J2 = Header->begin(),
         JE = Header->end(); J1 != JE && !MatchFailed; ++J1) {
      if (cast<Instruction>(J1) == RealIV)
        continue;
      if (cast<Instruction>(J1) == IV)
        continue;
      if (!BaseUseSet.count(J1))
        continue;
      if (PossibleRedPHISet.count(J1)) // Skip reduction PHIs.
        continue;

      while (J2 != JE && (!RootUseSet.count(J2) ||
             std::find(Roots[i].begin(), Roots[i].end(), J2) !=
               Roots[i].end())) {
        // As we iterate through the instructions, instructions that don't
        // belong to previous iterations (or the base case), must belong to
        // future iterations. We want to track the alias set of writes from
        // previous iterations.
        if (!isa<PHINode>(J2) && !BaseUseSet.count(J2) &&
            !AllRootUses.count(J2)) {
          if (J2->mayWriteToMemory())
            AST.add(J2);

          // Note: This is specifically guarded by a check on isa<PHINode>,
          // which while a valid (somewhat arbitrary) micro-optimization, is
          // needed because otherwise isSafeToSpeculativelyExecute returns
          // false on PHI nodes.
          if (!isSimpleLoadStore(J2) && !isSafeToSpeculativelyExecute(J2, DL))
            FutureSideEffects = true; 
        }

        ++J2;
      }

      if (!J1->isSameOperationAs(J2)) {
        DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                        " vs. " << *J2 << "\n");
        MatchFailed = true;
        break;
      }

      // Make sure that this instruction, which is in the use set of this
      // root instruction, does not also belong to the base set or the set of
      // some previous root instruction.
      if (BaseUseSet.count(J2) || AllRootUses.count(J2)) {
        DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                        " vs. " << *J2 << " (prev. case overlap)\n");
        MatchFailed = true;
        break;
      }

      // Make sure that we don't alias with any instruction in the alias set
      // tracker. If we do, then we depend on a future iteration, and we
      // can't reroll.
      if (J2->mayReadFromMemory()) {
        for (AliasSetTracker::iterator K = AST.begin(), KE = AST.end();
             K != KE && !MatchFailed; ++K) {
          if (K->aliasesUnknownInst(J2, *AA)) {
            DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                            " vs. " << *J2 << " (depends on future store)\n");
            MatchFailed = true;
            break;
          }
        }
      }

      // If we've past an instruction from a future iteration that may have
      // side effects, and this instruction might also, then we can't reorder
      // them, and this matching fails. As an exception, we allow the alias
      // set tracker to handle regular (simple) load/store dependencies.
      if (FutureSideEffects &&
            ((!isSimpleLoadStore(J1) && !isSafeToSpeculativelyExecute(J1)) ||
             (!isSimpleLoadStore(J2) && !isSafeToSpeculativelyExecute(J2)))) {
        DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                        " vs. " << *J2 <<
                        " (side effects prevent reordering)\n");
        MatchFailed = true;
        break;
      }

      // For instructions that are part of a reduction, if the operation is
      // associative, then don't bother matching the operands (because we
      // already know that the instructions are isomorphic, and the order
      // within the iteration does not matter). For non-associative reductions,
      // we do need to match the operands, because we need to reject
      // out-of-order instructions within an iteration!
      // For example (assume floating-point addition), we need to reject this:
      //   x += a[i]; x += b[i];
      //   x += a[i+1]; x += b[i+1];
      //   x += b[i+2]; x += a[i+2];
      bool InReduction = Reductions.isPairInSame(J1, J2);

      if (!(InReduction && J1->isAssociative())) {
        bool Swapped = false, SomeOpMatched = false;;
        for (unsigned j = 0; j < J1->getNumOperands() && !MatchFailed; ++j) {
          Value *Op2 = J2->getOperand(j);

	  // If this is part of a reduction (and the operation is not
	  // associatve), then we match all operands, but not those that are
	  // part of the reduction.
          if (InReduction)
            if (Instruction *Op2I = dyn_cast<Instruction>(Op2))
              if (Reductions.isPairInSame(J2, Op2I))
                continue;

          DenseMap<Value *, Value *>::iterator BMI = BaseMap.find(Op2);
          if (BMI != BaseMap.end())
            Op2 = BMI->second;
          else if (std::find(Roots[i].begin(), Roots[i].end(),
                             (Instruction*) Op2) != Roots[i].end())
            Op2 = IV;

          if (J1->getOperand(Swapped ? unsigned(!j) : j) != Op2) {
	    // If we've not already decided to swap the matched operands, and
	    // we've not already matched our first operand (note that we could
	    // have skipped matching the first operand because it is part of a
	    // reduction above), and the instruction is commutative, then try
	    // the swapped match.
            if (!Swapped && J1->isCommutative() && !SomeOpMatched &&
                J1->getOperand(!j) == Op2) {
              Swapped = true;
            } else {
              DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                              " vs. " << *J2 << " (operand " << j << ")\n");
              MatchFailed = true;
              break;
            }
          }

          SomeOpMatched = true;
        }
      }

      if ((!PossibleRedLastSet.count(J1) && hasUsesOutsideLoop(J1, L)) ||
          (!PossibleRedLastSet.count(J2) && hasUsesOutsideLoop(J2, L))) {
        DEBUG(dbgs() << "LRR: iteration root match failed at " << *J1 <<
                        " vs. " << *J2 << " (uses outside loop)\n");
        MatchFailed = true;
        break;
      }

      if (!MatchFailed)
        BaseMap.insert(std::pair<Value *, Value *>(J2, J1));

      AllRootUses.insert(J2);
      Reductions.recordPair(J1, J2, i+1);

      ++J2;
    }
  }

  if (MatchFailed)
    return false;

  DEBUG(dbgs() << "LRR: Matched all iteration increments for " <<
                  *RealIV << "\n");

  DenseSet<Instruction *> LoopIncUseSet;
  collectInLoopUserSet(L, LoopIncs, SmallInstructionSet(),
                       SmallInstructionSet(), LoopIncUseSet);
  DEBUG(dbgs() << "LRR: Loop increment set size: " <<
                  LoopIncUseSet.size() << "\n");

  // Make sure that all instructions in the loop have been included in some
  // use set.
  for (BasicBlock::iterator J = Header->begin(), JE = Header->end();
       J != JE; ++J) {
    if (isa<DbgInfoIntrinsic>(J))
      continue;
    if (cast<Instruction>(J) == RealIV)
      continue;
    if (cast<Instruction>(J) == IV)
      continue;
    if (BaseUseSet.count(J) || AllRootUses.count(J) ||
        (LoopIncUseSet.count(J) && (J->isTerminator() ||
                                    isSafeToSpeculativelyExecute(J, DL))))
      continue;

    if (AllRoots.count(J))
      continue;

    if (Reductions.isSelectedPHI(J))
      continue;

    DEBUG(dbgs() << "LRR: aborting reroll based on " << *RealIV <<
                    " unprocessed instruction found: " << *J << "\n");
    MatchFailed = true;
    break;
  }

  if (MatchFailed)
    return false;

  DEBUG(dbgs() << "LRR: all instructions processed from " <<
                  *RealIV << "\n");

  if (!Reductions.validateSelected())
    return false;

  // At this point, we've validated the rerolling, and we're committed to
  // making changes!

  Reductions.replaceSelected();

  // Remove instructions associated with non-base iterations.
  for (BasicBlock::reverse_iterator J = Header->rbegin();
       J != Header->rend();) {
    if (AllRootUses.count(&*J)) {
      Instruction *D = &*J;
      DEBUG(dbgs() << "LRR: removing: " << *D << "\n");
      D->eraseFromParent();
      continue;
    }

    ++J; 
  }

  // Insert the new induction variable.
  const SCEV *Start = RealIVSCEV->getStart();
  if (Inc == 1)
    Start = SE->getMulExpr(Start,
                           SE->getConstant(Start->getType(), Scale));
  const SCEVAddRecExpr *H =
    cast<SCEVAddRecExpr>(SE->getAddRecExpr(Start,
                           SE->getConstant(RealIVSCEV->getType(), 1),
                           L, SCEV::FlagAnyWrap));
  { // Limit the lifetime of SCEVExpander.
    SCEVExpander Expander(*SE, "reroll");
    Value *NewIV = Expander.expandCodeFor(H, IV->getType(), Header->begin());

    for (DenseSet<Instruction *>::iterator J = BaseUseSet.begin(),
         JE = BaseUseSet.end(); J != JE; ++J)
      (*J)->replaceUsesOfWith(IV, NewIV);

    if (BranchInst *BI = dyn_cast<BranchInst>(Header->getTerminator())) {
      if (LoopIncUseSet.count(BI)) {
        const SCEV *ICSCEV = RealIVSCEV->evaluateAtIteration(IterCount, *SE);
        if (Inc == 1)
          ICSCEV =
            SE->getMulExpr(ICSCEV, SE->getConstant(ICSCEV->getType(), Scale));
        // Iteration count SCEV minus 1
        const SCEV *ICMinus1SCEV =
          SE->getMinusSCEV(ICSCEV, SE->getConstant(ICSCEV->getType(), 1));

        Value *ICMinus1; // Iteration count minus 1
        if (isa<SCEVConstant>(ICMinus1SCEV)) {
          ICMinus1 = Expander.expandCodeFor(ICMinus1SCEV, NewIV->getType(), BI);
        } else {
          BasicBlock *Preheader = L->getLoopPreheader();
          if (!Preheader)
            Preheader = InsertPreheaderForLoop(L, this);

          ICMinus1 = Expander.expandCodeFor(ICMinus1SCEV, NewIV->getType(),
                                            Preheader->getTerminator());
        }
 
        Value *Cond = new ICmpInst(BI, CmpInst::ICMP_EQ, NewIV, ICMinus1,
                                   "exitcond");
        BI->setCondition(Cond);

        if (BI->getSuccessor(1) != Header)
          BI->swapSuccessors();
      }
    }
  }

  SimplifyInstructionsInBlock(Header, DL, TLI);
  DeleteDeadPHIs(Header, TLI);
  ++NumRerolledLoops;
  return true;
}