IfConversion.cpp

static void UpdatePredRedefs(MachineInstr *MI, SmallSet<unsigned,4> &Redefs,
                             const TargetRegisterInfo *TRI,
                             bool AddImpUse = false) {
  SmallVector<unsigned, 4> Defs;
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI->getOperand(i);
    if (!MO.isReg())
      continue;
    unsigned Reg = MO.getReg();
    if (!Reg)
      continue;
    if (MO.isDef())
      Defs.push_back(Reg);
    else if (MO.isKill()) {
      Redefs.erase(Reg);
      for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
        Redefs.erase(*SR);
    }
  }
  for (unsigned i = 0, e = Defs.size(); i != e; ++i) {
    unsigned Reg = Defs[i];
    if (Redefs.count(Reg)) {
      if (AddImpUse)
        // Treat predicated update as read + write.
        MI->addOperand(MachineOperand::CreateReg(Reg, false/*IsDef*/,
                                                true/*IsImp*/,false/*IsKill*/));
    } else {
      Redefs.insert(Reg);
      for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
        Redefs.insert(*SR);
    }
  }
}

static void UpdatePredRedefs(MachineBasicBlock::iterator I,
                             MachineBasicBlock::iterator E,
                             SmallSet<unsigned,4> &Redefs,
                             const TargetRegisterInfo *TRI) {
  while (I != E) {
    UpdatePredRedefs(I, Redefs, TRI);
    ++I;
  }
}

/// IfConvertSimple - If convert a simple (split, no rejoin) sub-CFG.
///
bool IfConverter::IfConvertSimple(BBInfo &BBI, IfcvtKind Kind) {
  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
  BBInfo *CvtBBI = &TrueBBI;
  BBInfo *NextBBI = &FalseBBI;

  SmallVector<MachineOperand, 4> Cond(BBI.BrCond.begin(), BBI.BrCond.end());
  if (Kind == ICSimpleFalse)
    std::swap(CvtBBI, NextBBI);

  if (CvtBBI->IsDone ||
      (CvtBBI->CannotBeCopied && CvtBBI->BB->pred_size() > 1)) {
    // Something has changed. It's no longer safe to predicate this block.
    BBI.IsAnalyzed = false;
    CvtBBI->IsAnalyzed = false;
    return false;
  }

  if (Kind == ICSimpleFalse)
    if (TII->ReverseBranchCondition(Cond))
      assert(false && "Unable to reverse branch condition!");

  // Initialize liveins to the first BB. These are potentiall redefined by
  // predicated instructions.
  SmallSet<unsigned, 4> Redefs;
  InitPredRedefs(CvtBBI->BB, Redefs, TRI);
  InitPredRedefs(NextBBI->BB, Redefs, TRI);

  if (CvtBBI->BB->pred_size() > 1) {
    BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
    // Copy instructions in the true block, predicate them, and add them to
    // the entry block.
    CopyAndPredicateBlock(BBI, *CvtBBI, Cond, Redefs);
  } else {
    PredicateBlock(*CvtBBI, CvtBBI->BB->end(), Cond, Redefs);

    // Merge converted block into entry block.
    BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
    MergeBlocks(BBI, *CvtBBI);
  }

  bool IterIfcvt = true;
  if (!canFallThroughTo(BBI.BB, NextBBI->BB)) {
    InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
    BBI.HasFallThrough = false;
    // Now ifcvt'd block will look like this:
    // BB:
    // ...
    // t, f = cmp
    // if t op
    // b BBf
    //
    // We cannot further ifcvt this block because the unconditional branch
    // will have to be predicated on the new condition, that will not be
    // available if cmp executes.
    IterIfcvt = false;
  }

  RemoveExtraEdges(BBI);

  // Update block info. BB can be iteratively if-converted.
  if (!IterIfcvt)
    BBI.IsDone = true;
  InvalidatePreds(BBI.BB);
  CvtBBI->IsDone = true;

  // FIXME: Must maintain LiveIns.
  return true;
}

/// IfConvertTriangle - If convert a triangle sub-CFG.
///
bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
  BBInfo *CvtBBI = &TrueBBI;
  BBInfo *NextBBI = &FalseBBI;
  DebugLoc dl;  // FIXME: this is nowhere

  SmallVector<MachineOperand, 4> Cond(BBI.BrCond.begin(), BBI.BrCond.end());
  if (Kind == ICTriangleFalse || Kind == ICTriangleFRev)
    std::swap(CvtBBI, NextBBI);

  if (CvtBBI->IsDone ||
      (CvtBBI->CannotBeCopied && CvtBBI->BB->pred_size() > 1)) {
    // Something has changed. It's no longer safe to predicate this block.
    BBI.IsAnalyzed = false;
    CvtBBI->IsAnalyzed = false;
    return false;
  }

  if (Kind == ICTriangleFalse || Kind == ICTriangleFRev)
    if (TII->ReverseBranchCondition(Cond))
      assert(false && "Unable to reverse branch condition!");

  if (Kind == ICTriangleRev || Kind == ICTriangleFRev) {
    if (ReverseBranchCondition(*CvtBBI)) {
      // BB has been changed, modify its predecessors (except for this
      // one) so they don't get ifcvt'ed based on bad intel.
      for (MachineBasicBlock::pred_iterator PI = CvtBBI->BB->pred_begin(),
             E = CvtBBI->BB->pred_end(); PI != E; ++PI) {
        MachineBasicBlock *PBB = *PI;
        if (PBB == BBI.BB)
          continue;
        BBInfo &PBBI = BBAnalysis[PBB->getNumber()];
        if (PBBI.IsEnqueued) {
          PBBI.IsAnalyzed = false;
          PBBI.IsEnqueued = false;
        }
      }
    }
  }

  // Initialize liveins to the first BB. These are potentially redefined by
  // predicated instructions.
  SmallSet<unsigned, 4> Redefs;
  InitPredRedefs(CvtBBI->BB, Redefs, TRI);
  InitPredRedefs(NextBBI->BB, Redefs, TRI);

  bool HasEarlyExit = CvtBBI->FalseBB != NULL;
  if (CvtBBI->BB->pred_size() > 1) {
    BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
    // Copy instructions in the true block, predicate them, and add them to
    // the entry block.
    CopyAndPredicateBlock(BBI, *CvtBBI, Cond, Redefs, true);
  } else {
    // Predicate the 'true' block after removing its branch.
    CvtBBI->NonPredSize -= TII->RemoveBranch(*CvtBBI->BB);
    PredicateBlock(*CvtBBI, CvtBBI->BB->end(), Cond, Redefs);

    // Now merge the entry of the triangle with the true block.
    BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);
    MergeBlocks(BBI, *CvtBBI, false);
  }

  // If 'true' block has a 'false' successor, add an exit branch to it.
  if (HasEarlyExit) {
    SmallVector<MachineOperand, 4> RevCond(CvtBBI->BrCond.begin(),
                                           CvtBBI->BrCond.end());
    if (TII->ReverseBranchCondition(RevCond))
      assert(false && "Unable to reverse branch condition!");
    TII->InsertBranch(*BBI.BB, CvtBBI->FalseBB, NULL, RevCond, dl);
    BBI.BB->addSuccessor(CvtBBI->FalseBB);
  }

  // Merge in the 'false' block if the 'false' block has no other
  // predecessors. Otherwise, add an unconditional branch to 'false'.
  bool FalseBBDead = false;
  bool IterIfcvt = true;
  bool isFallThrough = canFallThroughTo(BBI.BB, NextBBI->BB);
  if (!isFallThrough) {
    // Only merge them if the true block does not fallthrough to the false
    // block. By not merging them, we make it possible to iteratively
    // ifcvt the blocks.
    if (!HasEarlyExit &&
        NextBBI->BB->pred_size() == 1 && !NextBBI->HasFallThrough) {
      MergeBlocks(BBI, *NextBBI);
      FalseBBDead = true;
    } else {
      InsertUncondBranch(BBI.BB, NextBBI->BB, TII);
      BBI.HasFallThrough = false;
    }
    // Mixed predicated and unpredicated code. This cannot be iteratively
    // predicated.
    IterIfcvt = false;
  }

  RemoveExtraEdges(BBI);

  // Update block info. BB can be iteratively if-converted.
  if (!IterIfcvt)
    BBI.IsDone = true;
  InvalidatePreds(BBI.BB);
  CvtBBI->IsDone = true;
  if (FalseBBDead)
    NextBBI->IsDone = true;

  // FIXME: Must maintain LiveIns.
  return true;
}

/// IfConvertDiamond - If convert a diamond sub-CFG.
///
bool IfConverter::IfConvertDiamond(BBInfo &BBI, IfcvtKind Kind,
                                   unsigned NumDups1, unsigned NumDups2) {
  BBInfo &TrueBBI  = BBAnalysis[BBI.TrueBB->getNumber()];
  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
  MachineBasicBlock *TailBB = TrueBBI.TrueBB;
  // True block must fall through or end with an unanalyzable terminator.
  if (!TailBB) {
    if (blockAlwaysFallThrough(TrueBBI))
      TailBB = FalseBBI.TrueBB;
    assert((TailBB || !TrueBBI.IsBrAnalyzable) && "Unexpected!");
  }

  if (TrueBBI.IsDone || FalseBBI.IsDone ||
      TrueBBI.BB->pred_size() > 1 ||
      FalseBBI.BB->pred_size() > 1) {
    // Something has changed. It's no longer safe to predicate these blocks.
    BBI.IsAnalyzed = false;
    TrueBBI.IsAnalyzed = false;
    FalseBBI.IsAnalyzed = false;
    return false;
  }

  // Put the predicated instructions from the 'true' block before the
  // instructions from the 'false' block, unless the true block would clobber
  // the predicate, in which case, do the opposite.
  BBInfo *BBI1 = &TrueBBI;
  BBInfo *BBI2 = &FalseBBI;
  SmallVector<MachineOperand, 4> RevCond(BBI.BrCond.begin(), BBI.BrCond.end());
  if (TII->ReverseBranchCondition(RevCond))
    assert(false && "Unable to reverse branch condition!");
  SmallVector<MachineOperand, 4> *Cond1 = &BBI.BrCond;
  SmallVector<MachineOperand, 4> *Cond2 = &RevCond;

  // Figure out the more profitable ordering.
  bool DoSwap = false;
  if (TrueBBI.ClobbersPred && !FalseBBI.ClobbersPred)
    DoSwap = true;
  else if (TrueBBI.ClobbersPred == FalseBBI.ClobbersPred) {
    if (TrueBBI.NonPredSize > FalseBBI.NonPredSize)
      DoSwap = true;
  }
  if (DoSwap) {
    std::swap(BBI1, BBI2);
    std::swap(Cond1, Cond2);
  }

  // Remove the conditional branch from entry to the blocks.
  BBI.NonPredSize -= TII->RemoveBranch(*BBI.BB);

  // Initialize liveins to the first BB. These are potentially redefined by
  // predicated instructions.
  SmallSet<unsigned, 4> Redefs;
  InitPredRedefs(BBI1->BB, Redefs, TRI);

  // Remove the duplicated instructions at the beginnings of both paths.
  MachineBasicBlock::iterator DI1 = BBI1->BB->begin();
  MachineBasicBlock::iterator DI2 = BBI2->BB->begin();
  MachineBasicBlock::iterator DIE1 = BBI1->BB->end();
  MachineBasicBlock::iterator DIE2 = BBI2->BB->end();
  // Skip dbg_value instructions
  while (DI1 != DIE1 && DI1->isDebugValue())
    ++DI1;
  while (DI2 != DIE2 && DI2->isDebugValue())
    ++DI2;
  BBI1->NonPredSize -= NumDups1;
  BBI2->NonPredSize -= NumDups1;

  // Skip past the dups on each side separately since there may be
  // differing dbg_value entries.
  for (unsigned i = 0; i < NumDups1; ++DI1) {
    if (!DI1->isDebugValue())
      ++i;
  }
  while (NumDups1 != 0) {
    ++DI2;
    if (!DI2->isDebugValue())
      --NumDups1;
  }

  UpdatePredRedefs(BBI1->BB->begin(), DI1, Redefs, TRI);
  BBI.BB->splice(BBI.BB->end(), BBI1->BB, BBI1->BB->begin(), DI1);
  BBI2->BB->erase(BBI2->BB->begin(), DI2);

  // Predicate the 'true' block after removing its branch.
  BBI1->NonPredSize -= TII->RemoveBranch(*BBI1->BB);
  DI1 = BBI1->BB->end();
  for (unsigned i = 0; i != NumDups2; ) {
    // NumDups2 only counted non-dbg_value instructions, so this won't
    // run off the head of the list.
    assert (DI1 != BBI1->BB->begin());
    --DI1;
    // skip dbg_value instructions
    if (!DI1->isDebugValue())
      ++i;
  }
  BBI1->BB->erase(DI1, BBI1->BB->end());
  PredicateBlock(*BBI1, BBI1->BB->end(), *Cond1, Redefs);

  // Predicate the 'false' block.
  BBI2->NonPredSize -= TII->RemoveBranch(*BBI2->BB);
  DI2 = BBI2->BB->end();
  while (NumDups2 != 0) {
    // NumDups2 only counted non-dbg_value instructions, so this won't
    // run off the head of the list.
    assert (DI2 != BBI2->BB->begin());
    --DI2;
    // skip dbg_value instructions
    if (!DI2->isDebugValue())
      --NumDups2;
  }
  PredicateBlock(*BBI2, DI2, *Cond2, Redefs);

  // Merge the true block into the entry of the diamond.
  MergeBlocks(BBI, *BBI1, TailBB == 0);
  MergeBlocks(BBI, *BBI2, TailBB == 0);

  // If the if-converted block falls through or unconditionally branches into
  // the tail block, and the tail block does not have other predecessors, then
  // fold the tail block in as well. Otherwise, unless it falls through to the
  // tail, add a unconditional branch to it.
  if (TailBB) {
    BBInfo TailBBI = BBAnalysis[TailBB->getNumber()];
    bool CanMergeTail = !TailBBI.HasFallThrough;
    // There may still be a fall-through edge from BBI1 or BBI2 to TailBB;
    // check if there are any other predecessors besides those.
    unsigned NumPreds = TailBB->pred_size();
    if (NumPreds > 1)
      CanMergeTail = false;
    else if (NumPreds == 1 && CanMergeTail) {
      MachineBasicBlock::pred_iterator PI = TailBB->pred_begin();
      if (*PI != BBI1->BB && *PI != BBI2->BB)
        CanMergeTail = false;
    }
    if (CanMergeTail) {
      MergeBlocks(BBI, TailBBI);
      TailBBI.IsDone = true;
    } else {
      BBI.BB->addSuccessor(TailBB);
      InsertUncondBranch(BBI.BB, TailBB, TII);
      BBI.HasFallThrough = false;
    }
  }

  // RemoveExtraEdges won't work if the block has an unanalyzable branch,
  // which can happen here if TailBB is unanalyzable and is merged, so
  // explicitly remove BBI1 and BBI2 as successors.
  BBI.BB->removeSuccessor(BBI1->BB);
  BBI.BB->removeSuccessor(BBI2->BB);
  RemoveExtraEdges(BBI);

  // Update block info.
  BBI.IsDone = TrueBBI.IsDone = FalseBBI.IsDone = true;
  InvalidatePreds(BBI.BB);

  // FIXME: Must maintain LiveIns.
  return true;
}

/// PredicateBlock - Predicate instructions from the start of the block to the
/// specified end with the specified condition.
void IfConverter::PredicateBlock(BBInfo &BBI,
                                 MachineBasicBlock::iterator E,
                                 SmallVectorImpl<MachineOperand> &Cond,
                                 SmallSet<unsigned, 4> &Redefs) {
  for (MachineBasicBlock::iterator I = BBI.BB->begin(); I != E; ++I) {
    if (I->isDebugValue() || TII->isPredicated(I))
      continue;
    if (!TII->PredicateInstruction(I, Cond)) {
#ifndef NDEBUG
      dbgs() << "Unable to predicate " << *I << "!\n";
#endif
      llvm_unreachable(0);
    }

    // If the predicated instruction now redefines a register as the result of
    // if-conversion, add an implicit kill.
    UpdatePredRedefs(I, Redefs, TRI, true);
  }

  std::copy(Cond.begin(), Cond.end(), std::back_inserter(BBI.Predicate));

  BBI.IsAnalyzed = false;
  BBI.NonPredSize = 0;

  ++NumIfConvBBs;
}

/// CopyAndPredicateBlock - Copy and predicate instructions from source BB to
/// the destination block. Skip end of block branches if IgnoreBr is true.
void IfConverter::CopyAndPredicateBlock(BBInfo &ToBBI, BBInfo &FromBBI,
                                        SmallVectorImpl<MachineOperand> &Cond,
                                        SmallSet<unsigned, 4> &Redefs,
                                        bool IgnoreBr) {
  MachineFunction &MF = *ToBBI.BB->getParent();

  for (MachineBasicBlock::iterator I = FromBBI.BB->begin(),
         E = FromBBI.BB->end(); I != E; ++I) {
    const TargetInstrDesc &TID = I->getDesc();
    // Do not copy the end of the block branches.
    if (IgnoreBr && TID.isBranch())
      break;

    MachineInstr *MI = MF.CloneMachineInstr(I);
    ToBBI.BB->insert(ToBBI.BB->end(), MI);
    ToBBI.NonPredSize++;
    unsigned NumOps = TII->getNumMicroOps(MI, InstrItins);
    if (NumOps > 1)
      ToBBI.ExtraCost += NumOps-1;

    if (!TII->isPredicated(I) && !MI->isDebugValue()) {
      if (!TII->PredicateInstruction(MI, Cond)) {
#ifndef NDEBUG
        dbgs() << "Unable to predicate " << *I << "!\n";
#endif
        llvm_unreachable(0);
      }
    }

    // If the predicated instruction now redefines a register as the result of
    // if-conversion, add an implicit kill.
    UpdatePredRedefs(MI, Redefs, TRI, true);
  }

  if (!IgnoreBr) {
    std::vector<MachineBasicBlock *> Succs(FromBBI.BB->succ_begin(),
                                           FromBBI.BB->succ_end());
    MachineBasicBlock *NBB = getNextBlock(FromBBI.BB);
    MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : NULL;

    for (unsigned i = 0, e = Succs.size(); i != e; ++i) {
      MachineBasicBlock *Succ = Succs[i];
      // Fallthrough edge can't be transferred.
      if (Succ == FallThrough)
        continue;
      ToBBI.BB->addSuccessor(Succ);
    }
  }

  std::copy(FromBBI.Predicate.begin(), FromBBI.Predicate.end(),
            std::back_inserter(ToBBI.Predicate));
  std::copy(Cond.begin(), Cond.end(), std::back_inserter(ToBBI.Predicate));

  ToBBI.ClobbersPred |= FromBBI.ClobbersPred;
  ToBBI.IsAnalyzed = false;

  ++NumDupBBs;
}

/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
/// This will leave FromBB as an empty block, so remove all of its
/// successor edges except for the fall-through edge.  If AddEdges is true,
/// i.e., when FromBBI's branch is being moved, add those successor edges to
/// ToBBI.
void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI, bool AddEdges) {
  ToBBI.BB->splice(ToBBI.BB->end(),
                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());

  std::vector<MachineBasicBlock *> Succs(FromBBI.BB->succ_begin(),
                                         FromBBI.BB->succ_end());
  MachineBasicBlock *NBB = getNextBlock(FromBBI.BB);
  MachineBasicBlock *FallThrough = FromBBI.HasFallThrough ? NBB : NULL;

  for (unsigned i = 0, e = Succs.size(); i != e; ++i) {
    MachineBasicBlock *Succ = Succs[i];
    // Fallthrough edge can't be transferred.
    if (Succ == FallThrough)
      continue;
    FromBBI.BB->removeSuccessor(Succ);
    if (AddEdges)
      ToBBI.BB->addSuccessor(Succ);
  }

  // Now FromBBI always falls through to the next block!
  if (NBB && !FromBBI.BB->isSuccessor(NBB))
    FromBBI.BB->addSuccessor(NBB);

  std::copy(FromBBI.Predicate.begin(), FromBBI.Predicate.end(),
            std::back_inserter(ToBBI.Predicate));
  FromBBI.Predicate.clear();

  ToBBI.NonPredSize += FromBBI.NonPredSize;
  ToBBI.ExtraCost += FromBBI.ExtraCost;
  FromBBI.NonPredSize = 0;
  FromBBI.ExtraCost = 0;

  ToBBI.ClobbersPred |= FromBBI.ClobbersPred;
  ToBBI.HasFallThrough = FromBBI.HasFallThrough;
  ToBBI.IsAnalyzed = false;
  FromBBI.IsAnalyzed = false;
}