BranchFolding.cpp

/// branched to and do not fall through. Tail-duplicate their instructions
/// into their predecessors to eliminate (dynamic) branches.
bool BranchFolder::TailDuplicateBlocks(MachineFunction &MF) {
  bool MadeChange = false;

  // Make sure blocks are numbered in order
  MF.RenumberBlocks();

  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
    MachineBasicBlock *MBB = I++;

    // Only duplicate blocks that end with unconditional branches.
    if (CanFallThrough(MBB))
      continue;

    MadeChange |= TailDuplicate(MBB, MF);

    // If it is dead, remove it.
    if (MBB->pred_empty()) {
      RemoveDeadBlock(MBB);
      MadeChange = true;
      ++NumDeadBlocks;
    }
  }
  return MadeChange;
}

/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
/// of its predecessors.
bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB,
                                 MachineFunction &MF) {
  // Don't try to tail-duplicate single-block loops.
  if (TailBB->isSuccessor(TailBB))
    return false;

  // Set the limit on the number of instructions to duplicate, with a default
  // of one less than the tail-merge threshold. When optimizing for size,
  // duplicate only one, because one branch instruction can be eliminated to
  // compensate for the duplication.
  unsigned MaxDuplicateCount =
    MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ?
    1 : TII->TailDuplicationLimit(*TailBB, TailMergeSize - 1);

  // Check the instructions in the block to determine whether tail-duplication
  // is invalid or unlikely to be profitable.
  unsigned i = 0;
  bool HasCall = false;
  for (MachineBasicBlock::iterator I = TailBB->begin();
       I != TailBB->end(); ++I, ++i) {
    // Non-duplicable things shouldn't be tail-duplicated.
    if (I->getDesc().isNotDuplicable()) return false;
    // Don't duplicate more than the threshold.
    if (i == MaxDuplicateCount) return false;
    // Remember if we saw a call.
    if (I->getDesc().isCall()) HasCall = true;
  }
  // Heuristically, don't tail-duplicate calls if it would expand code size,
  // as it's less likely to be worth the extra cost.
  if (i > 1 && HasCall)
    return false;

  // Iterate through all the unique predecessors and tail-duplicate this
  // block into them, if possible. Copying the list ahead of time also
  // avoids trouble with the predecessor list reallocating.
  bool Changed = false;
  SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
                                               TailBB->pred_end());
  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
       PE = Preds.end(); PI != PE; ++PI) {
    MachineBasicBlock *PredBB = *PI;

    assert(TailBB != PredBB &&
           "Single-block loop should have been rejected earlier!");
    if (PredBB->succ_size() > 1) continue;

    MachineBasicBlock *PredTBB, *PredFBB;
    SmallVector<MachineOperand, 4> PredCond;
    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
      continue;
    if (!PredCond.empty())
      continue;
    // EH edges are ignored by AnalyzeBranch.
    if (PredBB->succ_size() != 1)
      continue;
    // Don't duplicate into a fall-through predecessor (at least for now).
    if (PredBB->isLayoutSuccessor(TailBB) && CanFallThrough(PredBB))
      continue;

    DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB
                 << "From Succ: " << *TailBB);

    // Remove PredBB's unconditional branch.
    TII->RemoveBranch(*PredBB);
    // Clone the contents of TailBB into PredBB.
    for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end();
         I != E; ++I) {
      MachineInstr *NewMI = MF.CloneMachineInstr(I);
      PredBB->insert(PredBB->end(), NewMI);
    }

    // Update the CFG.
    PredBB->removeSuccessor(PredBB->succ_begin());
    assert(PredBB->succ_empty() &&
           "TailDuplicate called on block with multiple successors!");
    for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
         E = TailBB->succ_end(); I != E; ++I)
       PredBB->addSuccessor(*I);

    Changed = true;
  }

  // If TailBB was duplicated into all its predecessors except for the prior
  // block, which falls through unconditionally, move the contents of this
  // block into the prior block.
  MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(TailBB));
  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
  SmallVector<MachineOperand, 4> PriorCond;
  bool PriorUnAnalyzable =
    TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
  // This has to check PrevBB->succ_size() because EH edges are ignored by
  // AnalyzeBranch.
  if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB &&
      TailBB->pred_size() == 1 && PrevBB.succ_size() == 1 &&
      !TailBB->hasAddressTaken()) {
    DEBUG(errs() << "\nMerging into block: " << PrevBB
          << "From MBB: " << *TailBB);
    PrevBB.splice(PrevBB.end(), TailBB, TailBB->begin(), TailBB->end());
    PrevBB.removeSuccessor(PrevBB.succ_begin());;
    assert(PrevBB.succ_empty());
    PrevBB.transferSuccessors(TailBB);
    Changed = true;
  }

  return Changed;
}

/// OptimizeBlock - Analyze and optimize control flow related to the specified
/// block.  This is never called on the entry block.
bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
  bool MadeChange = false;
  MachineFunction &MF = *MBB->getParent();
ReoptimizeBlock:

  MachineFunction::iterator FallThrough = MBB;
  ++FallThrough;

  // If this block is empty, make everyone use its fall-through, not the block
  // explicitly.  Landing pads should not do this since the landing-pad table
  // points to this block.  Blocks with their addresses taken shouldn't be
  // optimized away.
  if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
    // Dead block?  Leave for cleanup later.
    if (MBB->pred_empty()) return MadeChange;

    if (FallThrough == MF.end()) {
      // TODO: Simplify preds to not branch here if possible!
    } else {
      // Rewrite all predecessors of the old block to go to the fallthrough
      // instead.
      while (!MBB->pred_empty()) {
        MachineBasicBlock *Pred = *(MBB->pred_end()-1);
        Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
      }
      // If MBB was the target of a jump table, update jump tables to go to the
      // fallthrough instead.
      MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, FallThrough);
      MadeChange = true;
    }
    return MadeChange;
  }

  // Check to see if we can simplify the terminator of the block before this
  // one.
  MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));

  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
  SmallVector<MachineOperand, 4> PriorCond;
  bool PriorUnAnalyzable =
    TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
  if (!PriorUnAnalyzable) {
    // If the CFG for the prior block has extra edges, remove them.
    MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
                                              !PriorCond.empty());

    // If the previous branch is conditional and both conditions go to the same
    // destination, remove the branch, replacing it with an unconditional one or
    // a fall-through.
    if (PriorTBB && PriorTBB == PriorFBB) {
      TII->RemoveBranch(PrevBB);
      PriorCond.clear();
      if (PriorTBB != MBB)
        TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
      MadeChange = true;
      ++NumBranchOpts;
      goto ReoptimizeBlock;
    }

    // If the previous block unconditionally falls through to this block and
    // this block has no other predecessors, move the contents of this block
    // into the prior block. This doesn't usually happen when SimplifyCFG
    // has been used, but it can happen if tail merging splits a fall-through
    // predecessor of a block.
    // This has to check PrevBB->succ_size() because EH edges are ignored by
    // AnalyzeBranch.
    if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
        PrevBB.succ_size() == 1 &&
        !MBB->hasAddressTaken()) {
      DEBUG(errs() << "\nMerging into block: " << PrevBB
                   << "From MBB: " << *MBB);
      PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
      PrevBB.removeSuccessor(PrevBB.succ_begin());;
      assert(PrevBB.succ_empty());
      PrevBB.transferSuccessors(MBB);
      MadeChange = true;
      return MadeChange;
    }

    // If the previous branch *only* branches to *this* block (conditional or
    // not) remove the branch.
    if (PriorTBB == MBB && PriorFBB == 0) {
      TII->RemoveBranch(PrevBB);
      MadeChange = true;
      ++NumBranchOpts;
      goto ReoptimizeBlock;
    }

    // If the prior block branches somewhere else on the condition and here if
    // the condition is false, remove the uncond second branch.
    if (PriorFBB == MBB) {
      TII->RemoveBranch(PrevBB);
      TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
      MadeChange = true;
      ++NumBranchOpts;
      goto ReoptimizeBlock;
    }

    // If the prior block branches here on true and somewhere else on false, and
    // if the branch condition is reversible, reverse the branch to create a
    // fall-through.
    if (PriorTBB == MBB) {
      SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
      if (!TII->ReverseBranchCondition(NewPriorCond)) {
        TII->RemoveBranch(PrevBB);
        TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
        MadeChange = true;
        ++NumBranchOpts;
        goto ReoptimizeBlock;
      }
    }

    // If this block has no successors (e.g. it is a return block or ends with
    // a call to a no-return function like abort or __cxa_throw) and if the pred
    // falls through into this block, and if it would otherwise fall through
    // into the block after this, move this block to the end of the function.
    //
    // We consider it more likely that execution will stay in the function (e.g.
    // due to loops) than it is to exit it.  This asserts in loops etc, moving
    // the assert condition out of the loop body.
    if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
        MachineFunction::iterator(PriorTBB) == FallThrough &&
        !CanFallThrough(MBB)) {
      bool DoTransform = true;

      // We have to be careful that the succs of PredBB aren't both no-successor
      // blocks.  If neither have successors and if PredBB is the second from
      // last block in the function, we'd just keep swapping the two blocks for
      // last.  Only do the swap if one is clearly better to fall through than
      // the other.
      if (FallThrough == --MF.end() &&
          !IsBetterFallthrough(PriorTBB, MBB))
        DoTransform = false;

      // We don't want to do this transformation if we have control flow like:
      //   br cond BB2
      // BB1:
      //   ..
      //   jmp BBX
      // BB2:
      //   ..
      //   ret
      //
      // In this case, we could actually be moving the return block *into* a
      // loop!
      if (DoTransform && !MBB->succ_empty() &&
          (!CanFallThrough(PriorTBB) || PriorTBB->empty()))
        DoTransform = false;


      if (DoTransform) {
        // Reverse the branch so we will fall through on the previous true cond.
        SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
        if (!TII->ReverseBranchCondition(NewPriorCond)) {
          DEBUG(errs() << "\nMoving MBB: " << *MBB
                       << "To make fallthrough to: " << *PriorTBB << "\n");

          TII->RemoveBranch(PrevBB);
          TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);

          // Move this block to the end of the function.
          MBB->moveAfter(--MF.end());
          MadeChange = true;
          ++NumBranchOpts;
          return MadeChange;
        }
      }
    }
  }

  // Analyze the branch in the current block.
  MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
  SmallVector<MachineOperand, 4> CurCond;
  bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
  if (!CurUnAnalyzable) {
    // If the CFG for the prior block has extra edges, remove them.
    MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());

    // If this is a two-way branch, and the FBB branches to this block, reverse
    // the condition so the single-basic-block loop is faster.  Instead of:
    //    Loop: xxx; jcc Out; jmp Loop
    // we want:
    //    Loop: xxx; jncc Loop; jmp Out
    if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
      SmallVector<MachineOperand, 4> NewCond(CurCond);
      if (!TII->ReverseBranchCondition(NewCond)) {
        TII->RemoveBranch(*MBB);
        TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
        MadeChange = true;
        ++NumBranchOpts;
        goto ReoptimizeBlock;
      }
    }

    // If this branch is the only thing in its block, see if we can forward
    // other blocks across it.
    if (CurTBB && CurCond.empty() && CurFBB == 0 &&
        MBB->begin()->getDesc().isBranch() && CurTBB != MBB &&
        !MBB->hasAddressTaken()) {
      // This block may contain just an unconditional branch.  Because there can
      // be 'non-branch terminators' in the block, try removing the branch and
      // then seeing if the block is empty.
      TII->RemoveBranch(*MBB);

      // If this block is just an unconditional branch to CurTBB, we can
      // usually completely eliminate the block.  The only case we cannot
      // completely eliminate the block is when the block before this one
      // falls through into MBB and we can't understand the prior block's branch
      // condition.
      if (MBB->empty()) {
        bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB);
        if (PredHasNoFallThrough || !PriorUnAnalyzable ||
            !PrevBB.isSuccessor(MBB)) {
          // If the prior block falls through into us, turn it into an
          // explicit branch to us to make updates simpler.
          if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
              PriorTBB != MBB && PriorFBB != MBB) {
            if (PriorTBB == 0) {
              assert(PriorCond.empty() && PriorFBB == 0 &&
                     "Bad branch analysis");
              PriorTBB = MBB;
            } else {
              assert(PriorFBB == 0 && "Machine CFG out of date!");
              PriorFBB = MBB;
            }
            TII->RemoveBranch(PrevBB);
            TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
          }

          // Iterate through all the predecessors, revectoring each in-turn.
          size_t PI = 0;
          bool DidChange = false;
          bool HasBranchToSelf = false;
          while(PI != MBB->pred_size()) {
            MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
            if (PMBB == MBB) {
              // If this block has an uncond branch to itself, leave it.
              ++PI;
              HasBranchToSelf = true;
            } else {
              DidChange = true;
              PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
              // If this change resulted in PMBB ending in a conditional
              // branch where both conditions go to the same destination,
              // change this to an unconditional branch (and fix the CFG).
              MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
              SmallVector<MachineOperand, 4> NewCurCond;
              bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
                      NewCurFBB, NewCurCond, true);
              if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
                TII->RemoveBranch(*PMBB);
                NewCurCond.clear();
                TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
                MadeChange = true;
                ++NumBranchOpts;
                PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
              }
            }
          }

          // Change any jumptables to go to the new MBB.
          MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, CurTBB);
          if (DidChange) {
            ++NumBranchOpts;
            MadeChange = true;
            if (!HasBranchToSelf) return MadeChange;
          }
        }
      }

      // Add the branch back if the block is more than just an uncond branch.
      TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
    }
  }

  // If the prior block doesn't fall through into this block, and if this
  // block doesn't fall through into some other block, see if we can find a
  // place to move this block where a fall-through will happen.
  if (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
                      PriorTBB, PriorFBB, PriorCond)) {

    // Now we know that there was no fall-through into this block, check to
    // see if it has a fall-through into its successor.
    bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
                                       CurCond);

    if (!MBB->isLandingPad()) {
      // Check all the predecessors of this block.  If one of them has no fall
      // throughs, move this block right after it.
      for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
           E = MBB->pred_end(); PI != E; ++PI) {
        // Analyze the branch at the end of the pred.
        MachineBasicBlock *PredBB = *PI;
        MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
        MachineBasicBlock *PredTBB, *PredFBB;
        SmallVector<MachineOperand, 4> PredCond;
        if (PredBB != MBB && !CanFallThrough(PredBB) &&
            !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
            && (!CurFallsThru || !CurTBB || !CurFBB)
            && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
          // If the current block doesn't fall through, just move it.
          // If the current block can fall through and does not end with a
          // conditional branch, we need to append an unconditional jump to
          // the (current) next block.  To avoid a possible compile-time
          // infinite loop, move blocks only backward in this case.
          // Also, if there are already 2 branches here, we cannot add a third;
          // this means we have the case
          // Bcc next
          // B elsewhere
          // next:
          if (CurFallsThru) {
            MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
            CurCond.clear();
            TII->InsertBranch(*MBB, NextBB, 0, CurCond);
          }
          MBB->moveAfter(PredBB);
          MadeChange = true;
          goto ReoptimizeBlock;
        }
      }
    }

    if (!CurFallsThru) {
      // Check all successors to see if we can move this block before it.
      for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
           E = MBB->succ_end(); SI != E; ++SI) {
        // Analyze the branch at the end of the block before the succ.
        MachineBasicBlock *SuccBB = *SI;
        MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;

        // If this block doesn't already fall-through to that successor, and if
        // the succ doesn't already have a block that can fall through into it,
        // and if the successor isn't an EH destination, we can arrange for the
        // fallthrough to happen.
        if (SuccBB != MBB && &*SuccPrev != MBB &&
            !CanFallThrough(SuccPrev) && !CurUnAnalyzable &&
            !SuccBB->isLandingPad()) {
          MBB->moveBefore(SuccBB);
          MadeChange = true;
          goto ReoptimizeBlock;
        }
      }

      // Okay, there is no really great place to put this block.  If, however,
      // the block before this one would be a fall-through if this block were
      // removed, move this block to the end of the function.
      MachineBasicBlock *PrevTBB, *PrevFBB;
      SmallVector<MachineOperand, 4> PrevCond;
      if (FallThrough != MF.end() &&
          !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
          PrevBB.isSuccessor(FallThrough)) {
        MBB->moveAfter(--MF.end());
        MadeChange = true;
        return MadeChange;
      }
    }
  }

  return MadeChange;
}