Class: Furnace::AVM2::Transform::CFGReduce

Inherits:
Object
  • Object
show all
Defined in:
lib/furnace-avm2/transform/cfg_reduce.rb

Instance Method Summary collapse

Instance Method Details

#check_nonlocal_loop(loop_stack, block) ⇒ Object

Check if the control transfer is nonlocal according to the innermost loop and adjust @loop_nonlocal accordingly for labels to be inserted where appropriate.



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# File 'lib/furnace-avm2/transform/cfg_reduce.rb', line 227

def check_nonlocal_loop(loop_stack, block)
  if loop_stack.first != block
    @loop_nonlocal.add block

    yield [loop_label(block)]
  else
    yield []
  end
end

#completely_dominated?(block, dominator) ⇒ Boolean

Block B is completely dominated by another block D if it is dominated by D and no edges ever lead to block B from any other block, including those dominated by D, but excluding any back edges.

Returns:

  • (Boolean) 


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# File 'lib/furnace-avm2/transform/cfg_reduce.rb', line 186

def completely_dominated?(block, dominator)
  if @loops.include?(block)
    (block.sources - @loops[block].to_a) == [dominator]
  else
    block.sources == [dominator]
  end
end

#extended_block(block, stopgap = nil, loop_stack = []) ⇒ Object 



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# File 'lib/furnace-avm2/transform/cfg_reduce.rb', line 19

def extended_block(block, stopgap=nil, loop_stack=[])
  nodes = []

  while block
    if @loops.include?(block) && loop_stack.include?(block)
      # We have just arrived to loop head. Insert `continue'
      # and exit.
      check_nonlocal_loop(loop_stack, block) do |params|
        nodes << AST::Node.new(:continue, params)
      end
      break
    elsif @loop_tails.include?(block) &&
              loop_stack.include?(@loop_tails[block])
      # We have just arrived to loop tail. Insert `break'
      # and exit.
      loop = @loop_tails[block]
      check_nonlocal_loop(loop_stack, loop) do |params|
        nodes << AST::Node.new(:break, params)
      end
      break
    elsif block == stopgap
      # We have just arrived to a merge point of `if'
      # contidional. Exit.
      break
    end

    if @visited.include? block
      raise "failsafe: block #{block.label} already visited"
    elsif block != @cfg.exit
      @visited.add block
    end

    block.insns.each do |insn|
      next if insn == block.cti
      nodes << insn
    end

    if block.cti
      if block.cti.type == :lookup_switch
        # this is a switch

      elsif @loops.include?(block)
        # we're trapped in a strange loop
        reverse = !block.cti.children[0]
        in_root, out_root = block.targets

        # One of the branch targets should reside within
        # the loop.
        if !@loops[block].include?(in_root)
          in_root, out_root = out_root, in_root
          reverse = !reverse
        end

        # Mark the loop tail so we could detect `break' and
        # `continue' statements.
        @loop_tails[out_root] = block

        # If we reversed the roots or it was a (jump-if false),
        # then reverse the condition.
        expr = normalize_cti_expr(block, reverse)

        body = extended_block(in_root, nil, [ block, *loop_stack ])

        # [(label name)]
        # We first parse the body and then add the label before
        # the loop body if anything in the body requires that label
        # to be present.
        if @loop_nonlocal.include?(block)
          nodes << AST::Node.new(:label, [ loop_label(block) ])
        end

        # (while (condition)
        #   (body ...))
        nodes << AST::Node.new(:while, [
          expr,
          body
        ])

        block = out_root
      else
        # this is an `if', `break' or `continue'
        reverse = !block.cti.children[0]
        left_root, right_root = block.targets

        # (if (condition)
        #   (if-true ...)
        #  [(if-false ...)])
        # Note that you cannot reach expression if-true nor
        # expression if-false without evaluating condition.
        # Thus, to go inside the if body, a root has to be
        # completely dominated by this block--that is, does
        # not have edges coming to it even from other blocks
        # dominated by this block.

        # If the left root isn't dominated by block,
        # then it can't be `if' branch.
        if !completely_dominated?(left_root, block)
          left_root, right_root = right_root, left_root
          reverse = !reverse
        end

        # If the left root still isn't dominated by block,
        # then this is not a proper conditional.
        unless completely_dominated?(left_root, block)
          raise "not-well-formed if"
        end

        # If the right root is dominated by this block, which
        # means that we have an `else' part, and if the condition
        # is reversed, turn that back. This serves purely aesthetical
        # purposes and depends on behavior of ASC code generator.
        if completely_dominated?(right_root, block) && reverse
          left_root, right_root = right_root, left_root
          reverse = false
        end

        # If we reversed the roots or it was a (jump-if false),
        # then reverse the condition.
        expr = normalize_cti_expr(block, reverse)

        # Does this conditional have an `else' block?
        if completely_dominated?(right_root, block)
          # Yes. Find a merge point.
          merge = find_merge_point(block, left_root, right_root, loop_stack)

          # If the merge search did not yield a valid node, use
          # stopgap for the current block to avoid runaway code
          # synthesis.
          #
          # The stopgap block is actually an innermost block from
          # a stopgap block set implicitly represented by a set of
          # objects contained in arguments of recursive calls. As
          # the `if's are fully nested when well-formed, we can only
          # check for collision with innermost stopgap block.
          nodes << AST::Node.new(:if, [
            expr,
            extended_block(left_root,  merge || stopgap, loop_stack),
            extended_block(right_root, merge || stopgap, loop_stack)
          ])

          block = merge
        else
          # No. The "right root" is actually post-if code.
          nodes << AST::Node.new(:if, [
            expr,
            extended_block(left_root, right_root, loop_stack)
          ])

          block = right_root
        end
      end
    elsif block.targets.count == 1
      block = block.targets.first
    elsif block == @cfg.exit
      break
    else
      raise "invalid target count (#{block.targets.count})"
    end
  end

  AST::Node.new(:begin, nodes)
end

#find_merge_point(root, left, right, loop_stack) ⇒ Object

A merge point for blocks R (root), L (left) and D (right) is first block found with BFS starting at L,D so that it is dominated by R, but not L or D.



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# File 'lib/furnace-avm2/transform/cfg_reduce.rb', line 197

def find_merge_point(root, left, right, loop_stack)
  worklist = Set[left, right]
  visited  = Set[root, left, right]

  while worklist.any?
    node = worklist.first
    worklist.delete node

    visited.add node

    if (@dom[node].include?(root) &&
        !(@dom[node].include?(left) ||
          @dom[node].include?(right))) ||
        loop_stack.include?(node)
      return node
    end

    node.targets.each do |target|
      next if visited.include? target
      worklist.add target
    end
  end

  # The paths have diverged.
  nil
end

#loop_label(block) ⇒ Object 



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# File 'lib/furnace-avm2/transform/cfg_reduce.rb', line 237

def loop_label(block)
  "label#{block.label}"
end

#normalize_cti_expr(block, negate) ⇒ Object 



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# File 'lib/furnace-avm2/transform/cfg_reduce.rb', line 241

def normalize_cti_expr(block, negate)
  if negate
    AST::Node.new(:!, [ block.cti.children[1] ])
  else
    block.cti.children[1]
  end
end

#transform(cfg) ⇒ Object 



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# File 'lib/furnace-avm2/transform/cfg_reduce.rb', line 4

def transform(cfg)
  @cfg   = cfg

  @dom   = @cfg.dominators
  @loops = @cfg.identify_loops

  @visited       = Set.new
  @loop_tails    = {}
  @loop_nonlocal = Set.new

  ast, = extended_block(@cfg.entry)

  ast
end