Module: Furnace::CFG::Algorithms

Included in:
Graph
Defined in:
lib/furnace/cfg/algorithms.rb

Instance Method Summary collapse

Instance Method Details

#compute_generic_domination(start, forward) ⇒ Object

Shamelessly stolen from http://www.cs.colostate.edu/~mstrout/CS553/slides/lecture04.pdf



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# File 'lib/furnace/cfg/algorithms.rb', line 92

def compute_generic_domination(start, forward)
  # values of β will give rise to dom!
  dom = { start => Set[start] }

  @nodes.each do |node|
    next if node == start
    dom[node] = @nodes.dup
  end

  change = true
  while change
    change = false
    @nodes.each do |node|
      next if node == start

      # Are we computing dominators or postdominators?
      if forward
        edges = node.sources + node.exception_sources
      elsif node.exception.nil?
        edges = node.targets
      else
        edges = node.targets + [ node.exception ]
      end

      #   Key Idea [for dominators]
      # If a node dominates all
      # predecessors of node n, then it
      # also dominates node n.
      pred = edges.map do |source|
        dom[source]
      end.reduce(:&)

      # An exception handler header node has no regular sources.
      pred = [] if pred.nil?

      current = Set[node].merge(pred)
      if current != dom[node]
        dom[node] = current
        change = true
      end
    end
  end

  dom
end

#dominators ⇒ Object 



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# File 'lib/furnace/cfg/algorithms.rb', line 138

def dominators
  @dominators ||= compute_generic_domination(@entry, true)
end

#eliminate_unreachable! ⇒ Object 



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# File 'lib/furnace/cfg/algorithms.rb', line 3

def eliminate_unreachable!
  worklist  = Set[ entry, exit ]
  reachable = Set[]

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

    node.targets.each do |target|
      unless reachable.include? target
        worklist.add target
      end
    end

    if node.exception
      unless reachable.include? node.exception
        worklist.add node.exception
      end
    end
  end

  @nodes.each do |node|
    unless reachable.include? node
      @nodes.delete node
      yield node if block_given?
    end
  end

  flush
end

#flush ⇒ Object 



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# File 'lib/furnace/cfg/algorithms.rb', line 186

def flush
  @dominators = nil
  @postdominators = nil

  super if defined?(super)
end

#identify_loops ⇒ Object

See also #dominators for references.



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# File 'lib/furnace/cfg/algorithms.rb', line 147

def identify_loops
  loops = Hash.new { |h,k| h[k] = Set.new }

  dom = dominators

  @nodes.each do |node|
    node.targets.each do |target|
      #   Back edges
      # A back edge of a natural loop is one whose
      # target dominates its source.
      if dom[node].include? target
        loops[target].add node
      end
    end
  end

  # At this point, +loops+ contains a list of all nodes
  # which have a back edge to the loop header. Expand
  # it to the list of all nodes in the loop.
  loops.each do |header, nodes|
    #   Natural loop
    # The natural loop of a back edge (m→n), where
    # n dominates m, is the set of nodes x such that n
    # dominates x and there is a path from x to m not
    # containing n.
    pre_header = dom[header]
    all_nodes  = Set[header]

    nodes.each do |node|
      all_nodes.merge(dom[node] - pre_header)
    end

    nodes.replace all_nodes
  end

  loops.default = nil
  loops
end

#merge_redundant! ⇒ Object 



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# File 'lib/furnace/cfg/algorithms.rb', line 35

def merge_redundant!
  worklist = @nodes.dup
  while worklist.any?
    node = worklist.first
    worklist.delete node

    target = node.targets[0]
    next if target == @exit

    # Skip explicitly non-redundant nodes
    if node.metadata[:keep]
      next
    end

    if node.targets.uniq == [target] &&
        target.sources.uniq == [node] &&
        node.exception == target.exception

      yield node, target if block_given?

      node.insns.delete node.cti
      @nodes.delete target
      worklist.delete target

      node.insns.concat target.insns
      node.cti           = target.cti
      node.target_labels = target.target_labels

      worklist.add node

      flush
    elsif node.targets.count == 1 &&
        node.insns.empty?

      target = node.targets.first

      yield target, node if block_given?

      node.sources.each do |source|
        index = source.targets.index(node)
        source.target_labels[index] = target.label
      end

      @nodes.delete node
      worklist.delete node

      if @entry == node
        @entry = target
      end

      flush
    end
  end
end

#postdominators ⇒ Object 



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# File 'lib/furnace/cfg/algorithms.rb', line 142

def postdominators
  @postdominators ||= compute_generic_domination(@exit, false)
end