[opt] net reader compaction

def compact_net_readers( nets ):

  ret = []
  all_reads = set()

  # First add normal update block reads
  for read in s._all_read_upblks:
    obj = s._id_obj[ read ]
    while obj:
      all_reads.add( id(obj) )
      obj = obj._nested

  # Then add net writers and top level output ports in the net!
  for writer, readers in nets:
    obj = writer
    while obj:
      all_reads.add( id(obj) )
      obj = obj._nested

    for reader in readers:
      if isinstance( reader, OutVPort ) and reader._host == s:
        obj = reader
        while obj:
          all_reads.add( id(obj) )
          obj = obj._nested

  # Now figure out if a reader can be safely removed from the net
  # Check if the reader itself, its ancestors, or sibling slices are
  # read somewhere else. If not, the reader can be moved from the net.

  for writer, readers in nets:
    new_readers = []

    for x in readers:
      flag = False
      obj = x
      while obj:
        if id(obj) in all_reads:
          flag = True
          break
        obj = obj._nested

      if x._slice:
        for obj in x._nested._slices.values(): # Check sibling slices
          if x != obj and _overlap(obj._slice, x._slice) and \
              id(obj) in all_reads:
            flag = True
            break

      if flag: new_readers.append( x ) # is read somewhere else

    ret.append( (writer, new_readers) )

  return ret