define count_blocks
  set var $block=$arg0
  set var $count=0
  set var $size=0
  set var $used=0
  while $block
    set var $count++
    set var $size += $block->end - $block->start
    set var $used += $block->free - $block->start
  end
  output $count 
  echo  blocks; 
  output $size
  echo b allocated;  
  output $used
  echo b used.
end

document count_blocks
Count the number of blocks in a chain, and compute their total size.
usage: count_blocks EXPR
EXPR should have type 'struct block *'.
example: (gdb) count_blocks generation[0].from_space
end