autolink body content to node
You may consider the FreeLinking module.

The Freelinking module adds a text filter for using Wiki-style markup for creating links automatically, by searching the node body for markup.  It supports a number of methods for creating the link, including Nodetitle (which sounds like the one you're looking for). You may even add methods by adding your own link plugin.

However, you need to indicate the text you want to be considered as a candidate for automatic linking by markup: i.e. by surrounding it with double square brackets.  This is how your example will look like:

The [[drupal]] is one of best CMS systems


It even lets you create placeholder links to pages that has not been created yet. Both internal and external links are supported by the module.

There tutorial is about using the Freelinking module, with a little help from the companion WikiTools module, here. 

If the way Freelinking does this is not good enough for you, here are a couple of alternative solutions:

Alternative solution #1: You can write your own custom text filter for this.  I suspect that to create something that works during node saves without freezing or killing your site will be very hard to do.  

Alternative solution #2: If you can accept a solution that delays the creation of links, you can place new and altered links in a queue, and then use a cron job  to parse them for links.  This allows you to space out the parsing of node bodies over time and will avoid the performance problems a real-time text filter would create.  For the record: There exists no module that does the parsing of node bodies and add links like this.  You'll need to write it yourself.