There's been quite a lot of work on the intermediate code optimiser since the last release and it's largely been rewritten. The main aim has been to reduce the number of cases where heap storage was allocated for argument tuples or closures.
There were a number of bugs that appeared during testing but I'm not aware of anything outstanding. Isabelle2013 and HOL4 both seem to build successfully and there does seem to be some speed-up. I'd like to release this version in the next month or so but I'd like to fix any other bugs that have been introduced. Please try out the SVN version and let me know how it goes. Don't forget that after the configure and make steps you need to run "make compiler" at least once and preferably twice. Otherwise you will still be using the 5.5 compiler.
David