I made a small change to the way Psyco handles existing code buffers, in
particular in the way it looks for an already-seen compiler state (which
took 70% of its execution time). Now the existing states are stored in
the "mergepoint" structure built in 'mergepoints.c': one array of
already-seen compiler states per merge point. This removes the quadratic
search time. It's probably enough to make it usable but really serious
solution would involve storing the already-seen states into a "decision
tree" against which we can do some kind of pattern-matching.