I'm resurrecting this thread.

Given the recent discussion about optimization, I've done some more thinking and simplification work on my proposal for a better ocaml header. Much of this is just adopting Goswin's proposal, which made a lot of sense. I tried to also reduce the variations while allowing for a lot of useful unboxing of floats, int64s etc. I've also tried to tackle tuples in this version -- basically, in order to be compatible with polymorphic functions, you really want tuples and all other polymorphic types (as in 'a embedded inside records) to be of uniform size. Expanding that size under some circumstances to 64 bits on 32 bit platforms makes sense if, for example, the tuple contains many floats. I don't think handling both what I call narrow and wide polymorphic types (for lack of a better name) will make the code much more complex -- I certainly don't want it to result in heavy register spilling.

Also, the dynamic parts of the header now appear BEFORE the header itself. Their highest bit is 0 to indicate that they're not the main header. This will be seen by GC code, which can afford to do the few extra comparisons. Mutator code will always have the main header available right before the data.