On Mon, Apr 10, 2006 at 10:10:18AM +0100, Simon Marlow wrote:
It's not *completely* straightforward to implement, at least in GHC, and at least if you want to implement it in a modular way (i.e. without touching lots of different parts of the system).
The obvious way to "add a bit to a closure" is to use the LSB of the info pointer, which currently is always 0. However, that means masking out this bit every time you want to get the info pointer of a closure, which means lots of changes to the runtime. The price seems pretty high.
An alternative is to have two info tables for every constructor, one normal one and one "deepSeq'd", and the normal one probably needs to point to the deepSeq'd version. This doesn't require masking out any bits, but it does increase code size (one extra info table + entry code for every constructor, except possibly those that don't contain any pointer fields), and one extra field in a constructor's info table. Plus associated cache pollution.
Yet another alternative is to store fully evaluated data in a segregated part of the heap. The garbage collector could do this - indeed we already do something similar, in that data that has no pointer fields is kept separate. Checking the "deepSeq" bit on a closure is then more complicated - but this has the advantage that only the GC and storage manager are affected.
None of these solutions is as simple and self-contained as I'd like :-(
it is unlikely it will even be possible to implement in jhc without radical changes to its internals. there is just no where to attach a bit to, and even if there were, there is no generic way to evaluate something to WHNF, or even a concept of WHNF in final grin. (grin code can look inside unevaluated closures, hopefully making the thunk non-updatable) John -- John Meacham - ⑆repetae.net⑆john⑈