deeqSeq proposal

[Andy Gill]

On Apr 10, 2006, at 2:25 AM, John Meacham wrote:

> 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)

I do not understand.

- (A) I'm calling for a recursive descent function that does seq. I  
could
write it in Haskell, for any specific type.  How is seq implemented jhs?

- (B) Once we have this recursive function, I'm advocating for an  
optimization
which will make it cheap. Why can't we just steal a bit in the (GHC)  
info table,
rather than mess with LSB of pointers, or have two info tables?

Yes, in grin this information would need to be used at compile time
  but the resulting code would be considerably faster. A deepSeq is
a gift to the compiler from the programmer.

Andy Gill