[m-rev.] for review: Add support for weak pointers to the C RTS

[Peter Wang]

On Fri, 18 Jul 2014 14:55:11 +1000, Paul Bone <paul at bone.id.au> wrote:
> On Tue, Apr 15, 2014 at 10:20:01PM +1000, Paul Bone wrote:
> > For review by anyone.
> > 
> 
> No-one has reviewed these changes in the last three months so I've committed
> them:
> 
>  + Make version_array's rewind code use constant stack space.
>  + Add missing MR_GC_malloc_atomic procedure
>  + Add support for weak pointers to the C RTS

Hi Paul,

Sorry for the late comments.  Please post a reminder next time.

I don't like that users who don't use rewind have to pay for it on every
update, in creating the weak pointer, and whatever overhead that might
add to GC elsewhere.  For users that retain old versions, the size of
each ML_va also increases from 4 words to 6 words after rounding.

An alternative solution would be to allocate a bit array to mark off the
elements that have been restored already, as you iterate forwards from
the oldest update to the newest.  If the i'th bit is set then the i'th
element of the version array was already restored to an older value, so
just ignore the current update.  No prev pointers required.

Peter