[Swift-devel] Changes to array closing semantics?

[Mihael Hategan]

On Thu, 2012-11-22 at 11:02 -0700, Justin M Wozniak wrote:
> On 11/22/2012 09:44 AM, Tim Armstrong wrote:
> >
> > Currently we do the dumb thing of incrementing once per loop 
> > iteration, but I'm going to make a change soon where, if we know how 
> > many loop iterations there are at the time the loop is started, we'll 
> > just increment the count by that amount at the start and decrement as 
> > each iteration finishes (or actually, since we chunk loops up and 
> > distribute them, as each chunk of the loop finishes).  This works well 
> > enough.
> 
> Just for clarity:  In general, we support conditional insertion: if (c > 
> 0) { A[i] = c; }.  The conditional code block executes some time in the 
> future when the condition is set (possibly somewhere else). We handle 
> this by allocating array "slots" before entering subordinate code and 
> dropping slots in all cases in the subordinate code.  When the array 
> runs out of slots, it closes itself.

I'm sticking that in using basic reference counting. Both branches of a
condition must have the same (writing) reference count, and if not, the
compiler inserts partial close statements to make them match. That's in
order to support things like:

if (c) {
  a[0]  = 1;
  a[1] = 1;
}
else {
  a[0] = 1;
}