local refs

[Matthew Fluet]

> I believe looking for any Thread_ primitive is a bit much, and all you
> need to look for is Thread_switchTo or Thread_switchToCont.  If those
> are not present, then there isn't any control-flow that's not in the
> graph.   Since there is no longer anything special about
> Thread_switchToCont, I propose to eliminate it, replacing it with
> Thread_switchTo, and to make both (1) and (2) above only look for
> Thread_switchTo.

I think I agree with this.

> On a related point, I believe that local-ref can be improved.  At
> present it does two different things:
> 
> 1. Move globals refs into functions that only execute once
> 2. Turns local refs into SSA vars
> 
> Of course (1) only makes sense if the once computation is correct.

Actually, it only moves localizable global refs into functions that only
execute once.

> But (2) seems like it's OK to do no matter what.  So, I guess I'm
> saying that local-ref shouldn't turn off all optimization when it
> determines that a program has threads/conts.  It should just turn off
> (1).  Does that make sense?

No, unless you're reading a lot more into (2) than I am.  What makes a ref
non-localizable is that it escapes.  The current model, in a single
threaded world, of "escape" is a ref that is used in any tuple/constructor
construction, control transfer, or primitive that is not a Ref_deref or
Ref_assign.  In a multi-threaded world, a ref can also "escape" to the
stack when it is copied, at a Thread_copy.  Take a look at callcc3.sml. 
The reference rr_0 looks like it is localizable, but it is live accross
the call to Thread_copyShrink.  If you try to localize it, then the
program doesn't terminate.  So, if you really wanted to do (2) in a
context where Thread primitives existed, you need to do liveness analysis
of the potentially localizable refs, and forbid localization of any live
across a block with a thread copy. (Note, this probably wouldn't be too
bad; there are generally only a few localizable refs, so doing the
propagation isn't too bad.)