bug#38492: 27.0.50; Warn pdumper users when pure space has been overflowed

[Daniel Colascione]

>> With portable dumping.  I think pure space provides no benefit.
>
> AFAICT, the purespace provides the same benefit with portable dumping as
> it does with unexec.  The benefit is to save time during the GC by not
> traversing the purespace.  It's basically an "old" generation in a kind
> of very restricted form of generational GC.
>
> Maybe purespace is too small compared to typical heap sizes to make
> a significant difference, nowadays.

I think the ratio of dumped-heap to runtime-heap is high enough these days
that the pure optimization you're describing doesn't really matter. I
think we need a better general-purpose GC that does generational tracking
even after the dump and without purespace's restriction against
intergenerational pointers. Whether that's something we reuse or write, I
don't know yet, but I have some ideas for how we can combine these
techniques with our conservative stack scanning.