RSSA

[Stephen Weeks]

The general principle that I would like to apply is to express as many
of the limit check optimizations as we can in RSSA/MACHINE, before the
codegens.  Using that principle ...

> What it basically comes down to is should I special case:
> 
>   Array {numElts, bytesPerElt as 0, extraBytes as 0}

This should be turned into a heap limit check by the backend.

> or will we turn this into a Heap limit check for 0 bytes and just special
> case
> 
>   Heap {bytes as 0, stackToo}

This should be special cased by the codegen to avoid the add.

> Likewise, I could imagine turning
> 
>   Array {numElts as Operand.Int numElts', bytesPerElt, extraBytes}
> 
> into a standard Heap check.

Agreed -- this will be handled by limit check insertion, so that the
codegen will never see such things.

> But, I don't know how this figures in with your notion of checking limit
> check insertions; I would imagine leaving Array kinds associated with
> Array allocations would make it easier.

I doubt that it matters much.  The code that inserts the limit checks
will see the Operand.Int's and the code that checks it can see them
just as easily.