CTFE Overhaul (Was: RFC: Thrift project proposal (draft))

[David Nadlinger]

On 3/26/11 9:16 PM, Don wrote:
> The basic problem with the current implementation of CTFE is that it
> uses copy-on-write. This means that references (including dynamic
> arrays) don't work properly -- they just copy a snapshot of the thing
> they are referencing. This is bug 1330. It also means it burns up memory
> like you wouldn't believe.
>
> I'm changing CTFE to use in-place modification. This fixes all those
> issues. But this is obviously a fairly intense change, and will take
> quite a lot of time to iron out all the corner cases. So that's all I'm
> planning on doing right now.
>
> But once that's done, it will be straightforward to implement other
> reference types, such as classes and pointers (pointer arithmetic will
> be restricted to pointers which point to array members). Once classes
> are implemented, it's straightforward to do exceptions. So, pretty much
> everything.

First of all, let me say again that I am really looking forward to your 
changes, as I even considered having a go at solving the referencing 
issue myself for a while (but without sound knowledge of the compiler 
internals, this is an even harder thing to pull off).

Do I understand correctly that your changes wouldn't introduce some form 
of real compile-time memory management, but alleviate the need for it by 
fixing bug 1330 and related ones, thus cutting down on the ridiculous 
amount of copying going on today?

And finally – I know such questions are tough to answer –, do you have a 
rough estimate on how long it will take you to get the basic set of 
changes ready for testing? This is somewhat relevant for me, as the 
coding period for GSoC is going to start in about two months from now, 
and I think that with current DMD, doing the Thrift compiler in CTFE 
might be infeasible due to memory usage.

Thanks a lot for your work,
David