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

[dsimcha]

On 3/26/2011 4:16 PM, Don wrote:
> dsimcha wrote:
>> On 3/26/2011 12:16 PM, Don wrote:
>>> I'm giving CTFE a *major* overhaul right now. I don't know if I'll be
>>> finished in time for the next compiler release, but definitely by the
>>> release after that. Most importantly, bug 1330, which is the root cause
>>> of almost all of the problems, will be fixed. I hope to move CTFE out
>>> the "experimental feature" category.
>>
>> This is great news, I'm looking forward to it. Thanks for the hard
>> work. Out of curiosity, can you give a brief overview of what new
>> things CTFE will be usable for?
>
> 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.

Right.  IIUC there's also no way to free the memory from copies that are 
no longer referenced.  I can see where this would leak memory like a sieve.

>
> 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.

This is a _huge_ improvement, but does it address the issue of freeing 
memory or is that beyond the scope?

>
> 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.

Excellent.

>
> I've been planning on doing this for over a year, but while Walter was
> working on 64-bit, I felt that I was the only one working on the
> showstopper wrong-code bugs and regressions, so I put this
> important-but-not-urgent stuff aside.

Agreed.  I love the 64-bit support (I've been using it for real work and 
it's surprisingly solid) but the pace of fixing miscellaneous bugs was 
understandably glacial while it was being implemented.