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

[Don]

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.

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.

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.