Ref counting for CTFE?

[Don via Digitalmars-d]

On Thursday, 29 May 2014 at 18:12:59 UTC, Timon Gehr wrote:
> On 05/29/2014 06:53 PM, Dylan Knutson wrote:
>> ...
>>
>> Is there anything so radically different in D than these other
>> languages, that prevents the implementation of a 
>> run-of-the-mill VM to
>> eval D code?
>
> No. (In fact, I've written a naive but mostly complete byte 
> code interpreter in half a week or so last year, as part of an 
> ongoing recreational D front end implementation effort.)
>
>> It just seems strange to me that it's such a problem when
>> this is basically solved by all scripting languages. And I'm 
>> really not
>> trying to downplay the difficulty in implementing CTFE in D, 
>> but rather
>> just figure out why it's so hard to implement in comparison.
>
> CTFE is somewhat intertwined with semantic analysis, which 
> makes it a little harder to specify/implement than usual 
> interpreters. However, the performance problem is mostly a 
> structural issue of the current implementation: DMDs CTFE 
> interpreter gradually grew out of its constant folder in some 
> kind of best effort fashion as far as I understand.
>
> It is feasible to do everything in the usual fashion and 
> occasionally just pause or restart interpretation at 
> well-defined points where it needs to interface with semantic 
> analysis.

Exactly. Historically, most of the work I've done on CTFE was in 
fixing up the relationship between CTFE and the rest of the 
compiler, ironing out all of the weird semantic interactions. 
Almost *nothing* has ever been done on the CTFE implementation 
itself.
The implementation is the crappiest thing you could imagine, it 
leaks memory like BP leaks oil. It's been hard to fix not because 
doing a JIT is hard, but because of the semantic interaction 
bugs. The good news is that most of those are fixed now.

But, it's worth mentioning that at dconf, CTFE and mixins were 
blamed for many things they aren't responsible for.
For example, Phobos takes forever to compile, but it's nothing to 
do with CTFE.
Phobos is slow to compile because everything imports everything 
else, and it instantiates nearly a million templates.
IE, an infinitely fast CTFE engine would make very little 
difference to Phobos compile times.