CTFE and DI: The Crossroads of D

[Adam Wilson]

On Thu, 10 May 2012 09:56:06 -0700, Steven Schveighoffer  
<schveiguy at yahoo.com> wrote:

> On Thu, 10 May 2012 12:04:44 -0400, deadalnix <deadalnix at gmail.com>  
> wrote:
>
>> Le 10/05/2012 17:54, Steven Schveighoffer a écrit :
>>> On Thu, 10 May 2012 10:47:59 -0400, Andrei Alexandrescu
>>> <SeeWebsiteForEmail at erdani.org> wrote:
>>>
>>>> On 5/10/12 6:17 AM, Steven Schveighoffer wrote:
>>>>> On Wed, 09 May 2012 23:00:07 -0400, Andrei Alexandrescu
>>>>> <SeeWebsiteForEmail at erdani.org> wrote:
>>>>>> Actually the point here is to still be able to benefit of di  
>>>>>> automated
>>>>>> generation while opportunistically marking certain functions as "put
>>>>>> the body in the .di file".
>>>>>
>>>>> If you aren't going to strip the files, I don't see the point in it.
>>>>
>>>> Inlining.
>>>
>>> No, I mean if dmd -H isn't going to strip the files, what is the point
>>> of dmd -H? I can already copy the .d to .di and have inlining/ctfe, or
>>> simply use the .d directly.
>>>
>>> At this point, in order to get CTFE to work, you have to keep just  
>>> about
>>> everything, including private imports. If we want to ensure CTFE works,
>>> dmd -H becomes a glorified cp. If we have some half-assed guess at what
>>> could be CTFE'd (which is growing by the day), then it's likely to not
>>> fit with the goals of the developer running dmd -H.
>>>
>>> -Steve
>>
>> If you can CTFE, you can know what is CTFEable. If it is currently half  
>> assed, then work on it and provide a better tool.
>
> There is already a better tool -- cp.  I ask again, what is the benefit  
> of .di generation if it is mostly a glorified (faulty?) copy operation?
>
> As Adam points out in his original post, ensuring CTFE availability may  
> not be (and is likely not) why you are creating a .di file.
>
> Plus, what isn't CTFEable today may be CTFEable tomorrow.
>
> inlining is one thing, because that's an optimization that has a valid  
> fallback.  CTFE does not.
>
> -Steve

Exactly this. I am currently in the process of changing the DRuntime  
makefiles such that some of the files are not processed as DI's. This  
allows Phobos CTFE dependencies on the DRT to remain valid while still  
allowing DI's to be generated for parts where they matter, with the goal  
of making both a shared and static library build of the DRT. The tool I am  
using to accomplish this feat? cp. It works, it delivers exactly what we  
need and it's *is not* a broken operation like the current DI generation.

Like Steve said, most people generating DI files are not really worried  
about CTFE working, in fact they almost undoubtedly *know* that they are  
breaking CTFE, yet they choose to do it anyways. They have their reasons,  
and frankly, it doesn't concern us as compiler writers if those reasons  
don't line up with our personal moral world-view. Our job is to provide a  
tool that DOES WHAT PEOPLE EXPECT. Otherwise they will move on to one that  
does. If people expected DI generation to be glorified (and not broken)  
copy operation, they would (and do) use cp.

-- 
Adam Wilson
IRC: LightBender
Project Coordinator
The Horizon Project
http://www.thehorizonproject.org/