Compiler as dll

[Daniel Keep]

grauzone wrote:
> John Reimer wrote:
>> ddl does not work for memory sharing like normal dll's, where multiple
>> applications have access to a single dll at runtime.  It appears that
>> such support would be quite difficult to implement and moves in the
>> direction of operating system features.
> 
> Couldn't this be achieved by simply mmap()-ing the file contents into
> memory? mmap() normally shared the memory pages with other processes.

I'm sure users of DDL would love for you to submit a patch. :)

> Of course, this wouldn't work if the code both isn't position
> independent, and needs to be relocated to a different base address. But
> that's also the case with operating system supported dynamic shared
> objects.
> 
>> It does do runtime linking, however, which is extremely useful for
>> certain situations... specifically any sort of application that needs
>> a plugin architecture for D (ie.. it can link with libraries and
>> object files at runtime) that is gc and exception friendly.  
> 
> I never understood why this is needed. Can't they simply compile the
> plugins into the main program?

A plugin architecture, by definition, is to let third parties add code
to your application.  This rather precludes being able to simply link it in.

> When it's a commercial program, the DLL plugin approach probably
> wouldn't work anyway: in order to enable others to compile plugins, you
> would need to expose your internal "headers" (D modules). Note that
> unlike in languages like C/C++, this would cause internal modules to be
> exposed too, even if they are not strictly needed. What would you do to
> avoid this? Maintain a separate set of import modules?

As Alexander Pánek said; you can just use .di files, which the compiler
can create from existing .d modules.

Heck, you can just use regular .d modules and stub out the
implementations.  There's no reason why you'd need to release your code
to third parties.

Templates are a different matter, but then C++ has the same problem.
Whether or not you want to release your templates as part of the SDK
really depends on what they are.  Templates + interfaces make a good pair.

> I think a purely extern(C) based interface would be better in these cases.
> 
> In fact, if you rely on the D ABI for dynamic linking, you'll probably
> have the same trouble as with C++ dynamic linking. For example, BeOS had
> to go through this to make sure their C++ based API maintains ABI
> compatibility:
> 
> http://homepage.corbina.net/~maloff/holy-wars/fbc.html
> 
> I'm not sure if the D ABI improves the situation. At any rate, it
> doesn't sound like a good idea.

There's some interesting problems at that article that maybe we should
ask Walter about.  For one, being able to control virtual function
ordering is an interesting idea.

But by and large, these are the same problems you'll get with ANY ABI.
If the size of a struct changes in a C ABI, you're hosed just as bad as
if the size of a struct in D, or the fields of a class.

  -- Daniel