Compile Imported Modules

[Jonathan Marler via Digitalmars-d]

On Thursday, 24 August 2017 at 17:37:12 UTC, Jonathan Marler 
wrote:
> On Thursday, 24 August 2017 at 16:49:08 UTC, Seb wrote:
>> On Thursday, 24 August 2017 at 16:32:32 UTC, Jonathan Marler 
>> wrote:
>>> On Thursday, 24 August 2017 at 15:56:32 UTC, H. S. Teoh wrote:
>>>> On Thu, Aug 24, 2017 at 03:53:05PM +0000, Jonathan Marler 
>>>> via Digitalmars-d wrote:
>>>>> Wanted to get peoples thoughts on this.  The idea is to 
>>>>> have a way to tell the compiler (probably with a command 
>>>>> line option) that you'd like to "compile imported modules".
>>>> [...]
>>>>
>>>> Isn't this what rdmd already does?
>>>>
>>>>
>>>> T
>>>
>>> That is one thing that rdmd does (as I mentioned in the 
>>> original post).
>>>
>>> I just looked through the rdmd code 
>>> (https://github.com/dlang/tools/blob/master/rdmd.d) and it 
>>> looks like it invokes the compiler using "dmd -v" to get the 
>>> list of modules and then invokes the compiler again with the 
>>> modules it found to perform the full compile.  So my original 
>>> thought that the logic to find modules is duplicated was 
>>> incorrect.  Instead we just pay a performance hit to get the 
>>> correct list of imports since running "dmd -v" seems to take 
>>> almost as long as the actual compile itself.  So this method 
>>> comes close to doubling the time it takes to compile than if 
>>> the feature was implemented in the compiler itself.
>>>
>>> In any case, the idea is to allow the compiler to resolve 
>>> this on it's own without help from rdmd.  This would remove 
>>> the need to invoke the compiler twice, once to find the 
>>> imports and once to compile.  It would also allow some 
>>> projects/applications that don't use rdmd to take advantage 
>>> of this feature, this may or may not include dub (not sure on 
>>> that one).
>>
>> rdmd is really bad in terms of performance. If you call a 
>> single D file with rdmd, it will always compile it twice. 
>> There was an attempt to fix this 
>> (https://github.com/dlang/tools/pull/194), but this has been 
>> reverted as it introduced a regression and no one had time to 
>> look at the regression.
>> Moving rdmd into DMD has been on the TODO list for quite a 
>> while and there is a consensus that the performance overhead 
>> if rdmd isn't nice. However, IIRC there was no clear consensus 
>> on how the integration should happen. I recall that the plan 
>> was to do try this with "dmd as a library", but I'm not sure 
>> whether that's really feasible ATM.
>
> Well this should solve the rdmd performance problem as well as 
> make other user cases easier that don't necessarilly use rdmd.
>
> I had another thought that instead of making this an "opt-in" 
> feature, it would probably make more sense to be an "opt-out" 
> feature.  So by default the compiler would compile missing 
> imported modules unless you indicate otherwise, maybe a command 
> line switch like "-dont-compile-imports".  And I don't see how 
> this would break anything.  Everything should work the same as 
> it did before, it's just now you can omit imported module files 
> from the command line and it should just work.

I've looked through the DMD code to see how this could be 
implemented and I've run into a problem.  The solution I came up 
with was to go through all the imported modules and then 
determine which ones need to be compiled that haven't been given 
on the command line. The problem is, I don't know how to 
determine whether a module was already compiled and given in an 
obj/lib file.  For example,

dmd something.obj anotherthing.lib prog.d

As far as I know, the compiler has no idea which modules are 
contained in "something.obj" and "anotherthing.lib".  It just 
compiles the source given on then command line, then passes all 
the object files and libraries to the linker, at which point the 
concept of modules is lost.

Am I correct in saying that the compiler has no idea which 
modules an obj/lib file contains?