[challenge] Linker surgery

[Rainer Schuetze]

On 18.05.2013 00:49, Dmitry Olshansky wrote:
> 17-May-2013 21:57, Rainer Schuetze пишет:
>>
>>
>> On 17.05.2013 14:29, Dmitry Olshansky wrote:
>>> 15-May-2013 04:17, IgorStepanov пишет:
>>>> Do this table linked, if you remove all functions, which use it?
>>>
>>> Thanks for this try, but they DO link in always.
>>> And I believe this is a key problem - each function goes into a separate
>>> object but globals are always pulled in!
>>>
>>
>> Yes, if you build a library the functions in a module are split into
>> separate object files, but data is always written into the object file
>> of the original module. The linker cannot split these afterwards if any
>> data in the module is referenced (which might happen by just importing
>> the module).
>
> And how then I would use these tables if even importing these modules
> then pulls in the data?
>

It depends. An imported module is referenced if it is part of the 
dependency chain for static constructors, i.e if it contains static 
this() or imports something that contains it. A module with just data in 
it should be fine.


> Local import doesn't help too.
>
>> A workaround could be to put the data into a different module.
>
> Then say I go for X files with tables and import these modules.
> It still doesn't work - evidently simply referencing a module pulls in
> the data.
>
> If I compile and link the following 3 modules:
>
>
> module fmod;
>
> public immutable int[] fable = [1,2,3];
>
>
> module mod;
>
> import fmod;
>
> int foo(int i)
> {
>         return fable[i];
> }
>
> //driver
> import mod;
>
> immutable byte[] bable = [1, 2, 3, 4, 5];
>
> byte boo(int ch){
>      return bable[ch];
> }
>
> void main(string[] args){
>      boo(0);
> }
>
> I still have this symbol in map file: _D4fmod5fableyAi
> That is immutable(int[]) fmod.fable after ddemangle.
>

If you compile all the files on the command line for the executable 
everything gets dragged in. You will have to build the first two modules 
into a library:

dmd -lib fmod.d mod.d -ofm.lib
dmd -map driver.d m.lib
grep fmod driver.map

Splitting functions into separate object files also only happens when 
-lib is specified.