Re: GCC IMA & general future proposal

["Giovanni Bajo" <giovannibajo at libero dot it>]

Jan Hubicka wrote:

>> It is still useful in such a framework for front ends to be able to
>> handle multiple translation units at once as C does, and pass off
>> GENERIC for all
>> of them to the next level at once, so that various
>> cross-translation-unit consistency checks can be made which depend
>> on language-specific rules.
>
> Also for C++ you don't need to end up with duplicated template bodies
> everywhere.

This is going to be complex though. If you look at the EDG frontend, they use
an iterative system to generate the minimum set of template instantiations (at
least, by default):

- When a file is first compiled, no templates are instantiated. Instead, a
separate text file (.ti IIRC) is generated with a list of the instantiations
that the file requires.
- At link time, all these files are collected and merged, getting an unique
lists of all the template instantiations needed.
- A new set of text files (.to IIRC) is generated, which contains the list of
instantiations which are requested for each file.
- The files with a .to list are recompiled. The presence of the .to file
automatically triggers the requested template instantiations. The .ti file is
generated again: it can be a superset of the previous one because the requested
instantiations might generate new dependencies on missing instantiations.
- The process is re-iterated: the .ti files are merged again, and if there are
missing instantiations, new .to files are generated, and so on.

Of course, special care must be taken at link time when assigning
instantiations to files, to avoid endless loops, and minimize recompilations.
EDG used to have bugs like this which are very hard to reduce and reproduce.

An additional good point of this system is that it is kind of "stable": if you
modify a single file, assuming you don't ask for more instantiations, you do
not have to recompile anything when you link. My experience is that building
big applications from scratch might requires several instantiation loops (up to
3 in my applications), but after that the compilation times after touching a
small set of files is much decreased.

I think that such a system could be added to GCC as well, by using a new option
(-fcool-template-processing) and a special collect2-like program to be run at
link time.

In other words, I believe a solution to the problem of minimizing the template
instantiations is orthogonal to IMA and its design.

Giovanni Bajo