Re: [PATCH v5 4/5] c++modules: report imported CMI files as dependencies

[Ben Boeckel <ben.boeckel@kitware.com>]

On Wed, Jul 19, 2023 at 17:11:08 -0400, Nathan Sidwell wrote:
> GCC is neither of these descriptions.  a CMI does not contain the transitive 
> closure of its imports.  It contains an import table.  That table lists the 
> transitive closure of its imports (it needs that closure to do remapping), and 
> that table contains the CMI pathnames of the direct imports.  Those pathnames 
> are absolute, if the mapper provded an absolute pathm or relative to the CMI repo.
> 
> The rationale here is that if you're building a CMI, Foo, which imports a bunch 
> of modules, those imported CMIs will have the same (relative) location in this 
> compilation and in compilations importing Foo (why would you move them?) Note 
> this is NOT inhibiting relocatable builds, because of the CMI repo.

But it is inhibiting distributed builds because the distributing tool
would need to know:

- what CMIs are actually imported (here, "read the module mapper file"
  (in CMake's case, this is only the modules that are needed; a single
  massive mapper file for an entire project would have extra entries) or
  "act as a proxy for the socket/program specified" for other
  approaches);
- read the CMIs as it sends to the remote side to gather any other CMIs
  that may be needed (recursively);

Contrast this with the MSVC and Clang (17+) mechanism where the command
line contains everything that is needed and a single bolus can be sent.

And relocatable is probably fine. How does it interact with reproducible
builds? Or are GCC CMIs not really something anyone should consider for
installation (even as a "here, maybe this can help consumers"
mechanism)?

> On 7/18/23 20:01, Ben Boeckel wrote:
> > Maybe I'm missing how this *actually* works in GCC as I've really only
> > interacted with it through the command line, but I've not needed to
> > mention `a.cmi` when compiling `use.cppm`. Is `a.cmi` referenced and
> > read through some embedded information in `b.cmi` or does `b.cmi`
> > include enough information to not need to read it at all? If the former,
> > distributed builds are going to have a problem knowing what files to
> > send just from the command line (I'll call this "implicit thin"). If the
> > latter, that is the "fat" CMI that I'm thinking of.
> 
> please don't use perjorative terms like 'fat' and 'thin'.

Sorry, I was internally analogizing to "thinLTO".

--Ben