C++20 module mapper

C++20 module mapper

[Alexander Christensen]

Hi, I hope I write to the right place.

I would like to know how to use the module mapper, documented here https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Module-Mapper.html,
to change the location of the expected gcm.cache directory. Given an example project layout:

src/
    Makefile
    project_1/
        // .cpp source files
        Makefile
    project_2/
        // .cpp source files
        Makefile

Compiling from inside project_1 and project_2 folders does not work with the default module mapper, as gcc expects the module cache
to be located in current working directory, which is a problem if project_2 references modules built in project_1.
However, I can specify a module-mapping file. But I can't get it to work.

What would be the correct syntax for such a file, located in project_2, to override the default module cache to read from modules in
src/gcm.cache/ and NOT src/project_2/gcm.cache/ ? This does not work:

// module_mapping.txt
'$root' ../gcm.cache/

And I do not want to prepend "cd .. && " to all build commands in src/project_2/Makefile.


Sincerely,
Alexander

Re: C++20 module mapper

[Andrew Jones]

Hi Alexander,

Actually, I've done some work on this:

      https://github.com/andrewvaughanj/cpp20_module_examples

and more specifically:

      https://github.com/andrewvaughanj/cpp20_module_examples/blob/main/do_gcc.sh

Please take a look and feel free to ask if you have any questions -- cheers,

Andrew

On Tue, Oct 19, 2021 at 4:58 PM Alexander Christensen via Gcc-help
<gcc-help@gcc.gnu.org> wrote:
>
> Hi, I hope I write to the right place.
>
> I would like to know how to use the module mapper, documented here https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Module-Mapper.html,
> to change the location of the expected gcm.cache directory. Given an example project layout:
>
> src/
>     Makefile
>     project_1/
>         // .cpp source files
>         Makefile
>     project_2/
>         // .cpp source files
>         Makefile
>
> Compiling from inside project_1 and project_2 folders does not work with the default module mapper, as gcc expects the module cache
> to be located in current working directory, which is a problem if project_2 references modules built in project_1.
> However, I can specify a module-mapping file. But I can't get it to work.
>
> What would be the correct syntax for such a file, located in project_2, to override the default module cache to read from modules in
> src/gcm.cache/ and NOT src/project_2/gcm.cache/ ? This does not work:
>
> // module_mapping.txt
> '$root' ../gcm.cache/
>
> And I do not want to prepend "cd .. && " to all build commands in src/project_2/Makefile.
>
>
> Sincerely,
> Alexander
>

Re: C++20 module mapper

[Andrew Jones]

Hi Alexander,

(re-added gcc-help in case this response piques any wider interests)

Ah, sorry, I'd misunderstood your original question then; I've never
used `-fmodules-cache-path` with `clang` -- does this allow you
specify where the `.pcm`s go (rather than the current working
directory)? That is, that `-emit-module-interface` automatically emits
into `-fmodules-cache-path`? If so, with what name?

Nonetheless, I am unaware of any such functionality inside of gcc to
match this, without, of course, writing your own mapper (which you
don't want to do). What you've done with symlinks clearly achieves it,
but I'm not sure if that's better/worse than writing to a mapper file.

As far as I know, making the mapper file is also the approach that
CMake takes: https://github.com/Kitware/CMake/blob/master/Help/dev/experimental.rst

Cheers,

Andrew

On Tue, Oct 19, 2021 at 6:24 PM Alexander Christensen
<alex_c007@hotmail.com> wrote:
>
> Hi Andrew,
>
> Thank you for the fast reply, and helpful source.
> Though that did not actually answer my question.
>
> I wanted to know if we could use the module mapping file with gcc to simulate what the flag "-fmodules-cache-path"
> (presumable) does for clang?
>
> The syntax "<$root ??> ../gcm.cache" (or permutations thereof) seems to have no effect.
> And I do not want to specify explicit mapping for each module.
> Much rather, I would like to use the default cache, but at a different location, both for reading and writing.
>
> My current hack involves symlinks as here: https://github.com/alexpanter/modules_testing/blob/master/gcc-testing/test_07_gameengine/init.sh
>
> Would you happen to know about that?
>
> Kind regards,
> Alexander
>
> ________________________________
> Fra: Andrew Jones <andrewvaughanj@gmail.com>
> Sendt: 19. oktober 2021 18:45
> Til: Alexander Christensen <alex_c007@hotmail.com>
> Cc: gcc-help@gcc.gnu.org <gcc-help@gcc.gnu.org>
> Emne: Re: C++20 module mapper
>
> Hi Alexander,
>
> Actually, I've done some work on this:
>
>       https://github.com/andrewvaughanj/cpp20_module_examples
>
> and more specifically:
>
>       https://github.com/andrewvaughanj/cpp20_module_examples/blob/main/do_gcc.sh
>
> Please take a look and feel free to ask if you have any questions -- cheers,
>
> Andrew
>
> On Tue, Oct 19, 2021 at 4:58 PM Alexander Christensen via Gcc-help
> <gcc-help@gcc.gnu.org> wrote:
> >
> > Hi, I hope I write to the right place.
> >
> > I would like to know how to use the module mapper, documented here https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Module-Mapper.html,
> > to change the location of the expected gcm.cache directory. Given an example project layout:
> >
> > src/
> >     Makefile
> >     project_1/
> >         // .cpp source files
> >         Makefile
> >     project_2/
> >         // .cpp source files
> >         Makefile
> >
> > Compiling from inside project_1 and project_2 folders does not work with the default module mapper, as gcc expects the module cache
> > to be located in current working directory, which is a problem if project_2 references modules built in project_1.
> > However, I can specify a module-mapping file. But I can't get it to work.
> >
> > What would be the correct syntax for such a file, located in project_2, to override the default module cache to read from modules in
> > src/gcm.cache/ and NOT src/project_2/gcm.cache/ ? This does not work:
> >
> > // module_mapping.txt
> > '$root' ../gcm.cache/
> >
> > And I do not want to prepend "cd .. && " to all build commands in src/project_2/Makefile.
> >
> >
> > Sincerely,
> > Alexander
> >