[cfe-dev] Modules TS Work

Whisperity via cfe-dev cfe-dev at lists.llvm.org
Sat Oct 13 04:40:51 PDT 2018

As I have now delved a bit into the Modules TS implementation from a
user standpoint (due to having implemented support for it in CMake,
see the patch here:
https://gitlab.kitware.com/cmake/cmake/merge_requests/2482 )
(Some of these concerns are already voiced by Boris in
https://build2.org/article/cxx-modules-misconceptions.xhtml#build )

 1. The first big issue is that while the "C++98 Modules" is somewhat
well-documented, the "Modules TS" has absolutely no workable
documentation available at all... I had to resort to looking up
Clang's option td files to figure out what this thing is supposed to

 2. Currently, Clang is wonky on how modules can be found. You need to
explicitly specify the modules' location which (the prebuilt, if you
want to allow caching of PCMs and no re-read of CPPM files all the
time...) has to be a single folder. The PCM must be available prior to
compilation so the main file is semantically correct, but *it also has
to be linked* against the resulting binary, otherwise implementations
will provide linker errors. (If I link .o instead of .pcm, it will
still work, and .o files are smaller than PCMs, but compilation will
have to semantically analyse the CPPM file providing no speedup on
module contents.) There is no direct sanity check that I'm linking the
same PCM that I used for compiling, and it looks like the PCM must be
linked *explicitly* (so the '-fprebuilt-modules-path' doesn't work

 3. This might be very CMake specific or I only found this issue
because working on CMake recently, but it is a real pain the behind to
figure out on which modules a translation unit depends. Unlike '-M'
which nicely spits out imports, the only option I saw so far is to
grep for the import statements and then try to map these to the module
files under the prebuild folder. (This is one of the issues I just
didn't succeed (perhaps yet?) in CMake because CMake is also
*somewhat* wonky, at dependency check time it does not know about
other targets and configuration, so I just can't implictly and
automatically say "Okay now please depend on
./whatever/path/mymodule.pcm or it's corresponding cppm input...)
I think this eventually needs to be considered because -M respects
compiler options when resolving the headers, so I think it's logical
that -M (or "-fmodules-list-dependencies" or whatever...) could also
respect where the prebuild dir is and spit out the PCM paths (or even
better the original CPPMs' -- I think PCM contains a backwards link to
where it was built from?) the TU depends on.

David Blaikie via cfe-dev <cfe-dev at lists.llvm.org> ezt írta (időpont:
2018. okt. 4., Cs, 20:02):
> Yeah, Richard seems to be working on modules TS features when he has time amongst other priorities.
> Any particular areas you've found gaps you've been interested in filling?
> On Sat, Sep 29, 2018 at 8:15 AM Matt Asplund (mwasplund) via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>> Hello,
>> I am curious if there is any active work being done on the Modules TS implementation. I would like to help fill in the gaps that I am finding while playing around with it and wanted to make sure I wasn’t duplicating work.
>> -Matt
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