[cfe-dev] Make command line support for C++20 module uniform with GCC

[Richard Smith via cfe-dev]

On Mon, 25 Oct 2021 at 01:57, chuanqi.xcq via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi all,
>
>    Recently I am playing with C++20 modules and I found that the command
> line support of GCC
> is much better than Clang. Here is an example:
>
> ```C++
> // say_hello.cpp
> module;
> #include <iostream>
> #include <string_view>
> export module Hello;
> export void SayHello
>   (std::string_view const &name)
> {
>   std::cout << "Hello " << name << "!\n";
> }
> // main.cpp
> #include <string_view>
> import Hello;
> int main() {
>   SayHello("world");
>   return 0;
> }
> ```
>
> To compile the example, in gcc we need:
> ```
> g++ -std=c++20 -fmodules-ts say_hello.cpp main.cpp
> ```
>
> And in clang, we need:
> ```
>
> clang++ -std=c++20 -fmodules-ts -Xclang -emit-module-interface -c
> say_hello.cpp -o Hello.pcm
>
> clang++ -std=c++20 -fmodules-ts -fprebuilt-module-path=. main.cpp
> say_hello.cpp
> ```
>

Your point is well-taken. However, some part of the extra work required
here is that you're not doing things in the expected way.

The above is not a correct way to enable C++20 modules in Clang:
-fmodules-ts enables the old Modules TS mode, not C++20 modules. -std=c++20
is enough to enable C++20 modules.

For the '-Xclang -emit-module-interface' portion, what Clang expects is
that files that define module interfaces are either named .cppm or are
specified with -x c++-module. With that file type, you can use --precompile
to produce a .pcm file (just like you'd use -E or -c to produce other kinds
of output). For example:

clang++ -std=c++20 say_hello.cppm --precompile -o Hello.pcm

The above commands are also parsing say_hello.cpp twice. You can avoid that
by using the precompiled form, say_hello.pcm, as a compilation input
instead:

clang++ -std=c++20 -fprebuilt-module-path=. say_hello.pcm main.cpp

However, this is all based on a model where the PCM file contains a
complete description of the input .cppm file, which is not a great model
for us to use moving forward due to all the extra stuff ending up in the
.pcm file. Currently, Clang lacks two important features here:

1) Produce a .pcm file and a .o file from a single compilation action.
2) Produce a .pcm file that contains only the information needed for an
importer, not a complete description of the input.

We will of course need some command-line support for those features, and
being compatible with GCC (which already provides these features) would
likely make sense.

As for building and using modules in a single clang command, I agree that'd
be nice to have, both for convenience and for GCC compatibility. But
ideally this shouldn't depend on what order the files are specified in on
the command line, which would require some kind of pre-scanning to find
which modules are defined in which files so they can be processed in
topological order. (Otherwise, specifying the files in the wrong order
would presumably result in stale .pcm files getting used, which would seem
quite user-hostile. I don't know if that's what you get from GCC or if it
does better somehow.) That kind of prescan might be more complexity than
we'd want in the compiler driver, though we can discuss that and figure out
where we want to draw that line.

In any case, I'm hoping we get some clear guidance from SG15 that we can
follow.

Yeah, in clang we need to another line to emit module interface explicitly
> and another option
> to tell the prebuilt-module-path. And in GCC, this happens by default,
> when GCC find it is compiling
> a c++20 module, it would generate the module interface automatically to
> the path:
> ```
> gcm.cache/filename.gcm
> ```
> It would create `gcm.cache` in case it doesn't exist.
>
> And GCC would search prebuilt module interface in `gcm.cache`
> automatically.
>
> It looks much more friendly to me. The intention of this mail is to ask if
> you think it is the right direction
> to make the clang's command line support for c++20 module more like GCC.
> The different I see now includes:
> - Generate prebuilt module interface automatically. (And generate it to a
> specific directory automatically)
> - Have a default value for prebuilt module path.
>
> I am wondering if any one more familiar with the clang's command line and
> file system would love to
> support this (I am not so familiar with it). Although It may take more
> time, I would love to support if others are busy.
>
> Thanks,
> Chuanqi
> _______________________________________________
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> cfe-dev at lists.llvm.org
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>
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