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

Thu Oct 28 18:05:54 PDT 2021

On Thu, Oct 28, 2021 at 5:51 PM Richard Smith <richard at metafoo.co.uk> wrote:

> On Thu, 28 Oct 2021 at 17:06, David Blaikie <dblaikie at gmail.com> wrote:
>
>> On Thu, Oct 28, 2021 at 4:58 PM Richard Smith via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> 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.
>>>
>>
>> Ah, that's good to know - didn't know you were inclined/supportive of
>> this direction (as the only way to build a module - or some mode that'd do
>> it as two-step too?) - one of the previous counterarguments was that
>> producing the .pcm without the .o unblocked consumers sooner/let the .o
>> generation be done in parallel with those consumers. Is that generally
>> known/considered to be too small of a benefit to be worth the build/support
>> complexity compared to the minimal-pcm+.o in-one-go mode & its benefits
>> (smaller .pcms)?
>>
>
> I think it's likely there'll be reasonable build strategies that want to
> build a minimal PCM and a .o file with two separate actions (to maximize
> throughput in highly parallel builds), and there'll be reasonable build
> strategies that want to build them as part of the same action (to minimize
> total time in a build with less parallelism). I expect people will want
> both options to be available. The option that we currently provide --
> producing a PCM file that can be used as an input to both .o generation and
> for import -- is probably not well aligned with what most build strategies
> will want.
>

Reckon there just aren't enough savings in reusing the PCM for .o
generation compared to parsing from scratch? Not enough to justify adding
an extra intermediate file (a full pcm that gets consumed for .o generation
and a slim pcm that gets consumed by uses). That we'll move away from
complete pcms entirely to only minimal pcms? Fair enough. Good to
know/think about.

>
> 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
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>
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>>
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