[PATCH] D134267: [C++] [Modules] Support one phase compilation model for named modules

Mon Oct 31 00:58:30 PDT 2022

ChuanqiXu added a comment.

In D134267#3892629 <https://reviews.llvm.org/D134267#3892629>, @ben.boeckel wrote:

> In D134267#3876071 <https://reviews.llvm.org/D134267#3876071>, @dblaikie wrote:
>
>> I'm getting a bit exhausted with all the words involved here & not sure how to simplify/clarify this.
>>
>> If @ben.boeckel has particular use cases, it might be easier for him to be here discussing them so we can discuss the tradeoffs directly rather than through intermediaries.
>
> Sorry, I've been lax on actually keeping up-to-date on all of these Clang threads.
>
> The current state is that CMake's test suite for C++ modules does not work with clang because it doesn't provide the control CMake needs to do its explicit builds. @ChuanqiXu's MyP1689 branch is close except that the `.pcm` output path can't seem to be controlled closely enough for reliable builds. Namely, I would like:
>
> - the ability to disable the module cache (completely); CMake doesn't need nor want it
> - the ability to say put the generated `.pcm` that this TU will generate goes *here*
>
> The latter is currently a juggle of flags and restrictions that I haven't been able to figure out. Without something like `-fmodule-output-path=`, convincing Clang to output its `.pcm` file to where CMake wants it to be is some combination of `-fmodules-cache-path=` and `-fmodule-name=` while hoping that the internal path computation inside of Clang makes what CMake wants. It's just far easier to have something like `-o` that gives the answer straight away.
>
> For the former, I use `-x c++module` as well so that I don't have to care about any extension-sniffing behaviors. I also need to give a bogus `-fmodule-cache-path=` for module consumers because as soon as `import` is seen, it wants to run off to look at the cache instead of noticing that there's a `-fmodule-file=` for everything it needs already and the cache is 100% unused.

BTW, the bogus `-fmodule-cache-path=` for module consumers can be addressed by the existing `-fprebuilt-module-path`. And I sent the new revisions in D137058 <https://reviews.llvm.org/D137058> and D137059 <https://reviews.llvm.org/D137059>.

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https://reviews.llvm.org/D134267