[C++][Modules] `-fmodule-output` does nothing

[davidstone]

Given some module in file a.cpp

export module b;

Compiling it with

clang++ -std=c++20 -x c++-module -fmodule-output=c.pcm --precompile a.cpp

I would expect to produce a file c.pcm. Instead it produces a file a.pcm. Am I misunderstanding the purpose of this option? The documentation states

-fmodule-output=<arg>
Save intermediate module file results when compiling a standard C++ module unit.

[davidstone]

It looks like if I remove --precompile (and add -c), it behaves as I expected. So if I have --precompile, I need to use -o to determine the name of the generated pcm file, but if I don't, then I use -o for the .o file and -fmodule-output= to determine the .pcm file name. This feels surprising to me -- I would expect -o to always apply to the .o file name (if it would be generated) and -fmodule-file= to always apply to the generated .pcm.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: David Stone (davidstone)

Given some module in file a.cpp

export module b;

Compiling it with

clang++ -std=c++20 -x c++-module -fmodule-output=c.pcm --precompile a.cpp

I would expect to produce a file c.pcm. Instead it produces a file a.pcm. Am I misunderstanding the purpose of this option? The documentation states

> -fmodule-output=<arg>
> Save intermediate module file results when compiling a standard C++ module unit.

[ChuanqiXu9]

Yes, -fmodule-output tend to be working with -c options. According to the document you cide:

-fmodule-output=
Save intermediate module file results when compiling a standard C++ module unit.

If we're --precompile, the .pcm file is not intermediate module file.

So I'd like to close this one as invalid.