clang 编译的模块 pcm 文件有什么用？

Question

I'm looking at the driver test cases for clang modules: https://github.com/llvm-mirror/clang/blob/master/test/Driver/modules.cpp

It includes steps to produce.pcm.o files. I'm wondering what they are for.

Given a c++20 module

// a-m.cc
module;
#include <iostream>
export module a;
export void do_a() { std::cout << "A\n"; }

You can compile it using

clang++ -std=c++20 -x c++-module --precompile a-m.cc -o a.pcm

which produces the precompiled module file a.pcm .

But there are also steps to compile.pcm files to.o files.

From the driver tests :

clang++ -std=c++20 a.pcm -S -o a.pcm.o

How are the.pcm.o files meant to be used?

If I write a main program

// main.cc
import a;

int main() {
    do_a();
    return 0;
}

Compile with

clang++ -std=c++20 -c main.cc -fmodule-file=a=a.pcm

And then try to link with the.pcm.o, I get

clang++ main.o a.pcm.o
/usr/bin/ld:a.pcm.o: file format not recognized; treating as linker script
/usr/bin/ld:a.pcm.o:2: syntax error
clang-13: error: linker command failed with exit code 1 (use -v to see invocation)

Note: you can compile am.cc

clang++ std=c++20 -c a-m.cc -o a.o

and link with ao, but what are the.pcm.o files for? and can they be used to avoid compiling am.cc again after you've precompiled it?

Answer 1

The pcm-files contains "half baked" information about a module. The file extension does not have to be pcm and i think Visual Studio for example uses .ifc as extension. (And a difference with Visual Studio is that it outputs the ifc-file and compiles the object file ( .obj ) at the same time, while clang does this in two separate steps)

The pcm file is storing the information about the module file in a format so that the compiler can easily import it or compile it to other object files.

The command from your link surprises me, I think that -S in the example is compiling to assembly, so i am surprised that it is listed in that clang repo that you linked to.

clang++ -std=c++20 a.pcm -S -o a.pcm.o # To mee this looks wrong (assembly output)

Try changing it to use -c instead

clang++ -std=c++20 a.pcm -c -o a.pcm.o # This is how you compile pcm-files to object files