[C++20] [Modules] Try to introduce thin BMI to exclude the things not necessary in an interface

[ChuanqiXu9]

Let's call the current BMI for named modules as fat BMI. Since it contains all the information needed to compile the BMI itself to an object file. And this is the foundation of 2 phase compilations.

However, it is a waste of time and space if we're using one phase compilations.

Then let's call the BMI which doesn't contain all the information to produce an object file, but is able to satisfy the requirement of consumers of the modules as thin BMI. Then it is clear that we should use thin BMI then fat BMI in one phase compilation.

Also in 2 phase compilations, the thin BMI may be helpful too if the build systems want to use a compare-and-swap strategy to generate the BMI based on the BMIDeclsHash (See #71618).

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Chuanqi Xu (ChuanqiXu9)

According to https://isocpp.org/files/papers/P3033R0.html, when we change the body of a non-inline functions in a module purview, the generated BMI shouldn't change.

For example,

export module a;
export int a() {
    return 43;
}

If we change the implementation from return 43; to return 44; the generated BMI shouldn't change. This is pretty helpful to improve the user's experience by avoiding unnecessary rebuildings as much as possible.

This covers: #70569.

Note that the later discussion in SG15 reveals that the paper is not conflicting with 2 phase compilation. Since we can generate 2 BMIs. One for users and one for building self.