[clang] [C++20] [Modules] Introduce -fgen-reduced-bmi (PR #85050)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> Do I miss something? The performance and file size is similar with and without `-fgen-reduced-bmi`. To reproduce clone https://github.com/koplas/clang-modules-test and run `build.sh` and `build_thin_bmi.sh`.

Currently, the Reduced BMI will only remove function bodies for non-inline functions and definitions for some variables...

And declarations in headers or GMF are primarily inline, so probably the current Reduced BMI won't produce a strong impact on modules whose contents are primarily in GMF as expectedly.

For example, I can observe some impact on https://github.com/alibaba/async_simple/tree/CXX20Modules, where the repo has more use codes in the module purview instead of the GMF.

Further, in my mind, we can reduce more things in Reduced BMI. An example may be https://github.com/llvm/llvm-project/pull/76930.

Thanks for testing : )

https://github.com/llvm/llvm-project/pull/85050