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[C++20] [Modules] Introduce thin BMI #71622

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[C++20] [Modules] Introduce thin BMI #71622

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Commits on Nov 8, 2023

  1. [C++20] [Modules] Introduce thin BMI 

    Close llvm#71034

This patch introduces thin BMI, which doesn't contain the definitions of
functions and variables if its definitions won't contribute to the ABI.

Testing is a big part of the patch. We want to make sure the thin BMI
contains the same behavior with the existing and relatively stable
fatBMI. This is pretty helpful for further reduction.

For user interfeaces, this patch introduces `-fthinBMI-output=`
arguments to specify the position of thin BMI. This should be used when
compiling a single module unit.

The design is helpful to use thin BMI in two phase compilations too.
With thin BMI, In two phase compilations, we'll generate 2 BMIs, one
thin BMI for being used by consumers, one fat BMI for compiling itself
to object files. Maybe it sounds confusing to have 2 BMIs for one module
unit. But only the thin BMI will be the BMI we're talking about
generally and the fat BMI is only visible by the module unit itself.

With one phase compilation, we may find the behavior of
`-fthinBMI-output=` is pretty similar with `-fmodule-output=`, except
one generating thin BMI and the other generating fat BMI. The design
here is based on 2 things:
(1) The serialization of C++ is pretty complex. We can't be sure we're handling
every detail correctly in the every beginning.
(2) The fat BMI is relatively widely used and relatively stable.
So it looks not good to replace the fat BMI immediately with thin BMI.

But, of course, in the end of the day, we want the consumers to use the
thin BMI only. When that day comes, the `-fmodule-output=` will be an alias to
`-fthinBMI-output=`.

Another design choice is to reuse `-fmodule-output=` and introduce a flag
`-femit-thin-BMI`. Then `-femit-thin-BMI -fmodule-output=` will have the same effect
with `-fthinBMI-output=` now.
The flag `-femit-thin-BMI` should be opt-in now and opt-off later
and finally deprecated.

The roadmap for thin BMI in my mind is:

(1) In clang18, release thin BMI and mark it as experimental. Also
encourage users and build systems to try this new mode.
(2) In clang19 or clang20 (based on the issue feedbacks), remove the
experimental mark for thin BMI and mark fat BMI as deprecated to be
used by consumers.
(3) In clang21 or clang22, error out if we found the users are trying to
import a fat BMI.
    @ChuanqiXu9
    ChuanqiXu9 committed Nov 8, 2023
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  2. fmt

    @ChuanqiXu9
    ChuanqiXu9 committed Nov 8, 2023
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