Close https://github.com/llvm/llvm-project/issues/71034
See
https://discourse.llvm.org/t/rfc-c-20-modules-introduce-thin-bmi-and-decls-hash/74755
This patch introduces reduced 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 reduced BMI
contains the same behavior with the existing and relatively stable
fatBMI. This is pretty helpful for further reduction.
The user interfaces part it left to following patches to ease the
reviewing.
With overriden input files, e,g,. the compiler get the file from an
in-memroy buffer, the compiler can't get correct modified time information
to indicate whehter the input files are changed or not. Then, the
semantics of ForceCheckCXX20ModulesInputFiles are broken.
In this patch, if both ForceCheckCXX20ModulesInputFiles and
ValidateASTInputFilesContent and enabled, the compiler will still check the
hash value of the contents even if their modification time is the same.
