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.
There were two assertions in DefaultArgStorage::setInherited previously.
It requires the DefaultArgument is either empty or an argument value. It
would crash if it has a pointer refers to the previous declaration or
contains a chain to the previous declaration.
But there are edge cases could hit them actually. One is
InheritDefaultArguments.cppm that I found recently. Another one is pr31469.cpp,
which was created fives years ago.
This patch tries to fix the two failures by handling full cases in
DefaultArgStorage::setInherited.
This is guaranteed to not introduce any breaking change since it lives
in the path we wouldn't touch before. And the added assertions for
sameness should keep the correctness.
Reviewed By: v.g.vassilev
Differential Revision: https://reviews.llvm.org/D128974
