Following of https://github.com/llvm/llvm-project/pull/76930
This follows the idea of "only writes what we writes", which I think is
the most natural and efficient way to implement this optimization.
We start writing the BMI from the first declaration in module purview
instead of the global module fragment, so that everything in the GMF
untouched won't be written in the BMI naturally.
The exception is, as I said in
https://github.com/llvm/llvm-project/pull/76930, when we write a
declaration we need to write its decl context, and when we write the
decl context, we need to write everything from it. So when we see
`std::vector`, we basically need to write everything under namespace
std. This violates our intention. To fix this, this patch delays the
writing of namespace in the GMF.
From my local measurement, the size of the BMI decrease to 90M from 112M
for a local modules build. I think this is significant.
This feature will be covered under the experimental reduced BMI so that
it won't affect any existing users. So I'd like to land this when the CI
gets green.
Documents will be added seperately.
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.
Close https://github.com/llvm/llvm-project/issues/59601.
This is actually a workaround for the issue. See the comments and the
test for example. The proper fix should make the placement allocation
functions acceptable based on the context. But it is harder and more
complex on the one side. On the other side, such workaround won't be too
bad in practice since users rarely call the placement allocation
functions directly.
So personally I prefer to address such problems in the simpler way.
Reviewed By: royjacobson
Differential Revision: https://reviews.llvm.org/D141023
