Currently we only implement this for the Itanium ABI since the correct
mangling for the initializers in other ABIs is not yet known.
For a module interface [which includes partition interface and implementation
units] (instead of the generic CXX initializer) we emit a module init that:
- wraps the contained initializations in a control variable to ensure that the inits only happen once, even if a module is imported many times by imports of the main unit.
- calls module initialisers for imported modules first. Note that the order of module import is not significant, and therefore neither is the order of imported module initializers.
- We then call initializers for the Global Module Fragment (if present)
- We then call initializers for the current module.
- We then call initializers for the Private Module Fragment (if present)
For a module implementation unit, or a non-module TU that imports at least one
module we emit a regular CXX init that:
- Calls the initializers for any imported modules first.
- Then proceeds as normal with remaining inits.
For all module unit kinds we include a global constructor entry, this allows
for the (in most cases unusual) possibility that a module object could be
included in a final binary without a specific call to its initializer.
- We provide the module pointer in the AST Context so that CodeGen can act on it and its sub-modules.
- We need to account for module build lines like this: clang -cc1 -std=c++20 Foo.pcm -emit-obj -o Foo.o or clang -cc1 -std=c++20 -xc++-module Foo.cpp -emit-obj -o Foo.o
- in order to do this, we add to ParseAST to set the module pointer in the ASTContext, once we establish that this is a module build and we know the module pointer. To be able to do this, we make the query for current module public in Sema.
- In CodeGen, we determine if the current build requires a CXX20-style module init and, if so, we defer any module initializers during the "Eagerly Emitted" phase.
- We then walk the module initializers at the end of the TU but before emitting deferred inits (which adds any hidden and static ones, fixing https://github.com/llvm/llvm-project/issues/51873 ).
- We then proceed to emit the deferred inits and continue to emit the CXX init function.