[C++20][Modules] Build module static initializers per P1874R1.

Currently we only implement this for the Itanium ABI since the correct
mangling for the initializers in other ABIs is not yet known.

Intended result:

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 initializers 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.

Implementation:

 - 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.

Differential Revision: https://reviews.llvm.org/D126189

1 files changed, 10 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaModule.cpp b/clang/lib/Sema/SemaModule.cpp
index e9a1ac17..f5c24bd 100644
--- a/clang/lib/Sema/SemaModule.cpp
+++ b/clang/lib/Sema/SemaModule.cpp
@@ -344,6 +344,16 @@ Sema::ActOnModuleDecl(SourceLocation StartLoc, SourceLocation ModuleLoc,
   // statements, so imports are allowed.
   ImportState = ModuleImportState::ImportAllowed;
 
+  // For an implementation, We already made an implicit import (its interface).
+  // Make and return the import decl to be added to the current TU.
+  if (MDK == ModuleDeclKind::Implementation) {
+    // Make the import decl for the interface.
+    ImportDecl *Import =
+        ImportDecl::Create(Context, CurContext, ModuleLoc, Mod, Path[0].second);
+    // and return it to be added.
+    return ConvertDeclToDeclGroup(Import);
+  }
+
   // FIXME: Create a ModuleDecl.
   return nullptr;
 }