[clang] [C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (PR #75912)

[David Blaikie via cfe-commits]

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@@ -1180,6 +1185,21 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
       TSK == TSK_ExplicitInstantiationDefinition)
     return false;
 
+  // Itanium C++ ABI [5.2.3]:
+  // Virtual tables for dynamic classes are emitted as follows:
+  //
+  // - If the class is templated, the tables are emitted in every object that
+  // references any of them.
+  // - Otherwise, if the class is attached to a module, the tables are uniquely
+  // emitted in the object for the module unit in which it is defined.
+  // - Otherwise, if the class has a key function (see below), the tables are
+  // emitted in the object for the translation unit containing the definition of
+  // the key function. This is unique if the key function is not inline.
+  // - Otherwise, the tables are emitted in every object that references any of
+  // them.
+  if (Module *M = RD->getOwningModule(); M && M->isNamedModule())
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dwblaikie wrote:

Rather than doing this based on a named module, I wonder if this could be powered by the same system that originally worked for clang header modular code generation. It worked/works by adding a bit to declarations about whether to home them to a modular object file

e6b7c28d17edf428c58ba65c549109c0a488a5be

Though I guess in 1ac9c98e6c40d9a1e6cb904b414161388af4a89c I used the bit on a type to mean "home this type in debug info" and the bit on a function to mean "home this function's code" - so I'm not sure what we'd do to track the vtable specifically... Maybe not worth it/not feasible, not sure.


https://github.com/llvm/llvm-project/pull/75912