[ABI] [C++20] [Modules] Don't generate vtable if the class is defined in other module unit

Close https://github.com/llvm/llvm-project/issues/61940.

The root cause is that clang will generate vtable as strong symbol now
even if the corresponding class is defined in other module units. After
I check the wording in Itanium ABI, I find this is not inconsistent.
Itanium ABI 5.2.3
(https://itanium-cxx-abi.github.io/cxx-abi/abi.html#vague-vtable) says:

> The virtual table for a class is emitted in the same object containing
> the definition of its key function, i.e. the first non-pure virtual
> function that is not inline at the point of class definition.

So the current behavior is incorrect. This patch tries to address this.
Also I think we need to do a similar change for MSVC ABI. But I don't
find the formal wording. So I don't address this in this patch.

Reviewed By: rjmccall, iains, dblaikie

Differential Revision: https://reviews.llvm.org/D150023

1 files changed, 8 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
index 32259d1..fb0276a 100644
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -1172,9 +1172,16 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
   if (!keyFunction)
     return false;
 
+  const FunctionDecl *Def;
   // Otherwise, if we don't have a definition of the key function, the
   // vtable must be defined somewhere else.
-  return !keyFunction->hasBody();
+  if (!keyFunction->hasBody(Def))
+    return true;
+
+  assert(Def && "The body of the key function is not assigned to Def?");
+  // If the non-inline key function comes from another module unit, the vtable
+  // must be defined there.
+  return Def->isInAnotherModuleUnit() && !Def->isInlineSpecified();
 }
 
 /// Given that we're currently at the end of the translation unit, and