rsmith added a comment.
In D99790#2681487 <https://reviews.llvm.org/D99790#2681487>, @lebedev.ri wrote:
> I think it should be fixed in `CodeGenVTables::maybeEmitThunk()`, likely in
> `arrangeGlobalDeclaration()`.
> Or is this wrong on the AST level, too?
The AST-level `ThunkInfo` doesn't track the `this` parameter type at all, so I
think this is probably a CodeGen-specific concern. I think this is a bug in
`maybeEmitThunk`:
const CGFunctionInfo &FnInfo =
IsUnprototyped ? CGM.getTypes().arrangeUnprototypedMustTailThunk(MD)
: CGM.getTypes().arrangeGlobalDeclaration(GD);
The problem here is that `GD` refers to the derived-class declaration (which
takes a `Derived*`) but what we actually want is the base class declaration
corresponding to the vtable entry for which we're trying to emit a thunk. So I
think we want to find the class that introduced the vtable slot, look up the
corresponding function in that class
(`CXXRecordDecl::getCorrespondingMethodInClass(ForVTable, true)`), and arrange
a declaration of *that* function.
I don't think there's an easy way to find the vtable slot owner from
`addVTableComponent`, so perhaps the easiest thing to do would be to track
either the effective `this` class or that class's corresponding method
declaration in `ThunkInfo`.
Note that this is also wrong for covariant return types. For example:
struct A {};
struct alignas(32) B : virtual A { char c[32]; };
struct Pad { char c[7]; };
struct C : B, Pad, virtual A {};
struct X { virtual A &f(); };
struct U { virtual ~U(); };
C c;
struct Y : U, X {
virtual B &f() override { return c; }
};
Y y;
... generates a thunk that claims its return value is `align 32
dereferenceable(40) %struct.B*`, which is nonsense; the return value should be
`align(1) dereferenceable(1)` because (as happens in this example) the `A`
virtual base subobject could be at a different offset and and alignment from
the `B` object. Using `X::f` instead of `Y::f` as the method whose declaration
we arrange should fix that too.
Repository:
rG LLVM Github Monorepo
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