[llvm-dev] Question: How to access c++ vtable pointer to use as Value* in LLVM pass

[Tim Northover via llvm-dev]

On Fri, 19 Apr 2019 at 05:13, K Jelesnianski via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> But this doesn't give me a Value* handle i can grab to and use later.
> How can I leverage this Value to get that contained ??

You need to get that from an instance rather than by iterating over
the Type; almost certainly from a pointer to an instance since classes
are hardly ever loaded as a whole in LLVM, just individual fields when
needed. It sounds like you'll have one lying around.

After that, you'd write:

    %vtable.ptr = getelementptr %MyStruct, %MyStruct* %obj, i32 0, i32 0
    %vtable = load i32(...)*, i32(...)** %vtable.ptr

The first index on that GEP is just because your object may be in an
array, the second selects the vtable pointer. Loading it gives you a
*pointer* to the vtable (so the object instance is a pointer to a
pointer to the vtable). It's essentially what you'd get if you'd had a
Value * from Clang's @_ZTV8MyStruct directly (via
Module::getGlobalVariable).

You have to bitcast it to the correct type, of course, because at the
moment it's pretending to be a i32(...)*. But that's probably what you
want to pass to your library function if it's expecting a vtable.

> 2nd question: What happens if the struct object is from a derived
> class; iterating over the struct again, it looks like the vtable ptr
> is tangled even deeper within the object:

It can get horribly complicated, with multiple vtables inside the
object at different locations and vtables within vtables; sometimes
even different vtables at different stages of the object's life. The
specification of what goes where is here:
https://itanium-cxx-abi.github.io/cxx-abi/abi.html

This is a good time to point out that all of this is platform
dependent. MSVC in particular does things very differently, and Clang
on Windows follows it.

Cheers.

Tim.