r226707 - Add the "thunk" attribute to MS ABI virtual member pointers

[Hans Wennborg]

On Wed, Jan 21, 2015 at 2:18 PM, Reid Kleckner <reid at kleckner.net> wrote:
> Author: rnk
> Date: Wed Jan 21 16:18:17 2015
> New Revision: 226707
>
> URL: http://llvm.org/viewvc/llvm-project?rev=226707&view=rev
> Log:
> Add the "thunk" attribute to MS ABI virtual member pointers
>
> This attribute implies indicates that the function musttail calls
> another function and returns whatever it returns. The return type of the
> thunk is meaningless, as the thunk can dynamically call different
> functions with different return types. So long as the callers bitcast
> the thunk with the correct type, behavior is well defined.
>
> This attribute was necessary to fix PR20944, where the indirect call
> combiner noticed that the thunk returned void and replaced the results
> of the indirect call instruction with undef.
>
> Over-the-shoulder reviewed by David Majnemer.

Is this a fix we want in 3.6?