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

[Reid Kleckner]

Yes, let's merge it, and David's instcombine change r226708.

On Thu, Jan 22, 2015 at 10:03 AM, Hans Wennborg <hans at chromium.org> wrote:

> 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?
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