[cfe-commits] MS pure virtual call ABI

[John McCall]

On Jun 26, 2012, at 11:28 PM, Chandler Carruth wrote:
> I'm sorry, but for now I would ask you to not submit patches for unpublished or undocumented ABI features.
> 
> There are some very limited places where we can take patches for such features, but it's not yet clear to me that this is one. I'll talk to some others about this particular instance.
> 
> We're still working out some aspects of the Windows support strategy for Clang, but it would help a lot to focus efforts on fixing correctness issues with the published (on MSDN) ABI specifications.

Chandler, this seems pretty well documented to me — the MSDN page that João linked (http://msdn.microsoft.com/en-us/library/ff798096.aspx) clearly specifies that we have to ensure that _purecall is called when a pure virtual call is made, and Timur demonstrated that the observable effect is indistinguishable from that call happening immediately.

Given that requirement, we could implement it in any of several different ways, but putting &_purecall in the appropriate vtable slot is the obviously best option.

John.