[PATCH v2] mscoree: Implement VTable fixup for x86_64 architecture.

[Vincent Povirk]

>> But I don't think it's possible for a .NET method to use __vectorcall
>> as its calling convention. The attribute used to specify it doesn't
>> have a value for this:
>> https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.unmanagedfunctionpointerattribute.unmanagedfunctionpointerattribute.aspx
>>
>> It's possible MS will extend this in the future, but it's also
>> possible they'll make up entirely new calling conventions. I don't
>> think they will, and for now I don't think it's worth the extra
>> complexity.
> Is it the same for managed C++? If you say __vectorcall support is not
> needed I will of course remove this code.

Managed C++ is limited to what the runtime can generate for vtable
fixups, and I can't find any indication that the runtime can generate
__vectorcall functions. So I don't think it is needed.

> If they do not support it they should just issue compiler warning, or am
> I missing something? Clang may hopefully add the suport. msvc is very
> unlikely to compile Wine soon I guess. Intel Compiler does not support
> ms_abi attribute at all and thus compiling wine 64bit with it is totally
> not feasible for now.

I don't know what compilers will be used in practice (some people
build parts of wine with msvc, but probably not mscoree). I think we
want GCC extensions to at least be inside a check for whether the
compiler is GCC.

> I can guess it might be just nearly as likely as without .NET. See this
> bug: https://bugs.winehq.org/show_bug.cgi?id=27680: there is 7-8 apps
> w/o .NET known so far. As I got from brief googling native .Net should
> not suffer from stack misalignment. So I thought it won't hurt to add an
> attribute to ReallyFixupVTable (which presumably will be universally
> supported some day) , but considered too much forcing alignment right in
> the thunk.

I think it's fine if we check for gcc (and I think clang may
conveniently claim to be gcc), but I also don't think it's required at
this point.

> Are you sure they are going to crash?

I'm not sure, I did not look carefully. But they do seem to put effort
into preserving alignment, in a way that assumes the stack is aligned
on entry.