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

[Paul Gofman]

On 01/22/2016 01:32 AM, Vincent Povirk wrote:
> In Wine with your patch, everything works except that the
> result is incorrect the first time add() is called. I suspect
> ReallyFixupVTable is changing some registers, and the thunk will have
> to preserve them.
>
You are right, I've made a dumb mistake forgetting that arguments regs
are actually volatile regs and ReallyFixupVTable would not preserve it.
I've updated the patch. I want to make a few notes on this:
1. XMM/YMM (0-5) registers save is required to work correctly if MS
vectorcall convention is used for method. Though wine & mono itself
should not touch SSE regs, app module init is finally called which can
do whatever.
2. The modern convention is to align stack on 16 bytes boundary on
ms_abi calls, ReallyFixupVTable originally relied on that. I could
potentially do a stack alignment in the thunk but I suppose it is not
desirable to complicate it further, and so added
__attribute__((force_align_arg_pointer)) to declaration (it is not
included in CDECL, and probably should not). I realize that some older
versions of gcc may somehow ignore this attr (I've read some bug reports
on that), but the problem may come only in case our thunk is called
unaligned (which potentially could be for some old code, but is not
common). If thunk is called correctly it passes the stack pointer aligned.
3. It looks like ReallyFixupVTableEntry has potential thread safety
issue. I mean the case when multiple vtable entries are present within
one fixup. I could possibly fix that but I think should be an extremely
rare case to occur, as application should start calling methods in
parallel right from start of module usage at the first place. So I am
not sure if it worth doing it.