[ms-cxxabi] Set proper SRet flags for most functions; also handle empty struct arguments correctly

[John McCall]

On Apr 12, 2013, at 8:44 AM, Timur Iskhodzhanov <timurrrr at google.com> wrote:
> 2013/4/9 John McCall <rjmccall at apple.com>
>> 
>>> If that looks OK - I assume I should do the same for other ABIInfos ?
>>> (I hope they are covered with tests well...)
> See the attached patch where I've applied the same approach to other ABIs.
> 
>>>> /// How should we pass a particular record type as an argument?
>>>> enum RecordArgABI {
>>>>   /// Pass it using the normal C aggregate rules for the ABI, potentially
>>>>   /// introducing extra copies and passing some or all of it in registers.
>>>>   RAA_DirectInRegisters,
>>>> 
>>>>   /// Pass it on the stack using its defined layout.  The argument must be
>>>>   /// evaluated directly into the correct stack position in the arguments area,
>>>>   /// and the call machinery must not move it or introduce extra copies.
>>>>   RAA_DirectInMemory,
>>>> 
>>>>   /// Pass it as a pointer to temporary memory.
>>>>   RAA_Indirect
>>>> };
>>> I'm not sure I can effectively use the first two cases right now and
>>> there should also be RAA_IndirectByval (see what I do for Microsoft
>>> ABI), so I've created a slightly simpler enum.
>> 
>> Despite how it looks in IR, "byval" is direct in memory.
>> 
>> Just to be clear, the Itanium ABI would use RAA_Indirect if the copy constructor
>> or destructor is non-trivial and RAA_DirectInRegisters otherwise,
> 
> OK, after re-reading the RAA_DirectInRegisters comment a few times I
> now understand it's exactly the same as I proposed for RAA_Default.
> Not sure why I had not seen that before...

RAA_Default is fine for the name, actually.  Go with that.

> I now use the same enum you've suggested, but I also explicitly set
> the RAA_DirectInRegisters's value to 0
> so now I can write like this:
> 
>  if (RecordArgABI RAA = getRecordArgABI(...))
>    ABIArgInfo::getIndirect(0, RAA == CGCXXABI::RAA_DirectInMemory);
>  // continue if RAA == RAA_DirectInRegisters.
> 
> Is it OK to do so?
> 
>> and the MS ABI would use RAA_DirectInMemory if there's any non-trivial
>> constructor or destructor and RAA_DirectInRegisters otherwise.
> I think you've meant just "MS ABI would use RAA_DirectInMemory.",
> otherwise my tests fail.

Are you saying that the MS ABI should just *always* use RAA_DirectInMemory?
Because I don't think that's true.  Microsoft's calling conventions are
definitely big on passing things on the stack, but some of them do use
registers, and it's at least possible that they'll promote sufficiently small
structs to registers as well.  At any rate, it should be up to the normal CC
code.

Test cases to consider about that would be:
  struct OnePointer { void *P; };
  __fastcall void test0(OnePointer op);
or:
  struct TwoPointers { void *P1, *P2; };
  __fastcall void test1(TwoPointers tp);

John.