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

[Timur Iskhodzhanov]

2013/3/26 John McCall <rjmccall at apple.com>:
> On Mar 26, 2013, at 6:27 PM, Timur Iskhodzhanov <timurrrr at google.com> wrote:
>> 2013/3/26 Timur Iskhodzhanov <timurrrr at google.com>:
>>> 2013/3/22 John McCall <rjmccall at apple.com>:
>>>>
>>>> On Mar 21, 2013, at 1:45 PM, Timur Iskhodzhanov <timurrrr at google.com> wrote:
>>>>
>>>>> Hi John,
>>>>>
>>>>> Please see the attached patch.
>>>>>
>>>>> It addresses most of the Clang-side change needed to fix http://llvm.org/PR13676
>>>>
>>>> +  bool IsMicrosoftABI = getContext().getTargetInfo().getCXXABI().isMicrosoft();
>>>> +
>>>>   if (isAggregateTypeForABI(RetTy)) {
>>>>     if (const RecordType *RT = RetTy->getAs<RecordType>()) {
>>>>       // Structures with either a non-trivial destructor or a non-trivial
>>>>       // copy constructor are always indirect.
>>>> -      if (hasNonTrivialDestructorOrCopyConstructor(RT))
>>>> +      if (hasNonTrivialDestructorOrCopyConstructor(RT) ||
>>>> +          (IsMicrosoftABI && hasNonTrivialDefaultConstructor(RT)))
>>>>
>>>> Please don't compute IsMicrosoftABI before it's needed.
>>> Done.
>>>
>>>> Are you sure it's just that the *default* constructor is non-trivial, or is it
>>>> the presence of *any* non-trivial constructor, or possibly even any
>>>> explicit constructor?
>>> I've made some more experiments and it seems the condition is rather
>>> "is this a POD?", e.g. we should use SRet if A has a virtual function
>>> or a destructor or an assignment operator (but not "void
>>> operator=(int);") or a private member or a base class.
>
> Interesting, okay.  I guess we'll assume that that means the C++98
> definition of POD.
>
>>> See the attached updated patch.
>>>
>>> I was a bit lazy to write tests for all these cases, will add them
>>> later if we ever find new incompatibilities.
>>> I've also added a couple of tests for the upcoming byval compatibility
>>> cleanup with FIXMEs.
>
> +      bool IsMicrosoftABI = getContext().getTargetInfo().getCXXABI().isMicrosoft();
>        // Structures with either a non-trivial destructor or a non-trivial
>        // copy constructor are always indirect.
> -      if (hasNonTrivialDestructorOrCopyConstructor(RT))
> +      if (hasNonTrivialDestructorOrCopyConstructor(RT) ||
> +          (IsMicrosoftABI && !isPOD(RT)))
>
> Let's just abstract this decision into the C++ ABI.  In both the argument
> and the return-type cases, when you're working with a RecordType and
> the decl is a CXXRecordDecl, just ask the current CGCXXABI.
Should I abstract out only the isAggregateTypeForABI() branches or the
classifyReturnType/classifyArgumentType functions completely?
What should the interface method return, an ABIArgInfo?

> There's a pointer to it on the CodeGenTypes.
Thanks for the hint!
Just for my own reference, CGT is defined in ABIInfo and CGCXXABI is
available as "CGT.getCXXABI()".

> Please refactor all the TargetInfos to do that.
You mean, in all the subclassesof ABIInfo, including ARM, PPC, etc?
I hope they are covered well with tests? :)

Most of the TargetInfos will only work with ItaniumCXXABI, right?
How will ItaniumCXXABI decide what to do differently in e.g. X86 vs ARM vs PPC ?

> Incidentally, IIRC the MS ABI does pass non-POD objects byval — that is,
> it passes them on the stack, not as a pointer to an object on the stack.  LLVM
> doesn't really support that, because it assumes that it can do extra memcpys
> of byval arguments.  Are you doing work on the LLVM side to make this work?
I'm working on the SRet support from LLVM;
I haven't seen a need for any specific LLVM handling of byvals so far,
will be aware of possible problems.

> John.