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

[John McCall]

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.

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?

For example, this class doesn't even have a default constructor:
  struct A {
    int x;
    A(int x) : x(x) {}
  };

And this C++11 class has a trivial default constructor, but it does still have
non-trivial constructors:
  struct A {
    int x;
    A() = default;
    A(int x) : x(x) {}
  };

And this C++11 class has no non-trivial constructors, but its constructor is
explicitly declared:
  struct A {
    int x;
    A() = default;
  };

John.