[MS ABI] Implement __declspec(empty_bases) and __declspec(layout_version)

The layout_version attribute is pretty straightforward: use the layout
rules from version XYZ of MSVC when used like
struct __declspec(layout_version(XYZ)) S {};

The empty_bases attribute is more interesting.  It tries to get the C++
empty base optimization to fire more often by tweaking the MSVC ABI
rules in subtle ways:
1. Disable the leading and trailing zero-sized object flags if a class
   is marked __declspec(empty_bases) and is empty.

   This means that given:
   struct __declspec(empty_bases) A {};
   struct __declspec(empty_bases) B {};
   struct C : A, B {};

   'C' will have size 1 and nvsize 0 despite not being annotated
   __declspec(empty_bases).

2. When laying out virtual or non-virtual bases, disable the injection
   of padding between classes if the most derived class is marked
   __declspec(empty_bases).

   This means that given:
   struct A {};
   struct B {};
   struct __declspec(empty_bases) C : A, B {};

   'C' will have size 1 and nvsize 0.

3. When calculating the offset of a non-virtual base, choose offset zero
   if the most derived class is marked __declspec(empty_bases) and the
   base is empty _and_ has an nvsize of 0.

   Because of the ABI rules, this does not mean that empty bases
   reliably get placed at offset 0!

   For example:
   struct A {};
   struct B {};
   struct __declspec(empty_bases) C : A, B { virtual ~C(); };

   'C' will be pointer sized to account for the vfptr at offset 0.
   'A' and 'B' will _not_ be at offset 0 despite being empty!
   Instead, they will be located right after the vfptr.

   This occurs due to the interaction betweeen non-virtual base layout
   and virtual function pointer injection: injection occurs after the
   nv-bases and shifts them down by the size of a pointer.

llvm-svn: 270457

0 files changed, 0 insertions, 0 deletions