forked from OSchip/llvm-project
cd3ebfe293
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
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| .. | ||
| itanium-union-bitfield.cpp | ||
| ms-vtordisp-local.cpp | ||
| ms-x86-alias-avoidance-padding.cpp | ||
| ms-x86-aligned-tail-padding.cpp | ||
| ms-x86-basic-layout.cpp | ||
| ms-x86-bitfields-vbases.cpp | ||
| ms-x86-declspec-empty_bases.cpp | ||
| ms-x86-empty-base-after-base-with-vbptr.cpp | ||
| ms-x86-empty-layout.c | ||
| ms-x86-empty-nonvirtual-bases.cpp | ||
| ms-x86-empty-virtual-base.cpp | ||
| ms-x86-lazy-empty-nonvirtual-base.cpp | ||
| ms-x86-member-pointers.cpp | ||
| ms-x86-misalignedarray.cpp | ||
| ms-x86-pack-and-align.cpp | ||
| ms-x86-primary-bases.cpp | ||
| ms-x86-size-alignment-fail.cpp | ||
| ms-x86-vfvb-alignment.cpp | ||
| ms-x86-vfvb-sharing.cpp | ||
| ms-x86-vtordisp.cpp | ||
| ms_struct-bitfields.c | ||