diff options
author | David Majnemer <david.majnemer@gmail.com> | 2016-05-23 17:16:12 +0000 |
---|---|---|
committer | David Majnemer <david.majnemer@gmail.com> | 2016-05-23 17:16:12 +0000 |
commit | 6b373d1daba236a6c672e8a4e21b519cda0540a4 (patch) | |
tree | 8146b61e6cce920f571cc747fa44cc2b55febc49 /lib/AST/RecordLayout.cpp | |
parent | df9c980345ef84b7c5669e80c0f264f9739ed0de (diff) |
[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.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270457 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/AST/RecordLayout.cpp')
-rw-r--r-- | lib/AST/RecordLayout.cpp | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/lib/AST/RecordLayout.cpp b/lib/AST/RecordLayout.cpp index b2c244e379..4cee2e0631 100644 --- a/lib/AST/RecordLayout.cpp +++ b/lib/AST/RecordLayout.cpp @@ -58,7 +58,7 @@ ASTRecordLayout::ASTRecordLayout(const ASTContext &Ctx, const CXXRecordDecl *PrimaryBase, bool IsPrimaryBaseVirtual, const CXXRecordDecl *BaseSharingVBPtr, - bool HasZeroSizedSubObject, + bool EndsWithZeroSizedObject, bool LeadsWithZeroSizedBase, const BaseOffsetsMapTy& BaseOffsets, const VBaseOffsetsMapTy& VBaseOffsets) @@ -82,7 +82,7 @@ ASTRecordLayout::ASTRecordLayout(const ASTContext &Ctx, CXXInfo->VBPtrOffset = vbptroffset; CXXInfo->HasExtendableVFPtr = hasExtendableVFPtr; CXXInfo->BaseSharingVBPtr = BaseSharingVBPtr; - CXXInfo->HasZeroSizedSubObject = HasZeroSizedSubObject; + CXXInfo->EndsWithZeroSizedObject = EndsWithZeroSizedObject; CXXInfo->LeadsWithZeroSizedBase = LeadsWithZeroSizedBase; |