[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

1 files changed, 25 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index aeacb4a6fe4..a1a1b9e7b5e 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -4968,6 +4968,24 @@ static void handleX86ForceAlignArgPointerAttr(Sema &S, Decl *D,
                                         Attr.getAttributeSpellingListIndex()));
 }
 
+static void handleLayoutVersion(Sema &S, Decl *D, const AttributeList &Attr) {
+  uint32_t Version;
+  Expr *VersionExpr = static_cast<Expr *>(Attr.getArgAsExpr(0));
+  if (!checkUInt32Argument(S, Attr, Attr.getArgAsExpr(0), Version))
+    return;
+
+  // TODO: Investigate what happens with the next major version of MSVC.
+  if (Version != LangOptions::MSVC2015) {
+    S.Diag(Attr.getLoc(), diag::err_attribute_argument_out_of_bounds)
+        << Attr.getName() << Version << VersionExpr->getSourceRange();
+    return;
+  }
+
+  D->addAttr(::new (S.Context)
+                 LayoutVersionAttr(Attr.getRange(), S.Context, Version,
+                                   Attr.getAttributeSpellingListIndex()));
+}
+
 DLLImportAttr *Sema::mergeDLLImportAttr(Decl *D, SourceRange Range,
                                         unsigned AttrSpellingListIndex) {
   if (D->hasAttr<DLLExportAttr>()) {
@@ -5749,6 +5767,12 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
     break;
 
   // Microsoft attributes:
+  case AttributeList::AT_EmptyBases:
+    handleSimpleAttribute<EmptyBasesAttr>(S, D, Attr);
+    break;
+  case AttributeList::AT_LayoutVersion:
+    handleLayoutVersion(S, D, Attr);
+    break;
   case AttributeList::AT_MSNoVTable:
     handleSimpleAttribute<MSNoVTableAttr>(S, D, Attr);
     break;
@@ -5767,6 +5791,7 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case AttributeList::AT_Thread:
     handleDeclspecThreadAttr(S, D, Attr);
     break;
+
   case AttributeList::AT_AbiTag:
     handleAbiTagAttr(S, D, Attr);
     break;