[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

2 files changed, 48 insertions, 18 deletions

diff --git a/clang/lib/AST/RecordLayout.cpp b/clang/lib/AST/RecordLayout.cpp
index b2c244e3790..4cee2e06311 100644
--- a/clang/lib/AST/RecordLayout.cpp
+++ b/clang/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;
 
 
diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index 6a7d5144c8c..d03273f9175 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -2228,7 +2228,8 @@ public:
   /// laid out.
   void initializeCXXLayout(const CXXRecordDecl *RD);
   void layoutNonVirtualBases(const CXXRecordDecl *RD);
-  void layoutNonVirtualBase(const CXXRecordDecl *BaseDecl,
+  void layoutNonVirtualBase(const CXXRecordDecl *RD,
+                            const CXXRecordDecl *BaseDecl,
                             const ASTRecordLayout &BaseLayout,
                             const ASTRecordLayout *&PreviousBaseLayout);
   void injectVFPtr(const CXXRecordDecl *RD);
@@ -2334,7 +2335,7 @@ MicrosoftRecordLayoutBuilder::getAdjustedElementInfo(
   if (!MaxFieldAlignment.isZero())
     Info.Alignment = std::min(Info.Alignment, MaxFieldAlignment);
   // Track zero-sized subobjects here where it's already available.
-  EndsWithZeroSizedObject = Layout.hasZeroSizedSubObject();
+  EndsWithZeroSizedObject = Layout.endsWithZeroSizedObject();
   // Respect required alignment, this is necessary because we may have adjusted
   // the alignment in the case of pragam pack.  Note that the required alignment
   // doesn't actually apply to the struct alignment at this point.
@@ -2369,7 +2370,7 @@ MicrosoftRecordLayoutBuilder::getAdjustedElementInfo(
     if (auto RT =
             FD->getType()->getBaseElementTypeUnsafe()->getAs<RecordType>()) {
       auto const &Layout = Context.getASTRecordLayout(RT->getDecl());
-      EndsWithZeroSizedObject = Layout.hasZeroSizedSubObject();
+      EndsWithZeroSizedObject = Layout.endsWithZeroSizedObject();
       FieldRequiredAlignment = std::max(FieldRequiredAlignment,
                                         Layout.getRequiredAlignment());
     }
@@ -2502,7 +2503,7 @@ MicrosoftRecordLayoutBuilder::layoutNonVirtualBases(const CXXRecordDecl *RD) {
       LeadsWithZeroSizedBase = BaseLayout.leadsWithZeroSizedBase();
     }
     // Lay out the base.
-    layoutNonVirtualBase(BaseDecl, BaseLayout, PreviousBaseLayout);
+    layoutNonVirtualBase(RD, BaseDecl, BaseLayout, PreviousBaseLayout);
   }
   // Figure out if we need a fresh VFPtr for this class.
   if (!PrimaryBase && RD->isDynamicClass())
@@ -2531,7 +2532,7 @@ MicrosoftRecordLayoutBuilder::layoutNonVirtualBases(const CXXRecordDecl *RD) {
       LeadsWithZeroSizedBase = BaseLayout.leadsWithZeroSizedBase();
     }
     // Lay out the base.
-    layoutNonVirtualBase(BaseDecl, BaseLayout, PreviousBaseLayout);
+    layoutNonVirtualBase(RD, BaseDecl, BaseLayout, PreviousBaseLayout);
     VBPtrOffset = Bases[BaseDecl] + BaseLayout.getNonVirtualSize();
   }
   // Set our VBPtroffset if we know it at this point.
@@ -2543,15 +2544,32 @@ MicrosoftRecordLayoutBuilder::layoutNonVirtualBases(const CXXRecordDecl *RD) {
   }
 }
 
+static bool recordUsesEBO(const RecordDecl *RD) {
+  if (!isa<CXXRecordDecl>(RD))
+    return false;
+  if (RD->hasAttr<EmptyBasesAttr>())
+    return true;
+  if (auto *LVA = RD->getAttr<LayoutVersionAttr>())
+    // TODO: Double check with the next version of MSVC.
+    if (LVA->getVersion() <= LangOptions::MSVC2015)
+      return false;
+  // TODO: Some later version of MSVC will change the default behavior of the
+  // compiler to enable EBO by default.  When this happens, we will need an
+  // additional isCompatibleWithMSVC check.
+  return false;
+}
+
 void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
+    const CXXRecordDecl *RD,
     const CXXRecordDecl *BaseDecl,
     const ASTRecordLayout &BaseLayout,
     const ASTRecordLayout *&PreviousBaseLayout) {
   // Insert padding between two bases if the left first one is zero sized or
   // contains a zero sized subobject and the right is zero sized or one leads
   // with a zero sized base.
-  if (PreviousBaseLayout && PreviousBaseLayout->hasZeroSizedSubObject() &&
-      BaseLayout.leadsWithZeroSizedBase())
+  bool MDCUsesEBO = recordUsesEBO(RD);
+  if (PreviousBaseLayout && PreviousBaseLayout->endsWithZeroSizedObject() &&
+      BaseLayout.leadsWithZeroSizedBase() && !MDCUsesEBO)
     Size++;
   ElementInfo Info = getAdjustedElementInfo(BaseLayout);
   CharUnits BaseOffset;
@@ -2560,14 +2578,23 @@ void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
   bool FoundBase = false;
   if (UseExternalLayout) {
     FoundBase = External.getExternalNVBaseOffset(BaseDecl, BaseOffset);
-    if (FoundBase)
+    if (FoundBase) {
       assert(BaseOffset >= Size && "base offset already allocated");
+      Size = BaseOffset;
+    }
   }
 
-  if (!FoundBase)
-    BaseOffset = Size.alignTo(Info.Alignment);
+  if (!FoundBase) {
+    if (MDCUsesEBO && BaseDecl->isEmpty() &&
+        BaseLayout.getNonVirtualSize() == CharUnits::Zero()) {
+      BaseOffset = CharUnits::Zero();
+    } else {
+      // Otherwise, lay the base out at the end of the MDC.
+      BaseOffset = Size = Size.alignTo(Info.Alignment);
+    }
+  }
   Bases.insert(std::make_pair(BaseDecl, BaseOffset));
-  Size = BaseOffset + BaseLayout.getNonVirtualSize();
+  Size += BaseLayout.getNonVirtualSize();
   PreviousBaseLayout = &BaseLayout;
 }
 
@@ -2746,8 +2773,9 @@ void MicrosoftRecordLayoutBuilder::layoutVirtualBases(const CXXRecordDecl *RD) {
     // with a zero sized base.  The padding between virtual bases is 4
     // bytes (in both 32 and 64 bits modes) and always involves rounding up to
     // the required alignment, we don't know why.
-    if ((PreviousBaseLayout && PreviousBaseLayout->hasZeroSizedSubObject() &&
-        BaseLayout.leadsWithZeroSizedBase()) || HasVtordisp) {
+    if ((PreviousBaseLayout && PreviousBaseLayout->endsWithZeroSizedObject() &&
+         BaseLayout.leadsWithZeroSizedBase() && !recordUsesEBO(RD)) ||
+        HasVtordisp) {
       Size = Size.alignTo(VtorDispAlignment) + VtorDispSize;
       Alignment = std::max(VtorDispAlignment, Alignment);
     }
@@ -2785,8 +2813,10 @@ void MicrosoftRecordLayoutBuilder::finalizeLayout(const RecordDecl *RD) {
     Size = Size.alignTo(RoundingAlignment);
   }
   if (Size.isZero()) {
-    EndsWithZeroSizedObject = true;
-    LeadsWithZeroSizedBase = true;
+    if (!recordUsesEBO(RD) || !cast<CXXRecordDecl>(RD)->isEmpty()) {
+      EndsWithZeroSizedObject = true;
+      LeadsWithZeroSizedBase = true;
+    }
     // Zero-sized structures have size equal to their alignment if a
     // __declspec(align) came into play.
     if (RequiredAlignment >= MinEmptyStructSize)
@@ -2919,7 +2949,7 @@ ASTContext::getASTRecordLayout(const RecordDecl *D) const {
       NewEntry = new (*this) ASTRecordLayout(
           *this, Builder.Size, Builder.Alignment, Builder.RequiredAlignment,
           Builder.HasOwnVFPtr, Builder.HasOwnVFPtr || Builder.PrimaryBase,
-          Builder.VBPtrOffset, Builder.NonVirtualSize,
+          Builder.VBPtrOffset, Builder.DataSize,
           Builder.FieldOffsets.data(), Builder.FieldOffsets.size(),
           Builder.NonVirtualSize, Builder.Alignment, CharUnits::Zero(),
           Builder.PrimaryBase, false, Builder.SharedVBPtrBase,