Perform zero-initialization of virtual base classes when emitting

a zero constant for a complete class.  rdar://problem/8424975

To make this happen, track the field indexes for virtual bases
in the complete object.  I'm curious whether we might be better
off making CGRecordLayoutBuilder *much* more reliant on
ASTRecordLayout;  we're currently duplicating an awful lot of the ABI
layout logic.

llvm-svn: 125555

1 files changed, 50 insertions, 21 deletions

diff --git a/clang/lib/CodeGen/CGRecordLayout.h b/clang/lib/CodeGen/CGRecordLayout.h
index 2a7aa359659..30da05f421f 100644
--- a/clang/lib/CodeGen/CGRecordLayout.h
+++ b/clang/lib/CodeGen/CGRecordLayout.h
@@ -173,12 +173,13 @@ class CGRecordLayout {
   void operator=(const CGRecordLayout&); // DO NOT IMPLEMENT
 
 private:
-  /// The LLVM type corresponding to this record layout.
-  llvm::PATypeHolder LLVMType;
+  /// The LLVM type corresponding to this record layout; used when
+  /// laying it out as a complete object.
+  llvm::PATypeHolder CompleteObjectType;
 
-  /// The LLVM type for the non-virtual part of this record layout, used for
-  /// laying out the record as a base.
-  llvm::PATypeHolder NonVirtualBaseLLVMType;
+  /// The LLVM type for the non-virtual part of this record layout;
+  /// used when laying it out as a base subobject.
+  llvm::PATypeHolder BaseSubobjectType;
 
   /// Map from (non-bit-field) struct field to the corresponding llvm struct
   /// type field no. This info is populated by record builder.
@@ -190,26 +191,41 @@ private:
 
   // FIXME: Maybe we could use a CXXBaseSpecifier as the key and use a single
   // map for both virtual and non virtual bases.
-  llvm::DenseMap<const CXXRecordDecl *, unsigned> NonVirtualBaseFields;
+  llvm::DenseMap<const CXXRecordDecl *, unsigned> NonVirtualBases;
 
-  /// Whether one of the fields in this record layout is a pointer to data
-  /// member, or a struct that contains pointer to data member.
+  /// Map from virtual bases to their field index in the complete object.
+  llvm::DenseMap<const CXXRecordDecl *, unsigned> CompleteObjectVirtualBases;
+
+  /// False if any direct or indirect subobject of this class, when
+  /// considered as a complete object, requires a non-zero bitpattern
+  /// when zero-initialized.
   bool IsZeroInitializable : 1;
 
-public:
-  CGRecordLayout(const llvm::StructType *LLVMType,
-                 const llvm::StructType *NonVirtualBaseLLVMType,
-                 bool IsZeroInitializable)
-    : LLVMType(LLVMType), NonVirtualBaseLLVMType(NonVirtualBaseLLVMType), 
-    IsZeroInitializable(IsZeroInitializable) {}
+  /// False if any direct or indirect subobject of this class, when
+  /// considered as a base subobject, requires a non-zero bitpattern
+  /// when zero-initialized.
+  bool IsZeroInitializableAsBase : 1;
 
-  /// \brief Return the LLVM type associated with this record.
+public:
+  CGRecordLayout(const llvm::StructType *CompleteObjectType,
+                 const llvm::StructType *BaseSubobjectType,
+                 bool IsZeroInitializable,
+                 bool IsZeroInitializableAsBase)
+    : CompleteObjectType(CompleteObjectType),
+      BaseSubobjectType(BaseSubobjectType),
+      IsZeroInitializable(IsZeroInitializable),
+      IsZeroInitializableAsBase(IsZeroInitializableAsBase) {}
+
+  /// \brief Return the "complete object" LLVM type associated with
+  /// this record.
   const llvm::StructType *getLLVMType() const {
-    return cast<llvm::StructType>(LLVMType.get());
+    return cast<llvm::StructType>(CompleteObjectType.get());
   }
 
-  const llvm::StructType *getNonVirtualBaseLLVMType() const {
-    return cast<llvm::StructType>(NonVirtualBaseLLVMType.get());
+  /// \brief Return the "base subobject" LLVM type associated with
+  /// this record.
+  const llvm::StructType *getBaseSubobjectLLVMType() const {
+    return cast<llvm::StructType>(BaseSubobjectType.get());
   }
 
   /// \brief Check whether this struct can be C++ zero-initialized
@@ -218,6 +234,12 @@ public:
     return IsZeroInitializable;
   }
 
+  /// \brief Check whether this struct can be C++ zero-initialized
+  /// with a zeroinitializer when considered as a base subobject.
+  bool isZeroInitializableAsBase() const {
+    return IsZeroInitializableAsBase;
+  }
+
   /// \brief Return llvm::StructType element number that corresponds to the
   /// field FD.
   unsigned getLLVMFieldNo(const FieldDecl *FD) const {
@@ -227,8 +249,15 @@ public:
   }
 
   unsigned getNonVirtualBaseLLVMFieldNo(const CXXRecordDecl *RD) const {
-    assert(NonVirtualBaseFields.count(RD) && "Invalid non-virtual base!");
-    return NonVirtualBaseFields.lookup(RD);
+    assert(NonVirtualBases.count(RD) && "Invalid non-virtual base!");
+    return NonVirtualBases.lookup(RD);
+  }
+
+  /// \brief Return the LLVM field index corresponding to the given
+  /// virtual base.  Only valid when operating on the complete object.
+  unsigned getVirtualBaseIndex(const CXXRecordDecl *base) const {
+    assert(CompleteObjectVirtualBases.count(base) && "Invalid virtual base!");
+    return CompleteObjectVirtualBases.lookup(base);
   }
 
   /// \brief Return the BitFieldInfo that corresponds to the field FD.
@@ -236,7 +265,7 @@ public:
     assert(FD->isBitField() && "Invalid call for non bit-field decl!");
     llvm::DenseMap<const FieldDecl *, CGBitFieldInfo>::const_iterator
       it = BitFields.find(FD);
-    assert(it != BitFields.end()  && "Unable to find bitfield info");
+    assert(it != BitFields.end() && "Unable to find bitfield info");
     return it->second;
   }