[ms-cxxabi] Implement MSVC virtual base adjustment

While we can't yet emit vbtables, this allows us to find virtual bases
of objects constructed in other TUs.

This make iostream hello world work, since basic_ostream virtually
inherits from basic_ios.

Differential Revision: http://llvm-reviews.chandlerc.com/D795

llvm-svn: 182870

1 files changed, 26 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/ItaniumCXXABI.cpp b/clang/lib/CodeGen/ItaniumCXXABI.cpp
index f7869568a47..6cf95b5ce99 100644
--- a/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -98,6 +98,11 @@ public:
                                       llvm::Value *ptr,
                                       QualType type);
 
+  llvm::Value *GetVirtualBaseClassOffset(CodeGenFunction &CGF,
+                                         llvm::Value *This,
+                                         const CXXRecordDecl *ClassDecl,
+                                         const CXXRecordDecl *BaseClassDecl);
+
   void BuildConstructorSignature(const CXXConstructorDecl *Ctor,
                                  CXXCtorType T,
                                  CanQualType &ResTy,
@@ -720,6 +725,27 @@ llvm::Value *ItaniumCXXABI::adjustToCompleteObject(CodeGenFunction &CGF,
   return CGF.Builder.CreateInBoundsGEP(ptr, offset);
 }
 
+llvm::Value *
+ItaniumCXXABI::GetVirtualBaseClassOffset(CodeGenFunction &CGF,
+                                         llvm::Value *This,
+                                         const CXXRecordDecl *ClassDecl,
+                                         const CXXRecordDecl *BaseClassDecl) {
+  llvm::Value *VTablePtr = CGF.GetVTablePtr(This, CGM.Int8PtrTy);
+  CharUnits VBaseOffsetOffset =
+    CGM.getVTableContext().getVirtualBaseOffsetOffset(ClassDecl, BaseClassDecl);
+
+  llvm::Value *VBaseOffsetPtr =
+    CGF.Builder.CreateConstGEP1_64(VTablePtr, VBaseOffsetOffset.getQuantity(),
+                                   "vbase.offset.ptr");
+  VBaseOffsetPtr = CGF.Builder.CreateBitCast(VBaseOffsetPtr,
+                                             CGM.PtrDiffTy->getPointerTo());
+
+  llvm::Value *VBaseOffset =
+    CGF.Builder.CreateLoad(VBaseOffsetPtr, "vbase.offset");
+
+  return VBaseOffset;
+}
+
 /// The generic ABI passes 'this', plus a VTT if it's initializing a
 /// base subobject.
 void ItaniumCXXABI::BuildConstructorSignature(const CXXConstructorDecl *Ctor,