Use 'offsetof(struct s, array) + p->count * sizeof(*p->array)' instead of sizeof, which could emit an erroneous answer when alignment and padding are taken into account.

Author: bwendling

clang/lib/CodeGen/CGBuiltin.cpp

@@ -877,14 +877,14 @@ CodeGenFunction::emitBuiltinObjectSize(const Expr *E, unsigned Type,
     //   2) bdos of the whole struct, including the flexible array:
     //
     //     __builtin_dynamic_object_size(p, 1) ==
-    //        sizeof(*p) + p->count * sizeof(*p->array)
+    //        offsetof(struct s, array) + p->count * sizeof(*p->array)
     //
     if (const ValueDecl *CountedByFD = FindCountedByField(E)) {
       // Find the flexible array member.
+      ASTContext &Ctx = getContext();
       const RecordDecl *OuterRD =
           CountedByFD->getDeclContext()->getOuterLexicalRecordContext();
-      const ValueDecl *FAM =
-          FindFlexibleArrayMemberField(getContext(), OuterRD);
+      const ValueDecl *FAM = FindFlexibleArrayMemberField(Ctx, OuterRD);
 
       // Find the outer struct expr (i.e. p in p->a.b.c.d).
       const Expr *CountedByExpr = BuildCountedByFieldExpr(E, CountedByFD);
@@ -894,9 +894,8 @@ CodeGenFunction::emitBuiltinObjectSize(const Expr *E, unsigned Type,
           EmitAnyExprToTemp(CountedByExpr).getScalarVal();
 
       // Get the size of the flexible array member's base type.
-      const auto *ArrayTy = getContext().getAsArrayType(FAM->getType());
-      CharUnits Size =
-          getContext().getTypeSizeInChars(ArrayTy->getElementType());
+      const auto *ArrayTy = Ctx.getAsArrayType(FAM->getType());
+      CharUnits Size = Ctx.getTypeSizeInChars(ArrayTy->getElementType());
       llvm::Constant *ArraySize =
           llvm::ConstantInt::get(CountedByInstr->getType(), Size.getQuantity());
 
@@ -905,10 +904,10 @@ CodeGenFunction::emitBuiltinObjectSize(const Expr *E, unsigned Type,
 
       if (const auto *DRE = dyn_cast<DeclRefExpr>(E->IgnoreParenImpCasts())) {
         // The whole struct is specificed in the __bdos.
-        QualType StructTy = DRE->getType()->getPointeeType();
-        llvm::Value *StructSize = ConstantInt::get(
-            ResType, getContext().getTypeSizeInChars(StructTy).getQuantity(),
-            true);
+        CharUnits FAMOffset = Ctx.toCharUnitsFromBits(Ctx.getFieldOffset(FAM));
+        llvm::Value *StructSize =
+            ConstantInt::get(ResType, FAMOffset.getQuantity(), true);
+
         ObjectSize = Builder.CreateAdd(StructSize, ObjectSize);
       }