Issue 250506.2: Improve Support for Downcasting Objects

Author:	Cary Coutant
Champion:	Cary Coutant
Date submitted:	2025-05-06
Date revised:	2025-11-20
Date closed:	2025-12-08
Type:	Enhancement
Status:	Accepted
DWARF version:	6

This is the second part of a three-part proposal. The first part, 250506.1, proposes a standard mechanism for locating the virtual function table (vtable) given an object of a polymorphic class. This second part proposes a standard mechanism for identifying the most-derived class of an object, given its vtable location, in order to support downcasting of pointers while debugging. The third part, 250506.3, proposes a fix to the DW_AT_vtable_elem_location attribute, which appears to be incorrectly implemented in compilers today.

Background

See 250506.1 for the full background and history.

Problem

The second request in PR 7081 was for a way to associate vtables with their class definitions, to make it easier to downcast pointers from the base class to the most-derived class, based on the vtable pointer. The clang and Rust approaches are similar, but not exactly the same, and the Concurrent approach puts a relocatable address of the vtable in the type DIE.

Adding an attribute directly to the type DIE that points to the vtable has the disadvantage that type de-duplication might discard all but one of several copies of a vtable, if multiple copies of the vtable remain after linking.

We also desire a fast lookup mechanism where a vtable address can be translated into the corresponding type DIE without requiring the consumer to parse all the DWARF info at once.

Proposal

To make the association between a vtable and a type DIE, we add a new DWARF tag for a vtable object, DW_TAG_vtable, whose DW_AT_location provides the address of the vtable, and whose (new) DW_AT_vtable_type attribute points to the type DIE corresponding to the vtable. Additional attributes may provide additional details about the vtable.

To provide for faster lookup of vtable addresses, we add vtable ranges to the DW_AT_ranges attribute of the compilation unit DIE.

In Section 2.1, "The Debugging Information Entry," add DW_TAG_vtable to Table 2.1.

In Section 2.2, "Attribute Types," add a row to Table 2.2:

Attribute	Usage
DW_AT_vtable_type	Type corresponding to a vtable

In Section 2.16, "Code Addresses, Ranges and Base Addresses," change the first paragraph as follows:

Any debugging information entry describing an entity that has a machine code address or range of machine code addresses or virtual tables (see Section 4.4), which includes compilation units, module initialization, subroutines, lexical blocks, try/catch blocks (see Section 4.8 on page 108), labels and the like, may have

In Section 4.1.2 [OLD 3.1.2], "Skeleton Compilation Unit Entries," change item 2 as follows:

2. Either a DW_AT_low_pc and DW_AT_high_pc pair of attributes or a DW_AT_ranges attribute whose values encode the contiguous or non-contiguous address ranges, respectively, of the machine instructions and the virtual tables generated for the compilation unit (see Section 2.16 on page 34).

In Chapter 5 [OLD 4], "Data Object and Object List Entries," add a new section:

5.4 Virtual Function Table (vtable) Entries

Virtual function tables (vtables) are represented by debugging information entries with the tag DW_TAG_vtable. It may have the following attributes:

1. A DW_AT_location attribute, whose value describes the location of the vtable.

2. A DW_AT_vtable_type attribute, which is a reference to the debugging information entry for the associated type.

3. A DW_AT_artificial attribute, which is a flag indicating that the vtable entry represents an object created by the compiler that does not correspond directly to source code.

Virtual Function Table entries allow a debugger to associate vtables with the corresponding class type, in order to downcast a pointer whose type is "pointer-to-base-class" to a pointer with type "pointer-to-derived-class," using the vtable pointer of the object.

In Section 8.5.3 [OLD 7.5.3], "Abbreviations Tables," add a row to Table 8.3:

TAG name	Value
DW_TAG_vtable	TBD

In Section 8.5.4 [OLD 7.5.4], "Attribute Encodings," add a row to Table 8.5:

Attribute name	Value	Classes
DW_AT_vtable_type	TBD	reference

In Appendix A, "Attribute by Tag," add a row for DW_TAG_vtable:

TAG name	Applicable attributes
DW_TAG_vtable	DW_AT_artificial DW_AT_location DW_AT_vtable_type

Optional Variations

• If useful, we could add additional attributes to the DW_TAG_vtable DIE to identify construction/destruction vtables, and A-in-B vtables.

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2025-11-20: Revised to put vtable ranges in the DW_AT_ranges attribute instead of a new attribute; updated chapter and section numbers.

2025-12-08: Accepted.