[Dwarf-discuss] Representing vtables in DWARF for downcasting

[Cary Coutant]

>
> In 250506.2, the use of a rnglist is throwing me.  I would expect the
> lifetime of a vtable to be the whole program.  Or did you envision the
> rnglist to be the range of data/rodata addresses of the vtable object?
> 2.17 clarifies that they're code addresses (i.e. text), though.
>

The DW_AT_vtable_ranges attribute is for the CU, and gives a set of ranges
of memory addresses for the vtables in that CU, not just one. (This is not
a loclist, but a rnglist.) Typically (at least in C++), vtables are emitted
in a specific section (where they can be relocated then made read-only),
and could be described by a single range list entry. With COMDATs, it might
take more than one range. I thought about adding DW_AT_vtable_low/high as
an alternative to the range list, but it seemed like overkill.

I may have to tweak the wording in 2.17 to cover the usage of range lists
for vtables. I think 2.17.3, "Non-Contiguous Address Ranges," is generic
enough to apply to vtable addresses as well as code addresses.

If it helps the design: there are languages where vtables are not
> necessarily statically allocated.


For vtables that are generated dynamically, I don't know that there's
anything we can do to help with accelerated access.

-cary
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