[clang] [llvm] [clang][DebugInfo] Add symbol for debugger with VTable information. (PR #130255)

[Carlos Alberto Enciso via llvm-commits]

CarlosAlbertoEnciso wrote:

> The DWARF currently provides access to the vtable location for /instances/ of the class, so curious what the distinction/need is for doing this from the class, without instances?
> 
> > Previously, this had to be done by searching for ELF symbols with matching names; something that was time-consuming and error-prone in certain edge cases.
> 
> (I can appreciate that, if we are at the point of searching the symbol table, it's not a great one - but could you talk more about the edge cases/error-prone situations?)

The simplest example of an edge-case is classes with VTables inside functions. This is because the demangled name of the VTable’s ELF symbol (e.g., `vtable for Func()::ClassA` ) is not a searchable name in the global context (in the SCE debugger at least). You could argue that it should be, but it is further complicated by things like template parameters, function overloading, anonymous namespaces etc.

Another example, the demangled ELF symbol: `vtable for int (anonymous namespace)::Func<((anonymous namespace)::E)0>(int)::A`. 

To work out which class `A` this refers to would involve parsing the template parameter correctly and matching to the correct anonymous namespace. While this technically isn’t impossible, it would involve the debugger keeping a lot of extra information around to disambiguate these rare cases, something we’re unlikely to be able to justify. 

Implementing such demangling-and-interpretation would be error-prone, and the whole point of DWARF is to present information in a structured manner, so making it easy to access + interpret is part of its purpose.


https://github.com/llvm/llvm-project/pull/130255