[llvm-dev] RFC: virtual-like methods via LLVM-style RTTI

[David Chisnall via llvm-dev]

On 3 May 2018, at 22:09, David Zarzycki via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello,
> 
> In an effort to help LLVM-style projects save memory, I’ve been toying with some macros that provide an alternative to C++ vtables that use LLVM-style RTTI design patterns instead. Is this something that LLVM or sub-projects think is worth pursuing? Or are the macros below too ugly/problematic? Feedback would be appreciated.

It would help to have a little bit of information about how you expect this to work.  LLVM-style RTTI works by adding a method to the vtable that returns the kind of the instance and can be used with templates to allow casting.  If you retain this, then every object still needs the vtable pointer, which appears to be what you are trying to avoid.  If you don’t have this, then how do you expect to be able to differentiate instance types?

From your code, it looks as if you’re implementing an inverted vtable structure, where there is a per-method vtable with per-subclass entries, rather than a traditional vtable where you have one per class and per-method entries.  Your total vtable space goes from being NxM to being MxN, which doesn’t sound like an improvement.  This kind of approach is generally popular in late-bound dynamic languages with duck typing where you have a lot of methods with few overrides and it’s worth having a slightly more expensive lookup to have a sparser structure than the full NxM matrix.  It doesn’t seem appropriate here.

David