I am trying to get the vtable pointer as a constant (!).
I've found
  auto vptr = typeid(A).vtbl.ptr
gets me the pointer, but because TypeInfo is not immutable (another forum thread), this will read the pointer from memory instead of loading a direct value.

Does anybody know how to get the class's vtable pointer without doing a memory read?

Thanks!
  Johan

On Thursday, 7 April 2016 at 20:43:04 UTC, Johan Engelen wrote:
> Does anybody know how to get the class's vtable pointer without doing a memory read?


I don't think you can... why do you want it though?

On Thursday, 7 April 2016 at 20:49:40 UTC, Adam D. Ruppe wrote:
> On Thursday, 7 April 2016 at 20:43:04 UTC, Johan Engelen wrote:
>> Does anybody know how to get the class's vtable pointer without doing a memory read?
>
> I don't think you can... why do you want it though?

I have implemented PGO (profile-guided optimization) of virtual calls in LDC. Now I am trying to find a way to express it in D, such that you can do the optimization without profiling (using your knowledge of your program):

        A a = getobject_mostlyA(...); // 80% of the time it will be an A object
        int num;
        if (a.__vptr == cast(void*)typeid(A).vtbl.ptr)
        {
            num = a.A.callVirtual(...);
        }
        else
        {
            num = a.callVirtual(...);
        }

This could be wrapped in a template "CallLikelyObject!A(...)" or something similar.
However, cast(void*)typeid(A).vtbl.ptr is suboptimal because TypeInfo is not constant and so it will always do a memory read. Unfortunately, recently LDC had to make TypeInfo mutable because of having to support "synchronize(typeid(A))", such that the load from memory cannot be optimized to the linker-resolved vtable address constant.

Is there a way to read a symbol if I myself specify the mangled name?
Pseudo-code:
   pragma(mangle, "LDC vtable mangled named for A")
   immutable void* vptrA;