Devirtualization in LLVM and Clang

The call itself is not very expensive, but the effect of having to jump through vtables can prevent other optimizations in a way similar to calling a function in a DLL.

Not a huge issue for business logic bits or if your virtual functions are quite large, but using virtual functions for small often used calls such as specific getters or some mathematical function can cause a big performance hit.

Virtual function calls can be expensive, just compare the out-of-the-box performance between std::sort (c++, no virtual calls) and qsort (c, "virtual call" via function pointer). This can make a 10x difference when sorting something trivial (e.g. integer array). std::sort will compile to a single instruction for the comparison, qsort will typically have to do a full virtual function call, not expensive by itself but still significantly worse than single integer comparison instruction.

I disagree, virtual function calls are somewhat costly. Removing a single virtual function call in any kind of hot code can result in measurable performance improvements.

Edit: it also depends a lot on if the virtual function can be predicted, e.g. which derived classes' method will actually end up being called. Mispredicted virtual functions are pretty slow.

It blocks other interprocedural optimization such inlining and interprocedural const prop.

Most polymorphism is actually statically determined, and most that isn't is static at compile time is static after program initialization. There is a lot of optimization you can do based on this knowledge.

Right but are those call sites typically monomorphic (i.e. devirtualizable?)

Should it really be done that way though? I have to think that if virtual calls are used like that there is a huge amount of performance squandered by the general structure.