LTO, Devirtualization, and Virtual Tables

Question

Comparing virtual functions in C++ and virtual tables in C, do compilers in general (and for sufficiently large projects) do as good a job at devirtualization?

Naive

Answer 1

I tried to summarize in http://hubicka.blogspot.ca/2014/01/devirtualization-in-c-part-2-low-level.html why generic optimizations have hard time to devirtualize. Your testcase gets inlined for me with GCC 4.8.1, but in slightly less trivial testcase where you pass pointer to your "object" out of main it will not.

The reason is that to prove that the virtual table pointer in obj and the virtual table itself did not change the alias analysis module has to track all possible places you can point to it. In a non-trivial code where you pass things outside of the current compilation unit this is often a lost game.

C++ gives you more information on when type of object may change and when it is known. GCC makes use of it and it will make a lot more use of it in the next release. (I will write on that soon, too).