Control-Flow Integrity. It's a bit of the new hotness in exploit mitigation, however it's quite complicated and there are various solutions that have different advantages and disadvantages. clang docs: http://clang.llvm.org/docs/ControlFlowIntegrity.html

Shorter CFI: when doing codegen for calls through function pointers (which will involve indirect calls through registers), emit extra code to make sure the register being jumped to is a legit function, thus breaking ROP payloads.

There's more to it, but that's the flavor of it.