使用虚函数而不是IF语句更快？

Question

I remember reading online somewhere that in EXTREMELY low latency situations its better to use virtual functions as a substitute for IF statements.

Is this true? Are they basically saying dynamic polymorphism is better for speed situations?

Do any users have any other C++ low latency "quirks" they could share?

Answer 1

I very much doubt that a single if/else statement would be slower than using a virtual function: the virtual function typically enforces a pipeline stall and limits the optimization opportunities. An if statement may stall the pipeline but if it is often executed the prediction may go the right way. However, if your alternative is between a cascade of a few if/else statements vs. just one virtual function call than that latter may be faster. Also, if the total code being executed via using virtual functions vs. branches is different functions ends up substantially smaller it may cause few cache misses on the instruction cache. That is, it depends on the situation. The best way is to measure. Note that measuring artificial code which is just attempting to investigate the difference between two approaches but doesn't really do any processing yields misleading results. However, when you need to produce very low latency code you typically can spend more time to come up with it, ie experimenting with multiple different approaches may be viable.

Although my colleagues tend to frown upon my template approaches for avoiding run-time branching, the code I end up with often is very slow to compile but very fast to run. Of course, this depends on the functions or branches being used to be known at compile time. In the areas I have used this eg for message processing it is often sufficient to have one dynamic decision eg one for each message (ie one virtual function call), followed by processing which doesn't involve any dynamic types (this are still conditionals, eg for the amount of values in a table).