Are there any performance test results for usage of likely/unlikely hints?

Question

gcc features likely/unlikely hints that help the compiler to generate machine code with better branch prediction.

Is there any data on how proper usage or failure to

Answer 1

Likely/Unlikely hints work by preloading the ICache with the branch code that is perceived as being generally correct by the programmer. Branch predictors are, by nature of relying upon limited historical data, effective in loops (or small codebases) only, and loops are not always the issue, with regards to branching performance -- for example, in a real-time simulation, or game, where large amounts of sim/game logic need to be processed for large numbers of objects, at a very high rate. Branch predictors cannot operate effectively in this context, and this is a serious performance concern for sim developers. This logic can consist of literally thousands of different, non-repeating conditionals each frame, completely disabling the ability of a branch predictor to operate effectively.

In answer to the original question, compilers tend to assume a conditional will be false, when generating the code to preload the Icache. You should check the assembly output in your code to verify that, and then you might be able to author a macro for conditionals you want to preload in a performant way, if you don't want to structure your code to fit a particular processor architecture.

Some studies have estimated that modern game engines, on modern processors, spend 60-80% of their time on cache misses, and that branch mis-predictions are approximately 15% of those misses. In order to accommodate a modern game engine, a branch predictor would need historical data for the entire game logic frame -- probably involving several MB of data for each pipeline.