My armchair speculation: - tightloop() does nothing useful and is not realistic assembly code, so modern CPU optimisations aren't targeted for it - loop_with_

My armchair speculation: - tightloop() does nothing useful and is not realistic assembly code, so modern CPU optimisations aren't targeted for it - loop_with_extra_call() does nothing useful either, but might look like a more realistic instruction sequence - the call could modify 'counter', making it appear to be a realistic sequence which the CPU optimisations are targeted for. End result: the CPU is better designed for handling cases like loop_with_extra_call(). In the real world this makes realistic code faster, and this benchmark is just a weird quirk. Disclaimer: I am not qualified enough to say I know what I'm talking about.