How does the benchmark tell how many branches were mispredicted? Is that something the processor exposes?

Yeah performance counters

	▲	cloudbonsai 33 minutes ago \| parent [-]
		To fill in the details, here is the code used for the measurement: https://github.com/lemire/counters/blob/main/include/counter... It fetches the number of mispredicted instructions from Linux's perf subsystem, which in turn gathers the metrics from CPU's PMU (Performance Monitoring Unit) interface.