Measuring the system clock frequency using loops (Intel and ARM)

In my previous post, Bitset decoding on Apple’s A12, I showed that Apple’s latest ARM-based processor can decode set bits out of a stream of words using 3 cycles per set bit. At best, this can run in one subtraction per cycle and Intel processors are good enough to achieve this rate:

In fact, Intel processors fuse the two instruction (dec and jnz) into a single microinstruction and they have special optimizations for tight loops. On the Apple A12, I do not need to do anything sophisticated to measure the frequency as I consistently get 2.5 GHz with a simple tight loop after I have stressed the processor enough.

Source: lemire.me