For hot functions that were carefully optimized, I have test harnesses that run them and measure their cycle counts, which are asserted to be equal to the cycle

For hot functions that were carefully optimized, I have test harnesses that run them and measure their cycle counts, which are asserted to be equal to the cycle count after hand-optimization (+/- some small margin if there is non-determinism). This is sensitive to regressions while having a very low false-positive rate due to being (approximately) deterministic.