Add cpa and snr benchmarks

[TrAyZeN]

No description provided.

[kingofpayne]

When testing (on my computer), criterion prints many warnings during the benchmark:
Warning: Unable to complete 100 samples in 5.0s. You may wish to increase target time to 22.7s, or reduce sample count to 20.
Can you configure it to increase that time limit ? (I would suggest 60 seconds)

Also when running it twice, many benches show regression or improvement above noise margin; this is probably because not enough samples are taken due to the time limit.

[TrAyZeN]

The measurement time if too low is automatically increased¹. Thus, in this case criterion will record 100 samples taking more than 5s. criterion performs measurements by sample, but a sample can contain multiple iterations of the function². From my understanding what changes if the measurement time is increased is the number of iterations per sample. In the case where the measurement time is too low, criterion with record 100 samples with 1 iterations. Increasing the measurement time would allow criterion to perform more iterations per samples. Increasing the measurement time seems indeed to yield more robust results.

To reduce benchmark variance, it is advised to disable CPU frequency scaling and to set task affinity³.

Footnotes

1. https://docs.rs/criterion/0.5.1/criterion/struct.Criterion.html#method.measurement_time ↩

2. https://bheisler.github.io/criterion.rs/book/analysis.html#measurement ↩

3. https://google.github.io/benchmark/reducing_variance.html ↩