Play with `core::hint::cold_path()`

[ounsworth]

https://doc.rust-lang.org/core/hint/fn.cold_path.html

It allows you to give the compiler hints about branches that are extremely unlikely, or where you don't care about performance (like an error path).

This ticket is a research task to try using core::hint::cold_path() on some of the performance-critical sections of algorithms and see if it gives enough of a performance boost to be worth littering the code with it. We would need solid data from our own benchmarks that this actually represents a positive change.

My intuition is that it probably does not because the parts of crypto that have expensive branches tend to be done many thousands of times, and therefore the CPU built-in branch predictors probably work fairly well.

[npajkovsky]

Yeah, using cold_path() for error paths in expensive operations may improve performance, but you'll need to test it. Unstable features like likely/unlikely could be better than cold_path as it can rearrange code during code gen, but that also must be tested.