donglora/vendor/crunchy/README.md

Crunchy

The crunchy unroller - deterministically unroll constant loops. For number "crunching".

The Rust optimizer will unroll constant loops that don't use the loop variable, like this:

for _ in 0..100 {
  println!("Hello!");
}

However, using the loop variable will cause it to never unroll the loop. This is unfortunate because it means that you can't constant-fold the loop variable, and if you end up stomping on the registers it will have to do a load for each iteration. This crate ensures that your code is unrolled and const-folded. It only works on literals, unfortunately, but there's a work-around:

debug_assert_eq!(MY_CONSTANT, 100);
unroll! {
  for i in 0..100 {
    println!("Iteration {}", i);
  }
}

This means that your tests will catch if you redefine the constant.

To default maximum number of loops to unroll is 128, but that can be easily decreased or increased using the cargo features:

• limit_64
• limit_128
• limit_256
• limit_512
• limit_1024
• limit_2048