About @branchHint

dimdin · December 18, 2024, 4:21pm

Assembly branch instructions, are conditional jumps. There are two paths to follow, the one that the condition holds and the opposite one.

CPU can predict the path with @branch(.none) or not a @branch hint at all.
@branch(.likely) means that this path is likely to be reached.
@branch(.unlikely) means that this path is unlikely to be reached.
@branch(.cold) means that this path is unlikely to ever be reached.
@branch(.unpredictable) means that it is difficult to predict if this path is reached or not.

For example:

    i = 1000;
    while (i > 0) {
        i -= 1;
        ...
    }

The statements inside while are .likely because they run 999 times, the other path (to skip while) is .unlikely because it runs only one time.