futexForAddress

fn futexForAddress(ev: *Evented, address: usize) *Futex {
    // Here we use Fibonacci hashing: the golden ratio can be used to evenly redistribute input
    // values across a range, giving a poor, but extremely quick to compute, hash.

    // This literal is the rounded value of '2^64 / phi' (where 'phi' is the golden ratio). The
    // shift then converts it to '2^b / phi', where 'b' is the pointer bit width.
    const fibonacci_multiplier = 0x9E3779B97F4A7C15 >> (64 - @bitSizeOf(usize));
    const hashed = address *% fibonacci_multiplier;
    comptime assert(std.math.isPowerOfTwo(ev.futexes.len));
    // The high bits of `hashed` have better entropy than the low bits.
    return &ev.futexes[hashed >> @clz(ev.futexes.len - 1)];
}