acquireThread

fn acquireThread(s: *SafeAllocator) *Thread {
    while (true) {
        const t = &s.threads[thread_index];
        if (t.mutex.tryLock()) {
            @branchHint(.likely);
            return t;
        }

        var max = @atomicLoad(usize, &n_threads, .unordered);
        if (max == 0) {
            @branchHint(.unlikely);
            max = @min(std.Thread.getCpuCount() catch s.threads.len, s.threads.len);
            max = @cmpxchgStrong(usize, &n_threads, 0, max, .monotonic, .monotonic) orelse max;
        }

        thread_index += 1;
        // thread_index may be greater than max if the zero is returned by getCpuCount
        thread_index *= @intFromBool(thread_index < max);
    }
}