If, as Robert Crovella suggests, the clock64() calls don't get optimized away, then this should be enough:

using clock_value_t = long long;

__device__ void sleep(clock_value_t sleep_cycles)
{
    auto start = clock64();
    auto cycles_elapsed;
    do {
        cycles_elapsed = clock64() - start;
     } while (cycles_elapsed < sleep_cycles);
}

If, as Robert Crovella suggests, the clock64() calls don't get optimized away, then this should be enough:

using clock_value_t = long long;

__device__ void sleep(clock_value_t sleep_cycles)
{
    clock_value_t start = clock64();
    clock_value_t cycles_elapsed;
    do { cycles_elapsed = clock64() - start; } 
    while (cycles_elapsed < sleep_cycles);
}