cuda unused threads

Question

say I have 64 threadds in a kernel

__global__ void kernel( ... )
{
    int i = threadIdx.x;
    ... ...
    if (i < 32)
    {
        ... ...
    }
}

basically after a certain point, I won't use threads 32 to 63 any more. What are they gonna do then? Are they gonna still consume processor power, or they are just dead.

Answer 1

They simply will not produce anymore instruction to be issued and executed. Let's say "Dead".

Answer 2

Every thread in a half-warp (or maybe warp depending on your architecture) executes the same instruction at the same time, so all the other threads in the half-warp continue to run, just with their output suppressed. All other half-warps (or maybe warps) are released back to the system as resources.