Latency of shuffle instructions in CUDA - cuda

Question

About the latency of __shfl() instruction:
Does the following instruction
c=__shfl(c, indi);
/*
where indi is any integer number(may be random (<32)),
and is different for different LaneID.
*/
has the same latency comparing to, lets say:
c=__shfl_down(c,1);

Answer 1

All warp-shuffle instructions have the same performance.

Answer 2

To provide a "quantitative" follow-up answer to Robert's answer, let us consider Mark Harris' reduction approach using CUDA shuffle operations detailed at Faster Parallel Reductions on Kepler.
In this approach, warp reduction is performed by using __shfl_down. An alternative approach to warp reduction is using __shfl_xor according to Lecture 4: warp shuffles, and reduction / scan operations. Below, I'm reporting the full code implementing both the approaches. If tested on a Kepler K20c, both take 0.044ms to reduce an array of N=200000 float elements. Relevantly, both the approaches outperform Thrust reduce by two orders of magnitude since the execution time for the Thrust case is 1.06ms for the same test.
Here is the full code:
#include <thrust\device_vector.h>
#define warpSize 32
/***********************************************/
/* warpReduceSum PERFORMING REDUCTION PER WARP */
/***********************************************/
__forceinline__ __device__ float warpReduceSum(float val) {
for (int offset = warpSize/2; offset > 0; offset /= 2) val += __shfl_down(val, offset);
//for (int i=1; i<warpSize; i*=2) val += __shfl_xor(val, i);
return val;
}
/*************************************************/
/* blockReduceSum PERFORMING REDUCTION PER BLOCK */
/*************************************************/
__forceinline__ __device__ float blockReduceSum(float val) {
// --- The shared memory is appointed to contain the warp reduction results. It is understood that the maximum number of threads per block will be
// 1024, so that there will be at most 32 warps per each block.
static __shared__ float shared[32];
int lane = threadIdx.x % warpSize; // Thread index within the warp
int wid = threadIdx.x / warpSize; // Warp ID
// --- Performing warp reduction. Only the threads with 0 index within the warp have the "val" value set with the warp reduction result
val = warpReduceSum(val);
// --- Only the threads with 0 index within the warp write the warp result to shared memory
if (lane==0) shared[wid]=val; // Write reduced value to shared memory
// --- Wait for all warp reductions
__syncthreads();
// --- There will be at most 1024 threads within a block and at most 1024 blocks within a grid. The partial sum is read from shared memory only
// the corresponding warp existed, otherwise the partial sum is set to zero.
val = (threadIdx.x < blockDim.x / warpSize) ? shared[lane] : 0;
// --- The first warp performs the final partial warp summation.
if (wid==0) val = warpReduceSum(val);
return val;
}
/********************/
/* REDUCTION KERNEL */
/********************/
__global__ void deviceReduceKernel(float *in, float* out, int N) {
float sum = 0.f;
// --- Reduce multiple elements per thread.
for (int i = blockIdx.x * blockDim.x + threadIdx.x; i < N; i += blockDim.x * gridDim.x) sum += in[i];
sum = blockReduceSum(sum);
if (threadIdx.x==0) out[blockIdx.x]=sum;
}
/********/
/* MAIN */
/********/
void main() {
const int N = 200000;
thrust::host_vector<float> h_out(N,0.f);
thrust::device_vector<float> d_in(N,3.f);
thrust::device_vector<float> d_out(N);
int threads = 512;
int blocks = min((N + threads - 1) / threads, 1024);
float time;
cudaEvent_t start, stop;
cudaEventCreate(&start);
cudaEventCreate(&stop);
// --- Performs the block reduction. It returns an output vector containig the block reductions as elements
cudaEventRecord(start, 0);
deviceReduceKernel<<<blocks, threads>>>(thrust::raw_pointer_cast(d_in.data()), thrust::raw_pointer_cast(d_out.data()), N);
// --- Performs a second block reduction with only one block. The input is an array of all 0's, except the first elements which are the
// block reduction results of the previous step.
deviceReduceKernel<<<1, 1024>>>(thrust::raw_pointer_cast(d_out.data()), thrust::raw_pointer_cast(d_out.data()), blocks);
cudaEventRecord(stop, 0);
cudaEventSynchronize(stop);
cudaEventElapsedTime(&time, start, stop);
printf("CUDA Shuffle - elapsed time: %3.5f ms \n", time);
h_out = d_out;
cudaEventRecord(start, 0);
float sum = thrust::reduce(d_in.begin(),d_in.end(),0.f,thrust::plus<float>());
cudaEventRecord(stop, 0);
cudaEventSynchronize(stop);
cudaEventElapsedTime(&time, start, stop);
printf("CUDA Thrust - elapsed time: %3.5f ms \n", time);
printf("Shuffle result = %f\n",h_out[0]);
printf("Thrust result = %f\n",sum);
getchar();
}