what does the width parameter of function __shfl_up(int var, unsigned int delta, int width) mean?

Question

What does the width parameter in __shfl_up(int var, unsigned int delta, int width = warpSize) mean? Assuming that var equals 4 in lane 4 and var equals 8 in lane 8. if I call __shfl_up(var, 4, 8) in lane 8, what it is returning?

Answer 1

The width parameter divides warpSize(typically 32) into partitions and each has width elements. Let's suppose the caller's warp id is laneId. calling __shfl_up(var, offset, width) will calculate source lane id, srclaneId = (laneId % width) - offset. if srclaneId is between 0 and width-1(inclusive) and the thread of srclaneId is active(see Active thread in a warp), __shfl_up(var, offset, width) returns the value of srclaneId's var, otherwise returns the value of laneid's var. In addition width must be power-of-2 (i.e. 2, 4, 8, 16 or 32).