Documentation

Split a segment descriptor across the gang, element wise. We try to put the same number of elements on each thread, which means that segments are sometimes split across threads.

Each thread gets a slice of segment descriptor, the segid of the first slice, and the offset of the first slice in its segment.

Example: In this picture each X represents 5 elements, and we have 5 segements in total.

   segs:    ----------------------- --- ------- --------------- -------------------
    elems:  |X X X X X X X X X|X X X X X X X X X|X X X X X X X X X|X X X X X X X X X|
            |     thread1     |     thread2     |     thread3     |     thread4     |
    segid:  0                 0                 3                 4
    offset: 0                 45                0                 5

pprp $ splitSegdOnElemsD theGang 
          $ lengthsToUSegd $ fromList [60, 10, 20, 40, 50 :: Int]

segd:    DUSegd lengths:  DVector lengths: [1,3,2,1]
                                        chunks:  [[45],[15,10,20],[40,5],[45]]
                     indices:  DVector lengths: [1,3,2,1]
                                        chunks:  [[0], [0,15,25], [0,40],[0]]
                    elements:  DInt [45,45,45,45]

segids: DInt [0,0,3,4]     (segment id of first slice on thread)
    offsets: DInt [0,45,0,5]    (offset of that slice in its segment)