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Thread: A bit confused regarding get_local_id and get_group_id

  1. #1
    Junior Member
    Join Date
    May 2011
    Posts
    5

    A bit confused regarding get_local_id and get_group_id

    Is the value returned by get_local_id indexed relative to the group id or is it globally indexed.



    Take the following example:
    I am splitting a matrix up into small 4x4 blocks for processing. Each thread will compute the results for a single 4x4 block. So to find the (i,j) of that matrix do I need to simply refer to the get_local_id or do I need to offset by the group id.

    I have the following to get group index:
    Code :
      // Block index
        int gx = get_group_id(0);
        int gy = get_group_id(1);
     
        // Thread index
        int tx = get_local_id(0);
        int ty = get_local_id(1);

    so which indexing should I do:
    Code :
        int mat_i =  4*tx;
        int mat_j =  4*ty;

    or
    Code :
        int mat_i =  4*tx + THREADS_PER_GROUP*4*gx;
        int mat_j =  4*ty + THREADS_PER_GROUP*4*gy;

  2. #2
    Senior Member
    Join Date
    May 2010
    Location
    Toronto, Canada
    Posts
    845

    Re: A bit confused regarding get_local_id and get_group_id

    Is the value returned by get_local_id() indexed relative to the group id or is it globally indexed
    For each work-group, the local IDs will go from 0 to get_local_size()-1. That is, from zero to the work-group size minus one.

    It's explained in section 3.2 of the spec.
    Disclaimer: Employee of Qualcomm Canada. Any opinions expressed here are personal and do not necessarily reflect the views of my employer. LinkedIn profile.

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