1. A method for debugging a high performance computing program, the method comprising: assigning a plurality of threads executing the program to groups in dependence upon addresses of calling instructions for the plurality of threads including iteratively assigning to a group threads having identical subsets of the lists of addresses of calling instructions; and displaying the groups to identify defective threads including displaying the groups to identify defective threads further comprises displaying a group for an iteration and a count of the threads assigned to the group for the iteration.