1. A method for debugging a high performance computing program, the method comprising: gathering lists of addresses of calling instructions for a plurality of threads of execution of the program; assigning the threads to groups in dependence upon the addresses 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.