1. A subsystem for performing an aggregation operation across multiple threads, the subsystem comprising: a barrier instruction execution unit that is configured to: receive a barrier aggregation instruction that specifies the aggregation operation for execution by a first thread of the multiple threads; generate an aggregation result based on the aggregation operation; communicate the aggregation result to each thread of the multiple threads; and waiting until each of the multiple threads has received the barrier aggregation instruction before any additional instructions are executed.