1. A parallel computer for conditionally updating shared variable directory (SVD) information, the parallel computer comprising a plurality of threads of execution, the threads organized into tasks, the parallel computer further comprising memory partitioned to provide each thread with a private memory and a shared memory, the parallel computer comprising computer processors operatively coupled to the memory having disposed within it computer program instructions that, when executed by the computer processors, cause the parallel computer to function by: receiving, by a runtime optimizer of a second task, a broadcast reduction operation header, the broadcast reduction operation header including an SVD key and a first SVD address, the first SVD address associated with the SVD key in a first SVD associated with a first task, wherein the first SVD address indicates a first location within a memory partition associated with the first SVD; in response to receiving the broadcast reduction operation header, retrieving from a remote address cache associated with the second task, by the runtime optimizer, a second SVD address indicating a second location within the memory partition associated with the first SVD; determining, by the runtime optimizer, that the first SVD address does not match the second SVD address; and in response to determining that the first SVD address does not match the second SVD address, updating, by the runtime optimizer, the remote address cache with the first SVD address.