Portals 4 and shmemlatency

[jdinan]

The shmemlatency test hangs when run on the Portals 4 build. This unit test is a bit sketchy. It only works on little endian systems because of how it updates the last element of buf through a char pointer and later waits on the same pointer through a call to shmem_long_wait_until. This only works because the lowest address of the long* is the least significant byte.

To do: Figure out why this is hanging and also whether to salvage this benchmark as a useful unit test or pitch it in favor of the perf suite.

First observed in #758