LibMem built with tunables enabled exposes two tunable parameters that will help you select the implementation of your choice:
LIBMEM_OPERATION: Instruction based on alignment and cacheability
LIBMEM_THRESHOLD: The threshold for ERMS and Non-Temporal instructions
The only mem functions supporting Tunable options are: memcpy, mempcpy, memmove, memset, and memcmp.
You should build a tunable binary to make use of the supported tunables.