It was introduced in PR https://github.com/open-mpi/ompi/pull/1228
in particular in commit 041a6a9f53033a12d1cbf5c1af36cb16c7cdcc36.
Original solution was using "flexible array member" called "mxm_base"
to "fall-through" to the "mxm" send/recv member that located in the
outer structure.
After changing number of elements in "mxm_base" from 0 to 1 we actually
allocating 2 mxm_req_base_t elements which leads to increased overal
size and harms cache performance.
It also brakes "mca_pml_yalla_check_request_state" function.
7d96f12fec