An implementation of R. Rabenseifner's algorithm for MPI_Iallreduce.
This algorithm is a combination of a reduce-scatter implemented with recursive vector halving
and recursive distance doubling, followed either by an allgather.
Limitations:
-- count >= 2^{\floor{\log_2 p}}
-- commutative operations only
-- intra-communicators only
Signed-off-by: Mikhail Kurnosov <mkurnosov@gmail.com>
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