Implements recursive doubling algorithm for MPI_Iscan. The algorithm preserves order of operations so it can be used both by commutative and non-commutative operations.
The MCA parameter coll_libnbc_iscan_algorithm was added for dynamic algorithm selection.
Signed-off-by: Mikhail Kurnosov <mkurnosov@gmail.com>
(cherry picked from commit 3d43ff0f3209d5bf4713c6696acb0acb8f1756e4)
Signed-off-by: Brian Barrett <bbarrett@amazon.com>
65990af3ad