[mpich-discuss] Using __float128 with MPI_Accumulate()?

[Jed Brown]

GCC 4.6 introduced support for quad precision using the __float128 type.
Since __float128 is not part of the MPI standard, PETSc defines new real
and complex MPI_Datatypes for __float128 and we define our own MPI_SUM,
MPI_MAX, etc. This works fine for "normal" reductions, but it's a problem
with MPI_Accumulate() because the standard does not allow user-defined
reduce operations.

Is there any way to work around this limitation other than to not use
MPI_Accumulate() in any context that might some day be asked to work with
__float128?
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