parallel_reduce/scan: Clarify identity in case no reproducer was given #487
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masterleinad
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| * If the ``functor`` is a lambda, ``ReducerArgument`` must satisfy the ``Reducer`` concept or ``ReducerArgumentNonConst`` must be a POD type with ``operator +=`` and ``operator =`` or a ``Kokkos::View``. In the latter case, a sum reduction is applied where the identity is assumed to be given by the default constructor of the value type (and not by ``reduction_identity```). If provided, the ``init``/ ``join``/ ``final`` member functions must not take a ``WorkTag`` argument even for tagged reductions. | ||
| * If ``ExecPolicy`` is ``TeamThreadRange`` a "reducing" ``functor`` is not allowed and the ``ReducerArgument`` must satisfy the ``Reducer`` concept or ``ReducerArgumentNonConst`` must be a POD type with ``operator +=`` and ``operator =`` or a ``Kokkos::View``. In the latter case, a sum reduction is applied where the identity is assumed to be given by the default constructor of the value type (and not by ``reduction_identity```). |
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I think it can be clearer to mention the case where reduction_identity is used and when default constructor value is.
For a user perspective it is not even clear why identity is needed. Are you talking about initial value for the reduction or for some other operations in the algorithms?
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The initial value is the identity (internally) so that we don't have to treat the first iteration as special.
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Path forward:
- accept as is, and document future behavior once refactor is in place
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Waiting for @crtrott's proposal for a different behavior. |
ajpowelsnl
approved these changes
Jul 30, 2024
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| * If the ``functor`` is a lambda, ``ReducerArgument`` must satisfy the ``Reducer`` concept or ``ReducerArgumentNonConst`` must be a POD type with ``operator +=`` and ``operator =`` or a ``Kokkos::View``. In the latter case, a sum reduction is applied where the identity is assumed to be given by the default constructor of the value type (and not by ``reduction_identity```). If provided, the ``init``/ ``join``/ ``final`` member functions must not take a ``WorkTag`` argument even for tagged reductions. | ||
| * If ``ExecPolicy`` is ``TeamThreadRange`` a "reducing" ``functor`` is not allowed and the ``ReducerArgument`` must satisfy the ``Reducer`` concept or ``ReducerArgumentNonConst`` must be a POD type with ``operator +=`` and ``operator =`` or a ``Kokkos::View``. In the latter case, a sum reduction is applied where the identity is assumed to be given by the default constructor of the value type (and not by ``reduction_identity```). |
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Path forward:
- accept as is, and document future behavior once refactor is in place
Co-authored-by: Nicolas Morales <[email protected]>
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Related to kokkos/kokkos#6760.