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Use MPI_BYTE for unknown types #81

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Use MPI_BYTE for unknown types #81

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8 changes: 4 additions & 4 deletions src/impl/KokkosComm_types.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -25,10 +25,7 @@ template <typename Scalar>
MPI_Datatype mpi_type() {
using T = std::decay_t<Scalar>;

if constexpr (std::is_same_v<T, std::byte>)
return MPI_BYTE;

else if constexpr (std::is_same_v<T, char>)
if constexpr (std::is_same_v<T, char>)
return MPI_CHAR;
else if constexpr (std::is_same_v<T, unsigned char>)
return MPI_UNSIGNED_CHAR;
Expand Down Expand Up @@ -99,6 +96,9 @@ MPI_Datatype mpi_type() {
else if constexpr (std::is_same_v<T, Kokkos::complex<double>>)
return MPI_DOUBLE_COMPLEX;

else if constexpr (std::is_trivially_copyable_v<T>)
return MPI_BYTE;
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I'm not sure that this fix alone solves your problem. I'm missing the part where we get the size of T. In https://github.com/kokkos/kokkos-comm/blob/develop/src/impl/KokkosComm_send.hpp#L72 we only get the type (MPI_BYTE here) and take the span, not the span*sizeof(T). Maybe mpi_type() should provide a count multiplier as well?

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Yes, @masterleinad identified this problem. I'm not sure what's the best way to proceed here. Your suggestion is one of the possible variants.


else {
static_assert(std::is_void_v<T>, "mpi_type not implemented");
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It might make sense to assert that T is trivially copyable at least.

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Good point.

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Looking ahead, I think trivially-copyable is a problem for Trilinos FAD types - they are actually trivially-copyable, but because of how they are implemented they have non-default ctors, etc that makes C++ think they're not trivially-copyable.

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Maybe if we run into this problem (a problem of success), we can make a special configure-time toggle to remove this trivially-copyable check.

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One thing I'm not a fan of is that if you have type T with large sizeof(T), n * sizeof(T) may exceed INT_MAX, while storing them in a view is totally fine.

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For now, maybe we can just send as MPI_BYTE as long as the size (MPI_Type_size) matches the extent (MPI_Type_get_extent) and the lower bound (MPI_Type_get_extent) is 0; is this the best way to check if a type is contiguous?

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Kokkos could offer a trait that overrides std::is_trivially_copyable so that users can tell Kokkos that they know what they are doing (e.g., not trying to copy pointers between processes).

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We discussed that also in the context of deep_copy where we are assuming that all types used in kernels are logically trivially copyable. There, we decided that this would be too big of a change to error out unless the user has specialized such a trait.

return MPI_CHAR; // unreachable
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