Custom init() function for parallel_reduce with array value_type

[etphipp]

I have a functor that does reduction on an array that requires a non-trivial init() function, however I am finding that my functor's init function is not getting called. Here are the relevant parts of the declaration:

struct MaxMinFunctor {
  typedef double value_type[];

   KOKKOS_INLINE_FUNCTION
   void init (value_type dst) const { ... }

By adding print and assert statements I can verify init() is not getting called. Looking at the Kokkos source code, I see this:

template< class FunctorType , class ArgTag >
struct FunctorValueTraits< FunctorType , ArgTag , true /* == exists FunctorType::value_type */ >
{
  typedef typename Impl::remove_extent< typename FunctorType::value_type >::type  value_type ;

  // If not an array then what is the sizeof(value_type)
  enum { StaticValueSize = Impl::is_array< typename FunctorType::value_type >::value ? 0 : sizeof(value_type) };

  typedef value_type                 * pointer_type ;

  // The reference_type for an array is 'value_type *'
  // The reference_type for a single value is 'value_type &'

  typedef typename Impl::if_c< ! StaticValueSize , value_type *
                                                 , value_type & >::type  reference_type ;

and this

// Function signatures for FunctorType::init function without a tag and is an array
template< class FunctorType >
struct FunctorValueInitFunction< FunctorType , void , true > {

  typedef typename FunctorValueTraits<FunctorType,void>::reference_type value_type ;

  KOKKOS_INLINE_FUNCTION static void enable_if( void (FunctorType::*)( value_type * ) const );

Shouldn't that be:

KOKKOS_INLINE_FUNCTION static void enable_if( void (FunctorType::*)( value_type ) const );

given that reference_type for an array is value_type *?

If I add an empty overload of "void init(double**) const {}" to my functor it works and the correct init function is called. This seems like a bug, or am I just doing something stupid?

[crtrott]

Can you try init(value_type& dat), I believe we expect a reference to the value_type.

[hcedwar]

The value_type is a dynamic length array, so appears to be a bug in the 'is there an init function' introspection.

[etphipp]

Changing it to a reference doesn't work, and in fact introduces a compiler error:

/home/etphipp/Trilinos/Trilinos/packages/trilinoscouplings/examples/fenl/SampleGrouping.hpp:260:33: error: parameter �dst� includes reference to array of unknown bound �Kokkos::Example::FENL::MaxAnisotropyGrouping<Scalar, MeshType, CoeffFunctionType>::MaxMinFunctor::value_type�
void init (value_type& dst) const

[etphipp]

My belief is the introspection is wrong and it should be

KOKKOS_INLINE_FUNCTION static void enable_if( void (FunctorType::*)( value_type ) const );

instead of

KOKKOS_INLINE_FUNCTION static void enable_if( void (FunctorType::*)( value_type* ) const );

since value_type is already a pointer.

[crtrott]

Not so sure. It looks like we need to test for both. If your value_type is a "double" then the signature is
value_type& not value_type for the init thing. Probably we need to do some clever thing about testing whether the value_type is a actual scalar type or an array ....

[crtrott]

Carter can you do this?

[etphipp]

Btw, changing it to a static-length array doesn't make it work either.

[crtrott]

Yeah I wouldn't expect it too.

[etphipp]

You actually do all of that stuff already. My point was the test for when value_type is an array is wrong. The separate test for when value_type is not an array is correct.

[crtrott]

Oh ok. So it should be easy to fix 👍

[etphipp]

Yes, I think I told you how to fix it 😃

[crtrott]

Pushed to master, closing the issue