composable_kernel/include/ck/utility/reduction_operator.hpp

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#ifndef CK_REDUCTION_OPERATOR_HPP
#define CK_REDUCTION_OPERATOR_HPP

#include "common_header.hpp"

namespace ck {

namespace reduce {

// Every binary operator used in reduction is represented by a templated functor class. Each functor
// class must provide at least
// three members:
// 1) GetReductionZeroVal() -- the interface to return the "identity element" for the binary
// operator, "identity element" is the unique
//                    element in the algebraic space that doesn't affect the value of other elements
//                    when operated against them, and the concept is similar to zero vector in
//                    vector space
//                    (http://pages.cs.wisc.edu/~matthewb/pages/notes/pdf/linearalgebra/VectorSpaces.pdf).
// 2) indexable -- boolean value indicating whether indices of the operated elements could be
// recorded. Usually, Min/Max operator could
//                 need to record the indices of elements. For operator like Add/Mul, no need to
//                 record the indices.
// 3) operator() -- the first argument of the operator must be both an input & output, and the
// corresponding variable usually stores
//                  the accumulated result of many operator() calls; the second argument is only an
//                  input. For indexable binary
//                  operator, the second version of operator() has third argument (which is an
//                  output) to indicate whether the
//                  accumulated value (the first argument) has changed, in which case the recorded
//                  accumulated index also need be
//                  changed.

template <class T>
struct Add
{
    using dataType = T;

    __host__ __device__ static constexpr T GetReductionZeroVal() { return static_cast<T>(0.0f); };

    __host__ __device__ inline constexpr void operator()(T& a, T b) const { a = a + b; }
};

template <class T>
struct Mul
{
    using dataType = T;

    __host__ __device__ static constexpr T GetReductionZeroVal() { return static_cast<T>(1.0f); };

    __host__ __device__ inline constexpr void operator()(T& a, T b) const { a = a * b; }
};

template <class T>
struct Max
{
    using dataType = T;

    __host__ __device__ static constexpr T GetReductionZeroVal()
    {
        return NumericLimits<T>::Lowest();
    };

    __host__ __device__ inline constexpr void operator()(T& a, T b) const
    {
        if(a < b)
            a = b;
    }

    __host__ __device__ inline constexpr void operator()(T& a, T b, bool& changed) const
    {
        if(a < b)
        {
            a       = b;
            changed = true;
        }
    }
};

template <class T>
struct Min
{
    using dataType = T;

    __host__ __device__ static constexpr T GetReductionZeroVal()
    {
        return NumericLimits<T>::Max();
    };

    __host__ __device__ inline constexpr void operator()(T& a, T b) const
    {
        if(a > b)
            a = b;
    }

    __host__ __device__ inline constexpr void operator()(T& a, T b, bool& changed) const
    {
        if(a > b)
        {
            a       = b;
            changed = true;
        }
    }
};

template <class T>
struct AMax
{
    using dataType = T;

    __host__ __device__ static constexpr T GetReductionZeroVal() { return static_cast<T>(0.0f); };

    __host__ __device__ inline constexpr void operator()(T& a, T b) const
    {
        if(a < b)
            a = b;
    }

    __host__ __device__ inline constexpr void operator()(T& a, T b, bool& changed) const
    {
        if(a < b)
        {
            a       = b;
            changed = true;
        }
    }
};

}; // end of namespace reduce

} // end of namespace ck

#endif