composable_kernel/include/ck/utility/amd_wave_read_first_lane.hpp

// SPDX-License-Identifier: MIT
// Copyright (c) 2018-2025, Advanced Micro Devices, Inc. All rights reserved.

#pragma once

#include "ck/ck.hpp"
#include "ck/utility/functional2.hpp"
#include "ck/utility/math.hpp"

#if !defined(__HIPCC_RTC__) || !defined(CK_CODE_GEN_RTC)
#include <array>
#include <cstddef>
#include <cstdint>
#include <type_traits>
#endif

namespace ck {
namespace detail {

template <unsigned SizeInBytes>
struct get_carrier;

template <>
struct get_carrier<1>
{
    using type = uint8_t;
};

template <>
struct get_carrier<2>
{
    using type = uint16_t;
};

template <>
struct get_carrier<3>
{
    using type = class carrier
    {
        using value_type = uint32_t;

        Array<ck::byte, 3> bytes;
        static_assert(sizeof(bytes) <= sizeof(value_type));

        // replacement of host std::copy_n()
        template <typename InputIterator, typename Size, typename OutputIterator>
        __device__ static OutputIterator copy_n(InputIterator from, Size size, OutputIterator to)
        {
            if(0 < size)
            {
                *to = *from;
                ++to;
                for(Size count = 1; count < size; ++count)
                {
                    *to = *++from;
                    ++to;
                }
            }

            return to;
        }

        // method to trigger template substitution failure
        __device__ carrier(const carrier& other) noexcept
        {
            copy_n(other.bytes.begin(), bytes.Size(), bytes.begin());
        }

        public:
        __device__ carrier& operator=(value_type value) noexcept
        {
            copy_n(reinterpret_cast<const ck::byte*>(&value), bytes.Size(), bytes.begin());

            return *this;
        }

        __device__ operator value_type() const noexcept
        {
            ck::byte result[sizeof(value_type)];

            copy_n(bytes.begin(), bytes.Size(), result);

            return *reinterpret_cast<const value_type*>(result);
        }
    };
};
static_assert(sizeof(get_carrier<3>::type) == 3);

template <>
struct get_carrier<4>
{
    using type = uint32_t;
};

template <unsigned SizeInBytes>
using get_carrier_t = typename get_carrier<SizeInBytes>::type;

} // namespace detail

__device__ inline uint32_t amd_wave_read_first_lane(uint32_t value)
{
    return __builtin_amdgcn_readfirstlane(value);
}

__device__ inline int32_t amd_wave_read_first_lane(int32_t value)
{
    return __builtin_amdgcn_readfirstlane(value);
}

__device__ inline int64_t amd_wave_read_first_lane(int64_t value)
{
    constexpr unsigned object_size        = sizeof(int64_t);
    constexpr unsigned second_part_offset = object_size / 2;
    auto* const from_obj                  = reinterpret_cast<const ck::byte*>(&value);
    alignas(int64_t) ck::byte to_obj[object_size];

    using Sgpr = uint32_t;

    *reinterpret_cast<Sgpr*>(to_obj) =
        amd_wave_read_first_lane(*reinterpret_cast<const Sgpr*>(from_obj));
    *reinterpret_cast<Sgpr*>(to_obj + second_part_offset) =
        amd_wave_read_first_lane(*reinterpret_cast<const Sgpr*>(from_obj + second_part_offset));

    return *reinterpret_cast<int64_t*>(to_obj);
}

template <typename Object,
          typename = ck::enable_if_t<ck::is_class_v<Object> && ck::is_trivially_copyable_v<Object>>>
__device__ auto amd_wave_read_first_lane(const Object& obj)
{
    using Size                = unsigned;
    constexpr Size SgprSize   = 4;
    constexpr Size ObjectSize = sizeof(Object);

    auto* const from_obj = reinterpret_cast<const ck::byte*>(&obj);
    alignas(Object) ck::byte to_obj[ObjectSize];

    constexpr Size RemainedSize             = ObjectSize % SgprSize;
    constexpr Size CompleteSgprCopyBoundary = ObjectSize - RemainedSize;
    for(Size offset = 0; offset < CompleteSgprCopyBoundary; offset += SgprSize)
    {
        using Sgpr = detail::get_carrier_t<SgprSize>;

        *reinterpret_cast<Sgpr*>(to_obj + offset) =
            amd_wave_read_first_lane(*reinterpret_cast<const Sgpr*>(from_obj + offset));
    }

    if constexpr(0 < RemainedSize)
    {
        using Carrier = detail::get_carrier_t<RemainedSize>;

        *reinterpret_cast<Carrier*>(to_obj + CompleteSgprCopyBoundary) = amd_wave_read_first_lane(
            *reinterpret_cast<const Carrier*>(from_obj + CompleteSgprCopyBoundary));
    }

    /// NOTE: Implicitly start object lifetime. It's better to use std::start_lifetime_at() in this
    /// scenario
    return *reinterpret_cast<Object*>(to_obj);
}

} // namespace ck