multiplyAndSplit64to32.hpp

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/* Copyright 2023 Jiří Vyskočil, Bernhard Manfred Gruber, Jan Stephan
 * SPDX-License-Identifier: MPL-2.0
 */
 
#pragma once
 
#include "alpaka/core/common.hpp"
 
#include <cstdint>
 
namespace alpaka::rand::engine::internal
{
    /// Get high 32 bits of a 64-bit number
    constexpr auto high32Bits(std::uint64_t const x) -> std::uint32_t
    {
        return static_cast<std::uint32_t>(x >> 32);
    }
 
    /// Get low 32 bits of a 64-bit number
    constexpr auto low32Bits(std::uint64_t const x) -> std::uint32_t
    {
        return static_cast<std::uint32_t>(x & 0xffff'ffff);
    }
 
    /** Multiply two 64-bit numbers and split the result into high and low 32 bits, also known as "mulhilo32"
     *
     * @param a first 64-bit multiplier
     * @param b second 64-bit multiplier
     * @param resultHigh high 32 bits of the product a*b
     * @param resultLow low 32 bits of the product a*b
     */
    // TODO: See single-instruction implementations in original Philox source code
    constexpr void multiplyAndSplit64to32(
        std::uint64_t const a,
        std::uint64_t const b,
        std::uint32_t& resultHigh,
        std::uint32_t& resultLow)
    {
        std::uint64_t const res64 = a * b;
        resultHigh = high32Bits(res64);
        resultLow = low32Bits(res64);
    }
} // namespace alpaka::rand::engine::internal