gem5/src/base/coroutine.hh

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#ifndef __BASE_COROUTINE_HH__
#define __BASE_COROUTINE_HH__

#include <functional>
#include <stack>

#include "base/fiber.hh"

namespace m5
{

/**
 * This template defines a Coroutine wrapper type with a Boost-like
 * interface. It is built on top of the gem5 fiber class.
 * The two template parameters (Arg and Ret) are the coroutine
 * argument and coroutine return types which are passed between
 * the coroutine and the caller via operator() and get() method.
 * This implementation doesn't support passing multiple values,
 * so a tuple must be used in that scenario.
 *
 * Most methods are templatized since it is relevant to distinguish
 * the cases where one or both of the template parameters are void
 */
template <typename Arg, typename Ret>
class Coroutine : public Fiber
{

    // This empty struct type is meant to replace coroutine channels
    // in case the channel should be void (Coroutine template parameters
    // are void. (See following ArgChannel, RetChannel typedef)
    struct Empty {};
    using ArgChannel = typename std::conditional<
        std::is_same<Arg, void>::value, Empty, std::stack<Arg>>::type;

    using RetChannel = typename std::conditional<
        std::is_same<Ret, void>::value, Empty, std::stack<Ret>>::type;

  public:
    /**
     * CallerType:
     * A reference to an object of this class will be passed
     * to the coroutine task. This is the way it is possible
     * for the coroutine to interface (e.g. switch back)
     * to the coroutine caller.
     */
    class CallerType
    {
        friend class Coroutine;
      protected:
        CallerType(Coroutine& _coro) : coro(_coro), callerFiber(nullptr) {}

      public:
        /**
         * operator() is the way we can jump outside the coroutine
         * and return a value to the caller.
         *
         * This method is generated only if the coroutine returns
         * a value (Ret != void)
         *
         * @ingroup api_coroutine
         */
        template <typename T = Ret>
        CallerType&
        operator()(typename std::enable_if<
                   !std::is_same<T, void>::value, T>::type param)
        {
            retChannel.push(param);
            callerFiber->run();
            return *this;
        }

        /**
         * operator() is the way we can jump outside the coroutine
         *
         * This method is generated only if the coroutine doesn't
         * return a value (Ret = void)
         *
         * @ingroup api_coroutine
         */
        template <typename T = Ret>
        typename std::enable_if<std::is_same<T, void>::value,
                                CallerType>::type&
        operator()()
        {
            callerFiber->run();
            return *this;
        }

        /**
         * get() is the way we can extrapolate arguments from the
         * coroutine caller.
         * The coroutine blocks, waiting for the value, unless it is already
         * available; otherwise caller execution is resumed,
         * and coroutine won't execute until a value is pushed
         * from the caller.
         *
         * @return arg coroutine argument
         *
         * @ingroup api_coroutine
         */
        template <typename T = Arg>
        typename std::enable_if<!std::is_same<T, void>::value, T>::type
        get()
        {
            auto& args_channel = coro.argsChannel;
            while (args_channel.empty()) {
                callerFiber->run();
            }

            auto ret = args_channel.top();
            args_channel.pop();
            return ret;
        }

      private:
        Coroutine& coro;
        Fiber* callerFiber;
        RetChannel retChannel;
    };

    /**
     * @ingroup api_coroutine
     * @{
     */
    Coroutine() = delete;
    Coroutine(const Coroutine& rhs) = delete;
    Coroutine& operator=(const Coroutine& rhs) = delete;
    /** @} */ // end of api_coroutine

    /**
     * Coroutine constructor.
     * The only way to construct a coroutine is to pass it the routine
     * it needs to run. The first argument of the function should be a
     * reference to the Coroutine<Arg,Ret>::caller_type which the
     * routine will use as a way for yielding to the caller.
     * The optional second boolean argument controls if the Coroutine
     * should be run on creation, which mimics Boost's Coroutine
     * semantics by default. This can be disabled as an optimization to
     * avoid unnecessary context switches on Coroutine creation.
     *
     * @param f task run by the coroutine
     * @param run_coroutine set to false to disable running the coroutine
     *                      immediately after it is created
     *
     * @ingroup api_coroutine
     */
    Coroutine(std::function<void(CallerType&)> f, bool run_coroutine = true)
      : Fiber(), task(f), caller(*this)
    {
        // When desired, run the Coroutine after it is created
        if (run_coroutine)
            this->call();
    }

    /**
     * @ingroup api_coroutine
     */
    virtual ~Coroutine() {}

  public:
    /** Coroutine interface */

    /**
     * operator() is the way we can jump inside the coroutine
     * and passing arguments.
     *
     * This method is generated only if the coroutine takes
     * arguments (Arg != void)
     *
     * @ingroup api_coroutine
     */
    template <typename T = Arg>
    Coroutine&
    operator()(typename std::enable_if<
               !std::is_same<T, void>::value, T>::type param)
    {
        argsChannel.push(param);
        this->call();
        return *this;
    }

    /**
     * operator() is the way we can jump inside the coroutine.
     *
     * This method is generated only if the coroutine takes
     * no arguments. (Arg = void)
     *
     * @ingroup api_coroutine
     */
    template <typename T = Arg>
    typename std::enable_if<std::is_same<T, void>::value, Coroutine>::type&
    operator()()
    {
        this->call();
        return *this;
    }

    /**
     * get() is the way we can extrapolate return values
     * (yielded) from the coroutine.
     * The caller blocks, waiting for the value, unless it is already
     * available; otherwise coroutine execution is resumed,
     * and caller won't execute until a value is yielded back
     * from the coroutine.
     *
     * @return ret yielded value
     *
     * @ingroup api_coroutine
     */
    template <typename T = Ret>
    typename std::enable_if<!std::is_same<T, void>::value, T>::type
    get()
    {
        auto& ret_channel = caller.retChannel;
        while (ret_channel.empty()) {
            this->call();
        }

        auto ret = ret_channel.top();
        ret_channel.pop();
        return ret;
    }

    /**
     * Check if coroutine is still running
     *
     * @ingroup api_coroutine
     */
    operator bool() const { return !this->finished(); }

  private:
    /**
     * Overriding base (Fiber) main.
     * This method will be automatically called by the Fiber
     * running engine and it is a simple wrapper for the task
     * that the coroutine is supposed to run.
     */
    void main() override { this->task(caller); }

    void
    call()
    {
        caller.callerFiber = currentFiber();
        run();
    }

  private:
    /** Arguments for the coroutine */
    ArgChannel argsChannel;

    /** Coroutine task */
    std::function<void(CallerType&)> task;

    /** Coroutine caller */
    CallerType caller;
};

} //namespace m5

#endif // __BASE_COROUTINE_HH__