gem5/src/base/sat_counter.hh

/*
 * Copyright (c) 2019, 2020 Inria
 * All rights reserved.
 *
 * The license below extends only to copyright in the software and shall
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 * property including but not limited to intellectual property relating
 * to a hardware implementation of the functionality of the software
 * licensed hereunder.  You may use the software subject to the license
 * terms below provided that you ensure that this notice is replicated
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 * All rights reserved.
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 * met: redistributions of source code must retain the above copyright
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#ifndef __BASE_SAT_COUNTER_HH__
#define __BASE_SAT_COUNTER_HH__

#include <cassert>
#include <cstdint>

#include "base/logging.hh"
#include "base/types.hh"

namespace gem5
{

/**
 * Implements an n bit saturating counter and provides methods to
 * increment, decrement, and read it.
 *
 * @tparam T The type of the underlying counter container.
 */
template <class T>
class GenericSatCounter
{
  public:
    /** The default constructor should never be used. */
    GenericSatCounter() = delete;

    /**
     * Constructor for the counter. The explicit keyword is used to make
     * sure the user does not assign a number to the counter thinking it
     * will be used as a counter value when it is in fact used as the number
     * of bits.
     *
     * @param bits How many bits the counter will have.
     * @param initial_val Starting value for the counter.
     *
     * @ingroup api_sat_counter
     */
    explicit GenericSatCounter(unsigned bits, T initial_val = 0)
        : initialVal(initial_val), maxVal((1ULL << bits) - 1),
          counter(initial_val)
    {
        fatal_if(bits > 8*sizeof(T),
                 "Number of bits exceeds counter size");
        fatal_if(initial_val > maxVal,
                 "Saturating counter's initial value exceeds max value.");
    }

    /**
     * Copy constructor.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter(const GenericSatCounter& other)
        : initialVal(other.initialVal), maxVal(other.maxVal),
          counter(other.counter)
    {
    }

    /**
     * Copy assignment.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter& operator=(const GenericSatCounter& other) {
        if (this != &other) {
            GenericSatCounter temp(other);
            this->swap(temp);
        }
        return *this;
    }

    /**
     * Move constructor.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter(GenericSatCounter&& other)
    {
        initialVal = other.initialVal;
        maxVal = other.maxVal;
        counter = other.counter;
        GenericSatCounter temp(0);
        other.swap(temp);
    }

    /**
     * Move assignment.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter& operator=(GenericSatCounter&& other) {
        if (this != &other) {
            initialVal = other.initialVal;
            maxVal = other.maxVal;
            counter = other.counter;
            GenericSatCounter temp(0);
            other.swap(temp);
        }
        return *this;
    }

    /**
     * Swap the contents of every member of the class. Used for the default
     * copy-assignment created by the compiler.
     *
     * @param other The other object to swap contents with.
     *
     * @ingroup api_sat_counter
     */
    void
    swap(GenericSatCounter& other)
    {
        std::swap(initialVal, other.initialVal);
        std::swap(maxVal, other.maxVal);
        std::swap(counter, other.counter);
    }

    /**
     * Pre-increment operator.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter&
    operator++()
    {
        if (counter < maxVal) {
            ++counter;
        }
        return *this;
    }

    /**
     * Post-increment operator.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter
    operator++(int)
    {
        GenericSatCounter old_counter = *this;
        ++*this;
        return old_counter;
    }

    /**
     * Pre-decrement operator.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter&
    operator--()
    {
        if (counter > 0) {
            --counter;
        }
        return *this;
    }

    /**
     * Post-decrement operator.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter
    operator--(int)
    {
        GenericSatCounter old_counter = *this;
        --*this;
        return old_counter;
    }

    /**
     * Shift-right-assignment.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter&
    operator>>=(const int& shift)
    {
        assert(shift >= 0);
        this->counter >>= shift;
        return *this;
    }

    /**
     * Shift-left-assignment.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter&
    operator<<=(const int& shift)
    {
        assert(shift >= 0);
        this->counter <<= shift;
        if (this->counter > maxVal) {
            this->counter = maxVal;
        }
        return *this;
    }

    /**
     * Add-assignment.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter&
    operator+=(const long long& value)
    {
        if (value >= 0) {
            if (maxVal - this->counter >= value) {
                this->counter += value;
            } else {
                this->counter = maxVal;
            }
        } else {
            *this -= -value;
        }
        return *this;
    }

    /**
     * Subtract-assignment.
     *
     * @ingroup api_sat_counter
     */
    GenericSatCounter&
    operator-=(const long long& value)
    {
        if (value >= 0) {
            if (this->counter > value) {
                this->counter -= value;
            } else {
                this->counter = 0;
            }
        } else {
            *this += -value;
        }
        return *this;
    }

    /**
     * Read the counter's value.
     *
     * @ingroup api_sat_counter
     */
    operator T() const { return counter; }

    /**
     * Reset the counter to its initial value.
     *
     * @ingroup api_sat_counter
     */
    void reset() { counter = initialVal; }

    /**
     * Calculate saturation percentile of the current counter's value
     * with regard to its maximum possible value.
     *
     * @return A value between 0.0 and 1.0 to indicate which percentile of
     *         the maximum value the current value is.
     *
     * @ingroup api_sat_counter
     */
    double calcSaturation() const { return (double) counter / maxVal; }

    /**
     * Whether the counter has achieved its maximum value or not.
     *
     * @return True if the counter saturated.
     *
     * @ingroup api_sat_counter
     */
    bool isSaturated() const { return counter == maxVal; }

    /**
     * Saturate the counter.
     *
     * @return The value added to the counter to reach saturation.
     *
     * @ingroup api_sat_counter
     */
    T saturate()
    {
        const T diff = maxVal - counter;
        counter = maxVal;
        return diff;
    }

  private:
    T initialVal;
    T maxVal;
    T counter;
};

/** @ingroup api_sat_counter
 *  @{
 */
typedef GenericSatCounter<uint8_t> SatCounter8;
typedef GenericSatCounter<uint16_t> SatCounter16;
typedef GenericSatCounter<uint32_t> SatCounter32;
typedef GenericSatCounter<uint64_t> SatCounter64;
/** @} */

} // namespace gem5

#endif // __BASE_SAT_COUNTER_HH__