gecko-dev/memory/build/Utils.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef Utils_h
#define Utils_h

#include <type_traits>

#include "mozilla/CheckedInt.h"
#include "mozilla/TemplateLib.h"

// Helper for log2 of powers of 2 at compile time.
template <size_t N>
struct Log2 : mozilla::tl::CeilingLog2<N> {
  using mozilla::tl::CeilingLog2<N>::value;
  static_assert(1ULL << value == N, "Number is not a power of 2");
};
#define LOG2(N) Log2<N>::value

enum class Order {
  eLess = -1,
  eEqual = 0,
  eGreater = 1,
};

// Compare two integers. Returns whether the first integer is Less,
// Equal or Greater than the second integer.
template <typename T>
Order CompareInt(T aValue1, T aValue2) {
  static_assert(std::is_integral_v<T>, "Type must be integral");
  if (aValue1 < aValue2) {
    return Order::eLess;
  }
  if (aValue1 > aValue2) {
    return Order::eGreater;
  }
  return Order::eEqual;
}

// Compare two addresses. Returns whether the first address is Less,
// Equal or Greater than the second address.
template <typename T>
Order CompareAddr(T* aAddr1, T* aAddr2) {
  return CompareInt(uintptr_t(aAddr1), uintptr_t(aAddr2));
}

// User-defined literals to make constants more legible
constexpr size_t operator"" _KiB(unsigned long long int aNum) {
  return size_t(aNum) * 1024;
}

constexpr size_t operator"" _KiB(long double aNum) {
  return size_t(aNum * 1024);
}

constexpr size_t operator"" _MiB(unsigned long long int aNum) {
  return size_t(aNum) * 1024_KiB;
}

constexpr size_t operator"" _MiB(long double aNum) {
  return size_t(aNum * 1024_KiB);
}

constexpr double operator""_percent(long double aPercent) {
  return double(aPercent) / 100;
}

// Helper for (fast) comparison of fractions without involving divisions or
// floats.
class Fraction {
 public:
  explicit constexpr Fraction(size_t aNumerator, size_t aDenominator)
      : mNumerator(aNumerator), mDenominator(aDenominator) {}

  MOZ_IMPLICIT constexpr Fraction(long double aValue)
      // We use an arbitrary power of two as denominator that provides enough
      // precision for our use case.
      : mNumerator(aValue * 4096), mDenominator(4096) {}

  inline bool operator<(const Fraction& aOther) const {
#ifndef MOZ_DEBUG
    // We are comparing A / B < C / D, with all A, B, C and D being positive
    // numbers. Multiplying both sides with B * D, we have:
    // (A * B * D) / B < (C * B * D) / D, which can then be simplified as
    // A * D < C * B. When can thus compare our fractions without actually
    // doing any division.
    // This however assumes the multiplied quantities are small enough not
    // to overflow the multiplication. We use CheckedInt on debug builds
    // to enforce the assumption.
    return mNumerator * aOther.mDenominator < aOther.mNumerator * mDenominator;
#else
    mozilla::CheckedInt<size_t> numerator(mNumerator);
    mozilla::CheckedInt<size_t> denominator(mDenominator);
    // value() asserts when the multiplication overflowed.
    size_t lhs = (numerator * aOther.mDenominator).value();
    size_t rhs = (aOther.mNumerator * denominator).value();
    return lhs < rhs;
#endif
  }

  inline bool operator>(const Fraction& aOther) const { return aOther < *this; }

  inline bool operator>=(const Fraction& aOther) const {
    return !(*this < aOther);
  }

  inline bool operator<=(const Fraction& aOther) const {
    return !(*this > aOther);
  }

  inline bool operator==(const Fraction& aOther) const {
#ifndef MOZ_DEBUG
    // Same logic as operator<
    return mNumerator * aOther.mDenominator == aOther.mNumerator * mDenominator;
#else
    mozilla::CheckedInt<size_t> numerator(mNumerator);
    mozilla::CheckedInt<size_t> denominator(mDenominator);
    size_t lhs = (numerator * aOther.mDenominator).value();
    size_t rhs = (aOther.mNumerator * denominator).value();
    return lhs == rhs;
#endif
  }

  inline bool operator!=(const Fraction& aOther) const {
    return !(*this == aOther);
  }

 private:
  size_t mNumerator;
  size_t mDenominator;
};

#endif