mfbt/TemplateLib.h
author Petru Lingurar <petru.lingurar@softvision.ro>
Fri, 21 Dec 2018 08:56:47 +0000
changeset 501492 65621d0fe1262af0643cec37c23b2d9ec42588ad
parent 391863 eb11f699091d1fa8ce5dc0ef671868feeb179778
child 508163 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1513938 - Enforce a Bundle size limit and drop `privateSession` if exceeds it. r=JanH, a=jcristau The `privateSession` key would normally allow persisting the Private Browsing session across OOMs in Activity's Bundle. We need to do that to avoid storing private, sensible data on disk like we do with the normal browsing session. In some cases `privateSession` would contain a lot of data which, along with other possible concurrent transactions could overflow Binder's buffer which has a limited fixed size, currently 1Mb. To avoid this, we will drop `privateSession` from the Bundle if the resulting size is greater than a _speculative_ size of 300KBs which would mean that in the case of an OOM all Private Browsing state would be lost. Bug 1515592 is filed to investigate for a better solution. Differential Revision: https://phabricator.services.mozilla.com/D15067

/* -*- 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/. */

/*
 * Reusable template meta-functions on types and compile-time values.  Meta-
 * functions are placed inside the 'tl' namespace to avoid conflict with non-
 * meta functions of the same name (e.g., mozilla::tl::FloorLog2 vs.
 * mozilla::FloorLog2).
 *
 * When constexpr support becomes universal, we should probably use that instead
 * of some of these templates, for simplicity.
 */

#ifndef mozilla_TemplateLib_h
#define mozilla_TemplateLib_h

#include <limits.h>
#include <stddef.h>

#include "mozilla/TypeTraits.h"

namespace mozilla {

namespace tl {

/** Compute min/max. */
template<size_t Size, size_t... Rest>
struct Min
{
  static constexpr size_t value =
    Size < Min<Rest...>::value
    ? Size
    : Min<Rest...>::value;
};

template<size_t Size>
struct Min<Size>
{
  static constexpr size_t value = Size;
};

template<size_t Size, size_t... Rest>
struct Max
{
  static constexpr size_t value =
    Size > Max<Rest...>::value
    ? Size
    : Max<Rest...>::value;
};

template<size_t Size>
struct Max<Size>
{
  static constexpr size_t value = Size;
};

/** Compute floor(log2(i)). */
template<size_t I>
struct FloorLog2
{
  static const size_t value = 1 + FloorLog2<I / 2>::value;
};
template<> struct FloorLog2<0> { /* Error */ };
template<> struct FloorLog2<1> { static const size_t value = 0; };

/** Compute ceiling(log2(i)). */
template<size_t I>
struct CeilingLog2
{
  static const size_t value = FloorLog2<2 * I - 1>::value;
};

/** Round up to the nearest power of 2. */
template<size_t I>
struct RoundUpPow2
{
  static const size_t value = size_t(1) << CeilingLog2<I>::value;
};
template<>
struct RoundUpPow2<0>
{
  static const size_t value = 1;
};

/** Compute the number of bits in the given unsigned type. */
template<typename T>
struct BitSize
{
  static const size_t value = sizeof(T) * CHAR_BIT;
};

/**
 * Produce an N-bit mask, where N <= BitSize<size_t>::value.  Handle the
 * language-undefined edge case when N = BitSize<size_t>::value.
 */
template<size_t N>
struct NBitMask
{
  // Assert the precondition.  On success this evaluates to 0.  Otherwise it
  // triggers divide-by-zero at compile time: a guaranteed compile error in
  // C++11, and usually one in C++98.  Add this value to |value| to assure
  // its computation.
  static const size_t checkPrecondition =
    0 / size_t(N < BitSize<size_t>::value);
  static const size_t value = (size_t(1) << N) - 1 + checkPrecondition;
};
template<>
struct NBitMask<BitSize<size_t>::value>
{
  static const size_t value = size_t(-1);
};

/**
 * For the unsigned integral type size_t, compute a mask M for N such that
 * for all X, !(X & M) implies X * N will not overflow (w.r.t size_t)
 */
template<size_t N>
struct MulOverflowMask
{
  static const size_t value =
    ~NBitMask<BitSize<size_t>::value - CeilingLog2<N>::value>::value;
};
template<> struct MulOverflowMask<0> { /* Error */ };
template<> struct MulOverflowMask<1> { static const size_t value = 0; };

/**
 * And<bool...> computes the logical 'and' of its argument booleans.
 *
 * Examples:
 *   mozilla::t1::And<true, true>::value is true.
 *   mozilla::t1::And<true, false>::value is false.
 *   mozilla::t1::And<>::value is true.
 */

template<bool...>
struct And;

template<>
struct And<> : public TrueType { };

template<bool C1, bool... Cn>
struct And<C1, Cn...>
  : public Conditional<C1, And<Cn...>, FalseType>::Type { };

} // namespace tl

} // namespace mozilla

#endif /* mozilla_TemplateLib_h */