DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Mercurial (3b823d058ef5)

VCS Links

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242
/* -*- 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 mozilla_Queue_h
#define mozilla_Queue_h

#include "mozilla/MemoryReporting.h"

namespace mozilla {

// define to turn on additional (DEBUG) asserts
// #define EXTRA_ASSERTS 1

// A queue implements a singly linked list of pages, each of which contains some
// number of elements. Since the queue needs to store a "next" pointer, the
// actual number of elements per page won't be quite as many as were requested.
//
// This class should only be used if it's valid to construct T elements from all
// zeroes. The class also fails to call the destructor on items. However, it
// will only destroy items after it has moved out their contents. The queue is
// required to be empty when it is destroyed.
//
// Each page consists of N entries.  We use the head buffer as a circular buffer
// if it's the only buffer; if we have more than one buffer when the head is
// empty we release it.  This avoids occasional freeing and reallocating buffers
// every N entries.  We'll still allocate and free every N if the normal queue
// depth is greated than N.  A fancier solution would be to move an empty Head
// buffer to be an empty tail buffer, freeing if we have multiple empty tails,
// but that probably isn't worth it.
//
// Cases:
//   a) single buffer, circular
//      Push: if not full:
//              Add to tail, bump tail and reset to 0 if at end
//            full:
//              Add new page, insert there and set tail to 1
//      Pop:
//            take entry and bump head, reset to 0 if at end
//   b) multiple buffers:
//      Push: if not full:
//              Add to tail, bump tail
//            full:
//              Add new page, insert there and set tail to 1
//      Pop:
//            take entry and bump head, reset to 0 if at end
//            if buffer is empty, free head buffer and promote next to head
//
template <class T, size_t RequestedItemsPerPage = 256>
class Queue {
 public:
  Queue() = default;

  ~Queue() {
    MOZ_ASSERT(IsEmpty());

    if (mHead) {
      free(mHead);
    }
  }

  T& Push(T&& aElement) {
#if defined(EXTRA_ASSERTS) && DEBUG
    size_t original_length = Count();
#endif
    if (!mHead) {
      mHead = NewPage();
      MOZ_ASSERT(mHead);

      mTail = mHead;
      T& eltLocation = mTail->mEvents[0];
      eltLocation = std::move(aElement);
      mOffsetHead = 0;
      mHeadLength = 1;
#ifdef EXTRA_ASSERTS
      MOZ_ASSERT(Count() == original_length + 1);
#endif
      return eltLocation;
    }
    if ((mHead == mTail && mHeadLength == ItemsPerPage) ||
        (mHead != mTail && mTailLength == ItemsPerPage)) {
      // either we have one (circular) buffer and it's full, or
      // we have multiple buffers and the last buffer is full
      Page* page = NewPage();
      MOZ_ASSERT(page);

      mTail->mNext = page;
      mTail = page;
      T& eltLocation = page->mEvents[0];
      eltLocation = std::move(aElement);
      mTailLength = 1;
#ifdef EXTRA_ASSERTS
      MOZ_ASSERT(Count() == original_length + 1);
#endif
      return eltLocation;
    }
    if (mHead == mTail) {
      // we have space in the (single) head buffer
      uint16_t offset = (mOffsetHead + mHeadLength++) % ItemsPerPage;
      T& eltLocation = mTail->mEvents[offset];
      eltLocation = std::move(aElement);
#ifdef EXTRA_ASSERTS
      MOZ_ASSERT(Count() == original_length + 1);
#endif
      return eltLocation;
    }
    // else we have space to insert into last buffer
    T& eltLocation = mTail->mEvents[mTailLength++];
    eltLocation = std::move(aElement);
#ifdef EXTRA_ASSERTS
    MOZ_ASSERT(Count() == original_length + 1);
#endif
    return eltLocation;
  }

  bool IsEmpty() const {
    return !mHead || (mHead == mTail && mHeadLength == 0);
  }

  T Pop() {
#if defined(EXTRA_ASSERTS) && DEBUG
    size_t original_length = Count();
#endif
    MOZ_ASSERT(!IsEmpty());

    T result = std::move(mHead->mEvents[mOffsetHead]);
    mOffsetHead = (mOffsetHead + 1) % ItemsPerPage;
    mHeadLength -= 1;

    // Check if mHead points to empty (circular) Page and we have more
    // pages
    if (mHead != mTail && mHeadLength == 0) {
      Page* dead = mHead;
      mHead = mHead->mNext;
      free(dead);
      mOffsetHead = 0;
      // if there are still >1 pages, the new head is full.
      if (mHead != mTail) {
        mHeadLength = ItemsPerPage;
      } else {
        mHeadLength = mTailLength;
        mTailLength = 0;
      }
    }

#ifdef EXTRA_ASSERTS
    MOZ_ASSERT(Count() == original_length - 1);
#endif
    return result;
  }

  T& FirstElement() {
    MOZ_ASSERT(!IsEmpty());
    return mHead->mEvents[mOffsetHead];
  }

  const T& FirstElement() const {
    MOZ_ASSERT(!IsEmpty());
    return mHead->mEvents[mOffsetHead];
  }

  T& LastElement() {
    MOZ_ASSERT(!IsEmpty());
    uint16_t offset =
        mHead == mTail ? mOffsetHead + mHeadLength - 1 : mTailLength - 1;
    return mTail->mEvents[offset];
  }

  const T& LastElement() const {
    MOZ_ASSERT(!IsEmpty());
    uint16_t offset =
        mHead == mTail ? mOffsetHead + mHeadLength - 1 : mTailLength - 1;
    return mTail->mEvents[offset];
  }

  size_t Count() const {
    // It is obvious count is 0 when the queue is empty.
    if (!mHead) {
      return 0;
    }

    // Compute full (intermediate) pages; Doesn't count first or last page
    int count = 0;
    // 1 buffer will have mHead == mTail; 2 will have mHead->mNext == mTail
    for (Page* page = mHead; page != mTail && page->mNext != mTail;
         page = page->mNext) {
      count += ItemsPerPage;
    }
    // add first and last page
    count += mHeadLength + mTailLength;
    MOZ_ASSERT(count >= 0);

    return count;
  }

  size_t ShallowSizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
    size_t n = 0;
    if (mHead) {
      for (Page* page = mHead; page != mTail; page = page->mNext) {
        n += aMallocSizeOf(page);
      }
    }
    return n;
  }

  size_t ShallowSizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
    return aMallocSizeOf(this) + ShallowSizeOfExcludingThis(aMallocSizeOf);
  }

 private:
  static_assert(
      (RequestedItemsPerPage & (RequestedItemsPerPage - 1)) == 0,
      "RequestedItemsPerPage should be a power of two to avoid heap slop.");

  // Since a Page must also contain a "next" pointer, we use one of the items to
  // store this pointer. If sizeof(T) > sizeof(Page*), then some space will be
  // wasted. So be it.
  static const size_t ItemsPerPage = RequestedItemsPerPage - 1;

  // Page objects are linked together to form a simple deque.
  struct Page {
    struct Page* mNext;
    T mEvents[ItemsPerPage];
  };

  static Page* NewPage() {
    return static_cast<Page*>(moz_xcalloc(1, sizeof(Page)));
  }

  Page* mHead = nullptr;
  Page* mTail = nullptr;

  uint16_t mOffsetHead = 0;  // Read position in head page
  uint16_t mHeadLength = 0;  // Number of items in the head page
  uint16_t mTailLength = 0;  // Number of items in the tail page
};

}  // namespace mozilla

#endif  // mozilla_Queue_h