1 files changed, 242 insertions, 0 deletions
diff --git a/xpcom/threads/Queue.h b/xpcom/threads/Queue.h
new file mode 100644
index 0000000000..754224ca7e
--- /dev/null
+++ b/xpcom/threads/Queue.h
@@ -0,0 +1,242 @@
+/* -*- 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