/// <summary>
/// A wait-free alternative to <see cref="TryEnqueue"/>, which fails on compare-and-swap failure.
/// </summary>
/// <param name="e">The item to enqueue.</param>
/// <returns><c>1</c> if next element cannot be filled, <c>-1</c> if CAS failed, and <c>0</c> if successful.</returns>
public int WeakEnqueue(T e)
{
Contract.Requires(e != null);
long mask = this.Mask;
long capacity = mask + 1;
long currentTail = this.ProducerIndex; // LoadLoad
long consumerIndexCache = this.ConsumerIndexCache; // LoadLoad
long wrapPoint = currentTail - capacity;
if (consumerIndexCache <= wrapPoint)
{
long currHead = this.ConsumerIndex; // LoadLoad
if (currHead <= wrapPoint)
{
return(1); // FULL :(
}
else
{
this.ConsumerIndexCache = currHead; // StoreLoad
}
}
// look Ma, no loop!
if (!this.TrySetProducerIndex(currentTail, currentTail + 1))
{
return(-1); // CAS FAIL :(
}
// Won CAS, move on to storing
long offset = RefArrayAccessUtil.CalcElementOffset(currentTail, mask);
this.SoElement(offset, e);
return(0); // AWESOME :)
}
/// <summary>
/// Lock free peek using ordered loads. As class name suggests access is limited to a single thread.
/// </summary>
/// <param name="item">The peeked item.</param>
/// <returns><c>true</c> if an item was retrieved, otherwise <c>false</c>.</returns>
/// <seealso cref="IQueue{T}.TryPeek"/>
public override bool TryPeek(out T item)
{
// Copy field to avoid re-reading after volatile load
T[] buffer = this.Buffer;
long consumerIndex = this.ConsumerIndex; // LoadLoad
long offset = this.CalcElementOffset(consumerIndex);
T e = RefArrayAccessUtil.LvElement(buffer, offset);
if (null == e)
{
// NOTE: Queue may not actually be empty in the case of a producer (P1) being interrupted after
// winning the CAS on offer but before storing the element in the queue. Other producers may go on
// to fill up the queue after this element.
if (consumerIndex != this.ProducerIndex)
{
do
{
e = RefArrayAccessUtil.LvElement(buffer, offset);
} while (e == null);
}
else
{
item = default(T);
return(false);
}
}
item = e;
return(true);
}
/// <summary>
/// Lock free Enqueue operation, using a single compare-and-swap. As the class name suggests, access is
/// permitted to many threads concurrently.
/// </summary>
/// <param name="e">The item to enqueue.</param>
/// <returns><c>true</c> if the item was added successfully, otherwise <c>false</c>.</returns>
/// <seealso cref="IQueue{T}.TryEnqueue"/>
public override bool TryEnqueue(T e)
{
Contract.Requires(e != null);
// use a cached view on consumer index (potentially updated in loop)
long mask = this.Mask;
long capacity = mask + 1;
long consumerIndexCache = this.ConsumerIndexCache; // LoadLoad
long currentProducerIndex;
do
{
currentProducerIndex = this.ProducerIndex; // LoadLoad
long wrapPoint = currentProducerIndex - capacity;
if (consumerIndexCache <= wrapPoint)
{
long currHead = this.ConsumerIndex; // LoadLoad
if (currHead <= wrapPoint)
{
return(false); // FULL :(
}
else
{
// update shared cached value of the consumerIndex
this.ConsumerIndexCache = currHead; // StoreLoad
// update on stack copy, we might need this value again if we lose the CAS.
consumerIndexCache = currHead;
}
}
} while (!this.TrySetProducerIndex(currentProducerIndex, currentProducerIndex + 1));
// NOTE: the new producer index value is made visible BEFORE the element in the array. If we relied on
// the index visibility to poll() we would need to handle the case where the element is not visible.
// Won CAS, move on to storing
long offset = RefArrayAccessUtil.CalcElementOffset(currentProducerIndex, mask);
this.SoElement(offset, e); // StoreStore
return(true); // AWESOME :)
}
/// <summary> /// A volatile load (load + LoadLoad barrier) of an element from a given offset. /// </summary> /// <param name="offset">Computed via <see cref="CalcElementOffset"/>.</param> /// <returns>The element at the offset.</returns> protected T LvElement(long offset) => RefArrayAccessUtil.LvElement(this.Buffer, offset);
/// <summary> /// An ordered store(store + StoreStore barrier) of an element to a given offset. /// </summary> /// <param name="offset">Computed via <see cref="CalcElementOffset"/>.</param> /// <param name="e">An orderly kitty.</param> protected void SoElement(long offset, T e) => RefArrayAccessUtil.SoElement(this.Buffer, offset, e);
/// <summary> /// Calculates an element offset based on a given array index. /// </summary> /// <param name="index">The desirable element index.</param> /// <returns>The offset in bytes within the array for a given index.</returns> protected long CalcElementOffset(long index) => RefArrayAccessUtil.CalcElementOffset(index, this.Mask);
