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Remove use of the modulo operator in Queue.Enumerator #11101

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merged 7 commits into from
Aug 27, 2016
Merged

Remove use of the modulo operator in Queue.Enumerator #11101

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9f01168
Remove use of the modulo operator in Queue
jamesqo Aug 24, 2016
7283d50
Add some comments to clarify things
jamesqo Aug 24, 2016
8e03f85
Add comment explaining why the % operator was removed
jamesqo Aug 25, 2016
082d16e
Remove bitwise and usage
jamesqo Aug 26, 2016
f4c6a48
PR feedback
jamesqo Aug 26, 2016
b886385
Revert to return ... instead of assigning to a variable
jamesqo Aug 26, 2016
42acb4b
Revert things to the way they were
jamesqo Aug 26, 2016
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27 changes: 21 additions & 6 deletions src/System.Collections/src/System/Collections/Generic/Queue.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -285,11 +285,6 @@ public bool Contains(T item)
return false;
}

private T GetElement(int i)
{
return _array[(_head + i) % _array.Length];
}

// Iterates over the objects in the queue, returning an array of the
// objects in the Queue, or an empty array if the queue is empty.
// The order of elements in the array is first in to last in, the same
Expand Down Expand Up @@ -399,12 +394,32 @@ public bool MoveNext()

if (_index == _q._size)
{
// We've run past the last element
_index = -2;
_currentElement = default(T);
return false;
}

_currentElement = _q.GetElement(_index);
// Cache some fields in locals to decrease code size
T[] array = _q._array;
int capacity = array.Length;

// _index represents the 0-based index into the queue, however the queue
// doesn't have to start from 0 and it may not even be stored contiguously in memory.

int arrayIndex = _q._head + _index; // this is the actual index into the queue's backing array
if (arrayIndex >= capacity)
{
// NOTE: Originally we were using the modulo operator here, however
// on Intel processors it has a very high instruction latency which
// was slowing down the loop quite a bit.
// Replacing it with simple comparison/subtraction operations sped up
// the average foreach loop by 2x.

arrayIndex -= capacity; // wrap around if needed
}

_currentElement = array[arrayIndex];
return true;
}

Expand Down