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Add Array.Reverse<T> #7132

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Sep 13, 2016
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Add Array.Reverse<T> #7132

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406ef90
Add Array.Reverse<T>
justinvp Sep 11, 2016
e270c04
Change List<T>.Reverse to use Array.Reverse<T>
justinvp Sep 11, 2016
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2 changes: 2 additions & 0 deletions src/mscorlib/model.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -286,6 +286,8 @@
<Member Name="Resize&lt;T&gt;(T[]@,System.Int32)" />
<Member Name="Reverse(System.Array)" />
<Member Name="Reverse(System.Array,System.Int32,System.Int32)" />
<Member Name="Reverse&lt;T&gt;(T[])" />
<Member Name="Reverse&lt;T&gt;(T[],System.Int32,System.Int32)" />
<Member Name="SetValue(System.Object,System.Int32)" />
<Member Name="SetValue(System.Object,System.Int32,System.Int32)" />
<Member Name="SetValue(System.Object,System.Int32,System.Int32,System.Int32)" />
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36 changes: 35 additions & 1 deletion src/mscorlib/src/System/Array.cs
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Expand Up @@ -1614,7 +1614,41 @@ public static void Reverse(Array array, int index, int length) {
[MethodImplAttribute(MethodImplOptions.InternalCall)]
[ReliabilityContract(Consistency.MayCorruptInstance, Cer.MayFail)]
private static extern bool TrySZReverse(Array array, int index, int count);


[ReliabilityContract(Consistency.MayCorruptInstance, Cer.MayFail)]
public static void Reverse<T>(T[] array)
{
if (array == null)
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
Contract.EndContractBlock();
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@jamesqo jamesqo Sep 11, 2016 •
edited
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Nit: EndContractBlock isn't needed.

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I know the code contract annotations aren't being maintained anymore going forward. I included them here for consistency with the existing code throughout this file, but I'd be fine with removing, if that's preferred. @jkotas do you have a preference? (I believe @weshaggard wanted to keep the annotations in existing code for now).

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consistency with the existing code throughout this file

Sounds good to me. The coding conventions say to use same style as the rest of the file.

Reverse(array, 0, array.Length);
}

[ReliabilityContract(Consistency.MayCorruptInstance, Cer.MayFail)]
public static void Reverse<T>(T[] array, int index, int length)
{
if (array == null)
ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
if (index < 0)
ThrowHelper.ThrowIndexArgumentOutOfRange_NeedNonNegNumException();
if (length < 0)
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This might increase the generated code size, but I think you can make this a teeny bit faster for the common case:

if ((index | length) < 0) // Will be true if either is negative
{
    if (index < 0)
        ThrowHelper.ThrowIndexArgumentOutOfRange_NeedNonNegNumException();
    else
        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.length, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
}

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I just aligned this with what @benaadams did for the non-generic Reverse in #7059.

ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.length, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
if (array.Length - index < length)
ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidOffLen);
Contract.EndContractBlock();

int i = index;
int j = index + length - 1;
while (i < j)
{
T temp = array[i];
array[i] = array[j];
array[j] = temp;
i++;
j--;
}
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This loop incurs range checks, while the native implementation does not. We may want to specialize if T is a primitive type and call into the native version if so, e.g.:

// After arg validation
if (typeof(T) == typeof(byte) || typeof(T) == typeof(sbyte) || ...)
{
    bool result = TrySZReverse(array, index, length);
    Contract.Assert(result);
    return;
}

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@justinvp justinvp Sep 11, 2016 •
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The jit doesn't eliminate the range checks in this case? (I haven't checked).

BTW, @mikedn says (comment):

I did a quick test in the related CoreCLR PR. And for very small arrays TrySZReverse will always loose due to the cost of another call and dispatching to the correct native implementation.

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The jit doesn't eliminate the range checks in this case?

I'd be surprised if it did- last I checked, the JIT was unable to do so except for very simple cases. For example, this will incur range checks:

int n = GetNumber();
var array = new int[n];

for (int i = 0;  i < n; i++) array[i] = n;

And for very small arrays

For larger arrays it will be a big win because that's potentially hundreds/thousands of range checks that are being bypassed. You could change the conditional to

if ((typeof(T) == ...) && length < SomeThreshold)

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@benaadams benaadams Sep 11, 2016 •
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Can always test the impact by switching them off https://github.com/dotnet/coreclr/issues/6790#issuecomment-241487380

There are some issues for range checks so hopefully this will be resolved,

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@benaadams benaadams Sep 11, 2016 •
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Could also do something like:

int j = index + length - 1;

for (var i = index; i < array.Length; i++)
{
    if (i >= j)
    {
        break;
    }

    T temp = array[i];
    array[i] = array[j];
    array[j] = temp;
    j--;
}

Which should cut some (for i indexing)?

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Could also do something like:

You're adding another branch for i >= j, so that should have the same performance characteristics as the range check?

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I would prefer the original code - the fact that the jitter cannot eliminate the range checks today doesn't mean that it cannot do that tomorrow and if we tried to do various optimization hacks, it would be both less readable and may end up being slower after the jitter starts eliminating the checks here.

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What I meant is that clean and easy to read implementation is preferrable in this case.

}

// Sorts the elements of an array. The sort compares the elements to each
// other using the IComparable interface, which must be implemented
// by all elements of the array.
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17 changes: 1 addition & 16 deletions src/mscorlib/src/System/Collections/Generic/List.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -930,22 +930,7 @@ public void Reverse(int index, int count) {
ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidOffLen);
Contract.EndContractBlock();

// The non-generic Array.Reverse is not used because it does not perform
// well for non-primitive value types.
// If/when a generic Array.Reverse<T> becomes available, the below code
// can be deleted and replaced with a call to Array.Reverse<T>.
int i = index;
int j = index + count - 1;
T[] array = _items;
while (i < j)
{
T temp = array[i];
array[i] = array[j];
array[j] = temp;
i++;
j--;
}

Array.Reverse(_items, index, count);
_version++;
}

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