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Optimize Enumerable Min/Max final reduction with shuffles #127995

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Merged
EgorBo merged 4 commits into from
May 10, 2026
Merged

Optimize Enumerable Min/Max final reduction with shuffles #127995

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6a10b48
Optimize Enumerable Min/Max final reduction with shuffles
EgorBo May 9, 2026
ec3708d
cleanup
EgorBo May 9, 2026
c573615
Merge branch 'main' into ai/linq-minmax-shuffle-reduction
EgorBo May 9, 2026
7f57c02
Merge branch 'main' into ai/linq-minmax-shuffle-reduction
EgorBo May 10, 2026
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52 changes: 43 additions & 9 deletions src/libraries/System.Linq/src/System/Linq/MaxMin.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections.Generic;
using System.Diagnostics;
using System.Numerics;
using System.Runtime.Intrinsics;
using System.Runtime.CompilerServices;
Expand Down Expand Up @@ -101,15 +102,7 @@ private static T MinMaxInteger<T, TMinMax>(this IEnumerable<T> source)
}

// Reduce to single value
// NOTE: this can be optimized further with shuffles.
value = best128[0];
for (int i = 1; i < Vector128<T>.Count; i++)
{
if (TMinMax.Compare(best128[i], value))
{
value = best128[i];
}
}
value = HorizontalMinMax<T, TMinMax>(best128);
}
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else
{
Expand All @@ -132,5 +125,46 @@ private static T MinMaxInteger<T, TMinMax>(this IEnumerable<T> source)

return value;
}

/// <summary>Reduces a <see cref="Vector128{T}"/> to a single element using <typeparamref name="TMinMax"/>.</summary>
[MethodImpl(MethodImplOptions.AggressiveInlining)]
private static T HorizontalMinMax<T, TMinMax>(Vector128<T> x)
where T : struct, IBinaryInteger<T>
where TMinMax : IMinMaxCalc<T>
{
// Perform log2(Vector128<T>.Count) reductions, each combining the vector with a shuffled
// copy of itself so that lane 0 ends up holding the min/max of all original lanes.
if (Vector128<T>.Count == 16)
{
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsByte(),
Vector128.Create((byte)8, 9, 10, 11, 12, 13, 14, 15, 0, 1, 2, 3, 4, 5, 6, 7)).As<byte, T>());
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsByte(),
Vector128.Create((byte)4, 5, 6, 7, 0, 1, 2, 3, 8, 9, 10, 11, 12, 13, 14, 15)).As<byte, T>());
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsByte(),
Vector128.Create((byte)2, 3, 0, 1, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)).As<byte, T>());
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsByte(),
Vector128.Create((byte)1, 0, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)).As<byte, T>());
}
else if (Vector128<T>.Count == 8)
{
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsInt16(),
Vector128.Create(4, 5, 6, 7, 0, 1, 2, 3)).As<short, T>());
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsInt16(),
Vector128.Create(2, 3, 0, 1, 4, 5, 6, 7)).As<short, T>());
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsInt16(),
Vector128.Create(1, 0, 2, 3, 4, 5, 6, 7)).As<short, T>());
}
else if (Vector128<T>.Count == 4)
{
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsInt32(), Vector128.Create(2, 3, 0, 1)).As<int, T>());
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsInt32(), Vector128.Create(1, 0, 3, 2)).As<int, T>());
}
else
{
Debug.Assert(Vector128<T>.Count == 2);
x = TMinMax.Compare(x, Vector128.Shuffle(x.AsInt64(), Vector128.Create(1, 0)).As<long, T>());
}
return x.ToScalar();
}
}
}