Implement remaining SIMD utils in JXL

src/ImageSharp/Formats/Jxl/Processing/JxlSimdUtils.cs

@@ -2,6 +2,7 @@
 // Licensed under the Six Labors Split License.
 
 using System.Numerics;
+using System.Numerics.Tensors;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Runtime.Intrinsics;
@@ -14,6 +15,8 @@ namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
 /// </summary>
 internal static partial class JxlSimdUtils
 {
+    public static int MaxVectorSize => Vector<float>.Count * sizeof(float);
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static Vector256<T> ConcatLowerLower<T>(Vector256<T> a, Vector256<T> b)
         where T : unmanaged => Vector256.Create(a.GetLower(), b.GetLower());
@@ -505,6 +508,39 @@ internal static partial class JxlSimdUtils
         Vector<float> exponent)
         => FastPow2f(Vector.Log2(@base) * exponent);
 
+    public static uint MaxValue(Span<uint> data)
+    {
+        int lanes = Vector<uint>.Count;
+        int lastFull = lanes * (data.Length / lanes);
+
+        Vector<uint> max = Vector<uint>.Zero;
+        ref uint dataRef = ref MemoryMarshal.GetReference(data);
+
+        for (int i = 0; i < lastFull; i += lanes)
+        {
+            max = Vector.Max(max, Vector.LoadUnsafe(ref Unsafe.Add(ref dataRef, i)));
+        }
+
+        if (lastFull < data.Length)
+        {
+            Vector<uint> stop = Vector.Create((uint)data.Length);
+            Vector<uint> fence = Iota((uint)lastFull);
+            Vector<uint> take = Vector.LessThan(fence, stop);
+            max = Vector.Max(
+                max,
+                Vector.ConditionalSelect(
+                    take,
+                    Vector.LoadUnsafe(ref Unsafe.Add(ref dataRef, lastFull)),
+                    Vector<uint>.Zero));
+        }
+
+        // The following part is to find the largest number in
+        // the vector.
+        Span<uint> copy = stackalloc uint[Vector<uint>.Count];
+        max.CopyTo(copy);
+        return TensorPrimitives.Max<uint>(copy);
+    }
+
     /// <summary>
     /// Incrementing values to compute the Iota function.
     /// </summary>