Comment change and implement ShuffleUnsafe for sbyte

- Implement `ShuffleUnsafe` for `sbyte`
- Remove remaining detailed comment on V128.ShuffleUnsafe (which specified which intrinsics were used): it is not good to only have it for this one, which could also theoretically change behaviour in the future e.g., with the introduction of RISC-V

src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector128.cs

@@ -2363,9 +2363,7 @@ public static Vector128<sbyte> Shuffle(Vector128<sbyte> vector, Vector128<sbyte>
         /// <param name="vector">The input vector from which values are selected.</param>
         /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>
         /// <returns>A new vector containing the values from <paramref name="vector" /> selected by the given <paramref name="indices" />.</returns>
-        /// <remarks>Unlike Shuffle, this method delegates to the underlying hardware intrinsic without ensuring that <paramref name="indices"/> are normalized to [0, 15].
-        /// On hardware with <see cref="Ssse3"/> support, indices are treated as modulo 16, and if the high bit is set, the result will be set to 0 for that element.
-        /// On hardware with <see cref="AdvSimd.Arm64"/> or <see cref="PackedSimd"/> support, this method behaves the same as Shuffle.</remarks>
+        /// <remarks>Unlike Shuffle, this method delegates to the underlying hardware intrinsic without ensuring that <paramref name="indices"/> are normalized to [0, 15].</remarks>
         [Intrinsic]
         [CompExactlyDependsOn(typeof(Ssse3))]
         [CompExactlyDependsOn(typeof(AdvSimd))]
@@ -2390,6 +2388,36 @@ public static Vector128<byte> ShuffleUnsafe(Vector128<byte> vector, Vector128<by
             return result;
         }
 
+        /// <summary>Creates a new vector by selecting values from an input vector using a set of indices.
+        /// Behavior is platform-dependent for out-of-range indices.</summary>
+        /// <param name="vector">The input vector from which values are selected.</param>
+        /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>
+        /// <returns>A new vector containing the values from <paramref name="vector" /> selected by the given <paramref name="indices" />.</returns>
+        /// <remarks>Unlike Shuffle, this method delegates to the underlying hardware intrinsic without ensuring that <paramref name="indices"/> are normalized to [0, 15].</remarks>
+        [Intrinsic]
+        [CompExactlyDependsOn(typeof(Ssse3))]
+        [CompExactlyDependsOn(typeof(AdvSimd))]
+        [CompExactlyDependsOn(typeof(AdvSimd.Arm64))]
+        [CompExactlyDependsOn(typeof(PackedSimd))]
+        public static Vector128<sbyte> ShuffleUnsafe(Vector128<sbyte> vector, Vector128<sbyte> indices)
+        {
+            Unsafe.SkipInit(out Vector128<sbyte> result);
+
+            for (int index = 0; index < Vector128<sbyte>.Count; index++)
+            {
+                byte selectedIndex = (byte)indices.GetElementUnsafe(index);
+                byte selectedValue = 0;
+
+                if (selectedIndex < Vector128<sbyte>.Count)
+                {
+                    selectedValue = vector.GetElementUnsafe(selectedIndex);
+                }
+                result.SetElementUnsafe(index, selectedValue);
+            }
+
+            return result;
+        }
+
         /// <summary>Creates a new vector by selecting values from an input vector using a set of indices.</summary>
         /// <param name="vector">The input vector from which values are selected.</param>
         /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>

src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector256.cs

@@ -2369,6 +2369,34 @@ public static Vector256<byte> ShuffleUnsafe(Vector256<byte> vector, Vector256<by
             return result;
         }
 
+        /// <summary>Creates a new vector by selecting values from an input vector using a set of indices.
+        /// Behavior is platform-dependent for out-of-range indices.</summary>
+        /// <param name="vector">The input vector from which values are selected.</param>
+        /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>
+        /// <returns>A new vector containing the values from <paramref name="vector" /> selected by the given <paramref name="indices" />.</returns>
+        /// <remarks>Unlike Shuffle, this method delegates to the underlying hardware intrinsic without ensuring that <paramref name="indices"/> are normalized to [0, 31].</remarks>
+        [Intrinsic]
+        [CompExactlyDependsOn(typeof(Avx2))]
+        [CompExactlyDependsOn(typeof(Avx512Vbmi.VL))]
+        public static Vector256<sbyte> ShuffleUnsafe(Vector256<sbyte> vector, Vector256<sbyte> indices)
+        {
+            Unsafe.SkipInit(out Vector256<sbyte> result);
+
+            for (int index = 0; index < Vector256<sbyte>.Count; index++)
+            {
+                byte selectedIndex = (byte)indices.GetElementUnsafe(index);
+                byte selectedValue = 0;
+
+                if (selectedIndex < Vector256<byte>.Count)
+                {
+                    selectedValue = vector.GetElementUnsafe(selectedIndex);
+                }
+                result.SetElementUnsafe(index, selectedValue);
+            }
+
+            return result;
+        }
+
         /// <summary>Creates a new vector by selecting values from an input vector using a set of indices.</summary>
         /// <param name="vector">The input vector from which values are selected.</param>
         /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>

src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector512.cs

@@ -2415,6 +2415,33 @@ public static Vector512<byte> ShuffleUnsafe(Vector512<byte> vector, Vector512<by
             return result;
         }
 
+        /// <summary>Creates a new vector by selecting values from an input vector using a set of indices.
+        /// Behavior is platform-dependent for out-of-range indices.</summary>
+        /// <param name="vector">The input vector from which values are selected.</param>
+        /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>
+        /// <returns>A new vector containing the values from <paramref name="vector" /> selected by the given <paramref name="indices" />.</returns>
+        /// <remarks>Unlike Shuffle, this method delegates to the underlying hardware intrinsic without ensuring that <paramref name="indices"/> are normalized to [0, 63].</remarks>
+        [Intrinsic]
+        [CompExactlyDependsOn(typeof(Avx512Vbmi))]
+        public static Vector512<sbyte> ShuffleUnsafe(Vector512<sbyte> vector, Vector512<sbyte> indices)
+        {
+            Unsafe.SkipInit(out Vector512<byte> result);
+
+            for (int index = 0; index < Vector512<sbyte>.Count; index++)
+            {
+                byte selectedIndex = (byte)indices.GetElementUnsafe(index);
+                byte selectedValue = 0;
+
+                if (selectedIndex < Vector512<byte>.Count)
+                {
+                    selectedValue = vector.GetElementUnsafe(selectedIndex);
+                }
+                result.SetElementUnsafe(index, selectedValue);
+            }
+
+            return result;
+        }
+
         /// <summary>Creates a new vector by selecting values from an input vector using a set of indices.</summary>
         /// <param name="vector">The input vector from which values are selected.</param>
         /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>

src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector64.cs

@@ -2284,6 +2284,33 @@ public static Vector64<byte> ShuffleUnsafe(Vector64<byte> vector, Vector64<byte>
             return result;
         }
 
+        /// <summary>Creates a new vector by selecting values from an input vector using a set of indices.
+        /// Behavior is platform-dependent for out-of-range indices.</summary>
+        /// <param name="vector">The input vector from which values are selected.</param>
+        /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>
+        /// <returns>A new vector containing the values from <paramref name="vector" /> selected by the given <paramref name="indices" />.</returns>
+        /// <remarks>Unlike Shuffle, this method delegates to the underlying hardware intrinsic without ensuring that <paramref name="indices"/> are normalized to [0, 7].</remarks>
+        [Intrinsic]
+        [CompExactlyDependsOn(typeof(AdvSimd))]
+        public static Vector64<sbyte> ShuffleUnsafe(Vector64<sbyte> vector, Vector64<sbyte> indices)
+        {
+            Unsafe.SkipInit(out Vector64<sbyte> result);
+
+            for (int index = 0; index < Vector64<byte>.Count; index++)
+            {
+                byte selectedIndex = (byte)indices.GetElementUnsafe(index);
+                byte selectedValue = 0;
+
+                if (selectedIndex < Vector64<byte>.Count)
+                {
+                    selectedValue = vector.GetElementUnsafe(selectedIndex);
+                }
+                result.SetElementUnsafe(index, selectedValue);
+            }
+
+            return result;
+        }
+
         /// <summary>Creates a new vector by selecting values from an input vector using a set of indices.</summary>
         /// <param name="vector">The input vector from which values are selected.</param>
         /// <param name="indices">The per-element indices used to select a value from <paramref name="vector" />.</param>