Add a fast path to IndexOfQuoteOrAnyControlOrBackSlash

src/libraries/System.Text.Json/src/System/Text/Json/Reader/JsonReaderHelper.net8.cs

@@ -2,7 +2,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Buffers;
+using System.Numerics;
 using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics;
+using System.Runtime.Intrinsics.Arm;
 
 namespace System.Text.Json
 {
@@ -19,7 +22,32 @@ internal static partial class JsonReaderHelper
             "\\"u8);
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static int IndexOfQuoteOrAnyControlOrBackSlash(this ReadOnlySpan<byte> span) =>
+        public static int IndexOfQuoteOrAnyControlOrBackSlash(this ReadOnlySpan<byte> span)
+        {
+            // For most inputs, we have a large span and we typically find a match within the first 16 bytes
+            // Usually, it's a quote in a property name.
+            if (!Vector128.IsHardwareAccelerated || !BitConverter.IsLittleEndian || span.Length < Vector128<byte>.Count)
+                return IndexOfQuoteOrAnyControlOrBackSlash_Fallback(span);
+
+            Vector128<byte> vec = Vector128.Create(span);
+
+            // Any control character (i.e. 0 to 31) or '"' or '\'
+            Vector128<byte> cmp = Vector128.LessThan(vec, Vector128.Create(JsonConstants.Space)) |
+                                  Vector128.Equals(vec, Vector128.Create(JsonConstants.Quote)) |
+                                  Vector128.Equals(vec, Vector128.Create(JsonConstants.BackSlash));
+
+            int idx = Vector128.IndexOfWhereAllBitsSet(cmp);
+            if (idx >= 0)
+            {
+                return idx;
+            }
+
+            int fallbackIndex = IndexOfQuoteOrAnyControlOrBackSlash_Fallback(span.Slice(Vector128<byte>.Count));
+            return fallbackIndex >= 0 ? fallbackIndex + Vector128<byte>.Count : fallbackIndex;
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private static int IndexOfQuoteOrAnyControlOrBackSlash_Fallback(ReadOnlySpan<byte> span) =>
             span.IndexOfAny(s_controlQuoteBackslash);
     }
 }