Intrinsicify SpanHelpers.IndexOf{Any}(byte, ...)

src/System.Private.CoreLib/shared/System.Private.CoreLib.Shared.projitems

@@ -47,6 +47,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\AttributeUsageAttribute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\BadImageFormatException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\BitConverter.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\BitOps.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Boolean.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\ArrayPool.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\ArrayPoolEventSource.cs" />

src/System.Private.CoreLib/shared/System/BitOps.cs

@@ -0,0 +1,33 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics.X86;
+
+namespace System
+{
+    internal static class BitOps
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int TrailingZeroCount(int matches)
+        {
+            if (Bmi1.IsSupported)
+            {
+                return (int)Bmi1.TrailingZeroCount((uint)matches);
+            }
+            else
+            {
+                // Fallback
+                // https://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup
+                return TrailingCountMultiplyDeBruijn[(int)(((uint)((matches & -matches) * 0x077CB531U)) >> 27)];
+            }
+        }
+
+        private static ReadOnlySpan<byte> TrailingCountMultiplyDeBruijn => new byte[32]
+        {
+            0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
+            31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
+        };
+    }
+}