Move FormattingHelpers.Count{Hex}Digits from Utf8Formatter into shared

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

@@ -52,6 +52,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\MemoryManager.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\TlsOverPerCoreLockedStacksArrayPool.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Utilities.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\FormattingHelpers.CountDigits.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Byte.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Char.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\CharEnumerator.cs" />

src/mscorlib/shared/System/Buffers/Text/FormattingHelpers.CountDigits.cs

@@ -0,0 +1,132 @@
+// 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.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace System.Buffers.Text
+{
+    internal static partial class FormattingHelpers
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int CountDigits(ulong value)
+        {
+            int digits = 1;
+            uint part;
+            if (value >= 10000000)
+            {
+                if (value >= 100000000000000)
+                {
+                    part = (uint)(value / 100000000000000);
+                    digits += 14;
+                }
+                else
+                {
+                    part = (uint)(value / 10000000);
+                    digits += 7;
+                }
+            }
+            else
+            {
+                part = (uint)value;
+            }
+
+            if (part < 10)
+            {
+                // no-op
+            }
+            else if (part < 100)
+            {
+                digits += 1;
+            }
+            else if (part < 1000)
+            {
+                digits += 2;
+            }
+            else if (part < 10000)
+            {
+                digits += 3;
+            }
+            else if (part < 100000)
+            {
+                digits += 4;
+            }
+            else if (part < 1000000)
+            {
+                digits += 5;
+            }
+            else
+            {
+                Debug.Assert(part < 10000000);
+                digits += 6;
+            }
+
+            return digits;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int CountDigits(uint value)
+        {
+            int digits = 1;
+            if (value >= 100000)
+            {
+                value = value / 100000;
+                digits += 5;
+            }
+
+            if (value < 10)
+            {
+                // no-op
+            }
+            else if (value < 100)
+            {
+                digits += 1;
+            }
+            else if (value < 1000)
+            {
+                digits += 2;
+            }
+            else if (value < 10000)
+            {
+                digits += 3;
+            }
+            else
+            {
+                Debug.Assert(value < 100000);
+                digits += 4;
+            }
+
+            return digits;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int CountHexDigits(ulong value)
+        {
+            // TODO: When x86 intrinsic support comes online, experiment with implementing this using lzcnt.
+            // return 16 - (int)((uint)Lzcnt.LeadingZeroCount(value | 1) >> 3);
+
+            int digits = 1;
+
+            if (value > 0xFFFFFFFF)
+            {
+                digits += 8;
+                value >>= 0x20;
+            }
+            if (value > 0xFFFF)
+            {
+                digits += 4;
+                value >>= 0x10;
+            }
+            if (value > 0xFF)
+            {
+                digits += 2;
+                value >>= 0x8;
+            }
+            if (value > 0xF)
+                digits++;
+
+            return digits;
+        }
+    }
+}