Optimize BigInteger formatting

src/libraries/Common/src/System/Number.Formatting.Common.cs

@@ -12,6 +12,8 @@ namespace System
 {
     internal static partial class Number
     {
+        private const int CharStackBufferSize = 32;
+
         private const int DefaultPrecisionExponentialFormat = 6;
 
         private const int MaxUInt32DecDigits = 10;

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

@@ -133,6 +133,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Base64Decoder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Base64Validator.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\FormattingHelpers.CountDigits.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\FormattingHelpers.CountDigits.Int128.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Constants.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\FormattingHelpers.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Buffers\Text\Utf8Formatter\Utf8Formatter.Boolean.cs" />

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.Int128.cs

@@ -0,0 +1,59 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace System.Buffers.Text
+{
+    internal static partial class FormattingHelpers
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int CountDigits(UInt128 value)
+        {
+            ulong upper = value.Upper;
+
+            // 1e19 is    8AC7_2304_89E8_0000
+            // 1e20 is  5_6BC7_5E2D_6310_0000
+            // 1e21 is 36_35C9_ADC5_DEA0_0000
+
+            if (upper == 0)
+            {
+                // We have less than 64-bits, so just return the lower count
+                return CountDigits(value.Lower);
+            }
+
+            // We have more than 1e19, so we have at least 20 digits
+            int digits = 20;
+
+            if (upper > 5)
+            {
+                // ((2^128) - 1) / 1e20 < 34_02_823_669_209_384_635 which
+                // is 18.5318 digits, meaning the result definitely fits
+                // into 64-bits and we only need to add the lower digit count
+
+                value /= new UInt128(0x5, 0x6BC7_5E2D_6310_0000); // value /= 1e20
+                Debug.Assert(value.Upper == 0);
+
+                digits += CountDigits(value.Lower);
+            }
+            else if ((upper == 5) && (value.Lower >= 0x6BC75E2D63100000))
+            {
+                // We're greater than 1e20, but definitely less than 1e21
+                // so we have exactly 21 digits
+
+                digits++;
+                Debug.Assert(digits == 21);
+            }
+
+            return digits;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int CountHexDigits(UInt128 value)
+        {
+            // The number of hex digits is log16(value) + 1, or log2(value) / 4 + 1
+            return ((int)UInt128.Log2(value) >> 2) + 1;
+        }
+    }
+}

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs

@@ -1,7 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Buffers.Binary;
 using System.Diagnostics;
 using System.Numerics;
 using System.Runtime.CompilerServices;
@@ -11,47 +10,6 @@ namespace System.Buffers.Text
 {
     internal static partial class FormattingHelpers
     {
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static int CountDigits(UInt128 value)
-        {
-            ulong upper = value.Upper;
-
-            // 1e19 is    8AC7_2304_89E8_0000
-            // 1e20 is  5_6BC7_5E2D_6310_0000
-            // 1e21 is 36_35C9_ADC5_DEA0_0000
-
-            if (upper == 0)
-            {
-                // We have less than 64-bits, so just return the lower count
-                return CountDigits(value.Lower);
-            }
-
-            // We have more than 1e19, so we have at least 20 digits
-            int digits = 20;
-
-            if (upper > 5)
-            {
-                // ((2^128) - 1) / 1e20 < 34_02_823_669_209_384_635 which
-                // is 18.5318 digits, meaning the result definitely fits
-                // into 64-bits and we only need to add the lower digit count
-
-                value /= new UInt128(0x5, 0x6BC7_5E2D_6310_0000); // value /= 1e20
-                Debug.Assert(value.Upper == 0);
-
-                digits += CountDigits(value.Lower);
-            }
-            else if ((upper == 5) && (value.Lower >= 0x6BC75E2D63100000))
-            {
-                // We're greater than 1e20, but definitely less than 1e21
-                // so we have exactly 21 digits
-
-                digits++;
-                Debug.Assert(digits == 21);
-            }
-
-            return digits;
-        }
-
         // Based on do_count_digits from https://github.com/fmtlib/fmt/blob/662adf4f33346ba9aba8b072194e319869ede54a/include/fmt/format.h#L1124
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static int CountDigits(ulong value)
@@ -149,13 +107,6 @@ public static int CountDigits(uint value)
             return (int)((value + tableValue) >> 32);
         }
 
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static int CountHexDigits(UInt128 value)
-        {
-            // The number of hex digits is log16(value) + 1, or log2(value) / 4 + 1
-            return ((int)UInt128.Log2(value) >> 2) + 1;
-        }
-
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static int CountHexDigits(ulong value)
         {

src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs

@@ -269,8 +269,6 @@ internal static partial class Number
         private const int SinglePrecisionCustomFormat = 7;
         private const int DoublePrecisionCustomFormat = 15;
 
-        private const int CharStackBufferSize = 32;
-
         /// <summary>The non-inclusive upper bound of <see cref="s_smallNumberCache"/>.</summary>
         /// <remarks>
         /// This is a semi-arbitrary bound. For mono, which is often used for more size-constrained workloads,

src/libraries/System.Runtime.Numerics/src/System.Runtime.Numerics.csproj

@@ -25,6 +25,8 @@
   </ItemGroup>
 
   <ItemGroup>
+    <Compile Include="$(CoreLibSharedDir)System\Buffers\Text\FormattingHelpers.CountDigits.cs"
+             Link="CoreLib\System\Buffers\Text\FormattingHelpers.CountDigits.cs" />
     <Compile Include="$(CoreLibSharedDir)System\Collections\Generic\ValueListBuilder.cs"
              Link="CoreLib\System\Collections\Generic\ValueListBuilder.cs" />
     <Compile Include="$(CommonPath)System\Text\ValueStringBuilder.cs"