Move StringBuilderCache to shared (dotnet#17964)

Author: stephentoub

src/System.Private.CoreLib/System.Private.CoreLib.csproj

@@ -317,7 +317,6 @@
     <Compile Include="$(BclSourcesRoot)\System\ThrowHelper.cs" />
     <Compile Include="$(BclSourcesRoot)\System\String.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Text\StringBuilder.CoreCLR.cs" />
-    <Compile Include="$(BclSourcesRoot)\System\Text\StringBuilderCache.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Exception.cs" />
     <Compile Include="$(BclSourcesRoot)\System\DateTime.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\OutOfMemoryException.cs" />

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

@@ -513,6 +513,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\StringSplitOptions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\SystemException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\ASCIIEncoding.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Text\StringBuilderCache.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\Decoder.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\DecoderNLS.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Text\DecoderBestFitFallback.cs" />

src/System.Private.CoreLib/shared/System/Text/StringBuilderCache.cs

@@ -0,0 +1,62 @@
+// 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.
+
+namespace System.Text
+{
+    /// <summary>Provide a cached reusable instance of stringbuilder per thread.</summary>
+    internal static class StringBuilderCache
+    {
+        // The value 360 was chosen in discussion with performance experts as a compromise between using
+        // as litle memory per thread as possible and still covering a large part of short-lived
+        // StringBuilder creations on the startup path of VS designers.
+        private const int MaxBuilderSize = 360;
+        private const int DefaultCapacity = 16; // == StringBuilder.DefaultCapacity
+
+        // WARNING: We allow diagnostic tools to directly inspect this member (t_cachedInstance).
+        // See https://github.com/dotnet/corert/blob/master/Documentation/design-docs/diagnostics/diagnostics-tools-contract.md for more details. 
+        // Please do not change the type, the name, or the semantic usage of this member without understanding the implication for tools. 
+        // Get in touch with the diagnostics team if you have questions.
+        [ThreadStatic]
+        private static StringBuilder t_cachedInstance;
+
+        /// <summary>Get a StringBuilder for the specified capacity.</summary>
+        /// <remarks>If a StringBuilder of an appropriate size is cached, it will be returned and the cache emptied.</remarks>
+        public static StringBuilder Acquire(int capacity = DefaultCapacity)
+        {
+            if (capacity <= MaxBuilderSize)
+            {
+                StringBuilder sb = t_cachedInstance;
+                if (sb != null)
+                {
+                    // Avoid stringbuilder block fragmentation by getting a new StringBuilder
+                    // when the requested size is larger than the current capacity
+                    if (capacity <= sb.Capacity)
+                    {
+                        t_cachedInstance = null;
+                        sb.Clear();
+                        return sb;
+                    }
+                }
+            }
+            return new StringBuilder(capacity);
+        }
+
+        /// <summary>Place the specified builder in the cache if it is not too big.</summary>
+        public static void Release(StringBuilder sb)
+        {
+            if (sb.Capacity <= MaxBuilderSize)
+            {
+                t_cachedInstance = sb;
+            }
+        }
+
+        /// <summary>ToString() the stringbuilder, Release it to the cache, and return the resulting string.</summary>
+        public static string GetStringAndRelease(StringBuilder sb)
+        {
+            string result = sb.ToString();
+            Release(sb);
+            return result;
+        }
+    }
+}