Add char.ToLower/Upper perf tests (#28765)

Author: stephentoub

src/System.Runtime/tests/Performance/Perf.Char.cs

@@ -2,20 +2,63 @@
 // 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.Collections.Generic;
+using System.Globalization;
 using Microsoft.Xunit.Performance;
+using Xunit;
 
 namespace System.Tests
 {
     public class Perf_Char
     {
-        [Benchmark]
-        public static char Char_ToLower()
+        private const int InnerIterations = 1_000_000;
+
+        public static IEnumerable<object[]> Char_ChangeCase_MemberData()
+        {
+            yield return new object[] { 'A', "en-US" }; // ASCII upper case
+            yield return new object[] { 'a', "en-US" }; // ASCII lower case
+            yield return new object[] { '\u0130', "en-US" }; // non-ASCII, English
+            yield return new object[] { '\u4F60', "zh-Hans" }; // non-ASCII, Chinese
+        }
+
+        [Benchmark(InnerIterationCount = InnerIterations)]
+        [MemberData(nameof(Char_ChangeCase_MemberData))]
+        public static char Char_ToLower(char c, string cultureName)
+        {
+            char ret = default(char);
+            CultureInfo culture = new CultureInfo(cultureName);
+
+            foreach (var iteration in Benchmark.Iterations)
+            {
+                using (iteration.StartMeasurement())
+                {
+                    for (int i = 0; i < InnerIterations; i++)
+                    {
+                        ret = Char.ToLower(c, culture);
+                    }
+                }
+            }
+
+            return ret;
+        }
+
+        [Benchmark(InnerIterationCount = InnerIterations)]
+        [MemberData(nameof(Char_ChangeCase_MemberData))]
+        public static char Char_ToUpper(char c, string cultureName)
         {
             char ret = default(char);
+            CultureInfo culture = new CultureInfo(cultureName);
 
             foreach (var iteration in Benchmark.Iterations)
+            {
                 using (iteration.StartMeasurement())
-                    ret = Char.ToLower('A');
+                {
+                    for (int i = 0; i < InnerIterations; i++)
+                    {
+                        ret = Char.ToUpper(c, culture);
+                    }
+                }
+            }
 
             return ret;
         }