Avoid bool[] allocation in MemberInfoCache.PopulateProperties (#66912)

Author: stephentoub

src/coreclr/System.Private.CoreLib/src/System/RuntimeType.CoreCLR.cs

@@ -1252,20 +1252,30 @@ private RuntimePropertyInfo[] PopulateProperties(Filter filter)
                         Dictionary<string, List<RuntimePropertyInfo>>? csPropertyInfos = filter.CaseSensitive() ? null :
                             new Dictionary<string, List<RuntimePropertyInfo>>();
 
-                        // All elements automatically initialized to false.
-                        bool[] usedSlots = new bool[RuntimeTypeHandle.GetNumVirtuals(declaringType)];
+                        // All elements initialized to false.
+                        int numVirtuals = RuntimeTypeHandle.GetNumVirtuals(declaringType);
+                        Span<bool> usedSlots = stackalloc bool[0];
+                        if (numVirtuals <= 128) // arbitrary stack limit
+                        {
+                            usedSlots = stackalloc bool[numVirtuals];
+                            usedSlots.Clear();
+                        }
+                        else
+                        {
+                            usedSlots = new bool[numVirtuals];
+                        }
 
                         // Populate associates off of the class hierarchy
                         do
                         {
-                            PopulateProperties(filter, declaringType, csPropertyInfos, usedSlots, ref list);
+                            PopulateProperties(filter, declaringType, csPropertyInfos, usedSlots, isInterface: false, ref list);
                             declaringType = RuntimeTypeHandle.GetBaseType(declaringType);
                         } while (declaringType != null);
                     }
                     else
                     {
                         // Populate associates for this interface
-                        PopulateProperties(filter, declaringType, null, null, ref list);
+                        PopulateProperties(filter, declaringType, null, default, isInterface: true, ref list);
                     }
 
                     return list.ToArray();
@@ -1275,7 +1285,8 @@ private void PopulateProperties(
                     Filter filter,
                     RuntimeType declaringType,
                     Dictionary<string, List<RuntimePropertyInfo>>? csPropertyInfos,
-                    bool[]? usedSlots,
+                    Span<bool> usedSlots,
+                    bool isInterface,
                     ref ListBuilder<RuntimePropertyInfo> list)
                 {
                     int tkDeclaringType = RuntimeTypeHandle.GetToken(declaringType);
@@ -1290,8 +1301,8 @@ private void PopulateProperties(
 
                     int numVirtuals = RuntimeTypeHandle.GetNumVirtuals(declaringType);
 
-                    Debug.Assert((declaringType.IsInterface && usedSlots == null && csPropertyInfos == null) ||
-                                    (!declaringType.IsInterface && usedSlots != null && usedSlots.Length >= numVirtuals));
+                    Debug.Assert((declaringType.IsInterface && isInterface && csPropertyInfos == null) ||
+                                 (!declaringType.IsInterface && !isInterface && usedSlots.Length >= numVirtuals));
 
                     for (int i = 0; i < tkProperties.Length; i++)
                     {
@@ -1313,7 +1324,7 @@ private void PopulateProperties(
                             tkProperty, declaringType, m_runtimeTypeCache, out bool isPrivate);
 
                         // If this is a class, not an interface
-                        if (usedSlots != null)
+                        if (!isInterface)
                         {
                             #region Remove Privates
                             if (declaringType != ReflectedType && isPrivate)