Speed up named type lookups in managed type system (#84285)

When the type system needs to resolve a named type in a module, it will do a `foreach` loop over all types in the module looking for the type. This can get mildly hot and I've seen it in CPU profiles but it never looked too important to address (despite the TODO).

But when MIBC files are passed to the compiler, this gets ridiculously hot. Compile Hello world by default: 0.98 seconds. Compile hello world with 5 MIBC files: 9.1 seconds.

This adds a hashtable to the lookup and drops the MIBC case to 1.4 seconds (we'll want to parallelize the MIBC loading on a background thread to get rid of the last mile, but first things first).

src/coreclr/tools/Common/TypeSystem/Ecma/EcmaModule.cs

@@ -14,8 +14,8 @@ namespace Internal.TypeSystem.Ecma
 {
     public partial class EcmaModule : ModuleDesc
     {
-        private PEReader _peReader;
-        protected MetadataReader _metadataReader;
+        private readonly PEReader _peReader;
+        protected readonly MetadataReader _metadataReader;
 
         internal interface IEntityHandleObject
         {
@@ -27,8 +27,8 @@ EntityHandle Handle
 
         private sealed class EcmaObjectLookupWrapper : IEntityHandleObject
         {
-            private EntityHandle _handle;
-            private object _obj;
+            private readonly EntityHandle _handle;
+            private readonly object _obj;
 
             public EcmaObjectLookupWrapper(EntityHandle handle, object obj)
             {
@@ -55,7 +55,7 @@ public object Object
 
         internal sealed class EcmaObjectLookupHashtable : LockFreeReaderHashtable<EntityHandle, IEntityHandleObject>
         {
-            private EcmaModule _module;
+            private readonly EcmaModule _module;
 
             public EcmaObjectLookupHashtable(EcmaModule module)
             {
@@ -178,8 +178,8 @@ private ModuleDesc ResolveModuleReference(ModuleReferenceHandle handle)
             return _moduleResolver.ResolveModule(this.Assembly, fileName);
         }
 
-        private LockFreeReaderHashtable<EntityHandle, IEntityHandleObject> _resolvedTokens;
-        private IModuleResolver _moduleResolver;
+        private readonly LockFreeReaderHashtable<EntityHandle, IEntityHandleObject> _resolvedTokens;
+        private readonly IModuleResolver _moduleResolver;
 
         internal EcmaModule(TypeSystemContext context, PEReader peReader, MetadataReader metadataReader, IAssemblyDesc containingAssembly, IModuleResolver customModuleResolver)
             : base(context, containingAssembly)
@@ -282,60 +282,80 @@ public bool IsPlatformNeutral
             }
         }
 
+        private Dictionary<(string Name, string Namespace), EntityHandle> _nameLookupCache;
+
+        private Dictionary<(string Name, string Namespace), EntityHandle> CreateNameLookupCache()
+        {
+            // TODO: it's not particularly efficient to materialize strings just to hash them and hold
+            // onto them forever. We could instead hash the UTF-8 bytes and hold the TypeDefinitionHandle
+            // so we can obtain the bytes again when needed.
+            // E.g. see the scheme explored in the first commit of https://github.com/dotnet/runtime/pull/84285.
+
+            var result = new Dictionary<(string Name, string Namespace), EntityHandle>();
+
+            MetadataReader metadataReader = _metadataReader;
+            foreach (TypeDefinitionHandle typeDefHandle in metadataReader.TypeDefinitions)
+            {
+                TypeDefinition typeDefinition = metadataReader.GetTypeDefinition(typeDefHandle);
+                if (typeDefinition.Attributes.IsNested())
+                    continue;
+
+                result.Add((metadataReader.GetString(typeDefinition.Name), metadataReader.GetString(typeDefinition.Namespace)), typeDefHandle);
+            }
+
+            foreach (ExportedTypeHandle exportedTypeHandle in metadataReader.ExportedTypes)
+            {
+                ExportedType exportedType = metadataReader.GetExportedType(exportedTypeHandle);
+                if (exportedType.Implementation.Kind == HandleKind.ExportedType)
+                    continue;
+
+                result.Add((metadataReader.GetString(exportedType.Name), metadataReader.GetString(exportedType.Namespace)), exportedTypeHandle);
+            }
+
+            return _nameLookupCache = result;
+        }
+
         public sealed override object GetType(string nameSpace, string name, NotFoundBehavior notFoundBehavior)
         {
             var currentModule = this;
             // src/coreclr/vm/clsload.cpp use the same restriction to detect a loop in the type forwarding.
             for (int typeForwardingChainSize = 0; typeForwardingChainSize <= 1024; typeForwardingChainSize++)
             {
-                var metadataReader = currentModule._metadataReader;
-                var stringComparer = metadataReader.StringComparer;
-                // TODO: More efficient implementation?
-                foreach (var typeDefinitionHandle in metadataReader.TypeDefinitions)
+                if ((currentModule._nameLookupCache ?? currentModule.CreateNameLookupCache()).TryGetValue((name, nameSpace), out EntityHandle foundHandle))
                 {
-                    var typeDefinition = metadataReader.GetTypeDefinition(typeDefinitionHandle);
-                    if (typeDefinition.Attributes.IsNested())
-                        continue;
+                    if (foundHandle.Kind == HandleKind.TypeDefinition)
+                        return currentModule.GetType((TypeDefinitionHandle)foundHandle);
 
-                    if (stringComparer.Equals(typeDefinition.Name, name) &&
-                        stringComparer.Equals(typeDefinition.Namespace, nameSpace))
+                    ExportedType exportedType = currentModule._metadataReader.GetExportedType((ExportedTypeHandle)foundHandle);
+                    if (exportedType.IsForwarder)
                     {
-                        return currentModule.GetType(typeDefinitionHandle);
-                    }
-                }
+                        object implementation = currentModule.GetObject(exportedType.Implementation, notFoundBehavior);
 
-                foreach (var exportedTypeHandle in metadataReader.ExportedTypes)
-                {
-                    var exportedType = metadataReader.GetExportedType(exportedTypeHandle);
-                    if (stringComparer.Equals(exportedType.Name, name) &&
-                        stringComparer.Equals(exportedType.Namespace, nameSpace))
-                    {
-                        if (exportedType.IsForwarder)
+                        if (implementation == null)
+                        {
+                            return null;
+                        }
+                        else if (implementation is EcmaModule ecmaModule)
+                        {
+                            currentModule = ecmaModule;
+                        }
+                        else if (implementation is ModuleDesc moduleDesc)
+                        {
+                            return moduleDesc.GetType(nameSpace, name, notFoundBehavior);
+                        }
+                        else if (implementation is ResolutionFailure)
+                        {
+                            // No need to check notFoundBehavior - the callee already handled ReturnNull and Throw
+                            return implementation;
+                        }
+                        else
                         {
-                            object implementation = currentModule.GetObject(exportedType.Implementation, notFoundBehavior);
-
-                            if (implementation == null)
-                            {
-                                return null;
-                            }
-                            if (implementation is EcmaModule ecmaModule)
-                            {
-                                currentModule = ecmaModule;
-                                break;
-                            }
-                            if (implementation is ModuleDesc moduleDesc)
-                            {
-                                return moduleDesc.GetType(nameSpace, name, notFoundBehavior);
-                            }
-                            if (implementation is ResolutionFailure failure)
-                            {
-                                // No need to check notFoundBehavior - the callee already handled ReturnNull and Throw
-                                return implementation;
-                            }
                             // TODO
                             throw new NotImplementedException();
                         }
-
+                    }
+                    else
+                    {
                         // TODO:
                         throw new NotImplementedException();
                     }