[Perf] Reduce allocations in Link generation code paths

src/Microsoft.AspNetCore.Routing/Internal/LinkGenerationDecisionTree.cs

@@ -22,10 +22,16 @@ namespace Microsoft.AspNetCore.Routing.Internal
 
         public IList<LinkGenerationMatch> GetMatches(VirtualPathContext context)
         {
-            var results = new List<LinkGenerationMatch>();
-            Walk(results, context, _root, isFallbackPath: false);
-            results.Sort(LinkGenerationMatchComparer.Instance);
-            return results;
+            // Perf: Avoid allocation for List if there aren't any Matches or Criteria
+            if (_root.Matches.Count > 0 || _root.Criteria.Count > 0)
+            {
+                var results = new List<LinkGenerationMatch>();
+                Walk(results, context, _root, isFallbackPath: false);
+                results.Sort(LinkGenerationMatchComparer.Instance);
+                return results;
+            }
+
+            return null;            
         }
 
         // We need to recursively walk the decision tree based on the provided route data

src/Microsoft.AspNetCore.Routing/RouteConstraintMatcher.cs

@@ -39,7 +39,7 @@ namespace Microsoft.AspNetCore.Routing
                 throw new ArgumentNullException(nameof(logger));
             }
 
-            if (constraints == null)
+            if (constraints == null || constraints.Count == 0)
             {
                 return true;
             }

src/Microsoft.AspNetCore.Routing/Template/TemplateBinder.cs

@@ -171,7 +171,8 @@ namespace Microsoft.AspNetCore.Routing.Template
 
             // Add any ambient values that don't match parameters - they need to be visible to constraints
             // but they will ignored by link generation.
-            var combinedValues = new RouteValueDictionary(context.AcceptedValues);
+            // Perf: Lazily allocate RouteValueDictionary only when needed as in most cases combined is same as accepted values
+            RouteValueDictionary combinedValues = null;
             if (ambientValues != null)
             {
                 foreach (var kvp in ambientValues)
@@ -181,6 +182,10 @@ namespace Microsoft.AspNetCore.Routing.Template
                         var parameter = GetParameter(kvp.Key);
                         if (parameter == null && !context.AcceptedValues.ContainsKey(kvp.Key))
                         {
+                            if (combinedValues == null)
+                            {
+                                combinedValues = new RouteValueDictionary(context.AcceptedValues);
+                            }
                             combinedValues.Add(kvp.Key, kvp.Value);
                         }
                     }
@@ -190,7 +195,7 @@ namespace Microsoft.AspNetCore.Routing.Template
             return new TemplateValuesResult()
             {
                 AcceptedValues = context.AcceptedValues,
-                CombinedValues = combinedValues,
+                CombinedValues = combinedValues ?? context.AcceptedValues
             };
         }
 

src/Microsoft.AspNetCore.Routing/Tree/TreeRouter.cs

@@ -138,9 +138,14 @@ namespace Microsoft.AspNetCore.Routing.Tree
             // order. We just need to iterate them and use the first one that can generate a link.
             var matches = _linkGenerationTree.GetMatches(context);
 
-            foreach (var match in matches)
+            if (matches == null)
             {
-                var path = GenerateVirtualPath(context, match.Entry);
+                return null;
+            }
+
+            for (var i = 0; i < matches.Count; i++)
+            {
+                var path = GenerateVirtualPath(context, matches[i].Entry);
                 if (path != null)
                 {
                     return path;