Reduce allocations again in ComputeReverseReferencesMap (#76417)

* Reduce allocations again in ComputeReverseReferencesMap

Speedometer results from the last PR didn't improve allocations in the ProjectDependencyGraph as much as I had hoped. This PR instead changes ComputeReverseReferencesMap to use pooled data structures and ImmutableHashSet.Builder to build up it's final result.

The speedometer test is still showing about 100 MB of allocations in this method during typing, my hope is that this should cut that in about half. Will create a test insertion for validation.

Author: ToddGrun

src/Workspaces/Core/Portable/Workspace/Solution/ProjectDependencyGraph.cs

@@ -8,6 +8,7 @@
 using System.Diagnostics;
 using System.Linq;
 using System.Threading;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis;
@@ -55,6 +56,9 @@ public partial class ProjectDependencyGraph
     private ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> _transitiveReferencesMap;
     private ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> _reverseTransitiveReferencesMap;
 
+    // Pool of ImmutableHashSet builders used in ComputeReverseReferencesMap to avoid temporary HashSet allocations.
+    private static readonly ObjectPool<ImmutableHashSet<ProjectId>.Builder> s_reverseReferencesBuilderPool = new(static () => ImmutableHashSet.CreateBuilder<ProjectId>(), size: 256);
+
     /// <remarks>
     ///   Intentionally created with a null reverseReferencesMap. Doing so indicates _lazyReverseReferencesMap
     ///    shouldn't be calculated until reverse reference information is requested. Once this information
@@ -209,17 +213,36 @@ private ImmutableHashSet<ProjectId> GetProjectsThatDirectlyDependOnThisProject_N
 
     private ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> ComputeReverseReferencesMap()
     {
-        var reverseReferencesMap = new Dictionary<ProjectId, HashSet<ProjectId>>();
-
+        using var _1 = PooledDictionary<ProjectId, ImmutableHashSet<ProjectId>.Builder>.GetInstance(out var reverseReferencesMapBuilders);
         foreach (var (projectId, references) in _referencesMap)
         {
             foreach (var referencedId in references)
-                reverseReferencesMap.MultiAdd(referencedId, projectId);
+            {
+                if (!reverseReferencesMapBuilders.TryGetValue(referencedId, out var builder))
+                {
+                    builder = s_reverseReferencesBuilderPool.Allocate();
+                    reverseReferencesMapBuilders.Add(referencedId, builder);
+                }
+
+                builder.Add(projectId);
+            }
+        }
+
+        // Convert all the populated ImmutableHashSet.Builder objects to ImmutableHashSets
+        var reverseReferencesBuilder = ImmutableDictionary.CreateBuilder<ProjectId, ImmutableHashSet<ProjectId>>();
+        foreach (var (projectId, builder) in reverseReferencesMapBuilders)
+        {
+            // Realize an ImmutableHashSet from the builder
+            var reverseReferencesForProject = builder.ToImmutableHashSet();
+
+            // Clear out the builder and release it back to the pool
+            builder.Clear();
+            s_reverseReferencesBuilderPool.Free(builder);
+
+            reverseReferencesBuilder.Add(projectId, reverseReferencesForProject);
         }
 
-        return reverseReferencesMap
-            .Select(kvp => KeyValuePairUtil.Create(kvp.Key, kvp.Value.ToImmutableHashSet()))
-            .ToImmutableDictionary();
+        return reverseReferencesBuilder.ToImmutable();
     }
 
     /// <summary>