Restore BytesAllocatedPerOperation for JSON and XML (#1919)

* Restore BytesAllocatedPerOperation for JSON and XML

Fix BytesAllocatedPerOperation not being output by the JSON and XML exporters.

* Add comment to GcStats

Add comment explaining what GcStats is for.

Author: martincostello

src/BenchmarkDotNet/Exporters/Json/JsonExporterBase.cs

@@ -65,7 +65,14 @@ public override void ExportToLog(Summary summary, ILogger logger)
                 // We show MemoryDiagnoser's results only if it is being used
                 if (report.BenchmarkCase.Config.HasMemoryDiagnoser())
                 {
-                    data.Add("Memory", report.GcStats);
+                    data.Add("Memory", new
+                    {
+                        report.GcStats.Gen0Collections,
+                        report.GcStats.Gen1Collections,
+                        report.GcStats.Gen2Collections,
+                        report.GcStats.TotalOperations,
+                        BytesAllocatedPerOperation = report.GcStats.GetBytesAllocatedPerOperation(report.BenchmarkCase)
+                    });
                 }
 
                 if (ExcludeMeasurements == false)

src/BenchmarkDotNet/Exporters/Xml/SummaryDto.cs

@@ -1,6 +1,5 @@
 using System.Collections.Generic;
 using System.Linq;
-using BenchmarkDotNet.Engines;
 using BenchmarkDotNet.Environments;
 using BenchmarkDotNet.Mathematics;
 using BenchmarkDotNet.Reports;
@@ -72,7 +71,14 @@ internal class BenchmarkReportDto
         public string MethodTitle => report.BenchmarkCase.Descriptor.WorkloadMethodDisplayInfo;
         public string Parameters => report.BenchmarkCase.Parameters.PrintInfo;
         public Statistics Statistics => report.ResultStatistics;
-        public GcStats Memory => report.GcStats;
+        public GcStats Memory => new GcStats()
+        {
+            Gen0Collections = report.GcStats.Gen0Collections,
+            Gen1Collections = report.GcStats.Gen1Collections,
+            Gen2Collections = report.GcStats.Gen2Collections,
+            TotalOperations = report.GcStats.TotalOperations,
+            BytesAllocatedPerOperation = report.GcStats.GetBytesAllocatedPerOperation(report.BenchmarkCase)
+        };
         [PublicAPI] public IEnumerable<Measurement> Measurements { get; }
 
         private readonly BenchmarkReport report;
@@ -83,4 +89,19 @@ public BenchmarkReportDto(BenchmarkReport report, bool excludeMeasurements = fal
             Measurements = excludeMeasurements ? null : report.AllMeasurements;
         }
     }
+
+    /// <summary>
+    /// This type is used to ensure that the allocated bytes are persisted in the XML
+    /// report when serialized, as the original <see cref="Engines.GcStats"/> type does
+    /// not contain a property for the value so the report would otherwise lack it.
+    /// See https://github.com/dotnet/BenchmarkDotNet/pull/1919 for more details.
+    /// </summary>
+    internal struct GcStats
+    {
+        public int Gen0Collections { get; set; }
+        public int Gen1Collections { get; set; }
+        public int Gen2Collections { get; set; }
+        public long TotalOperations { get; set; }
+        public long BytesAllocatedPerOperation { get; set; }
+    }
 }