Mzlib benchmarks (dotnet#2358)

* baseline to a benchmark
* Pre-allocate memory stream, so the cost of it's creation is not included in the benchmark (#1)
---------
Co-authored-by: Adam Sitnik <[email protected]>

src/Microsoft.ManagedZLib/benchmarks/ManagedZLibBenchmarks.cs

@@ -23,12 +23,14 @@ public static IEnumerable<string> UncompressedTestFileNames()
     }
 
     public CompressedFile? CompressedFile;
+    private MemoryStream _outputStream;
 
     [GlobalSetup]
     public void Setup()
     {
         Debug.Assert(File != null);
         CompressedFile = new CompressedFile(File, Level);
+        _outputStream = new MemoryStream(UncompressedData.Length); 
     }
 
 
@@ -44,29 +46,29 @@ public void Setup()
     [GlobalCleanup]
     public void Cleanup() => CompressedFile?.CompressedDataStream.Dispose();
 
-    [Benchmark]
-    public int DecompressNative()
+    [Benchmark(Baseline = true)]
+    public void DecompressNative()
     {
         CompressedFile!.CompressedDataStream.Position = 0;
-        MemoryStream expectedStream = new();
+        _outputStream.Position = 0;
+
         System.IO.Compression.DeflateStream decompressor = new System.IO.Compression.DeflateStream(CompressedFile.CompressedDataStream, System.IO.Compression.CompressionMode.Decompress);
-        decompressor.CopyTo(expectedStream);
-        return 0;
+        decompressor.CopyTo(_outputStream);
     }
 
     [Benchmark]
-    public int DecompressManaged()
+    public void DecompressManaged()
     {
         CompressedFile!.CompressedDataStream.Position = 0;
-        MemoryStream expectedStream = new();
+        _outputStream.Position = 0;
+
         DeflateStream decompressor = new DeflateStream(CompressedFile.CompressedDataStream, CompressionMode.Decompress);
-        decompressor.CopyTo(expectedStream);
-        return 0;
+        decompressor.CopyTo(_outputStream);
     }
 
     public class ProgramRun
     {
         static void Main() => BenchmarkRunner.Run<ManagedZLibBenchmark>();
     }
 
-}
+}