From 69c3475da115fb446aaac14ba952e0e1c197dc0a Mon Sep 17 00:00:00 2001
From: Eric Erhardt <eric.erhardt@microsoft.com>
Date: Thu, 26 May 2022 16:29:24 -0500
Subject: [PATCH] Fix Stream.ReadAtLeast perf regression in
 DataContractSerializer

#69272 changed DCS to no longer call Stream.Read inside a loop, but instead call the new ReadAtLeast method. ReadAtLeast only takes a Span<byte>, and not a byte[]. This caused a regression because the internal encoding stream wrapper classes don't override Read(Span<byte>), so the base implementation is used. The base implementation is slower because it needs to rent a byte[] from the pool, and do 2 copies.

Overriding Read(Span<byte>) on the internal encoding stream wrapper classes allows ReadAtLeast to be just as fast.

Fix #69730
---
 .../Serialization/Json/JsonEncodingStreamWrapper.cs   | 11 ++++++++---
 .../src/System/Xml/EncodingStreamWrapper.cs           | 11 ++++++++---
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/JsonEncodingStreamWrapper.cs b/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/JsonEncodingStreamWrapper.cs
index 8f30f153be140..f6a0516d24bac 100644
--- a/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/JsonEncodingStreamWrapper.cs
+++ b/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/Json/JsonEncodingStreamWrapper.cs
@@ -174,7 +174,10 @@ public override void Flush()
             _stream.Flush();
         }
 
-        public override int Read(byte[] buffer, int offset, int count)
+        public override int Read(byte[] buffer, int offset, int count) =>
+            Read(new Span<byte>(buffer, offset, count));
+
+        public override int Read(Span<byte> buffer)
         {
             try
             {
@@ -182,7 +185,7 @@ public override int Read(byte[] buffer, int offset, int count)
                 {
                     if (_encodingCode == SupportedEncoding.UTF8)
                     {
-                        return _stream.Read(buffer, offset, count);
+                        return _stream.Read(buffer);
                     }
 
                     Debug.Assert(_bytes != null);
@@ -206,11 +209,13 @@ public override int Read(byte[] buffer, int offset, int count)
                 }
 
                 // Give them bytes
+                int count = buffer.Length;
                 if (_byteCount < count)
                 {
                     count = _byteCount;
                 }
-                Buffer.BlockCopy(_bytes!, _byteOffset, buffer, offset, count);
+
+                _bytes.AsSpan(_byteOffset, count).CopyTo(buffer);
                 _byteOffset += count;
                 _byteCount -= count;
                 return count;
diff --git a/src/libraries/System.Private.DataContractSerialization/src/System/Xml/EncodingStreamWrapper.cs b/src/libraries/System.Private.DataContractSerialization/src/System/Xml/EncodingStreamWrapper.cs
index e0fe24ba58163..7d9bab87855fd 100644
--- a/src/libraries/System.Private.DataContractSerialization/src/System/Xml/EncodingStreamWrapper.cs
+++ b/src/libraries/System.Private.DataContractSerialization/src/System/Xml/EncodingStreamWrapper.cs
@@ -593,14 +593,17 @@ public override int ReadByte()
             return _byteBuffer[0];
         }
 
-        public override int Read(byte[] buffer, int offset, int count)
+        public override int Read(byte[] buffer, int offset, int count) =>
+            Read(new Span<byte>(buffer, offset, count));
+
+        public override int Read(Span<byte> buffer)
         {
             try
             {
                 if (_byteCount == 0)
                 {
                     if (_encodingCode == SupportedEncoding.UTF8)
-                        return _stream.Read(buffer, offset, count);
+                        return _stream.Read(buffer);
 
                     Debug.Assert(_bytes != null);
                     Debug.Assert(_chars != null);
@@ -622,9 +625,11 @@ public override int Read(byte[] buffer, int offset, int count)
                 }
 
                 // Give them bytes
+                int count = buffer.Length;
                 if (_byteCount < count)
                     count = _byteCount;
-                Buffer.BlockCopy(_bytes!, _byteOffset, buffer, offset, count);
+
+                _bytes.AsSpan(_byteOffset, count).CopyTo(buffer);
                 _byteOffset += count;
                 _byteCount -= count;
                 return count;