Utilize stackalloc and access static byte data directly (#767)

* Eliminate array allocation in MimeUtils.Unquote() * Use stackalloc in Header.Unfold() to reduce string allocations * Use stackalloc in Header constructor * Eliminate two buffer allocations in DecodeRfc2231 * Use static ReadOnlySpan<byte> compiler feature to reference to static data directly in the encoders See dotnet/roslyn#24621 & https://vcsjones.dev/csharp-readonly-span-bytes-static/ for more information about the ReadOnlySpan<byte> static array usage.
jstedfast · Feb 14, 2022 · 045f5c0 · 045f5c0
1 parent a19fa26
commit 045f5c0
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Showing 11 changed files with 42 additions and 18 deletions.
diff --git a/MimeKit/Encodings/Base64Decoder.cs b/MimeKit/Encodings/Base64Decoder.cs
@@ -37,7 +37,7 @@ namespace MimeKit.Encodings {
 	/// </remarks>
 	public class Base64Decoder : IMimeDecoder
 	{
-		static readonly byte[] base64_rank = new byte[256] {
+		static ReadOnlySpan<byte> base64_rank => new byte[256] {
 			255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,
 			255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,
 			255,255,255,255,255,255,255,255,255,255,255, 62,255,255,255, 63,

diff --git a/MimeKit/Encodings/Base64Encoder.cs b/MimeKit/Encodings/Base64Encoder.cs
@@ -37,7 +37,7 @@ namespace MimeKit.Encodings {
 	/// </remarks>
 	public class Base64Encoder : IMimeEncoder
 	{
-		static readonly byte[] base64_alphabet = {
+		static ReadOnlySpan<byte> base64_alphabet => new byte[] {
 			0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4A, 0x4B, 0x4C, 0x4D, 0x4E, 0x4F, 0x50,
 			0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5A, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66,
 			0x67, 0x68, 0x69, 0x6A, 0x6B, 0x6C, 0x6D, 0x6E, 0x6F, 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76,

diff --git a/MimeKit/Encodings/HexEncoder.cs b/MimeKit/Encodings/HexEncoder.cs
@@ -38,7 +38,7 @@ namespace MimeKit.Encodings {
 	/// </remarks>
 	public class HexEncoder : IMimeEncoder
 	{
-		static readonly byte[] hex_alphabet = new byte[16] {
+		static ReadOnlySpan<byte> hex_alphabet => new byte[16] {
 			0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, // '0' -> '7'
 			0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, // '8' -> 'F'
 		};

diff --git a/MimeKit/Encodings/QEncoder.cs b/MimeKit/Encodings/QEncoder.cs
@@ -58,7 +58,7 @@ public enum QEncodeMode : byte {
 	/// </remarks>
 	public class QEncoder : IMimeEncoder
 	{
-		static readonly byte[] hex_alphabet = new byte[16] {
+		static ReadOnlySpan<byte> hex_alphabet => new byte[16] {
 			0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, // '0' -> '7'
 			0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, // '8' -> 'F'
 		};

diff --git a/MimeKit/Encodings/QuotedPrintableEncoder.cs b/MimeKit/Encodings/QuotedPrintableEncoder.cs
@@ -39,7 +39,7 @@ namespace MimeKit.Encodings {
 	/// </remarks>
 	public class QuotedPrintableEncoder : IMimeEncoder
 	{
-		static readonly byte[] hex_alphabet = new byte[16] {
+		static ReadOnlySpan<byte> hex_alphabet => new byte[16] {
 			0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, // '0' -> '7'
 			0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, // '8' -> 'F'
 		};

diff --git a/MimeKit/Encodings/UUDecoder.cs b/MimeKit/Encodings/UUDecoder.cs
@@ -39,7 +39,7 @@ namespace MimeKit.Encodings {
 	/// </remarks>
 	public class UUDecoder : IMimeDecoder
 	{
-		static readonly byte[] uudecode_rank = new byte[256] {
+		static ReadOnlySpan<byte> uudecode_rank => new byte[256] {
 			 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
 			 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
 			  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15,

diff --git a/MimeKit/Header.cs b/MimeKit/Header.cs
@@ -312,9 +312,14 @@ protected Header (ParserOptions options, HeaderId id, string name, byte[] field,
 		/// <param name="fieldNameLength">The length of the field name (not including trailing whitespace).</param>
 		/// <param name="value">The raw value of the header.</param>
 		/// <param name="invalid"><c>true</c> if the header field is invalid; othereise, <c>false</c>.</param>
+#if NET5_0_OR_GREATER
+		[System.Runtime.CompilerServices.SkipLocalsInit]
+#endif
 		internal protected Header (ParserOptions options, byte[] field, int fieldNameLength, byte[] value, bool invalid)
 		{
-			var chars = new char[fieldNameLength];
+			Span<char> chars = fieldNameLength <= 32
+				? stackalloc char[32]
+				: new char[fieldNameLength];
 
 			for (int i = 0; i < fieldNameLength; i++)
 				chars[i] = (char) field[i];
@@ -323,7 +328,7 @@ internal protected Header (ParserOptions options, byte[] field, int fieldNameLen
 			rawField = field;
 			rawValue = value;
 
-			Field = new string (chars, 0, fieldNameLength);
+			Field = chars.Slice (0, fieldNameLength).ToString ();
 			Id = Field.ToHeaderId ();
 			IsInvalid = invalid;
 		}
@@ -340,9 +345,15 @@ internal protected Header (ParserOptions options, byte[] field, int fieldNameLen
 		/// <param name="field">The raw header field.</param>
 		/// <param name="value">The raw value of the header.</param>
 		/// <param name="invalid"><c>true</c> if the header field is invalid; othereise, <c>false</c>.</param>
+#if NET5_0_OR_GREATER
+		[System.Runtime.CompilerServices.SkipLocalsInit]
+#endif
 		internal protected Header (ParserOptions options, byte[] field, byte[] value, bool invalid)
 		{
-			var chars = new char[field.Length];
+			Span<char> chars = field.Length <= 32
+				? stackalloc char[32]
+				: new char[field.Length];
+
 			int count = 0;
 
 			while (count < field.Length && (invalid || !field[count].IsBlank ())) {
@@ -354,7 +365,7 @@ internal protected Header (ParserOptions options, byte[] field, byte[] value, bo
 			rawField = field;
 			rawValue = value;
 
-			Field = new string (chars, 0, count);
+			Field = chars.Slice (0, count).ToString ();
 			Id = Field.ToHeaderId ();
 			IsInvalid = invalid;
 		}
@@ -1306,6 +1317,9 @@ public override string ToString ()
 		/// </remarks>
 		/// <returns>The unfolded header value.</returns>
 		/// <param name="text">The header text.</param>
+#if NET5_0_OR_GREATER
+		[System.Runtime.CompilerServices.SkipLocalsInit]
+#endif
 		public static string Unfold (string text)
 		{
 			int startIndex;
@@ -1330,14 +1344,16 @@ public static string Unfold (string text)
 			}
 
 			int count = endIndex - startIndex;
-			char[] chars = new char[count];
+			Span<char> chars = count <= 32
+				? stackalloc char[32]
+				: new char[count];
 
 			for (i = startIndex, count = 0; i < endIndex; i++) {
 				if (text[i] != '\r' && text[i] != '\n')
 					chars[count++] = text[i];
 			}
 
-			return new string (chars, 0, count);
+			return chars.Slice (0, count).ToString ();
 		}
 
 		static bool IsAsciiAtom (byte c)

diff --git a/MimeKit/MimeParser.cs b/MimeKit/MimeParser.cs
@@ -106,7 +106,7 @@ enum MimeParserState : sbyte
 	/// </remarks>
 	public partial class MimeParser : IEnumerable<MimeMessage>
 	{
-		static readonly byte[] UTF8ByteOrderMark = { 0xEF, 0xBB, 0xBF };
+		static ReadOnlySpan<byte> UTF8ByteOrderMark => new byte[] { 0xEF, 0xBB, 0xBF };
 		const int ReadAheadSize = 128;
 		const int BlockSize = 4096;
 		const int PadSize = 4;

diff --git a/MimeKit/MimeReader.cs b/MimeKit/MimeReader.cs
@@ -51,7 +51,8 @@ enum MimeEntityType
 			Multipart
 		}
 
-		static readonly byte[] UTF8ByteOrderMark = { 0xEF, 0xBB, 0xBF };
+		static ReadOnlySpan<byte> UTF8ByteOrderMark => new byte[] { 0xEF, 0xBB, 0xBF };
+
 		static readonly Task CompletedTask;
 		const int HeaderBufferGrowSize = 64;
 		const int ReadAheadSize = 128;

diff --git a/MimeKit/ParameterList.cs b/MimeKit/ParameterList.cs
@@ -32,6 +32,7 @@
 
 using MimeKit.Encodings;
 using MimeKit.Utils;
+using System.Buffers;
 
 namespace MimeKit {
 	/// <summary>
@@ -906,17 +907,22 @@ static string DecodeRfc2231 (out Encoding encoding, ref Decoder decoder, HexDeco
 			}
 
 			int length = endIndex - index;
-			var decoded = new byte[hex.EstimateOutputLength (length)];
+			var decoded = ArrayPool<byte>.Shared.Rent(hex.EstimateOutputLength (length));
 
 			// hex decode...
 			length = hex.Decode (text, index, length, decoded);
 
 			int outLength = decoder.GetCharCount (decoded, 0, length, flush);
-			var output = new char[outLength];
+			var output = ArrayPool<char>.Shared.Rent (outLength);
 
 			outLength = decoder.GetChars (decoded, 0, length, output, 0, flush);
 
-			return new string (output, 0, outLength);
+			var result = new string (output, 0, outLength);
+
+			ArrayPool<char>.Shared.Return (output);
+			ArrayPool<byte>.Shared.Return (decoded);
+
+			return result;
 		}
 
 		internal static bool TryParse (ParserOptions options, byte[] text, ref int index, int endIndex, bool throwOnError, out ParameterList paramList)

diff --git a/MimeKit/Utils/MimeUtils.cs b/MimeKit/Utils/MimeUtils.cs
@@ -41,6 +41,7 @@ public static class MimeUtils
 	{
 		const string base36 = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
 		static string DefaultHostName = null;
+		static readonly char[] UnquoteChars = new[] { '\r', '\n', '\t', '\\', '"' };
 
 		/// <summary>
 		/// A string comparer that performs a case-insensitive ordinal string comparison.
@@ -524,7 +525,7 @@ public static string Unquote (string text)
 			if (text == null)
 				throw new ArgumentNullException (nameof (text));
 
-			int index = text.IndexOfAny (new [] { '\r', '\n', '\t', '\\', '"' });
+			int index = text.IndexOfAny (UnquoteChars);
 
 			if (index == -1)
 				return text;