Reduced intermediate allocations: Gif

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -22,7 +22,7 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
     /// <summary>
     /// The temp buffer used to reduce allocations.
     /// </summary>
-    private readonly byte[] buffer = new byte[16];
+    private ScratchBuffer buffer;   // mutable struct, don't make readonly
 
     /// <summary>
     /// The global color table.
@@ -249,13 +249,13 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
     /// <param name="stream">The <see cref="BufferedReadStream"/> containing image data.</param>
     private void ReadGraphicalControlExtension(BufferedReadStream stream)
     {
-        int bytesRead = stream.Read(this.buffer, 0, 6);
+        int bytesRead = stream.Read(this.buffer.Span, 0, 6);
         if (bytesRead != 6)
         {
             GifThrowHelper.ThrowInvalidImageContentException("Not enough data to read the graphic control extension");
         }
 
-        this.graphicsControlExtension = GifGraphicControlExtension.Parse(this.buffer);
+        this.graphicsControlExtension = GifGraphicControlExtension.Parse(this.buffer.Span);
     }
 
     /// <summary>
@@ -264,13 +264,13 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
     /// <param name="stream">The <see cref="BufferedReadStream"/> containing image data.</param>
     private void ReadImageDescriptor(BufferedReadStream stream)
     {
-        int bytesRead = stream.Read(this.buffer, 0, 9);
+        int bytesRead = stream.Read(this.buffer.Span, 0, 9);
         if (bytesRead != 9)
         {
             GifThrowHelper.ThrowInvalidImageContentException("Not enough data to read the image descriptor");
         }
 
-        this.imageDescriptor = GifImageDescriptor.Parse(this.buffer);
+        this.imageDescriptor = GifImageDescriptor.Parse(this.buffer.Span);
         if (this.imageDescriptor.Height == 0 || this.imageDescriptor.Width == 0)
         {
             GifThrowHelper.ThrowInvalidImageContentException("Width or height should not be 0");
@@ -283,13 +283,13 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
     /// <param name="stream">The <see cref="BufferedReadStream"/> containing image data.</param>
     private void ReadLogicalScreenDescriptor(BufferedReadStream stream)
     {
-        int bytesRead = stream.Read(this.buffer, 0, 7);
+        int bytesRead = stream.Read(this.buffer.Span, 0, 7);
         if (bytesRead != 7)
         {
             GifThrowHelper.ThrowInvalidImageContentException("Not enough data to read the logical screen descriptor");
         }
 
-        this.logicalScreenDescriptor = GifLogicalScreenDescriptor.Parse(this.buffer);
+        this.logicalScreenDescriptor = GifLogicalScreenDescriptor.Parse(this.buffer.Span);
     }
 
     /// <summary>
@@ -306,8 +306,8 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
         long position = stream.Position;
         if (appLength == GifConstants.ApplicationBlockSize)
         {
-            stream.Read(this.buffer, 0, GifConstants.ApplicationBlockSize);
+            stream.Read(this.buffer.Span, 0, GifConstants.ApplicationBlockSize);
-            bool isXmp = this.buffer.AsSpan().StartsWith(GifConstants.XmpApplicationIdentificationBytes);
+            bool isXmp = this.buffer.Span.StartsWith(GifConstants.XmpApplicationIdentificationBytes);
             if (isXmp && !this.skipMetadata)
             {
                 GifXmpApplicationExtension extension = GifXmpApplicationExtension.Read(stream, this.memoryAllocator);
@@ -331,8 +331,8 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
             // http://www.vurdalakov.net/misc/gif/netscape-buffering-application-extension
             if (subBlockSize == GifConstants.NetscapeLoopingSubBlockSize)
             {
-                stream.Read(this.buffer, 0, GifConstants.NetscapeLoopingSubBlockSize);
+                stream.Read(this.buffer.Span, 0, GifConstants.NetscapeLoopingSubBlockSize);
-                this.gifMetadata!.RepeatCount = GifNetscapeLoopingApplicationExtension.Parse(this.buffer.AsSpan(1)).RepeatCount;
+                this.gifMetadata!.RepeatCount = GifNetscapeLoopingApplicationExtension.Parse(this.buffer.Span.Slice(1)).RepeatCount;
                 stream.Skip(1); // Skip the terminator.
                 return;
             }
@@ -762,4 +762,11 @@ internal sealed class GifDecoderCore : IImageDecoderInternals
             }
         }
     }
+
+    private unsafe struct ScratchBuffer
+    {
+        private fixed byte scratch[16];
+
+        public Span<byte> Span => MemoryMarshal.CreateSpan(ref this.scratch[0], 16);
+    }
 }

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -28,11 +28,6 @@ internal sealed class GifEncoderCore : IImageEncoderInternals
     /// </summary>
     private readonly Configuration configuration;
 
-    /// <summary>
-    /// A reusable buffer used to reduce allocations.
-    /// </summary>
-    private readonly byte[] buffer = new byte[20];
-
     /// <summary>
     /// Whether to skip metadata during encode.
     /// </summary>
@@ -324,9 +319,10 @@ internal sealed class GifEncoderCore : IImageEncoderInternals
             backgroundColorIndex: unchecked((byte)transparencyIndex),
             ratio);
 
-        descriptor.WriteTo(this.buffer);
+        Span<byte> buffer = stackalloc byte[20];
+        descriptor.WriteTo(buffer);
 
-        stream.Write(this.buffer, 0, GifLogicalScreenDescriptor.Size);
+        stream.Write(buffer, 0, GifLogicalScreenDescriptor.Size);
     }
 
     /// <summary>
@@ -365,12 +361,14 @@ internal sealed class GifEncoderCore : IImageEncoderInternals
             return;
         }
 
+        Span<byte> buffer = stackalloc byte[2];
+
         for (int i = 0; i < metadata.Comments.Count; i++)
         {
             string comment = metadata.Comments[i];
-            this.buffer[0] = GifConstants.ExtensionIntroducer;
+            buffer[1] = GifConstants.CommentLabel;
-            this.buffer[1] = GifConstants.CommentLabel;
+            buffer[0] = GifConstants.ExtensionIntroducer;
-            stream.Write(this.buffer, 0, 2);
+            stream.Write(buffer);
 
             // Comment will be stored in chunks of 255 bytes, if it exceeds this size.
             ReadOnlySpan<char> commentSpan = comment.AsSpan();
@@ -437,22 +435,23 @@ internal sealed class GifEncoderCore : IImageEncoderInternals
     private void WriteExtension<TGifExtension>(TGifExtension extension, Stream stream)
         where TGifExtension : struct, IGifExtension
     {
-        IMemoryOwner<byte>? owner = null;
-        Span<byte> extensionBuffer;
         int extensionSize = extension.ContentLength;
 
         if (extensionSize == 0)
         {
             return;
         }
-        else if (extensionSize > this.buffer.Length - 3)
+
+        IMemoryOwner<byte>? owner = null;
+        Span<byte> extensionBuffer = stackalloc byte[0];    // workaround compiler limitation
+        if (extensionSize > 128)
         {
             owner = this.memoryAllocator.Allocate<byte>(extensionSize + 3);
             extensionBuffer = owner.GetSpan();
         }
         else
         {
-            extensionBuffer = this.buffer;
+            extensionBuffer = stackalloc byte[extensionSize + 3];
         }
 
         extensionBuffer[0] = GifConstants.ExtensionIntroducer;
@@ -489,9 +488,10 @@ internal sealed class GifEncoderCore : IImageEncoderInternals
             height: (ushort)image.Height,
             packed: packedValue);
 
-        descriptor.WriteTo(this.buffer);
+        Span<byte> buffer = stackalloc byte[20];
+        descriptor.WriteTo(buffer);
 
-        stream.Write(this.buffer, 0, GifImageDescriptor.Size);
+        stream.Write(buffer, 0, GifImageDescriptor.Size);
     }
 
     /// <summary>