Faster decoding

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -5,6 +5,7 @@ using System;
 using System.Buffers;
 using System.IO;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using System.Text;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
@@ -410,13 +411,12 @@ namespace SixLabors.ImageSharp.Formats.Gif
         private void ReadFrameColors<TPixel>(ref Image<TPixel> image, ref ImageFrame<TPixel> previousFrame, Span<byte> indices, Span<byte> colorTable, GifImageDescriptor descriptor)
             where TPixel : struct, IPixel<TPixel>
         {
+            ref byte indicesRef = ref MemoryMarshal.GetReference(indices);
             int imageWidth = this.logicalScreenDescriptor.Width;
             int imageHeight = this.logicalScreenDescriptor.Height;
 
             ImageFrame<TPixel> prevFrame = null;
-
             ImageFrame<TPixel> currentFrame = null;
-
             ImageFrame<TPixel> imageFrame;
 
             if (previousFrame == null)
@@ -479,7 +479,6 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     }
 
                     writeY = interlaceY + descriptor.Top;
-
                     interlaceY += interlaceIncrement;
                 }
                 else
@@ -487,14 +486,13 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     writeY = y;
                 }
 
-                Span<TPixel> rowSpan = imageFrame.GetPixelRowSpan(writeY);
+                ref TPixel rowRef = ref MemoryMarshal.GetReference(imageFrame.GetPixelRowSpan(writeY));
-
                 var rgba = new Rgba32(0, 0, 0, 255);
 
                 // #403 The left + width value can be larger than the image width
-                for (int x = descriptor.Left; x < descriptor.Left + descriptor.Width && x < rowSpan.Length; x++)
+                for (int x = descriptor.Left; x < descriptor.Left + descriptor.Width && x < imageWidth; x++)
                 {
-                    int index = indices[i];
+                    int index = Unsafe.Add(ref indicesRef, i);
 
                     if (this.graphicsControlExtension == null ||
                         this.graphicsControlExtension.TransparencyFlag == false ||
@@ -502,7 +500,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     {
                         int indexOffset = index * 3;
 
-                        ref TPixel pixel = ref rowSpan[x];
+                        ref TPixel pixel = ref Unsafe.Add(ref rowRef, x);
                         rgba.Rgb = colorTable.GetRgb24(indexOffset);
 
                         pixel.PackFromRgba32(rgba);

src/ImageSharp/Formats/Gif/LzwDecoder.cs

@@ -4,7 +4,8 @@
 using System;
 using System.Buffers;
 using System.IO;
-
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Memory;
 
 namespace SixLabors.ImageSharp.Formats.Gif
@@ -115,14 +116,14 @@ namespace SixLabors.ImageSharp.Formats.Gif
             int data = 0;
             int first = 0;
 
-            Span<int> prefixSpan = this.prefix.Span;
+            ref int prefixRef = ref MemoryMarshal.GetReference(this.prefix.Span);
-            Span<int> suffixSpan = this.suffix.Span;
+            ref int suffixRef = ref MemoryMarshal.GetReference(this.suffix.Span);
-            Span<int> pixelStackSpan = this.pixelStack.Span;
+            ref int pixelStackRef = ref MemoryMarshal.GetReference(this.pixelStack.Span);
+            ref byte pixelsRef = ref MemoryMarshal.GetReference(pixels);
 
             for (code = 0; code < clearCode; code++)
             {
-                prefixSpan[code] = 0;
+                Unsafe.Add(ref suffixRef, code) = (byte)code;
-                suffixSpan[code] = (byte)code;
             }
 
             byte[] buffer = new byte[255];
@@ -176,7 +177,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
                     if (oldCode == NullCode)
                     {
-                        pixelStackSpan[top++] = suffixSpan[code];
+                        Unsafe.Add(ref pixelStackRef, top++) = Unsafe.Add(ref suffixRef, code);
                         oldCode = code;
                         first = code;
                         continue;
@@ -185,27 +186,27 @@ namespace SixLabors.ImageSharp.Formats.Gif
                     int inCode = code;
                     if (code == availableCode)
                     {
-                        pixelStackSpan[top++] = (byte)first;
+                        Unsafe.Add(ref pixelStackRef, top++) = (byte)first;
 
                         code = oldCode;
                     }
 
                     while (code > clearCode)
                     {
-                        pixelStackSpan[top++] = suffixSpan[code];
+                        Unsafe.Add(ref pixelStackRef, top++) = Unsafe.Add(ref suffixRef, code);
-                        code = prefixSpan[code];
+                        code = Unsafe.Add(ref prefixRef, code);
                     }
 
-                    first = suffixSpan[code];
+                    int suffixCode = Unsafe.Add(ref suffixRef, code);
-
+                    first = suffixCode;
-                    pixelStackSpan[top++] = suffixSpan[code];
+                    Unsafe.Add(ref pixelStackRef, top++) = suffixCode;
 
                     // Fix for Gifs that have "deferred clear code" as per here :
                     // https://bugzilla.mozilla.org/show_bug.cgi?id=55918
                     if (availableCode < MaxStackSize)
                     {
-                        prefixSpan[availableCode] = oldCode;
+                        Unsafe.Add(ref prefixRef, availableCode) = oldCode;
-                        suffixSpan[availableCode] = first;
+                        Unsafe.Add(ref suffixRef, availableCode) = first;
                         availableCode++;
                         if (availableCode == codeMask + 1 && availableCode < MaxStackSize)
                         {
@@ -221,7 +222,7 @@ namespace SixLabors.ImageSharp.Formats.Gif
                 top--;
 
                 // Clear missing pixels
-                pixels[xyz++] = (byte)pixelStackSpan[top];
+                Unsafe.Add(ref pixelsRef, xyz++) = (byte)Unsafe.Add(ref pixelStackRef, top);
             }
         }
 
@@ -238,8 +239,9 @@ namespace SixLabors.ImageSharp.Formats.Gif
         /// </summary>
         /// <param name="buffer">The buffer to store the block in.</param>
         /// <returns>
-        /// The <see cref="T:byte[]"/>.
+        /// The <see cref="int"/>.
         /// </returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private int ReadBlock(byte[] buffer)
         {
             int bufferSize = this.stream.ReadByte();