Implement feedback and update benchmarks

Author: JimBobSquarePants

src/ImageSharp/Common/Helpers/Numerics.cs

@@ -474,8 +474,10 @@ private static void ClampImpl<T>(Span<T> span, T min, T max)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static void Premultiply(ref Vector4 source)
     {
-        Vector4 alpha = PermuteW(source);
-        source = WithW(source * alpha, alpha);
+        // Load into a local variable to prevent accessing the source from memory multiple times.
+        Vector4 src = source;
+        Vector4 alpha = PermuteW(src);
+        source = WithW(src * alpha, alpha);
     }
 
     /// <summary>

tests/ImageSharp.Benchmarks/Color/Bulk/PremultiplyVector4.cs

@@ -25,7 +25,19 @@ public void PremultiplyBaseline()
     }
 
     [Benchmark]
-    public void Premultiply() => Numerics.Premultiply(Vectors);
+    public void Premultiply()
+    {
+        ref Vector4 baseRef = ref MemoryMarshal.GetReference<Vector4>(Vectors);
+
+        for (int i = 0; i < Vectors.Length; i++)
+        {
+            ref Vector4 v = ref Unsafe.Add(ref baseRef, i);
+            Numerics.Premultiply(ref v);
+        }
+    }
+
+    [Benchmark]
+    public void PremultiplyBulk() => Numerics.Premultiply(Vectors);
 
     [MethodImpl(InliningOptions.ShortMethod)]
     private static void Premultiply(ref Vector4 source)

tests/ImageSharp.Benchmarks/Color/Bulk/UnPremultiplyVector4.cs

@@ -20,12 +20,25 @@ public void UnPremultiplyBaseline()
         for (int i = 0; i < Vectors.Length; i++)
         {
             ref Vector4 v = ref Unsafe.Add(ref baseRef, i);
+
             UnPremultiply(ref v);
         }
     }
 
     [Benchmark]
-    public void UnPremultiply() => Numerics.UnPremultiply(Vectors);
+    public void UnPremultiply()
+    {
+        ref Vector4 baseRef = ref MemoryMarshal.GetReference<Vector4>(Vectors);
+
+        for (int i = 0; i < Vectors.Length; i++)
+        {
+            ref Vector4 v = ref Unsafe.Add(ref baseRef, i);
+            Numerics.UnPremultiply(ref v);
+        }
+    }
+
+    [Benchmark]
+    public void UnPremultiplyBulk() => Numerics.UnPremultiply(Vectors);
 
     [MethodImpl(InliningOptions.ShortMethod)]
     private static void UnPremultiply(ref Vector4 source)