Fix BitOperations

src/ImageSharp/Common/Helpers/Numerics.cs

@@ -23,6 +23,28 @@ namespace SixLabors.ImageSharp
         private const int ShuffleAlphaControl = 0b_11_11_11_11;
 #endif
 
+#if !SUPPORTS_BITOPERATIONS
+        /// <summary>
+        /// Gets the counts the number of bits needed to hold an integer.
+        /// </summary>
+        private static ReadOnlySpan<byte> BitCountLut => new byte[]
+        {
+            0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5,
+            5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+            6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+            7, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+            8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+            8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+            8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+            8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+            8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+            8, 8, 8,
+        };
+#endif
+
         /// <summary>
         /// Determine the Greatest CommonDivisor (GCD) of two numbers.
         /// </summary>
@@ -756,7 +778,7 @@ namespace SixLabors.ImageSharp
         /// widening them to 32-bit integers and performing four additions.
         /// </summary>
         /// <remarks>
-        /// <code>byte(1, 2, 3, 4,  5, 6, 7, 8,  9, 10, 11, 12,  13, 14, 15, 16)</code>
+        /// <c>byte(1, 2, 3, 4,  5, 6, 7, 8,  9, 10, 11, 12,  13, 14, 15, 16)</c>
         /// is widened and added onto <paramref name="accumulator"/> as such:
         /// <code>
         ///  accumulator += i32(1, 2, 3, 4);
@@ -834,8 +856,17 @@ namespace SixLabors.ImageSharp
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static int MinimumBitsToStore(uint number)
         {
+#if !SUPPORTS_BITOPERATIONS
+            if (number < 0x100)
+            {
+                return BitCountLut[(int)number];
+            }
+
+            return 8 + BitCountLut[(int)number >> 8];
+#else
             const int bitInUnsignedInteger = sizeof(uint) * 8;
             return bitInUnsignedInteger - BitOperations.LeadingZeroCount(number);
+#endif
         }
     }
 }