[C#] Improve Span<> utilization (#8588)

There are a couple instances where the ByteBuffer's Span property was accessed in a loop.
 + Extracted the use of the property outside of the loop to save a few cpu cycles.

Access to the allocator's internal buffer isn't exposed as a ReadOnlySpan<byte> from the ByteBuffer
or the FlatBufferBuilder.
 + Added a few convenience functions to access the buffer using a ReadOnlySpan<byte>.

There are a few cases where built in Span extensions can be used to run optimized code.
 + Added the use of Span.Fill() and ReadOnlySpan.SequenceCompareTo to replace existing loops.

Co-authored-by: Björn Harrtell <bjornharrtell@users.noreply.github.com>
Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

net/FlatBuffers/ByteBuffer.cs

@@ -275,6 +275,11 @@ namespace Google.FlatBuffers
         }
 
 #if ENABLE_SPAN_T && UNSAFE_BYTEBUFFER
+        public ReadOnlySpan<byte> ToSizedReadOnlySpan()
+        {
+            return _buffer.ReadOnlySpan.Slice(Position, Length - Position);
+        }
+
         public ReadOnlyMemory<byte> ToReadOnlyMemory(int pos, int len)
         {
             return _buffer.ReadOnlyMemory.Slice(pos, len);
@@ -289,6 +294,11 @@ namespace Google.FlatBuffers
         {
             return _buffer.Span.Slice(pos, len);
         }
+
+        public ReadOnlySpan<byte> ToReadOnlySpan(int pos, int len)
+        {
+            return _buffer.ReadOnlySpan.Slice(pos, len);
+        }
 #else
         public ArraySegment<byte> ToArraySegment(int pos, int len)
         {
@@ -380,18 +390,19 @@ namespace Google.FlatBuffers
 #elif ENABLE_SPAN_T
         protected void WriteLittleEndian(int offset, int count, ulong data)
         {
+            Span<byte> span = _buffer.Span.Slice(offset, count);
             if (BitConverter.IsLittleEndian)
             {
                 for (int i = 0; i < count; i++)
                 {
-                    _buffer.Span[offset + i] = (byte)(data >> i * 8);
+                    span[i] = (byte)(data >> i * 8);
                 }
             }
             else
             {
                 for (int i = 0; i < count; i++)
                 {
-                    _buffer.Span[offset + count - 1 - i] = (byte)(data >> i * 8);
+                    span[count - 1 - i] = (byte)(data >> i * 8);
                 }
             }
         }
@@ -399,19 +410,20 @@ namespace Google.FlatBuffers
         protected ulong ReadLittleEndian(int offset, int count)
         {
             AssertOffsetAndLength(offset, count);
+            ReadOnlySpan<byte> span = _buffer.ReadOnlySpan.Slice(offset, count);
             ulong r = 0;
             if (BitConverter.IsLittleEndian)
             {
                 for (int i = 0; i < count; i++)
                 {
-                    r |= (ulong)_buffer.Span[offset + i] << i * 8;
+                    r |= (ulong)span[i] << i * 8;
                 }
             }
             else
             {
                 for (int i = 0; i < count; i++)
                 {
-                    r |= (ulong)_buffer.Span[offset + count - 1 - i] << i * 8;
+                    r |= (ulong)span[count - 1 - i] << i * 8;
                 }
             }
             return r;
@@ -445,8 +457,7 @@ namespace Google.FlatBuffers
         {
             AssertOffsetAndLength(offset, sizeof(byte) * count);
             Span<byte> span = _buffer.Span.Slice(offset, count);
-            for (var i = 0; i < span.Length; ++i)
+            span.Fill(value);
-                span[i] = value;
         }
 #else
         public void PutSbyte(int offset, sbyte value)

net/FlatBuffers/FlatBufferBuilder.cs

@@ -957,6 +957,21 @@ namespace Google.FlatBuffers
             return _bb.ToSizedArray();
         }
 
+#if ENABLE_SPAN_T && UNSAFE_BYTEBUFFER
+        /// <summary>
+        /// A utility function return the ByteBuffer data as a
+        /// `ReadOnlySpan<byte>`.
+        /// </summary>
+        /// <returns>
+        /// A `ReadOnlySpan<byte>` that references the internal 
+        /// ByteBuffer data.
+        /// </returns>
+        public ReadOnlySpan<byte> SizedReadOnlySpan()
+        {
+            return _bb.ToSizedReadOnlySpan();
+        }
+#endif
+
         /// <summary>
         /// Finalize a buffer, pointing to the given `rootTable`.
         /// </summary>

net/FlatBuffers/Table.cs

@@ -183,6 +183,12 @@ namespace Google.FlatBuffers
             var len_2 = bb.GetInt(offset_2);
             var startPos_1 = offset_1 + sizeof(int);
             var startPos_2 = offset_2 + sizeof(int);
+
+#if ENABLE_SPAN_T && UNSAFE_BYTEBUFFER
+            var span_1 = bb.ToReadOnlySpan(startPos_1, len_1);
+            var span_2 = bb.ToReadOnlySpan(startPos_2, len_2);
+            return span_1.SequenceCompareTo(span_2);
+#else
             var len = Math.Min(len_1, len_2);
             for(int i = 0; i < len; i++) {
                 byte b1 = bb.Get(i + startPos_1);
@@ -191,6 +197,7 @@ namespace Google.FlatBuffers
                     return b1 - b2;
             }
             return len_1 - len_2;
+#endif
         }
 
         // Compare string from the ByteBuffer with the string object
@@ -200,6 +207,11 @@ namespace Google.FlatBuffers
             var len_1 = bb.GetInt(offset_1);
             var len_2 = key.Length;
             var startPos_1 = offset_1 + sizeof(int);
+#if ENABLE_SPAN_T && UNSAFE_BYTEBUFFER
+            ReadOnlySpan<byte> span = bb.ToReadOnlySpan(startPos_1, len_1);
+            ReadOnlySpan<byte> keySpan = key;
+            return span.SequenceCompareTo(keySpan);
+#else
             var len = Math.Min(len_1, len_2);
             for (int i = 0; i < len; i++) {
                 byte b = bb.Get(i + startPos_1);
@@ -207,6 +219,7 @@ namespace Google.FlatBuffers
                     return b - key[i];
             }
             return len_1 - len_2;
+#endif
         }
     }
 }