Tons of very "interesting" attributes like this:
// prevent inlining into main to decrease JIT time to generate main
[SkipLocalsInit][MethodImpl(NoInlining)]
[SkipLocalsInit][StructLayout(LayoutKind.Explicit, Pack = 32)]
[SkipLocalsInit][MethodImpl(AggressiveOptimization | NoInlining)]
[FieldOffset(32)]
and tons of "unchecked" blocks.Not to mention that the entire file is using explicit vectorization, which I consider to be a very high degree of optimization -- tons of software never bothers to implement explicit vectorization, and does just fine.
If all of this is "nothing out of the ordinary", then "ordinary" C# has changed a lot since I last spent much time with it.
[0]: https://benchmarksgame-team.pages.debian.net/benchmarksgame/...