Its possible the JIT would have optimized this but that optimization isn't making its way into the native AOT build.

Also possible if it were more idiomatic C# it would have optimized it better. RyuJIT does in fact do BCE and other loop optimizations. It's improving a lot every year too.

The optimization paths from C# though Roslyn and RyuJIT probably don't start with C# written like it were C. Maybe iterating over Spans and slices would have optimized to fewer instructions than raw dogging nested for loops.