Why is BitConverter slower than doing the bitwise operations directly?

Question

I recently did some profiling on some code and found that the largest CPU usage was being consumed by calls to BitConverter such as:

return BitConverter.ToInt16(new byte[] { byte1, byte2 });

when switching to something like:

return (short)(byte1 << 8 | byte2);

I noticed a huge improvement in performance.

My question is why is using BitConverter so much slower? I would have assumed that BitConverter was essentially doing the same kind of bit shifting internally.

Answer 1

The call to BitConverter involves the allocation and initialisation of a new object. And then a method call. And inside the method call is parameter validation.

The bitwise operations can be compiled right down to a handful of CPU opcodes to do a shift followed by the or.

The latter will surely be faster because it removes all of the overhead of the former.