only real way to make sure it's accurate

You never know which and how many registers the compiler puts on the stack. Yes, there are some compilers out there who push/pop every single register for each function. This might be a non-issue if you're only looking for millisecond accuracy.

Also, the function has _two_ return statements and will most likely crash horribly if used with a compiler that pushes anything on the stack at the beginning of the function.

And then of course there's popular compilers that don't support simple in-line assembly.