It's not even guaranteed to be functionally equivalent when compiled on the same hardware with the same compiler etc. Undefined behaviour can do what it wants. (And implementation defined behaviour also has a lot of leeway.)
However, if you stick to only defined behaviour, they are 'functionally equivalent', if your compiler doesn't have a bug.
However, if you stick to only defined behaviour, they are 'functionally equivalent', if your compiler doesn't have a bug.