How not to optimize away - mechanics of a folly class

I was searching for a programming technique that would ensure variables used for benchmarking (without observable side effects) won't be optimized away by the compiler

This gives some info, but I ended up using folly and the following function

/**
 * Call doNotOptimizeAway(var) against variables that you use for
 * benchmarking but otherwise are useless. The compiler tends to do a
 * good job at eliminating unused variables, and this function fools
 * it into thinking var is in fact needed.
 */
#ifdef _MSC_VER

#pragma optimize("", off)

template <class T>
void doNotOptimizeAway(T&& datum) {
  datum = datum;
}

#pragma optimize("", on)

#else
template <class T>
void doNotOptimizeAway(T&& datum) {
  asm volatile("" : "+r" (datum));
}
#endif

I want to use the above, but I have little understanding of its workings. I'm mostly interested in the non VC++ portion and why/how the line

asm volatile("" : "+r" (datum));

creates a non optimizable context or why is this something one would choose to implement such a thing. Also a comparison between the 2 methods would be interesting (I don't know how pragma optimize works but it looks like a cleaner solution - non portable though)