Could a C++ implementation, in theory, parallelise the evaluation of two function arguments?

Question

Given the following function call:

f(g(), h())

since the order of evaluation of function arguments is unspecified (still the case in C++11

Answer 1

Easy answer: when the functions are sequenced, even if indeterminately, there is no possibility for a race condition between the two, which is not true if they are parallelized. Even a pair of one line "trivial" functions could do it.

void g()
{
    *p = *p + 1;
}


void h()
{
    *p = *p - 1;
}

If p is a name shared by g and h, then a sequential calling of g and h in any order will result in the value pointed to by p not changing. If they are parallelized, the reading of *p and the assigning of it could be interleaved arbitrarily between the two:

1. g reads *p and finds the value 1.
2. f reads *p and also finds the value 1.
3. g writes 2 to *p.
4. f, still using the value 1 it read before will write 0 to *p.

Thus, the behavior is different when they are parallelized.