K&R C gives compilers a license to rearrange expressions involving adjacent operators that are mathematically commutative and associative, even in the presence of parentheses. However, ISO C does not grant compilers this same freedom.
This section discusses the differences between these two definitions of C and clarifies the distinctions between an expression’s side effects, grouping, and evaluation by considering the expression statement from the following code fragment.
int i, *p, f(void), g(void); /*...*/ i = *++p + f() + g();