Evaluation Order of C Language

Question

1. Is x+=x*=x undefined behavior?
2. Can anyone explain this rule in Order of evaluation? What is "single evaluation"? What is the opposite of "single evaluation"?

   14) With respect to an indeterminately-sequenced function call, the operation of compound assignment operators, and both prefix and postfix forms of increment and decrement operators are single evaluations.

Answer 1

x+=x*=x has undefined behaviour because x is assigned to twice between sequence points.

---

The text corresponding to 14) in C11, C17 says

A compound assignment of the form E1 op= E2 is equivalent to the simple assignment expression E1 = E1 op (E2), except that the lvalue E1 is evaluated only once, and with respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation.

What I believe it means is that

int x = 0;

int foo(void) {
    x = 5;
    return x;
}

int main(void) {
    int y = foo() + (x += 2); 
}

will have either the behaviour of

int main(void) {
    int _tmp = x += 2;
    int y = foo() + _tmp; 
}

or

int main(void) {
    int _tmp = foo();
    int y = _tmp + (x += 2); 
}

and cannot be split to for example

int main(void) {
    int _tmp = x;
    int _tmp2 = foo();
    x = _tmp + 2;
    int y = _tmp2 + x; 
}

Note that this guarantee is new in C11, and it does not exist in C89, C99.