The nature of operators and priority calculation

There are three influencing factors that affect the evaluation of expressions:
1. The precedence of operators
2. The associativity of operators
3. Whether to control the order of evaluation.
So when we calculate complex expressions, we must clarify the nature and nature of each operator priority.

Classification (dendrogram)

Detailed form

Priority decreases

Priority calculation problem

We understand that after learning the priority of operators, there are simple calculation paths for some complex expressions, but some problems also arise, such as:
a b + c d + e f
How to calculate the above expression? ?
It is possible
a b
c d
a b + c d
e f
a b + c d + e f It
is also possible
a b
c d
e f
a b + c d
a b + c d + e*f

So how to determine the calculation path?
Let's test it with a bunch of codes:

int main()
{
    
    
 int i = 10;
 i = i-- - --i * ( i = -3 ) * i++ + ++i;
 printf("i = %d\n", i);
 return 0; 
 }

There are different results under different compilers:

It shows that the calculation path cannot be determined by the compiler itself.
Summary:
If the expression of the program we write cannot determine the unique calculation path through the attributes of the operator, then this expression is problematic.