C 中没有赋值或语句的数字

Question

What happens to the values or text without assignment. How does compiler handle them? Do they impact performance of the code (What does the CPU/processor do with them)?
Here is the example code: I just put 3, 5 and "abcd", code doesn't throw any error.

int main() 
{
    // Here I declared the variables
    int a, b;
    
    // Here there are some numbers
    3;
    5;
    "abcd";
    
    return 0;
}

Answer 1

The compiler will tell you:

<source>:9:5: warning: expression result unused [-Wunused-value]

If the expression result is not used then the compiler will not generate any resulting code. It is very likely that the string literal will be not present in the object and executable files

As @EricPostpischil stated if the expression fas side effect it will be evaluated.

examples:

p++;
1 + foo(p);
1 + p++;
foo(p);

also if you declare objects as side effects prone this kind of expressions will be evaluated.

example

    volatile int a, b;
    a;  //this will generate the code
    b;  //this will generate the code

https://godbolt.org/z/fa4sa51nq

Answer 2

C 2018 6.8.3 says:

The expression in an expression statement is evaluated as a void expression for its side effects.

Thus no use is made of the value. Only side effects are useful. In a statement such as printf("Hello, world.\n");, a so-called side effect is to send the string to standard output.

Statements such as 3; and 5; have no side effects, and the main value is ignored, so they have no effects.

All expression statements you think of as “doing something”, such as:

printf("x = %d.\n", x);
b = 4;
FindMatchingThings(a, b, c);
++y;

actually do those things by way of their side effects: Sending data to a stream, updating the value of an object, or calling a function which has its own side effects.