If .. else number is smaller than, unreachable code

Question

Let's say we have the following code in C (or similar language):

if (x < 10)
  do_work1();
else if (x < 5)
  do_work2();

Will the second branch of this conditional be executed in some cases? Will the compiler warn about unreachable code?

Answer 1

Will the second branch of condition be executed in some case?

• Yes, it could be possible, it depends on what else is going on in the code and what the compiler chooses to do with your code.

Shouldn't compiler warn about unreachable code?

• No, it can't because there's no guarantee that it's unreachable

Take this for example:

int x = 11;

void* change_x(){
   while(1)
      x = 3;
}

int main(void) 
{
    pthread_t cxt;
    int y = 0;
    pthread_create(&cxt, NULL, change_x, NULL);
    while(1){
        if(x < 10)
            printf("x is less than ten!\n");
        else if (x < 5){
            printf("x is less than 5!\n");
            exit(1);
        }
        else if(y == 0){    // The check for y is only in here so we don't kill
                            // ourselves reading "x is greater than 10" while waiting
                            // for the race condition
            printf("x is greater than 10!\n");
            y = 1;
        }
        x = 11;
    }

    return 0;
}

And the output:

mike@linux-4puc:~> ./a.out 
x is greater than 10!
x is less than 5!      <-- Look, we hit the "unreachable code"

Answer 2

• If x is a local variable then I don't see any way that do_work2 could be executed.
• If x is a global variable or is shared between multiple threads then do_work2 could be executed.

It is not possible to prove in general whether or not code is reachable. The compiler can have some simple, understandable and fast-to-check rules that can detect simple cases of unreachable code. It should not include a slow and complex solving system that only sometimes works.

If you want extra checking then use an external tool.

Answer 3

The second branch will not be executed and the compiler shouldn't warn about unreachable code.