c - Why is do...while slower than while when using gcc optimization

I was playing around with optimization for C programs and used some code for (naively) calculating primes:

#include 

int is_prime(int x) {
    int divisor = 2;

    if(x <= 1)
        return(0);
    if(x == 2)

        return(1);
    while(divisor * divisor <= x) {
        if(x % divisor ==  0)
            return(0);
        divisor++;
    }
    return(1);
}

int main(void) {

    for(int i = 0; i <= 10000000; i++)
        if(is_prime(i))
            printf("%d is a prime number.\n", i);
}

I thought I could boost the performance a little bit by using a do...while loop instead of the while loop, so that the while condition will never be executed for an even x. So I changed the is_prime() function to this:

int is_prime(int x) {
    int divisor = 2;


    if(x <= 1)
        return(0);
    if(x == 2)
        return(1);
    do {
        if(x % divisor ==  0)
            return(0);
        divisor++;
    } while(divisor * divisor <= x);

    return(1);
}

When I compile both version without optimizations (gcc main.c -o main) time ./main > /dev/null takes about 5.5s for both, where it looks like the do..while version performs a tiny bit better (maybe ~40ms).

When I optimize (gcc main.c -O3 -o main), I get a strong difference between the two. time ./main > /dev/null gives me ~5.4s for the do...while version and ~4.9s for the while version.

How can this be explained? Is the gcc compiler just not as well optimized for do..while loops? Should I thus always use while loops instead?

---

My CPU is an Intel i5-4300M.

My gcc version is gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0.

I have now tested the clang compiler (clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final); clang main.c -O3 -o main) as well. Here the results are as expected: time ./main > /dev/null takes about 6.5s for the do...while version and 6.8s for the while version.