The compiler cannot detect the trip count statically. To fix: Specify the expected number of iterations using a directive: #pragma loop_count.

Example

...
// Iterate through a loop a minimum of three, maximum of ten, and average of five times
#pragma loop_count min(3), max(10), avg(5)
for (int i=start;i<=end;i++)
...

#include <stdio.h>

int mysum(int start, int end, int a)
{
    int iret=0;
    // Iterate through a loop a minimum of three, maximum of ten, and average of five times
    #pragma loop_count min(3), max(10), avg(5)
    for (int i=start;i<=end;i++)
        iret += a;
    return iret;
}

int main()
{
    int t;
    t = mysum(1, 10, 3);
    printf("t1=%d\r\n",t);
    t = mysum(2, 6, 2);
    printf("t2=%d\r\n",t);
    t = mysum(5, 12, 1);
    printf("t3=%d\r\n",t);
}