SPRUI04 User guide

SPRUI04F july 2015 – april 2023

4.15.11 Advice #30005 Prevent Loop Disqualification due to arithmetic operation

advice #30005: Loop at line 5 cannot be scheduled efficiently, as it 
              contains a "division" operation. Rewrite using simpler 
              operations if possible.

The compiler can insert calls to special functions in the run-time support library (RTS) to support operations that are not natively supported by the ISA. For example, the compiler calls __c6xabi_divi() function to perform 32-bit integer divide operation. Such functions are called compiler helper functions, and result in a function call within the loop body. In the example below, the compiler will accomplish the division operation by calling the compiler helper function "_divi" :

void func(float *p, float n)
{
        int i;
 
        for (i = 1; i < 1000; i++)   {
            p[i] /= n;
        }
}

However if we modify this loop, like below, the loop pipelines :

void func_adjusted(float *p, float n)
{
        int i;
 
        float inv = 1/n;
 
        for (i = 1; i < 1000; i++)    {
            p[i] *= inv;
        }
}