Typical Advice

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

Why is the Compiler giving this Advice ?

The compiler is inserting a special function call within a loop, which prevents the software pipelining optimization. Software pipelining is a key compiler optimization for achieving good performance. You are getting this Advice so you can take steps to potentially prevent this pipeline disqualification.

What it means:

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() (_divi() in COFF) function to perform 32-bit integer divide operation. Such functions are called compiler helper functions, and result in a function call withing 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;
     }
}

Any call in a loop, including calls to compiler helper functions, disables software pipelining, and this may result in a drop in performance.

Risks, Severity

If this condition occurs, the compiler cannot perform the software pipelining loop optimization, which is crucial to getting good performance.

Suggested Action

Rewrite code using simpler operation(s).

More Resources

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