Question

I’m calling a function quite frequently in a core audio callback to apply eq to my audio samples. I’m having very strange performance results in this method measured in instruments time profiles.

I’ve done 3 tests. The first calls the function and just returns a zero value.
Instruments reports this as 1%.

inline AudioUnitSampleType LowPassProcess (struct EQEffect *obj , AudioUnitSampleType input)
{
return 0;
}

The second test is actually doing EQ calculations in the function by accessing a struct containing eq parameters.

When doing this instruments reports it as *40%* !

struct globaldata

    {
        float cutoff;
        float fs;
        float f;
        float q;
        float scale;

        AudioUnitSampleType low;
        AudioUnitSampleType high;
        AudioUnitSampleType band;
        AudioUnitSampleType notch;
    };



struct globaldata global;

inline AudioUnitSampleType LowPassProcess (struct EQEffect *obj , AudioUnitSampleType input)
{

    global.low= global.low + (global.f * global.band);
    global.high= global.scale * input -global.low - global.q * global.band;
    global.band = global.f * global.high +global.band;
    global.notch = global.high + global.low;

    return global.low;

};

Finally I tried calling the function again but in this case not accessing the EQ struct but still performing the same number of calculations.

When doing this instruments reports it as 7 %

struct globaldata

        {
            float cutoff;
            float fs;
            float f;
            float q;
            float scale;

            AudioUnitSampleType low;
            AudioUnitSampleType high;
            AudioUnitSampleType band;
            AudioUnitSampleType notch;
        };


    struct globaldata global;

inline AudioUnitSampleType LowPassProcess (struct EQEffect *obj , AudioUnitSampleType input)
{


    float x =10+(50*8);
    float y = ((10 *5) -50)- (6*40);
    float z=10 *(6+9);
    float j=60+0;


    return 0;

};

So my question is why does my function spend up to 5 times longer to execute when I perform calculations on struct members and take far less time when I simply just perform calculations on variables?

Answer 1

I’m guessing the compiler just optimizes away your constant calculations.

In any case, remember that it doesn’t matter if something takes 40% if it’s still fast enough (and I would expect it to be if the constant calculations take 7%). =)