Unexpected Optimization #1: avoiding denormals

I’ve been noticing a strange spike in CPU usage while playing with Twytch. When a synth voice rings out and is not producing sound anymore, it is killed and reused later to save CPU cycles. However, there’s a significant increase in CPU usage right before a voice dies. For a while this was a mystery, but I was recently reading some filter code in another project that added a very small alternating current to the input of the filter. In the comment it said “Hack to prevent denormals”.

A denormal is a type of really small floating point number. Wikipedia give a better explanation, but in short the structure of floating point numbers cause the gap from 0 to the smallest positive number to be much larger than the gap between the two smallest positive numbers. Architectures compensate for this by setting aside some numbers that are more evenly spaced really close to zero called “denormal numbers” or “subnormal numbers”. The problem with this in synth development, is that processing subnormal numbers is much slower than normal floating point numbers. Since we create really small numbers any time there’s an exponential decay like in an envelope or a filter, we need to disable denormals.

The alternative to having denormals is flushing those numbers to zero. Since these numbers are so small these jumps are imperceptible in an audio signal. All we have to do is slap an environment variable setting and we should be good to go! (on some machines)

#include <fenv.h>
// ...