This is the next tutorial in my Learning XS series. This post explores one of Perl's most powerful optimisation techniques: Custom Ops . We'll build a real-world example, reimplementing one of my first XS cpan modules - a Shannon entropy calculator - and explain how to bypass Perl's subroutine call overhead entirely.

So firstly... What are Custom Ops?

When you call a Perl subroutine, Perl executes an entersub op that:

Sets up a new stack frame

Pushes arguments onto the stack

Magical things happens

Jumps to the subroutine code

More Magical things can happens

Cleans up and returns

For simple functions called millions of times (like mathematical operations), this overhead dominates execution time. Custom ops let you replace the entire entersub call with a single, specialised op - eliminating that overhead.

Next..…