Functions don't need to be accessed through them symbol. A file compiler can assume that a function does not change inside a file, etc. Inlining and being called via a symbol are slightly different things...

I would also think that lexical functions are not called via symbols...

I understand what they do, and I appreciate that it helps performance to inline them. What I don't understand is the (setf vref) syntax. I thought that after a defun statement you had to have a symbol, not an S-expression? Something like so:

    (defun-inline set-vref (new-value chip x y)
       ...)

And how would you actually call the original function to set a pixel? (I'm not very familiar with inlines, maybe that's my problem here.)

Common lisp has this universal setter-macro, SETF, which can be used to set values to all sorts of places.

In this case you would use

    (setf (vref chip x y) new-value)

to call the function (just like with AREF above). Using SETF has the advantage that you can define "modify macros" that read and write a place. For example,

    (incf (vref chip x y) delta)

would increment the place by DELTA. Or

    (rotatef (vref chip x1 y1)
             (vref chip x2 y2))

would swap two pixels.

This is special syntax in Common Lisp. (SETF F) actually names a function too. It allows you to define a setter (a setf-expansion) with a normal DEFUN.

It would be interesting to do a measurement of inline vs non-inline to see how much of a speed difference it makes.