bug#42147: 28.0.50; pure vs side-effect-free, missing optimizations?

[Andrea Corallo via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi all,

I was looking in byte-opt.el to how are classified a number of
functions.

My understanding is that pure functions should be technically a subset
of side-effect-and-error-free for which the environment has no influence
on the value the function evaluates to.  For this reason they can be
constant folded in the compile time if possible.

Now in pure functions I see we do not have a lot of functions that (to
my understanding) would classify for that.  I'm thinking to simple
predicates acting on immutable objects as consp or fixnump to give an
example.

Shouldn't we move these into the pure class?

ATM for instance this does not get optimized:

(defun foo ()
  (fixnump 3))

In case this makes sense I'll be happy to work on it and prepare a
patch.

Thanks

  Andrea