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

[Mattias Engdegård]

3 juli 2020 kl. 21.05 skrev Andrea Corallo <andrea_corallo@yahoo.it>:

> attached the updated version of the patch updating the pure function
> classification.

Thanks Andrea! Philipp Stephani raised the interesting question of 
(essentially) whether 'car' is pure. For the purposes of the current constant 
folding in the byte compiler the answer is yes, but perhaps you have wider 
ambitions in your work?

Clearly, (car X) cannot be moved past some operations with side-effects if X is 
aliased:

(let* ((x (list 'a))
       (y (car x)))
  (f x)
  y)

Here, (car x) cannot be sunk past the call to f despite x remaining unchanged 
(assuming lexical binding).
It would be useful to know more exactly what notion of purity you require.