[rurban]

But this could be solved with escape analysis. Rarely do such let locals shadow outer locals, and as such don't need to be restored.

[dan-robertson]

You can’t know whether you are shadowing an outer variable. With dynamic scope:

  (defun get-x () x)
  (defun foo (let ((x 7)) (get-x)))
  (foo) ;; => 7 (not shadowing x)
  (let ((x 4)) (foo)) ;; => 7 (is shadowing x)
  (setq old-get-x #'get-x)
  (defun get-x () (let ((x 10)) (funcall old-get-x)))
  (foo) ;; => 10

[kazinator]

Emacs Lisp has supported lexical scope in its byte code compiler as an option for eons, and is starting to make more use of that. No reason why a brand new JIT couldn't follow suit.

[rurban]

Ouch, you are right. So the only strategy would be to switch to lexical compilation blockwise, while fixing the remaining dynamic extent vars.

[pzone]

Which is more or less the strategy in the Emacs codebase - the intention is to incrementally port to lexical binding on a per-file basis. (This is seen through the annotation -- lexical-binding:t -- at the top of many .el files.)