Attached is a new patch which modifies the byte-interpreter to use C functions for comparing list length to numbers at run-time. It does this by checking if the next bytecode is a comparison op when it encounters a length op. It does not have the problem of incompatibly changing = like the last method. The only remaining incompatible change is forms like (= 0 (length some-long-circular-list)) will now not throw an error, instead they will return nil in byte-compiled code. Similarly for dotted lists. The rare code which does require the error to be thrown can switch to (= 0 (safe-length some-long-circular-list)) to regain the behavior.

This change also makes length= and related then either worse than not having them, or they become trivial:

(defun length= (number list)
(= number (length list)))

Regardless of if this change is acceptable, I also need to get my copyright assignment done. I'm working on another unrelated patch as well.


-- Graham Dobbins