scratch/bytecode-speedup 712237cab6 11/11: ; * lisp/emacs-lisp/byte-opt.el: Remove outdated comments

[Mattias Engdegård]

branch: scratch/bytecode-speedup
commit 712237cab6bc1c4fc326818a5c4e1f9bbdf45de7
Author: Mattias Engdegård <mattiase@acm.org>
Commit: Mattias Engdegård <mattiase@acm.org>

    ; * lisp/emacs-lisp/byte-opt.el: Remove outdated comments
    
    These were optimisation ideas that have been implemented, have become
    irrelevant, or were impractical to begin with.
---
 lisp/emacs-lisp/byte-opt.el | 118 +-------------------------------------------
 1 file changed, 2 insertions(+), 116 deletions(-)

diff --git a/lisp/emacs-lisp/byte-opt.el b/lisp/emacs-lisp/byte-opt.el
index bd57e2b203..4c0b69abeb 100644
--- a/lisp/emacs-lisp/byte-opt.el
+++ b/lisp/emacs-lisp/byte-opt.el
@@ -37,125 +37,11 @@
 
 ;; TO DO:
 ;;
-;; (apply (lambda (x &rest y) ...) 1 (foo))
-;;
-;; maintain a list of functions known not to access any global variables
-;; (actually, give them a 'dynamically-safe property) and then
-;;   (let ( v1 v2 ... vM vN ) <...dynamically-safe...> )  ==>
-;;   (let ( v1 v2 ... vM ) vN <...dynamically-safe...> )
-;; by recursing on this, we might be able to eliminate the entire let.
-;; However certain variables should never have their bindings optimized
-;; away, because they affect everything.
-;;   (put 'debug-on-error 'binding-is-magic t)
-;;   (put 'debug-on-abort 'binding-is-magic t)
-;;   (put 'debug-on-next-call 'binding-is-magic t)
-;;   (put 'inhibit-quit 'binding-is-magic t)
-;;   (put 'quit-flag 'binding-is-magic t)
-;;   (put 't 'binding-is-magic t)
-;;   (put 'nil 'binding-is-magic t)
-;; possibly also
-;;   (put 'gc-cons-threshold 'binding-is-magic t)
-;;   (put 'track-mouse 'binding-is-magic t)
-;; others?
-;;
-;; Simple defsubsts often produce forms like
-;;    (let ((v1 (f1)) (v2 (f2)) ...)
-;;       (FN v1 v2 ...))
-;; It would be nice if we could optimize this to
-;;    (FN (f1) (f2) ...)
-;; but we can't unless FN is dynamically-safe (it might be dynamically
-;; referring to the bindings that the lambda arglist established.)
-;; One of the uncountable lossages introduced by dynamic scope...
-;;
-;; Maybe there should be a control-structure that says "turn on
-;; fast-and-loose type-assumptive optimizations here."  Then when
-;; we see a form like (car foo) we can from then on assume that
-;; the variable foo is of type cons, and optimize based on that.
-;; But, this won't win much because of (you guessed it) dynamic
-;; scope.  Anything down the stack could change the value.
-;; (Another reason it doesn't work is that it is perfectly valid
-;; to call car with a null argument.)  A better approach might
-;; be to allow type-specification of the form
-;;   (put 'foo 'arg-types '(float (list integer) dynamic))
-;;   (put 'foo 'result-type 'bool)
-;; It should be possible to have these types checked to a certain
-;; degree.
-;;
-;; collapse common subexpressions
-;;
-;; It would be nice if redundant sequences could be factored out as well,
-;; when they are known to have no side-effects:
-;;   (list (+ a b c) (+ a b c))   -->  a b add c add dup list-2
-;; but beware of traps like
-;;   (cons (list x y) (list x y))
-;;
-;; Tail-recursion elimination is not really possible in Emacs Lisp.
-;; Tail-recursion elimination is almost always impossible when all variables
-;; have dynamic scope, but given that the "return" byteop requires the
-;; binding stack to be empty (rather than emptying it itself), there can be
-;; no truly tail-recursive Emacs Lisp functions that take any arguments or
-;; make any bindings.
-;;
-;; Here is an example of an Emacs Lisp function which could safely be
-;; byte-compiled tail-recursively:
-;;
-;;  (defun tail-map (fn list)
-;;    (cond (list
-;;           (funcall fn (car list))
-;;           (tail-map fn (cdr list)))))
-;;
-;; However, if there was even a single let-binding around the COND,
-;; it could not be byte-compiled, because there would be an "unbind"
-;; byte-op between the final "call" and "return."  Adding a
-;; Bunbind_all byteop would fix this.
-;;
-;;   (defun foo (x y z) ... (foo a b c))
-;;   ... (const foo) (varref a) (varref b) (varref c) (call 3) END: (return)
-;;   ... (varref a) (varbind x) (varref b) (varbind y) (varref c) (varbind z) 
(goto 0) END: (unbind-all) (return)
-;;   ... (varref a) (varset x) (varref b) (varset y) (varref c) (varset z) 
(goto 0) END: (return)
-;;
-;; this also can be considered tail recursion:
-;;
-;;   ... (const foo) (varref a) (call 1) (goto X) ... X: (return)
-;; could generalize this by doing the optimization
-;;   (goto X) ... X: (return)  -->  (return)
-;;
-;; But this doesn't solve all of the problems: although by doing tail-
-;; recursion elimination in this way, the call-stack does not grow, the
-;; binding-stack would grow with each recursive step, and would eventually
-;; overflow.  I don't believe there is any way around this without lexical
-;; scope.
-;;
-;; Wouldn't it be nice if Emacs Lisp had lexical scope.
-;;
-;; Idea: the form (lexical-scope) in a file means that the file may be
-;; compiled lexically.  This proclamation is file-local.  Then, within
-;; that file, "let" would establish lexical bindings, and "let-dynamic"
-;; would do things the old way.  (Or we could use CL "declare" forms.)
-;; We'd have to notice defvars and defconsts, since those variables should
-;; always be dynamic, and attempting to do a lexical binding of them
-;; should simply do a dynamic binding instead.
-;; But!  We need to know about variables that were not necessarily defvared
-;; in the file being compiled (doing a boundp check isn't good enough.)
-;; Fdefvar() would have to be modified to add something to the plist.
-;;
-;; A major disadvantage of this scheme is that the interpreter and compiler
-;; would have different semantics for files compiled with (dynamic-scope).
-;; Since this would be a file-local optimization, there would be no way to
-;; modify the interpreter to obey this (unless the loader was hacked
-;; in some grody way, but that's a really bad idea.)
-
-;; Other things to consider:
-
-;; ;; Associative math should recognize subcalls to identical function:
-;; (disassemble (lambda (x) (+ (+ (foo) 1) (+ (bar) 2))))
-;; ;; This should generate the same as (1+ x) and (1- x)
-
-;; (disassemble (lambda (x) (cons (+ x 1) (- x 1))))
 ;; ;; An awful lot of functions always return a non-nil value.  If they're
 ;; ;; error free also they may act as true-constants.
-
+;;
 ;; (disassemble (lambda (x) (and (point) (foo))))
+
 ;; ;; When
 ;; ;;   - all but one arguments to a function are constant
 ;; ;;   - the non-constant argument is an if-expression (cond-expression?)