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; Improved commentary in the variable constprop mechanism

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* lisp/emacs-lisp/byte-opt.el (byte-optimize--lexvars)
(byte-optimize--vars-outside-condition)
(byte-optimize-form-code-walker, byte-optimize-let-form):
Clarify various aspects in the variable constant-propagation code,
as kindly pointed out by Stefan Monnier.
This commit is contained in:
Mattias Engdegård 2021-02-07 10:35:36 +01:00
parent 06e1e5eeac
commit 765ffeb545
1 changed files with 13 additions and 8 deletions
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21
lisp/emacs-lisp/byte-opt.el
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@ -378,16 +378,17 @@ very chatty, but can be useful for debugging.")
(defvar byte-optimize--lexvars nil
"Lexical variables in scope, in reverse order of declaration.
Each element is on the form (NAME CHANGED [VALUE]), where:
Each element is on the form (NAME KEEP [VALUE]), where:
NAME is the variable name,
CHANGED is a boolean indicating whether it's been changed (with setq),
KEEP is a boolean indicating whether the binding must be retained,
VALUE, if present, is a substitutable expression.
Earlier variables shadow later ones with the same name.")
(defvar byte-optimize--vars-outside-condition nil
"Alist of variables lexically bound outside conditionally executed code.
Variables here are sensitive to mutation inside the condition, since such
changes may not be effective for all code paths.
Variables here are sensitive to mutation inside the conditional code,
since their contents in sequentially later code depends on the path taken
and may no longer be statically known.
Same format as `byte-optimize--lexvars', with shared structure and contents.")
(defvar byte-optimize--vars-outside-loop nil
@ -507,7 +508,9 @@ Same format as `byte-optimize--lexvars', with shared structure and contents.")
(`(,(or 'and 'or) . ,exps) ; Remember, and/or are control structures.
;; FIXME: We have to traverse the expressions in left-to-right
;; order, but doing so we miss some optimisation opportunities:
;; order (because that is the order of evaluation and variable
;; mutations must be found prior to their use), but doing so we miss
;; some optimisation opportunities:
;; consider (and A B) in a for-effect context, where B => nil.
;; Then A could be optimised in a for-effect context too.
(let ((tail exps)
@ -592,7 +595,7 @@ Same format as `byte-optimize--lexvars', with shared structure and contents.")
;; Needed as long as we run byte-optimize-form after cconv.
(`(internal-make-closure . ,_)
;; Look up free vars and mark them as changed, so that they
;; Look up free vars and mark them to be kept, so that they
;; won't be optimised away.
(dolist (var (caddr form))
(let ((lexvar (assq var byte-optimize--lexvars)))
@ -627,10 +630,11 @@ Same format as `byte-optimize--lexvars', with shared structure and contents.")
;; We are in conditional code and the variable was
;; bound outside: cancel substitutions.
(setcdr (cdr lexvar) nil)
;; Set a new value (if substitutable).
(setcdr (cdr lexvar)
(and (byte-optimize--substitutable-p value)
(list value))))
(setcar (cdr lexvar) t)) ; Mark variable as changed.
(setcar (cdr lexvar) t)) ; Mark variable to be kept.
(push var var-expr-list)
(push value var-expr-list))
(setq args (cddr args)))
@ -735,8 +739,9 @@ Same format as `byte-optimize--lexvars', with shared structure and contents.")
(let* ((opt-body (byte-optimize-body (cdr form) for-effect))
(bindings nil))
(dolist (var let-vars)
;; VAR is (NAME EXPR [CHANGED [VALUE]])
;; VAR is (NAME EXPR [KEEP [VALUE]])
(if (and (nthcdr 3 var) (not (nth 2 var)))
;; Value present and not marked to be kept: eliminate.
(when byte-optimize-warn-eliminated-variable
(byte-compile-warn "eliminating local variable %S" (car var)))
(push (list (nth 0 var) (nth 1 var)) bindings)))