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#78394
31.0.50; Questions about native-comp-speed and type decl
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Message #8 received at 78394 <at> debbugs.gnu.org (full text, mbox):
> Date: Mon, 12 May 2025 23:55:01 +0800
> From: "Yue Yi" via "Bug reports for GNU Emacs,
> the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
>
> When modifying an org HTML export backend, I tentatively added type
> annotations to a function to make the code run faster. However, during
> unit testing, byte compilation produced correct results, while native
> compilation did not. Specifically, the following cleaned-up code
> illustrates the issue:
>
> --------------------------->8<-----------------------------
> (defconst my/plist '(:html-checkbox-type unicode))
> (defconst t-checkbox-types
> '(( unicode .
> ((on . "�7�1") (off . "�7�0")
> (trans . "�7�2")))))
>
> (let ((native-comp-speed 2))
> (defun t--checkbox (checkbox info)
> "Format CHECKBOX into HTML."
> (declare (ftype (function (t plist) string))
> (side-effect-free t) (important-return-value t))
> (cdr (assq checkbox
> (cdr (assq (plist-get info :html-checkbox-type)
> t-checkbox-types)))))
>
> (defun t--format-checkbox (checkbox info)
> "Format a CHECKBOX option to string.
>
> CHECKBOX can be `on', `off', `trans', or anything else.
> Returns an empty string if CHECKBOX is not one of the these three."
> (declare (ftype (function (t plist) string))
> (side-effect-free t) (important-return-value t))
> (let ((a (t--checkbox checkbox info)))
> (concat a (and a " "))))
> ;; native comp
> (native-compile 't--checkbox)
> (native-compile 't--format-checkbox))
> --------------------------->8<-----------------------------
>
> AFACK, the default value of `native-comp-speed' is 2. In this case, the
> behavior of the following code is inconsistent with that of the
> byte-compiled or non-compiled code:
>
> --------------------------->8<-----------------------------
> ;; normal
> (t--format-checkbox nil my/plist) ;;=> ""
> ;; byte-code
> (t--format-checkbox nil my/plist) ;;=> ""
> ;; speed 1
> (t--format-checkbox nil my/plist) ;;=> ""
> ;; speed 2
> (t--format-checkbox nil my/plist) ;;=> " "
> --------------------------->8<-----------------------------
>
> Of course, we can notice that the type declaration for `t--checkbox' is
> problematic --- its return type should be (or null string) instead of
> string. After correcting this mistake, the function works properly under
> speed=2.
>
> In C, we generally avoid aggressive optimizations because they can lead
> to unpredictable behavior. What I��d like to ask is whether similar
> situations can occur in Emacs Lisp��s native compilation as well --- like
> the issue I encountered here?
>
> Another question is about `compilation-safety'. As I understand it, when
> set to 1, it prevents Emacs from crashing due to faulty
> optimizations. Does this mean the variable only guards against the most
> severe cases, rather than ensuring the correctness of optimizations in
> general?
I hope Andrea (CC'ed) will be able to answer your questions.
This bug report was last modified 5 days ago.
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