GNU bug report logs - #18968
25.0.50; unhelpful "unused" warning from byte compiler

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Tom Tromey <tom <at> tromey.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 25.0.50; unhelpful "unused" warning from byte compiler
Date: Wed, 05 Nov 2014 19:45:36 -0700

Byte-compile this program:

(defvar q-x)
(defun z ()
  (pop (aref q-x 0))
  nil)

I get:

q.el:2:8:Warning: value returned from (aref v 0) is unused

I think this warning is not very useful; while it is true that the aref
result is not used, it's still convenient to use 'pop', there isn't a
convenient substitute, and so there isn't anything useful to be done
about this warning.




In GNU Emacs 25.0.50.7 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.9)
 of 2014-11-02 on bapiya
Repository revision: 118270 larsi <at> gnus.org-20141103010307-bxryy53hat500i4w
Windowing system distributor `Fedora Project', version 11.0.11404000
Configured using:
 `configure --prefix=/home/tromey/Emacs/install'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  shell-dirtrack-mode: t
  diff-auto-refine-mode: t
  flyspell-mode: t
  which-function-mode: t
  global-auto-revert-mode: t
  desktop-save-mode: t
  erc-services-mode: t
  erc-list-mode: t
  erc-menu-mode: t
  erc-autojoin-mode: t
  erc-ring-mode: t
  erc-networks-mode: t
  erc-pcomplete-mode: t
  erc-track-mode: t
  erc-match-mode: t
  erc-button-mode: t
  erc-fill-mode: t
  erc-stamp-mode: t
  erc-netsplit-mode: t
  erc-irccontrols-mode: t
  erc-noncommands-mode: t
  erc-move-to-prompt-mode: t
  erc-readonly-mode: t
  savehist-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  auto-fill-function: do-auto-fill

Recent input:
- a l l SPC ' ( p o p SPC ( a r e f SPC v e c SPC 0 
) ) M-b M-d 6 C-e <backspace> ) ) C-j C-z n C-x b b 
y t <tab> <tab> - c o <backspace> <backspace> c <backspace> 
<backspace> c o m <tab> <return> C-s v a l u e SPC 
r e t u r n e d C-s M-x l g r e p <return> v a l u 
e SPC r e t u r n e d <return> <return> <return> C-x 
o C-x 1 C-u C-u C-n C-u C-n C-n C-c C-c C-x C-g C-s 
C-w C-w C-a C-u C-p C-x 1 C-n C-l C-s e r r o r - f 
r e e C-s C-s C-r C-r C-r C-r C-x b q . e <tab> <return> 
C-g C-x C-f M-p M-p M-p M-p M-p M-p M-n <return> C-e 
C-a C-o ( d e f u n SPC z SPC ( ) C-n TAB C-e <return> 
C-p C-p C-a C-o ( d e f v a r SPC x ) C-n C-n C-e M-b 
5 SPC M-d x C-e <return> n i l ) <return> C-f C-x C-o 
C-x C-s <switch-frame> <switch-frame> M-< C-k C-k C-n 
C-x C-s C-z n C-z n C-p C-o ( d e f u n <backspace> 
v <backspace> <backspace> v a r SPC q q <backspace> 
- x ) C-n C-n C-b C-b q - C-x C-s C-a C-k <tab> ( p 
o p SPC q - x ) C-x C-s <switch-frame> C-z n M-b ( 
<backspace> M-b ( a r e f SPC C-e C-b C-e SPC 0 ) C-x 
C-s <switch-frame> <switch-frame> M-b C-b C-t C-t C-x 
C-s C-z n M-x r e p o r <tab> <return>

Recent messages:
Wrote /home/tromey/Emacs/ET/q.el
Saving file /home/tromey/Emacs/ET/q.el...
Wrote /home/tromey/Emacs/ET/q.el
Auto-saving...done
Saving file /home/tromey/Emacs/ET/q.el...
Wrote /home/tromey/Emacs/ET/q.el
Saving file /home/tromey/Emacs/ET/q.el...
Wrote /home/tromey/Emacs/ET/q.el
Saving file /home/tromey/Emacs/ET/q.el...
Wrote /home/tromey/Emacs/ET/q.el

Load-path shadows:
/home/tromey/.emacs.d/elpa/css-mode-1.0/css-mode hides /home/tromey/Emacs/install/share/emacs/25.0.50/lisp/textmodes/css-mode
/home/tromey/.emacs.d/elpa/bubbles-0.5/bubbles hides /home/tromey/Emacs/install/share/emacs/25.0.50/lisp/play/bubbles

Features:
(mailalias view xterm shadow emacsbug python json elcomp mail-hist
find-dired pcmpl-unix pcmpl-gnu shell make-mode flow-fill mule-util
bbdb-gui bbdb-hooks sort gnus-cite smiley shr-color color shr browse-url
mm-archive gnus-async gnus-bcklg qp gnus-ml disp-table gnus-topic
nndraft nnmh nnfolder utf-7 bbdb-gnus bbdb-snarf mail-extr bbdb-com
warnings cl gnutls network-stream starttls gnus-agent gnus-srvr
gnus-score score-mode nnvirtual gnus-msg nntp gnus-cache gnus-registry
registry eieio-base gnus-art mm-uu mml2015 mm-view mml-smime smime dig
mailcap gnus-sum gnus-group gnus-undo smtpmail sendmail gnus-start
gnus-cloud nnimap nnmail mail-source tls utf7 netrc nnoo parse-time
gnus-spec gnus-int gnus-range gnus-win gnus gnus-ems nnheader idutils
derived log-edit message rfc822 mml mml-sec mm-decode mm-bodies
mm-encode mail-parse rfc2231 rfc2047 rfc2045 ietf-drums mailabbrev
mail-utils gmm-utils mailheader pcvs-util eieio-opt speedbar sb-image
ezimage dframe dabbrev help-mode debug copyright misearch multi-isearch
edebug texinfo sh-script smie executable sgml-mode org-element org-rmail
org-mhe org-irc org-info org-gnus org-docview doc-view image-mode
org-bibtex bibtex org-bbdb org-w3m org org-macro org-footnote
org-pcomplete org-list org-faces org-entities noutline outline
org-version ob-emacs-lisp ob ob-tangle ob-ref ob-lob ob-table ob-exp
org-src ob-keys ob-comint ob-core ob-eval org-compat org-macs
org-loaddefs smerge-mode diff-mode easy-mmode bug-reference goto-addr
add-log vc-arch vc-mtn vc-hg jka-compr dired vc-sccs vc-svn vc-cvs
vc-rcs vc-bzr cc-mode cc-fonts cc-guess cc-menus cc-cmds vc-git flyspell
ispell diminish projectile edmacro kmacro pkg-info find-func lisp-mnt
epl grep compile dash s appt diary-lib diary-loaddefs cal-menu calendar
cal-loaddefs which-func imenu minimap autorevert filenotify desktop
frameset cus-start cus-load status erc-services erc-list erc-menu
erc-join erc-ring erc-networks erc-pcomplete pcomplete erc-track
erc-match erc-button wid-edit erc-fill erc-stamp erc-netsplit
erc-goodies erc erc-backend erc-compat format-spec auth-source cl-macs
gv eieio byte-opt bytecomp byte-compile cl-extra cconv eieio-core
cl-loaddefs cl-lib gnus-util mm-util mail-prsvr password-cache thingatpt
pp advice help-fns vc-dir ewoc vc vc-dispatcher cc-styles cc-align
cc-engine cc-vars cc-defs bbdb timezone ange-ftp comint ansi-color ring
server savehist dwarf-mode-autoloads gdb-shell-autoloads
jabber-autoloads lisppaste-autoloads pydoc-info-autoloads info-look info
easymenu weblogger-autoloads package epg-config bbdb-autoloads time-date
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel x-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core frame cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote make-network-process dbusbind
gfilenotify dynamic-setting system-font-setting font-render-setting
move-toolbar gtk x-toolkit x multi-tty emacs)

Memory information:
((conses 16 1079236 147373)
 (symbols 48 101416 18)
 (miscs 40 42437 4907)
 (strings 32 315780 71503)
 (string-bytes 1 7486012)
 (vectors 16 97717)
 (vector-slots 8 2099698 83288)
 (floats 8 573 1002)
 (intervals 56 64171 4427)
 (buffers 976 232)
 (heap 1024 260330 33179))

Tom

Message #8 received at 18968 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Tom Tromey <tom <at> tromey.com>
Cc: 18968 <at> debbugs.gnu.org
Subject: Re: bug#18968: 25.0.50; unhelpful "unused" warning from byte compiler
Date: Wed, 05 Nov 2014 22:28:50 -0500

> Byte-compile this program:

> (defvar q-x)
> (defun z ()
>   (pop (aref q-x 0))
>   nil)

> I get:

> q.el:2:8:Warning: value returned from (aref v 0) is unused

> I think this warning is not very useful;

I'd tend to agree, tho it turned out it was useful because it pointed
out that (aref v 0) was called twice by the macro-expanded code instead
of only once.

So I installed the patch below which should fix the underlying problem.


        Stefan


=== modified file 'lisp/subr.el'
--- lisp/subr.el	2014-10-30 05:15:28 +0000
+++ lisp/subr.el	2014-11-06 03:25:36 +0000
@@ -169,7 +169,8 @@
          ;; So we can use `pop' in the bootstrap before `gv' can be used.
          (list 'prog1 place (list 'setq place (list 'cdr place)))
        (gv-letplace (getter setter) place
-         `(prog1 ,getter ,(funcall setter `(cdr ,getter)))))))
+         (macroexp-let2 macroexp-copyable-p x getter
+           `(prog1 ,x ,(funcall setter `(cdr ,x))))))))
 
 (defmacro when (cond &rest body)
   "If COND yields non-nil, do BODY, else return nil.

Message #11 received at 18968 <at> debbugs.gnu.org (full text, mbox):

From: Tom Tromey <tom <at> tromey.com>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Tom Tromey <tom <at> tromey.com>, 18968 <at> debbugs.gnu.org
Subject: Re: bug#18968: 25.0.50; unhelpful "unused" warning from byte compiler
Date: Thu, 06 Nov 2014 07:37:14 -0700

Stefan> I'd tend to agree, tho it turned out it was useful because it pointed
Stefan> out that (aref v 0) was called twice by the macro-expanded code instead
Stefan> of only once.

Thanks for looking deeper.
That hadn't occurred to me.

Stefan> So I installed the patch below which should fix the underlying problem.

It worked for me.

Tom

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