bug#51075: 29.0.50; Uninitialised variable warning in src/term.c

bug#51075: 29.0.50; Uninitialised variable warning in src/term.c

[Basil L. Contovounesios via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Severity: minor

I'm currently seeing the following warning with GCC 10.3.0:

In file included from term.c:30:
term.c: In function ‘encode_terminal_code’:
lisp.h:1853:35: warning: ‘gstring’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 1853 |   return XVECTOR (array)->contents[idx];
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
term.c:553:16: note: ‘gstring’ was declared here
  553 |    Lisp_Object gstring;
      |                ^~~~~~~

I'm guessing it started with the following recent change:

Remove encode_terminal_code UNINITs
7e871dcd27 2021-10-03 17:52:28 -0700
https://git.sv.gnu.org/cgit/emacs.git/commit/?id=7e871dcd27

What's the best way to pacify it?  Do we care about doing so in GCC 10?

Is

  cmp = composition_table[src->u.cmp.id];

indeed guaranteed to be non-NULL?  If so, maybe gstring could be
initialised as nil and then easserted as non-nil in the !cmp branch
(eassuming cmp is non-NULL in the !src->u.cmp.automatic branch does not
make the warning go away).  If not, could there indeed be a bug?

Thanks,

-- 
Basil

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo version 1.16.0, Xaw3d scroll bars)
 of 2021-10-07 built on tia
Repository revision: e338effc774f9a7cfe13d4a35dc1cf493a22c972
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-Og -ggdb3' --config-cache --prefix
 /home/blc/.local --enable-checking=structs --with-x-toolkit=lucid
 --with-file-notification=yes --with-x'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
X11 XAW3D XDBE XIM XPM LUCID ZLIB

Important settings:
  value of $LANG: en_IE.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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  electric-indent-mode: t
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Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg rfc6068 epg-config gnus-util rmail
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Memory information:
((conses 16 50810 6940)
 (symbols 48 6621 1)
 (strings 32 18974 1858)
 (string-bytes 1 613497)
 (vectors 16 13616)
 (vector-slots 8 184446 10452)
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 (intervals 56 231 0)
 (buffers 992 10))

bug#51075: 29.0.50; Uninitialised variable warning in src/term.c

[Eli Zaretskii]

> Cc: Paul Eggert <eggert@cs.ucla.edu>
> Date: Thu, 07 Oct 2021 12:18:14 +0100
> From:  "Basil L. Contovounesios" via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> I'm currently seeing the following warning with GCC 10.3.0:
> 
> In file included from term.c:30:
> term.c: In function ‘encode_terminal_code’:
> lisp.h:1853:35: warning: ‘gstring’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>  1853 |   return XVECTOR (array)->contents[idx];
>       |          ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
> term.c:553:16: note: ‘gstring’ was declared here
>   553 |    Lisp_Object gstring;
>       |                ^~~~~~~

It's a bogus warning.  It might be worth reporting to the GCC folks
(unless GCC 11 already fixed that).

> Is
> 
>   cmp = composition_table[src->u.cmp.id];
> 
> indeed guaranteed to be non-NULL?

If it's ever NULL, the very next line will segfault:

	      cmp = composition_table[src->u.cmp.id];
	      required = cmp->glyph_len;

> If so, maybe gstring could be
> initialised as nil and then easserted as non-nil in the !cmp branch
> (eassuming cmp is non-NULL in the !src->u.cmp.automatic branch does not
> make the warning go away).  If not, could there indeed be a bug?

Feel free to assign some value to gstring in the 'else' clause, to
shut up GCC in this case.

bug#51075: 29.0.50; Uninitialised variable warning in src/term.c

[Paul Eggert]

[-- Attachment #1: Type: text/plain, Size: 719 bytes --]

On 10/7/21 06:01, Eli Zaretskii wrote:

> It's a bogus warning.  It might be worth reporting to the GCC folks
> (unless GCC 11 already fixed that).

Yes, I believed it's fixed in GCC 11. GCC 11.2.1 20210728 (Red Hat 
11.2.1-1) does not warn me about it.

> Feel free to assign some value to gstring in the 'else' clause, to
> shut up GCC in this case.

I installed into the emacs-28 branch the attached, which fixes it in a 
different way to make it a bit clearer to the reader that we're 
pacifying an inadequate compiler rather than executing odd code for an 
unknown reason.

There are a handful of other bogus warnings with GCC 11.2.1, which I'll 
try to get around to pacifying (I hope before GCC 12 comes out :-).

[-- Attachment #2: 0001-Pacify-GCC-10.3-Wmaybe-uninitialized.patch --]
[-- Type: text/x-patch, Size: 781 bytes --]

From ac7bd3f3ec1d12ea255312778d5be674e5fbe293 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Thu, 7 Oct 2021 13:09:19 -0700
Subject: [PATCH] Pacify GCC 10.3 -Wmaybe-uninitialized

Problem reported by Basil L. Contovounesios (Bug#51075).
* src/term.c (encode_terminal_code):
Add an UNINIT to pacify GCC 10 bug.
---
 src/term.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/term.c b/src/term.c
index 0858f81685..6f0b827cfc 100644
--- a/src/term.c
+++ b/src/term.c
@@ -550,7 +550,7 @@ encode_terminal_code (struct glyph *src, int src_len,
       if (src->type == COMPOSITE_GLYPH)
 	{
 	  struct composition *cmp;
-	  Lisp_Object gstring;
+	  Lisp_Object gstring UNINIT;
 	  int i;
 
 	  nbytes = buf - encode_terminal_src;
-- 
2.31.1

bug#51075: 29.0.50; Uninitialised variable warning in src/term.c

[Basil L. Contovounesios via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii [2021-10-07 09:02 -0400] wrote:

>> Cc: Paul Eggert <eggert@cs.ucla.edu>
>> Date: Thu, 07 Oct 2021 12:18:14 +0100
>> From:  "Basil L. Contovounesios" via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>>
>> Is
>> 
>>   cmp = composition_table[src->u.cmp.id];
>> 
>> indeed guaranteed to be non-NULL?
>
> If it's ever NULL, the very next line will segfault:
>
> 	      cmp = composition_table[src->u.cmp.id];
> 	      required = cmp->glyph_len;

Right, I didn't read carefully enough.

Thanks,

-- 
Basil

bug#51075: 29.0.50; Uninitialised variable warning in src/term.c

[Basil L. Contovounesios via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Paul Eggert [2021-10-07 13:13 -0700] wrote:

> On 10/7/21 06:01, Eli Zaretskii wrote:
>
>> It's a bogus warning.  It might be worth reporting to the GCC folks
>> (unless GCC 11 already fixed that).
>
> Yes, I believed it's fixed in GCC 11. GCC 11.2.1 20210728 (Red Hat 11.2.1-1)
> does not warn me about it.
>
>> Feel free to assign some value to gstring in the 'else' clause, to
>> shut up GCC in this case.
>
> I installed into the emacs-28 branch the attached, which fixes it in a different
> way to make it a bit clearer to the reader that we're pacifying an inadequate
> compiler rather than executing odd code for an unknown reason.
>
> There are a handful of other bogus warnings with GCC 11.2.1, which I'll try to
> get around to pacifying (I hope before GCC 12 comes out :-).

Thanks!

-- 
Basil