Re: warnings compiling emacs28 with gcc 12.1

[Eli Zaretskii]

> From: "Jose A. Ortega Ruiz" <jao@gnu.org>
> Date: Thu, 28 Jul 2022 04:32:39 +0100
> 
> I compiled today the emacs28 branch with gcc version
> 'gcc (Debian 12.1.0-7) 12.1.0' and configured with

The lesson to be learned here is never use NN.1 version of GCC with
too high level of warnings: you get a lot of noise due to bugs in an
immature GCC version.

>    In file included from intervals.c:45:
>    intervals.c: In function ‘graft_intervals_into_buffer’:
>    intervals.h:101:29: warning: potential null pointer dereference 
> [-Wnull-dereference]
>      101 | #define TOTAL_LENGTH(i) ((i)->total_length)
>          |                         ~~~~^~~~~~~~~~~~~~~

Nonsense.

> and there's another one about free that looks a bit scary:
> 
>    alloc.c: In function ‘lisp_free’:
>    alloc.c:1052:3: warning: pointer ‘block’ used after ‘free’ 
> [-Wuse-after-free]
>     1052 |   mem_delete (mem_find (block));
>          |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    alloc.c:1050:3: note: call to ‘free’ here
>     1050 |   free (block);
>          |   ^~~~~~~~~~~~

Another nonsense.  Look in mem_find, and you will see that it uses its
argument as just a number, it never dereferences it.

> There are also several out-of-bounds access in xdisp.c that look like
> this one:
> 
>    xdisp.c: In function ‘append_space_for_newline’:
>    xdisp.c:22011:7: warning: ‘memset’ offset [2352, 2359] from the object at 
> ‘it’ is out of the bounds of referenced subobject ‘charpos’ with type ‘long 
> int’ at offset 2344 [-Warray-bounds]
>    22011 |       memset (&it->position, 0, sizeof it->position);
>          |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    In file included from composite.h:29,
>                     from xdisp.c:441:
>    dispextern.h:214:13: note: subobject ‘charpos’ declared here
>      214 |   ptrdiff_t charpos;
>          |             ^~~~~~~

I see no problem there.

> And so on.  I guess the whole list is easily reproducible by anyone
> compiling git gcc 12.1 in a GNU/Linux system, so i'm not sending it in
> full.  I can do that in a bug report if you think it's needed.
> 
> Apologies if all this is already well-known/spurious.

I wouldn't worry about these.

Thanks.