Wireshark · Wireshark-dev: Re: [Wireshark-dev] error: logical && with non-zero constant will always evaluate as true

[Evan Huus <eapache@xxxxxxxxx>]

While 'separators' is not immediately constant either,
wtap_file_read_till_separator (the function containing the strchr) is
called in only one place, and that call passes in a string constant to
the 'separators' parameter.

GCC is smart enough to figure that out, so __builtin_constant_p [1]
returns 1, leading to the warning you're seeing.

>From my perspective, this is a compiler bug. The code as written is
perfectly correct, and GCC is optimizing it and then warning on the
partially-optimized code before completing the optimization (which is
to evaluate the big string of &&'s to false because
__builtin_constant_p(c) is false, resulting in a simple call to
__builtin_strchr (s, c)).

The GCC in squeeze is two patch versions shy of latest in that series
(4.4.5 vs 4.4.7). I don't see any reference to this particular bug in
the changelogs, but it might help to try a newer compiler. If not, I
would suggest filing a bug against Debian's GCC package and let them
figure out who ought to deal with it.

In the meantime, I'm not really sure what the best workaround is...

[1] http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Other-Builtins.html#index-g_t_005f_005fbuiltin_005fconstant_005fp-2868

On Sat, Dec 29, 2012 at 10:17 AM, Martin Kaiser <lists@xxxxxxxxx> wrote:
> Dear all,
>
> I get this warning (error) when I compile svn head
>
> peektagged.c: In function ‘wtap_file_read_till_separator’:
> peektagged.c:150: error: logical ‘&&’ with non-zero constant
> will always
> evaluate as true
> make[2]: *** [libwiretap_la-peektagged.lo] Error 1
>
>
> The offending line is
>
>    if (strchr (separators, c) != NULL)
>
> which seems to evaluate on my system (debian squeeze) to something like
>
>   (__extension__ (__builtin_constant_p (c) && !__builtin_constant_p (s) \
>         && (c) == '\0'                    \
>         ? (char *) __rawmemchr (s, c)                 \
>         : __builtin_strchr (s, c)))
>
>
> I still don't understand the problem. c is not constant, it's read from
> a file.
>
> Commenting out
>
> AC_WIRESHARK_GCC_CFLAGS_CHECK(-Wlogical-op)
>
> in configure.ac will stop the warning.
>
> Any ideas how to fix this properly?
>
> Thanks,
>
>    Martin
>
>
>
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