bug#17754: Gcc 4.8.2 -Werror=maybe-unused warning in dfa.c

[Jim Meyering]

On Wed, Jun 11, 2014 at 7:09 AM, behoffski <address@hidden> wrote:
> G'day,
>
> Just upgraded to a recent release of gcc (previously I was running
> 4.7.x):
>     $ gcc --version
>     gcc (Gentoo 4.8.2 p1.3r1, pie-0.5.8r1) 4.8.2
>     Copyright (C) 2013 Free Software Foundation, Inc.
>     This is free software; see the source for copying conditions.  There is
> NO
>     warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
> PURPOSE.
>
> It complains about an stack variable, mbclen, possibly being
> used uninitialised in a call to transit_state_consume_1char.
>
> The warning message is (I'm still at 2.20 post-admin commit
> ....78f07b8c8e26):
>
>     make[2]: Entering directory `/home/grep-gnu/grep/src'
>       CC       dfa.o
>     dfa.c: In function 'transit_state':
>     dfa.c:3213:31: error: 'mbclen' may be used uninitialized in this
> function [-Werror=maybe-uninitialized]
>        transit_state_consume_1char (d, s, pp, wc, mbclen, match_lens);
>                                    ^
>     cc1: all warnings being treated as errors
>     make[2]: *** [dfa.o] Error 1
>     make[2]: Leaving directory `/home/grep-gnu/grep/src
>
> The earlier version of gcc did not complain about this code.

Thanks for the report, but...
With gcc-4.9, I don't see that warning, so I suspect gcc's flow
analysis has improved enough that it determines there is no
risk of use-uninitialized there. Also, gcc's 4.8.2->4.8.3 delta
fixed many bugs, so if you require 4.8.x, it's probably
better to use 4.8.3.

When enabling warnings, it's best to use the most recent version
of gcc and e.g., glibc header files possible.  Otherwise, we end
up with false positives like this.