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

[behoffski]

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.

Reading through the code, it looks that the offending line is
only reached if (nelem > 0), and, if so, mbclen is initialised
earlier.  At the expense of some code duplication, or perhaps
the judicious use of a couple of goto's, the
(nelem == 0 || maxlen == 0) block could be split up and rearranged
such that the connection between the preceding paragraph
(nelem > 0) and the following paragraphs (transit_state_consume_1char)
would become more apparent, and the fact that mbclen is always
initialised would become clear, both to the reader and the
compiler.

cheers,

behoffski (Brenton Hoff)
Programmer, Grouse Software