Bug#627795: g++-4.6 miscompiles reverse_iterator

[Jakub Wilk <jwilk@debian.org>]

Package: g++-4.6
Version: 4.6.0-8
Severity: important

The attached test program is supposed to print -42, and does so when 
compiled with g++-4.4 or g++-4.5. However, it outputs random junk when 
compiled with g++-4.6:

$ g++ -Wall -O2 test.cxx
$ ./a.out
31
$ ./a.out
207
$ ./a.out
79


We've discussed this code on #debian-devel and nobody could find any bug 
in it, so the only explanation is that it's g++ that miscompiles it.


-- System Information:
Debian Release: wheezy/sid
  APT prefers unstable
  APT policy: (990, 'unstable'), (500, 'experimental')
Architecture: i386 (x86_64)

Kernel: Linux 2.6.38-2-amd64 (SMP w/2 CPU cores)
Locale: LANG=C, LC_CTYPE=pl_PL.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages g++-4.6 depends on:
ii  gcc-4.6                   4.6.0-8        The GNU C compiler
ii  gcc-4.6-base              4.6.0-8        The GNU Compiler Collection (base 
ii  libc6                     2.13-4         Embedded GNU C Library: Shared lib
ii  libcloog-ppl0             0.15.9-3       the Chunky Loop Generator (runtime
ii  libgmp10                  2:5.0.1+dfsg-7 Multiprecision arithmetic library
ii  libgmpxx4ldbl             2:5.0.1+dfsg-7 Multiprecision arithmetic library 
ii  libmpc2                   0.9-3          multiple precision complex floatin
ii  libmpfr4                  3.0.1-3        multiple precision floating-point 
ii  libppl-c4                 0.11.2-3       Parma Polyhedra Library (C interfa
ii  libppl9                   0.11.2-3       Parma Polyhedra Library (runtime l
ii  libstdc++6-4.6-dev        4.6.0-8        The GNU Standard C++ Library v3 (d
ii  zlib1g                    1:1.2.5.dfsg-1 compression library - runtime

Versions of packages g++-4.6 suggests:
pn  g++-4.6-multilib              <none>     (no description available)
pn  gcc-4.6-doc                   <none>     (no description available)
ii  libstdc++6-4.6-dbg            4.6.0-8    The GNU Standard C++ Library v3 (d

--
Jakub Wilk

#include <iostream>

int f()
{
    unsigned char data[3] = { 42, 42, 42 };
    unsigned char *begin = data;
    unsigned char *end = data + 3;
    std::reverse_iterator<unsigned char *> reversed(end);
    for (; begin < end; begin++, reversed++)
        if (*reversed != 42)
            return *reversed;
    return -42;
}

int main(int argc, char ** argv)
{
    std::cout << f() << "\n";
}