[Bug c/62031] New: Different results between O2 and O3 for gcc-4.7.2-5 (Debian 4.7.2-5)

["tomasz.ostaszewski at interia dot pl" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62031

            Bug ID: 62031
           Summary: Different results between O2 and O3 for gcc-4.7.2-5
                    (Debian 4.7.2-5)
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tomasz.ostaszewski at interia dot pl

Created attachment 33255
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33255&action=edit
problematic code, preprocessed

Hello, 

My little program produces different results when compiled with O2 and O3
options. Turning on/off '-fstrict-aliasing' does not make any difference. 

#include <stdlib.h>
#include <stdio.h>

#define NUM_OF_STATES 4
typedef unsigned int entry_t[2];
typedef struct entries_item { entry_t metricEntries_[0]; } entries_item_t;

void test_00(size_t numOfStates, entries_item_t* p_bm, const unsigned int*
polyArray,
                      size_t polyArraySize) {
    size_t idx;
    unsigned int hlp0, hlp1;
    for (idx = 0; idx < numOfStates; ++idx) {
        size_t idy;

        hlp0 = (idx << 1) | 0x00;
        hlp1 = (idx << 1) | 0x01;
        p_bm->metricEntries_[idx][0] = 0;
        p_bm->metricEntries_[idx][1] = 0;
        for (idy = 0; idy < polyArraySize; ++idy) {
            p_bm->metricEntries_[idx][0] |= __builtin_parity(hlp0 &
polyArray[idy]) << idy;
            p_bm->metricEntries_[idx][1] |= __builtin_parity(hlp1 &
polyArray[idy]) << idy;
#if 0
            fprintf(stderr, "%4.4u : %8.8x %8.8x\n", __LINE__,
p_bm->metricEntries_[idx][0], p_bm->metricEntries_[idx][1]);
#endif
        }
    }
    fprintf(stderr, "%4.4u :", __LINE__);
    for (idx = 0; idx < numOfStates; ++idx) {
        fprintf(stderr, "%2u:%2u ", p_bm->metricEntries_[idx][0],
p_bm->metricEntries_[idx][1]);
    }
    fprintf(stderr, "\n");
}

int main(__attribute__((unused)) int argc, __attribute__((unused)) char** argv)
{
    unsigned int polyArray[] = { 0x07, 0x05 };
    entries_item_t* pBranchMetrics;

    pBranchMetrics = malloc(sizeof(entry_t) * NUM_OF_STATES);
    test_00(NUM_OF_STATES, pBranchMetrics, polyArray, sizeof(polyArray) /
sizeof(polyArray[0]));
    free(pBranchMetrics);
    return 0;
}

When compiled with O2 produces a correct result:
0029 : 0: 3  1: 2  3: 0  2: 1 

When compiled with O3 produces an incorrect result:
0029 : 0: 2  0: 2  2: 0  2: 0 

Suprisingly, changing #if 0 to #if 1 forces program to produce correct results.
Also adding the 'static' keyword before 'void test_00' also changes program to
produce a correct result. Also, changing zero sized array to fixed array (size
1) also makes the problem go away. 

Valgrind does not report any violations in either case (O2/O3). Both gcc-4.6.3
and clang 3.5.0 produce correct results with O3. 

problematic gcc version: 
$ gcc --verbose
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i486-linux-gnu/4.7/lto-wrapper
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.2-5'
--with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --enable-objc-gc --enable-targets=all --with-arch-32=i586
--with-tune=generic --enable-checking=release --build=i486-linux-gnu
--host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.7.2 (Debian 4.7.2-5) 

Assembler:
$as --version
GNU assembler (GNU Binutils for Debian) 2.22
Copyright 2011 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or later.
This program has absolutely no warranty.
This assembler was configured for a target of `i486-linux-gnu'.

System:
Linux debian-vm-x86 3.2.0-4-686-pae #1 SMP Debian 3.2.60-1+deb7u3 i686
GNU/Linux

LibC:
 /lib/i386-linux-gnu/libc-2.13.so 
GNU C Library (Debian EGLIBC 2.13-38+deb7u3) stable release version 2.13, by
Roland McGrath et al.
Copyright (C) 2011 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.
Compiled by GNU CC version 4.4.7.
Compiled on a Linux 3.2.57 system on 2014-07-09.
Available extensions:
    crypt add-on version 2.1 by Michael Glad and others
    GNU Libidn by Simon Josefsson
    Native POSIX Threads Library by Ulrich Drepper et al
    BIND-8.2.3-T5B
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
<http://www.debian.org/Bugs/>.