[Bug c/43794] New: miscompile semantically equivalent C source files using -O2

["wuwei08 at iscas dot ac dot cn" <gcc-bugzilla at gcc dot gnu dot org>]

Two semantically equivalent files output different results using gcc 4.5.0
release,while gcc 4.4.1 produced correct results.

here is the command line option used to compile the files:
gcc -O2 MTest-1.c -o MTest-1.exe
gcc -O2 MTest-7.c -o MTest-7.exe
#and the results 
./MTest-1.exe # generate MTest-1.out
./MTest-7.exe # generate MTest-7.out
diff MTest-1.out MTest-7.out

use 
gcc -O2 -fno-strict-aliasing
or
gcc -O2 -fno-strict-pre
can avoid this problem.

I have read the Non-bugs section in http://gcc.gnu.org/bugs/ and I'm sure our
files does not violate the aliasing rule.
the gcc 4.4.1 works correctly using fstrict-aliasing:
gcc-4.4 -O2 -fstrict-aliasing MTest-1.c -o MTest-1.exe

Here is the gcc configure&build options
$../exec/gcc -v
Using built-in specs.
COLLECT_GCC=../exec/bin/gcc
COLLECT_LTO_WRAPPER=/sda3/test-gcc-4.5.0/exec/bin/../libexec/gcc/i686-pc-linux-gnu/4.5.0/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: ../src/configure --prefix=/sda3/test-gcc-4.5.0/obj/../exec
--enable-languages=c
Thread model: posix
gcc version 4.5.0 (GCC) 

I compiled it with gcc 4.4.1:
$gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.1-4ubuntu9'
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--enable-multiarch --enable-linker-build-id --with-system-zlib
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc
--enable-targets=all --disable-werror --with-arch-32=i486 --with-tune=generic
--enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu
--target=i486-linux-gnu
Thread model: posix
gcc version 4.4.1 (Ubuntu 4.4.1-4ubuntu9)


-- 
           Summary: miscompile semantically equivalent C source files using
                    -O2
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: wuwei08 at iscas dot ac dot cn
 GCC build triplet: x86_32-Ubuntu-Linux
  GCC host triplet: x86_32-Ubuntu-Linux
GCC target triplet: x86_32-Ubuntu-Linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43794