[Bug c/49863] New: -Wunused-result not behaving correctly

["antomicx at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49863

           Summary: -Wunused-result not behaving correctly
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: antomicx@gmail.com


With a snippet as simple as the following

# cat main.c

#include <stdlib.h>

int main() {
  system("ls");

  return 0;
}

I'm getting these, different, compilation results:

1) # gcc main.c
Output: empty

2) # gcc -Wunused-result main.c
Output: empty

3) # gcc -W -Wall -Wextra main.c
Output: empty

4) # gcc -W -Wall -Wextra -Wunused-result main.c
Output: empty

5) # gcc -O0 -W -Wall -Wextra -Wunused-result main.c 
Output: empty

6) # gcc -O1 main.c # same thing with -O2 -O3
Output: 
main.c: In function ‘main’:
main.c:4:9: warning: ignoring return value of ‘system’, declared with attribute
warn_unused_result

Why is putting -Ox with x > 0 the only way to get the correct compilation
warning when invoking the 'system' call without parsing its result?

# gcc -v

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.5.2-8ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-4.5/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.5 --enable-shared --enable-multiarch
--with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.5 --libdir=/usr/lib/x86_64-linux-gnu
--enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-plugin --enable-gold --enable-ld=default
--with-plugin-ld=ld.gold --enable-objc-gc --disable-werror --with-arch-32=i686
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4)