[Bug c/29605] New: [4.0 Regression] optimization breaks global variable affectation

["clement dot vasseur at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

The following code does not produce the same result when compiled with
optimization.

Looking at the generated assembler code, it looks like the "value = 0"
affectation in the "test" function is not emitted when using -Os, -O1 and -O2.
It works fine with -O0 and -O3.

----------------------
int value = 0;

void modify(int arg)
{
 if (arg != 42)
  return;
 value = 1;
}

void test(void)
{
 if (value)
  return;
 modify(42);
 value = 0;
}

int main(void)
{
 test();
 return value;
}
----------------------

Usage:
$ gcc -O0 test.c ; ./a.out ; echo $?
0
$ gcc -O1 test.c ; ./a.out ; echo $?
1

More information about the gcc build:

Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --enable-languages=c,c++,java,f95
--prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--program-suffix=-4.0 --enable-__cxa_atexit --enable-clocale=gnu
--enable-libstdcxx-debug --enable-java-awt=gtk-default --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-4.0-1.4.2.0/jre --enable-mpfr
--enable-checking=release x86_64-linux-gnu
Thread model: posix
gcc version 4.0.4 20060630 (prerelease) (Ubuntu 4.0.3-4)


GCC versions 3.4.6 and 4.1.1 appear to work fine.


-- 
           Summary: [4.0 Regression] optimization breaks global variable
                    affectation
           Product: gcc
           Version: 4.0.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: clement dot vasseur at gmail dot com
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29605