[Bug c/43736] New: Invalid uninitialized variable warning at -O3 -Wall

[dougsemler at gmail dot com]

Invalid warnings about uninitialized variables are given when compiling the
following code at -O3 with gcc 4.4.3 (I see it in 4.5 as well).  If I remove
the loop then the warning disappears.  This is a contrived example based on
binutils/dwarf.c (which errors out when building with -Werror -Wall -O3).  The
order of the check in the !do_loc && do_types if statement is important. 
Obviously, the do_types code path would be taken in both cases.

problem.c:

int do_types;
int do_loc;
unsigned char storage[8];

void testfunc(void)
{
  int i;
  int j;
  unsigned char array[8];

  for (j = 0; j < 1; j++)
  {
    if (do_types)
      for (i = 0; i < 8; i++)
        array[i] = storage[i];
    if (!do_loc && do_types)
      for (i = 0; i < 8; i++)
        storage[i] = array[i];
  }
}
EOF

prompt> gcc -c -O3 problem.c -Wall
problem.c: In function ‘testfunc’:
problem.c:9: warning: ‘array[7]’ may be used uninitialized in this function
problem.c:9: warning: ‘array[6]’ may be used uninitialized in this function
problem.c:9: warning: ‘array[5]’ may be used uninitialized in this function
problem.c:9: warning: ‘array[4]’ may be used uninitialized in this function
problem.c:9: warning: ‘array[3]’ may be used uninitialized in this function
problem.c:9: warning: ‘array[2]’ may be used uninitialized in this function
problem.c:9: warning: ‘array[1]’ may be used uninitialized in this function
problem.c:9: warning: ‘array[0]’ may be used uninitialized in this function

prompt> gcc -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-ppl --with-cloog --with-tune=generic --with-arch_32=i686
--build=x86_64-redhat-linux
Thread model: posix
gcc version 4.4.3 20100127 (Red Hat 4.4.3-4) (GCC)


-- 
           Summary: Invalid uninitialized variable warning at -O3 -Wall
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dougsemler at gmail dot com
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43736