[Bug c/52979] New: likely wrong code bug w/packed bitfields

[regehr at cs dot utah.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52979

             Bug #: 52979
           Summary: likely wrong code bug w/packed bitfields
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: regehr@cs.utah.edu
                CC: chenyang@cs.utah.edu


[regehr@dyson r30]$ current-gcc -O2 small.c ; ./a.out 
0
[regehr@dyson r30]$ current-gcc -O3 small.c ; ./a.out 
1
[regehr@dyson r30]$ cat small.c
int printf (const char *, ...);
int c, d, e;

void fn1 ()
{
}

#pragma pack(1)
struct S1
{
    int f0:31;
    int f1:6;
} 
a = { 1 };

static struct S1 b = { 1 };

void fn2 ()
{
    for (;;)
    {
        a.f1 = 1;
        struct S1 f = { };
        b = f;
        e = 0;
        if (d)
            c = a.f0;
        break;
    }
}

void fn3 ()
{
    fn2 ();
    a = b;
}

int main (void)
{
    fn3 ();
    printf ("%d\n", a.f0);
    return 0;
}
[regehr@dyson r30]$ current-gcc -v
Using built-in specs.
COLLECT_GCC=current-gcc
COLLECT_LTO_WRAPPER=/uusoc/exports/scratch/regehr/z/compiler-install/gcc-r186403-install/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.8.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --with-libelf=/usr/local --enable-lto
--prefix=/home/regehr/z/compiler-install/gcc-r186403-install
--program-prefix=r186403- --enable-languages=c,c++
Thread model: posix
gcc version 4.8.0 20120413 (experimental) (GCC)