[Bug c/47796] New: The code to write to a bit_field data strucuture will be removed unexpectedly with gcc 4.5.1 -O2 option

["qihua.dai at intel dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47796

           Summary: The code to write to a bit_field data strucuture will
                    be removed unexpectedly with gcc 4.5.1 -O2 option
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: qihua.dai@intel.com


Created attachment 23393
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23393
sample code for this problem

The code "tmp.a = 0xc;" and "tmp.d = 0x1;" is to write to a bit_field data
strucuture. They will be removed unexpectedly with gcc 4.5.1 -O2 option if
there is the same code in "if" condition before them, and using unsigned int
pointer to access the bit_field data strcuture after them. See below:
{
    ......
    if(rev==0x0)
    {
        ......
    }
    else
    {
        tmp.a = 0xc;
        tmp.d = 0x1;
        pData = (unsigned int *)pTmp;
        data = *pData;
        ......
    }
    tmp.a = 0xc; // this code line is removed unexpected after -O2 optimization
    tmp.d = 0x1; // this code line is removed unexpected after -O2 optimization
    pData = (unsigned int *)pTmp;
    data = *pData;
    ......
}

This issue only occurs when using -O2 gcc 4.5 option for bit_field c code.

On Fedora 11 (kernel 2.6.29.4-167). gcc 4.4.0 has no this problem.
On Fedora 14 (kernel 2.6.35.6-45). gcc 4.5.1 has no this problem.

Attached file includes the sample code tmp.c, and its assemle files built with
-O0 and -O2.
tmp_bit_field_O0.s is assemble file generated by -O0
tmp_bit_field_O2.s is assemble file generated by -O2

reproduce steps:
1, gcc -O0 tmp.c
2, ./a.out
3, the correct result below is printed:
data = 0x8000000c
rev_data = 0x11

4, gcc -O2 tmp.c
5, ./a.out
6, the incorrect result below is printed:
data = 0x0
rev_data = 0x11