[Bug optimization/15256] New: [tree-ssa] Optimize manual bitfield manipilation.

["kazu at cs dot umass dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

It would be nice if we can convert foo into bar.

unsigned int
foo (unsigned int eax)
{
  unsigned int edx = eax & 1;
  edx ^= 1;
  eax &= -2;
  eax |= edx;
  return eax;
}

unsigned int
bar (unsigned int eax)
{
  return eax ^ 1;
}

The last SSA form looks like:

foo (eax)
{
  unsigned int edx;

<bb 0>:
  edx_2 = eax_1 & 1;
  edx_3 = edx_2 ^ 1;
  eax_4 = eax_1 & 0fffffffe;
  eax_5 = eax_4 | edx_3;
  return eax_5;

}

This testcase is inspired by

http://gcc.gnu.org/ml/gcc-patches/2004-05/msg00058.html

This sequence of four instructions is a little too tough for the combiner to
chew as it only considers three insturctions at one time.

Note that some people manually implement bitfields
without relying on ":" in a struct.

-- 
           Summary: [tree-ssa] Optimize manual bitfield manipilation.
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: enhancement
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15256