[Bug tree-optimization/29789] New: Missed optimization in libquantum

[rguenth at gcc dot gnu dot org]

In SPEC 2k6 libquantum we can find the following (simplified) hot loop

void
quantum_toffoli(int control1, int control2, int target, unsigned long *state,
int size)
{
  int i;

  for(i=0; i<size; i++)
    {
       if (state[i] & ((unsigned long) 1 << control1))
         if (state[i] & ((unsigned long) 1 << control2))
           state[i] ^= ((unsigned long) 1 << target);
    }
}

where we miss that
  - The masks of the form ((unsigned long) 1 << X) are loop invariant
  - We can combine the two ifs to
      if (state[i] & C == C)
    with C = ((unsigned long) 1 << control1) | ((unsigned long) 1 << control2)

The first missed-optimization is also caused by fold at

      /* If this is an EQ or NE comparison with zero and ARG0 is
         (1 << foo) & bar, convert it to (bar >> foo) & 1.  Both require
         two operations, but the latter can be done in one less insn
         on machines that have only two-operand insns or on which a
         constant cannot be the first operand.  */

which causes the bit-tests to be written with a not loop-invariant shift.


-- 
           Summary: Missed optimization in libquantum
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at gcc dot gnu dot org
OtherBugsDependingO 15357,26163
             nThis:


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29789