[Bug tree-optimization/51049] New: A regression caused by "Improve handling of conditional-branches on targets with high branch costs"

["liujiangning at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51049

             Bug #: 51049
           Summary: A regression caused by "Improve handling of
                    conditional-branches on targets with high branch
                    costs"
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: liujiangning@gcc.gnu.org


int f(char *i, int j)
{
        if (*i && j!=2)
                return *i;
        else
                return j;
}

Before the check-in r180109, we have

  D.4710 = *i;
  D.4711 = D.4710 != 0;
  D.4712 = j != 2;
  D.4713 = D.4711 & D.4712;
  if (D.4713 != 0) goto <D.4714>; else goto <D.4715>;
  <D.4714>:
  D.4710 = *i;
  D.4716 = (int) D.4710;
  return D.4716;
  <D.4715>:
  D.4716 = j;
  return D.4716;

After check-in r180109, we have

  D.4711 = *i;
  if (D.4711 != 0) goto <D.4712>; else goto <D.4710>;
  <D.4712>:
  if (j != 2) goto <D.4713>; else goto <D.4710>;
  <D.4713>:
  D.4711 = *i;
  D.4714 = (int) D.4711;
  return D.4714;
  <D.4710>:
  D.4714 = j;
  return D.4714;

the code below in function fold_truth_andor makes difference,

      /* Transform (A AND-IF B) into (A AND B), or (A OR-IF B)     into (A OR
B).
     For sequence point consistancy, we need to check for trapping,
     and side-effects.  */
      else if (code == icode && simple_operand_p_2 (arg0)
               && simple_operand_p_2 (arg1))
         return fold_build2_loc (loc, ncode, type, arg0, arg1);

for "*i != 0" simple_operand_p(*i) returns false. Originally this is not
checked by the code. 

Please refer to http://gcc.gnu.org/ml/gcc-patches/2011-10/msg02445.html for
discussion details.

This change accidently made some benchmarks significantly improved due to some
other reasons, but Michael gave the comments below.

======Michael's comment======

It's nice that it caused a benchmark to improve significantly, but that should
be done via a proper analysis and patch, not as a side effect of a supposed
non-change.

======End of Michael's comment======

The potential impact would be hurting other scenarios on performance.

The key point is for this small case I gave RHS doesn't have side effect at
all, so the optimization of changing it to AND doesn't violate C specification. 

======Kai's comment======

As for the case that left-hand side has side-effects but right-hand not, we
aren't allowed to do this AND/OR merge.  For example 'if ((f = foo ()) != 0 &&
f < 24)' we aren't allowed to make this transformation.

This shouldn't be that hard.  We need to provide to simple_operand_p_2 an
additional argument for checking trapping or not.

======End of Kai's comment======

This optimization change is blocking some other optimizations I am working on
in back-ends. For example, the problem I described at
http://gcc.gnu.org/ml/gcc/2011-09/msg00175.html disappeared. But it is not a
proper behavior.

Thanks,
-Jiangning