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: unassig...@gcc.gnu.org
ReportedBy: liujiangn...@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
