------- Comment #4 from rguenth at gcc dot gnu dot org 2008-11-01 17:11 -------
We need to be careful with simplifying to SSA_NAMEs as the following example
shows:
int foo (int i, int b)
{
int mask;
int result;
if (b)
mask = -1;
else
mask = 0;
result = i + 1;
result = result & mask;
return result;
}
we have a phi-translation for result & mask that is 0 or result dependent
on the path through the CFG. But we cannot insert this as a PHI as one
argument (result with value i + 1) is not available there.
The PRE of 4.3 inserts i + 1, re-generating the expression recursively and
exposing a code hoisting opportunity:
<bb 2>:
if (b_2(D) != 0)
goto <bb 5>;
else
goto <bb 3>;
<bb 5>:
pretmp.6_10 = i_5(D) + 1;
pretmp.6_11 = pretmp.6_10;
goto <bb 4>;
<bb 3>:
pretmp.6_13 = i_5(D) + 1;
<bb 4>:
# prephitmp.7_14 = PHI <pretmp.6_10(5), pretmp.6_13(3)>
# prephitmp.7_12 = PHI <pretmp.6_11(5), 0(3)>
# mask_1 = PHI <-1(5), 0(3)>
result_6 = prephitmp.7_14;
result_7 = prephitmp.7_12;
return result_7;
I don't think we want to do this now (without code hoisting implemented), but
for cases where result_6 is available we surely want it.
I'm trying to find a way to detect whether it is safe to phi-translate to
result_6.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |dberlin at gcc dot gnu dot
| |org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37542
