http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54717
--- Comment #12 from Sergey Ostanevich <sergos.gnu at gmail dot com> 2012-11-14
18:56:22 UTC ---
Actually, it is not.
I found that PRE did not collected a memory access within the loop that caused
later missing vectorization. Here is dump before (good one) and after the
commit (bad one)
<bb 88>:
pretmp_263 = (integer(kind=8)) ival2_82;
pretmp_264 = pretmp_263 + -1;
pretmp_265 = *xxtrt_46(D)[pretmp_264];
<bb 28>:
# ival2_10 = PHI <ival2_63(88), ival2_89(92)>
# ival2_14 = PHI <ival2_82(88), ival2_15(92)>
# prephitmp_266 = PHI <pretmp_265(88), prephitmp_237(92)>
_83 = (integer(kind=8)) ival2_10;
_84 = _83 + -1;
_85 = *xxtrt_46(D)[_84];
_86 = (integer(kind=8)) ival2_14;
_87 = _86 + -1;
_88 = prephitmp_266;
if (_85 < _88)
goto <bb 29>;
else
goto <bb 90>;
<bb 90>:
goto <bb 30>;
<bb 29>:
<bb 30>:
# ival2_15 = PHI <ival2_14(90), ival2_10(29)>
# prephitmp_237 = PHI <_88(90), _85(29)>
ival2_89 = ival2_10 + -1;
if (ival2_10 == ipos1_12)
goto <bb 91>;
else
goto <bb 92>;
<bb 92>:
goto <bb 28>;
---------------------------------
<bb 88>:
<bb 28>:
# ival2_10 = PHI <ival2_63(88), ival2_89(92)>
# ival2_14 = PHI <ival2_82(88), ival2_15(92)>
_83 = (integer(kind=8)) ival2_10;
_84 = _83 + -1;
_85 = *xxtrt_46(D)[_84];
_86 = (integer(kind=8)) ival2_14;
_87 = _86 + -1;
_88 = *xxtrt_46(D)[_87];
if (_85 < _88)
goto <bb 29>;
else
goto <bb 90>;
<bb 90>:
goto <bb 30>;
<bb 29>:
<bb 30>:
# ival2_15 = PHI <ival2_14(90), ival2_10(29)>
ival2_89 = ival2_10 + -1;
if (ival2_10 == ipos1_12)
goto <bb 91>;
else
goto <bb 92>;
<bb 92>:
goto <bb 28>;
-------------------------
So for the loop that starting at bb 28 you can see the xxtrt_46 access was not
put into pretemp. Possible reason is exactly as it was mentioned by Richard -
there were extra candidates collected and this one become less anticipatable
Skipping partial partial redundancy for expression
{array_ref<pretmp_8,0,4>,mem_ref<0B>,xxtrt_46(D)}@.MEM_30(D) (0165)
not partially anticipated on any to be optimized for speed edges
-----------------------------------------------------------------------
Found partial partial redundancy for expression
{array_ref<pretmp_8,0,4>,mem_ref<0B>,xxtrt_46(D)}@.MEM_30(D) (0165)
Created phi prephitmp_237 = PHI <_88(90), _85(29)>
in block 30
