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Subject: [Bug tree-optimization/54717] [4.8 Regression] Runtime regression: polyhedron test "rnflow" degraded
Date: Wed, 14 Nov 2012 18:56:00 -0000
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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