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Subject: [Bug tree-optimization/54717] [4.8 Regression] Runtime regression: polyhedron test "rnflow" degraded
Date: Thu, 15 Nov 2012 10:52:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54717

--- Comment #16 from Jan Hubicka <hubicka at gcc dot gnu.org> 2012-11-15 10:52:13 UTC ---
OK, 4.7 vectorize two loops in the function in cptrf2

loop at ../a.f90:3538

      if (nxtr < 4) then
         kerr = 1
         do ixtr = 1, nxtr - 1
           ixtrt (ixtr) = ixtr + 1
         enddo
         goto 9000
      endif


and 

loop at ../a.f90:3530


         ixtrt = 0


The second loop is recognized as memset by mainline, so it remains to figure
out what is wrong with the first loop.  It is unrolled:

Analyzing # of iterations of loop 9
  exit condition [1, + , 1](no_overflow) != ival2_27 + -1
  bounds on difference of bases: 0 ... 1
  result:
    # of iterations (unsigned int) ival2_27 + 4294967294, bounded by 1
Loop 9 iterates at most 1 times.
Estimating sizes for loop 9
 BB: 8, after_exit: 0
  size:   0 _38 = (integer(kind=8)) ixtr_12;
   Induction variable computation will be folded away.
  size:   1 _39 = _38 + -1;
   Induction variable computation will be folded away.
  size:   1 ixtr_40 = ixtr_12 + 1;
   Induction variable computation will be folded away.
  size:   1 *ixtrt_33(D)[_39] = ixtr_40;
  size:   2 if (ixtr_12 == _37)
   Exit condition will be eliminated in last copy.
 BB: 79, after_exit: 1
size: 5-2, last_iteration: 5-4
  Loop size: 5
  Estimated size after unrolling: 2
Unrolled loop 9 completely (duplicated 1 times).

I do not quite see why it iterates at most once, but if seems to work. So I
would say that it is good idea to unroll rather than vectorize.

Is the slowdown still reproducing with my patch?