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Date: Thu, 19 Jul 2007 17:11:00 -0000
Subject: [Bug tree-optimization/32825]  New: Reduction with nonzero start (arbitrary also) causes an extra add to happen
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Testcase (Compile at -O2 -maltivec -ftree-vectorize):
int a[16*100];
int f(int e)
{
  int i;
  for(i = 0;i<16*100;i++)
    e += a[i];
  return e;
}
--------- Cut -----
Currently we get:
  ivtmp.42 = (long unsigned int) &a;
  vect_var_.36 = { 0, 0, 0, 0 };

<bb 3>:
  vect_var_.36 = MEM[index: ivtmp.42] + vect_var_.36;
  ivtmp.42 = ivtmp.42 + 16;
  if (ivtmp.42 != (long unsigned int) (&a + 6400))
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 4>:
  vect_var_.39 = vect_var_.36 v>> 64;
  vect_var_.47 = vect_var_.39 + vect_var_.36;
  vect_var_.48 = vect_var_.47 v>> 32;
  stmp_var_.38 = BIT_FIELD_REF <vect_var_.48 + vect_var_.47, 32, 96>;
  return stmp_var_.38 + e;

Though the last add is extra and does not need to be done, we can get rid of it
by having vect_var_.36 being set initially to {e, 0, 0, 0} .

Note this happens with a non zero start also, that is:
int a[16*100];
int f(int e)
{
  int i;
  e = 1;
  for(i = 0;i<16*100;i++)
    e += a[i];
  return e;
}


-- 
           Summary: Reduction with nonzero start (arbitrary also) causes an
                    extra add to happen
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32825