[Bug tree-optimization/32825] New: Reduction with nonzero start (arbitrary also) causes an extra add to happen

[Bug tree-optimization/32825] New: Reduction with nonzero start (arbitrary also) causes an extra add to happen

[pinskia at gcc dot gnu dot org]

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

[Bug tree-optimization/32825] Reduction with nonzero start (arbitrary also) causes an extra add to happen

[dorit at gcc dot gnu dot org]

------- Comment #1 from dorit at gcc dot gnu dot org  2007-07-19 18:15 -------
...
> 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} .

The problem is that often initializing a vector to {e, 0, 0, 0} is (much?) more
expensive than initializing a vector to {0, 0, 0, 0} and then adding e to the
final scalar result. We actually had both options in the vectorizer for a while
(guarded by ADJUST_IN_EPILOG hard-coded #define), however we didn't know how to
choose between the two options (cost wise), so we just arbitrarily chose one.
Now that we're starting to build a cost model we may try to evaluate which of
the two options to generate. 


-- 


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

[Bug tree-optimization/32825] Reduction with nonzero start (arbitrary also) causes an extra add to happen

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2007-07-19 18:32 -------
> The problem is that often initializing a vector to {e, 0, 0, 0} is (much?) more
On SPU, it is not:

        cwd     $2,0($sp)
        shufb   $5,$3,$5,$2

vs:
        ori     $7,$3,0
        il      $5,0
...
        a       $8,$9,$7

Also it increases register pressure by long gating incomming argument.


-- 


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

[Bug tree-optimization/32825] Reduction with nonzero start (arbitrary also) causes an extra add to happen

[dorit at gcc dot gnu dot org]

------- Comment #3 from dorit at gcc dot gnu dot org  2007-07-24 13:05 -------
(In reply to comment #1)
> ... We actually had both options in the vectorizer for a while
> (guarded by ADJUST_IN_EPILOG hard-coded #define), however we didn't know how to
> choose between the two options (cost wise), so we just arbitrarily chose one.
> Now that we're starting to build a cost model we may try to evaluate which of
> the two options to generate. 

for the record - this was the patch that removed the second option:

2007-04-18  Dorit Nuzman  <dorit@il.ibm.com>

        * tree-vect-transform.c (get_initial_def_for_reduction): Clean away
        the unused code for reduction without adjust-in-epilog to simplify the
        function.


-- 


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