[Bug c/43425] New: enhance scalar expansion to vectorize this loop

[Bug c/43425] New: enhance scalar expansion to vectorize this loop

[changpeng dot fang at amd dot com]

chfang@pathscale:~/gcc$ cat foo.c
int a[100], b[100];

void foo(int n, int mid)
{
  int i, t = 0;
  for(i=0; i<n; i++)
    {
      a[i] = b[i] + t; 
      t = b[i];
    }
}
chfang@pathscale:~/gcc$ gcc -O3 -ftree-vectorizer-verbose=7 -c foo.c

foo.c:6: note: not vectorized: unsupported use in stmt.
foo.c:3: note: vectorized 0 loops in function.

scalar expansion of t into array to carry the values accross iteration.


-- 
           Summary: enhance scalar expansion to vectorize this loop
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: changpeng dot fang at amd dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43425

[Bug tree-optimization/43425] enhance scalar expansion to vectorize this loop

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2010-03-18 18:31 -------
Basically undoing predictive commoning (which we switched the order for 4.5 to
fix a different issue).  Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|c                           |tree-optimization
     Ever Confirmed|0                           |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2010-03-18 18:31:30
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43425

[Bug tree-optimization/43425] enhance scalar expansion to vectorize this loop

[irar at il dot ibm dot com]

------- Comment #2 from irar at il dot ibm dot com  2010-03-28 08:59 -------
I think PR 35229 covers this issue.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43425

[Bug tree-optimization/43425] gcc should vectorize this loop by substitution

[changpeng dot fang at amd dot com]

------- Comment #3 from changpeng dot fang at amd dot com  2010-05-07 21:33 -------
I just found that the test case in the same as (similar to) bug 35229.
The subject of this bug is wrong. Scalar expansion is not appropriate
for this case.

Actually the loop can be transform to:

void foo(int n)
{
  int i;
  a[0] = b[0]; /* + t if t live before this point */
  for(i=1; i<n; i++)
    {
      a[i] = b[i] + b[i-1]; 
    }
  /* t = b[n-1]; is t live after this point */
}

The this loop can be vectorized.

In open64, this optimization is called forward (backward) substitution, i.e.
substitute t with b[i-1].

I am not clear whether bug 35229 addresses the same issue. Maybe we should
close
one of them.


-- 

changpeng dot fang at amd dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|enhance scalar expansion to |gcc should vectorize this
                   |vectorize this loop         |loop by substitution


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43425