[Bug tree-optimization/33707] New: missed optimization with dependency checker

[victork at gcc dot gnu dot org]

Dependency checker fails to build distance vector for the following example:

int
foo (char *a, unsigned n)
{
    int i;
    a[0] = 0;
    for (i = 16; i < n; i++)
      a[i] = a[i-16];
}

Compiled with 
gcc -O2 -ftree-vectorize -maltivec -fdump-tree-vect-details -S t.c

>From the vectorizer dump:

...
  (overlap_iterations_b = not known
)
)
(dependence classified: scev_not_known)
)
)
...

However, it succeeds to calculate distance vector if number of iterations is
known constant:

int
foo (char *a, unsigned n)
{
    int i;

    a[0] = 0;

    for (i = 16; i < 128; i++)
    {
      a[i] = a[i-16];
    }
}
>From dump of vectorizer:

....
)
(build_classic_dist_vector
  dist_vector = ( 16
  )
)
)
)
....

-- Victor


-- 
           Summary: missed optimization with dependency checker
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: minor
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: victork at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33707