[Bug tree-optimization/31995] New: can't determine dependence between p->a[x+i] and p->a[x+i+1]

["pinskia at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

While looking into some failures on the pointer plus branch after fixing up
forwprop, I noticed that we would get a failure in the vectorizer testsuite and
the reason is because we no longer could determine dependence for the two data
accesses.

Here is the testcase which is a modified version of vect-102.c which gets us
the same IR as what the pointer plus branch gives:
/* { dg-require-effective-target vect_int } */

#include <stdlib.h>
#include <stdarg.h>
#include "tree-vect.h"

#define N 9

struct extraction
{
  int a[N];
  int b[N];
};

static int a[N] = {1,2,3,4,5,6,7,8,9};
static int b[N] = {2,3,4,5,6,7,8,9,9};

int main1 (int x, int y) {
  int i;
  struct extraction *p;
  p = (struct extraction *) malloc (sizeof (struct extraction));

  for (i = 0; i < N; i++)
    {
       p->a[i] = a[i];
       if (x == 135)
         abort (); /* to avoid vectorization  */
    }

  /* Not vectorizable: distance 1.  */
  for (i = 0; i < N - 1; i++)
    {
       p->a[x + i] = p->a[x + i + 1];
    }

  /* check results: */
  for (i = 0; i < N; i++)
    {
       if (p->a[i] != b[i])
         abort();
    }
  return 0;
}

int main (void)
{ 
  check_vect ();

  return main1 (0, N);
}

/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 0 "vect" } } */
/* { dg-final { scan-tree-dump-times "possible dependence between data-refs" 1
"vect" } } */
/* { dg-final { cleanup-tree-dump "vect" } } */

-------------------------- cut ---------------
if you change the 1 to 8, we should be able to vectorize that loop too.


-- 
           Summary: can't determine dependence between p->a[x+i] and p-
                    >a[x+i+1]
           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=31995