[Bug tree-optimization/18400] New: wrong unrolling after vectorization due to invalid loop->nb_iterations

["dorit at il dot ibm dot com" <gcc-bugzilla at gcc dot gnu dot org>]

When a loop like the following:

=============================
#define N 8
int main ()
{
  int b[N] = {0,3,6,9,12,15,18,21};
  int a[N];
  int i;
  for (i = 0; i < N; i++){
      a[i] = b[i];
  }
  /* check results:  */
  for (i = 0; i < N; i++){
      if (a[i] != b[i])
        abort ();
  }
  return 0;
}
=============================

is compiled with:
-O2 -ftree-vectorize -maltivec -funroll-loops 

it gets vectorized and then completely unrolled. The complete unroller looks at 
loop->nb_iterations to get the number of iterations; the problem is that loop-
>nb_iterations is not always updated after vectorization, so the loop ends up 
being unrolled by 8 instead of by 2, and we get an execution failure.

The following patch will fix the problem:

Index: tree-vectorizer.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/tree-vectorizer.c,v
retrieving revision 2.25
diff -c -3 -p -r2.25 tree-vectorizer.c
*** tree-vectorizer.c   8 Nov 2004 13:54:41 -0000       2.25
--- tree-vectorizer.c   9 Nov 2004 13:16:54 -0000
*************** make_loop_iterate_ntimes (struct loop *l
*** 635,640 ****
--- 635,642 ----

    if (vect_debug_stats (loop) || vect_debug_details (loop))
      print_generic_expr (dump_file, cond_stmt, TDF_SLIM);
+   
+   loop->nb_iterations = niters;
  }

  
*************** vect_transform_loop_bound (loop_vec_info
*** 2881,2886 ****
--- 2883,2890 ----

    if (vect_debug_details (NULL))
      print_generic_expr (dump_file, cond_stmt, TDF_SLIM);
+   
+   loop->nb_iterations = new_loop_bound;
  }

-- 
           Summary: wrong unrolling after vectorization due to invalid loop-
                    >nb_iterations
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: dorit at il dot ibm dot com
        ReportedBy: dorit at il dot ibm dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: powerpc-apple-darwin7.0.0
  GCC host triplet: powerpc-apple-darwin7.0.0
GCC target triplet: powerpc-apple-darwin7.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18400