[Bug tree-optimization/48172] [4.5/4.6/4.7 Regression] incorrect vectorization of loop in GCC 4.5.* with -O3

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48172

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-18 
09:48:39 UTC ---
(compute_affine_dependence
  (stmt_a =
D.3677_12 = array[D.3676_11];
)
  (stmt_b =
array[D.3675_10] = D.3680_16;
)
(subscript_dependence_tester
(analyze_overlapping_iterations
  (chrec_a = {0, +, 2}_2)
  (chrec_b = {514, +, 1}_2)
(analyze_siv_subscript
(analyze_subscript_affine_affine
  (overlaps_a = [257 + 1 * x_1]
)
  (overlaps_b = [0 + 2 * x_1]
)
)
)
  (overlap_iterations_a = [257 + 1 * x_1]
)
  (overlap_iterations_b = [0 + 2 * x_1]
)
)
(Dependence relation cannot be represented by distance vector.)
)
(compute_affine_dependence
  (stmt_a =
D.3679_15 = array[D.3678_14];
)
  (stmt_b =
array[D.3675_10] = D.3680_16;
)
(subscript_dependence_tester
(analyze_overlapping_iterations
  (chrec_a = {1, +, 2}_2)
  (chrec_b = {514, +, 1}_2)
(analyze_siv_subscript
(analyze_subscript_affine_affine
  (overlaps_a = [257 + 1 * x_1]
)
  (overlaps_b = [1 + 2 * x_1]
)
)
)
  (overlap_iterations_a = [257 + 1 * x_1]
)
  (overlap_iterations_b = [1 + 2 * x_1]
  (overlap_iterations_b = [1 + 2 * x_1]
)
)
(Dependence relation cannot be represented by distance vector.)
)
)

...

t.c:23: note: versioning for alias required: bad dist vector for
array[D.3676_11] and array[D.3675_10]
t.c:23: note: mark for run-time aliasing test between array[D.3676_11] and
array[D.3675_10]
t.c:23: note: versioning for alias required: bad dist vector for
array[D.3678_14] and array[D.3675_10]
t.c:23: note: mark for run-time aliasing test between array[D.3678_14] and
array[D.3675_10]

and the alias check looks like

  vect_parray.14_32 = &array;
  vect_parray.17_31 = &array[514];
  D.3707_29 = vect_parray.14_32 + 32;
  D.3708_28 = D.3707_29 < vect_parray.17_31;
  D.3709_63 = vect_parray.17_31 + 16;
  D.3710_64 = D.3709_63 < vect_parray.14_32;
  D.3711_65 = D.3708_28 || D.3710_64;
  D.3713_78 = !D.3711_65;
  if (D.3713_78 != 0)
    goto <bb 12>;
  else
    goto <bb 9>;

which doesn't at all test something sensible.

Shortened non-runtime testcase:

#define ASIZE 1028
#define HALF (ASIZE/2)
unsigned int array[ASIZE];

void foo(void)
{
  int i;
  for (i = 0; i < HALF-1; i++)
    array[HALF+i] = array[2*i] + array[2*i + 1];
}