[Bug tree-optimization/32500] [4.2 Regression] Loop optimization limits range to size of array used inside loop

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

------- Comment #12 from rguenth at gcc dot gnu dot org  2007-07-04 10:16 -------
This is SCEV.  From

<L2>:;
  i_7 = ASSERT_EXPR <i_20, i_20 > 4>;
  i.10_10 = (unsigned int) i_7;
  D.2489_11 = i.10_10 - 7;
  if (D.2489_11 <= 2) goto <L3>; else goto <L4>;

we have

Found new range for i.10_10: [5, 12]


Visiting statement:
D.2489_11 = i.10_10 - 7;

(analyze_scalar_evolution
  (loop_nb = 1)
  (scalar = D.2489_11)
(get_scalar_evolution
  (scalar = D.2489_11)
  (scalar_evolution = {4294967290, +, 1}_1))
(set_scalar_evolution 
  (scalar = D.2489_11)
  (scalar_evolution = {4294967290, +, 1}_1))
) 
(instantiate_parameters
  (loop_nb = 1)
  (chrec = {4294967290, +, 1}_1)
  (res = {4294967290, +, 1}_1))
Found new range for D.2489_11: [4294967290, +INF]

which is wrong.  The new range should be ~[5, 4294967290].

scev_probably_wraps_p() returns false for the above chrec because for
the loop in question estimated_nb_iterations is 4(!) which is derived
from infer_loop_bounds_from_undefined.  On the trunk this is fixed
by rewriting number of iterations analysis.  On the 4.2 branch we
can fix this conservatively by

Index: tree-ssa-loop-niter.c
===================================================================
--- tree-ssa-loop-niter.c       (revision 126260)
+++ tree-ssa-loop-niter.c       (working copy)
@@ -1747,6 +1747,12 @@ infer_loop_bounds_from_undefined (struct
     {
       bb = bbs[i];

+      /* If BB is not executed in each iteration of the loop, we cannot
+        use the operations in it to infer reliable upper bound on the
+        # of iterations of the loop.  */
+      if (!dominated_by_p (CDI_DOMINATORS, loop->latch, bb))
+       continue;
+
       for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
         {
          tree stmt = bsi_stmt (bsi);

I'm going to test this.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rguenth at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2007-06-26 10:45:47         |2007-07-04 10:16:51
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32500