[Bug tree-optimization/102659] -O2 vectorization if-conversion produces wrong code for gcc.dg/torture/pr69760.c

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102659

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-10-11
            Summary|[false diagnosis] extra     |-O2 vectorization
                   |warning info after O2       |if-conversion produces
                   |vectorization for           |wrong code for
                   |gcc.dg/torture/pr69760.c    |gcc.dg/torture/pr69760.c
          Component|debug                       |tree-optimization
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
           Keywords|diagnostic                  |wrong-code

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
With -flto we have range info on the test_func function arguments (w/o -flto as
well if you make the function static).

  # RANGE [10000000, 10000000] NONZERO 10000000
  int L_16(D) = L;
  # RANGE [0, 400] NONZERO 508
  int m_13(D) = m;
  # RANGE [4, 4] NONZERO 4
  int n_15(D) = n;
  # RANGE [400, 400] NONZERO 400
  int N_12(D) = N;

the warning is emitted from the max_loop_iterations call in
vect_truncate_gather_scatter_offset, so it's not because something is
vectorized but because we run the vectorizer.  It's also emitted on
the if-converted code only because only then the address computation
of the a[L * k] = 0.0; store can be used to bound the number of iterations.

And indeed the address calculation overflows 32bit memory so this is a problem
with if-conversion since n == 4 and thus

      if (k >= 0 && k < n)
        a[L * k] = 0.0;

will never trigger in iteration 54.

if-conversion doesn't check whether it makes conditional integer overflow
non-conditional.  I think we might have a duplicate report of the problem
somewhere though.