[Bug tree-optimization/113467] [14 regression] libgcrypt-1.10.3 is miscompiled

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113467

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
I do wonder whether LOOP_VINFO_EARLY_BREAKS_VECT_PEELED actually works (since
without early exits we cannot handle a non-empty latch because of correctness
issues).  I'd very much have preferred to deal with these by loop rotation
(there's the loop_ch pass).  We're still doing this, even when
LOOP_VINFO_EARLY_BREAKS_VECT_PEELED:

  /* We assume that the loop exit condition is at the end of the loop. i.e,
     that the loop is represented as a do-while (with a proper if-guard
     before the loop if needed), where the loop header contains all the
     executable statements, and the latch is empty.  */
  if (!empty_block_p (loop->latch)
      || !gimple_seq_empty_p (phi_nodes (loop->latch)))
    return opt_result::failure_at (vect_location,
                                   "not vectorized: latch block not empty.\n");

so that's a bit odd (but loop_ch tries to ensure the latch is empty anyway).

Does the following fix the issue?

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 330c4571c8d..b67ee783002 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -998,6 +1000,9 @@ vec_init_loop_exit_info (class loop *loop)
        }
     }

+  if (candidate->src != single_pred (loop->latch))
+    return NULL;
+
   return candidate;
 }