[Bug tree-optimization/65217] __builtin_unreachable in if statement causes bad assembly generation

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65217

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization, xfail
             Status|RESOLVED                    |REOPENED
   Last reconfirmed|                            |2015-04-27
                 CC|                            |law at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
          Component|c                           |tree-optimization
         Resolution|FIXED                       |---
     Ever confirmed|0                           |1

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
It was only fixed by luck due to us folding all stmts in forwprop which turns

  if (_3 != n_1(D))

into the canonical

  if (n_1(D) != _3)

and then DOM recording the "proper" equality via record_equality by _always_
swapping operands due to

  else if (is_gimple_min_invariant (x)
           /* ???  When threading over backedges the following is important
              for correctness.  See PR61757.  */
           || (loop_depth_of_name (x) <= loop_depth_of_name (y)))

and its '==' part.

For non-canonical order (in GCC 4.9 and earlier) that swapping turned it into
the unfortunate direction.

That equality decides whether we propagate _3 into the return value on the
else branch (where n_1(D) == _3) or leave it as n_1(D) (good for this case).

I am about to commit a patch that does not perform this kind of random swapping
(and will XFAIL the testcase).