[Bug tree-optimization/78731] Possible bug with switch when optimization is turned on.

[wilson at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78731

Jim Wilson <wilson at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-12-08
            Version|5.4.1                       |5.4.0
     Ever confirmed|0                           |1

--- Comment #1 from Jim Wilson <wilson at gcc dot gnu.org> ---
I can reproduce with gcc-5.4 on x86_64 and aarch64.  But not with gcc-6.

The problem appears in the dom2 tree dump, where the switch statement is
optimized incorrectly.

git bisect tracked the fix down to a patch from Jeff Law.  However, this patch
appears to be an optimization, not a bug fix, so it isn't obvious why it fixes
the problem.  I haven't tried debugging dom to see what the problem is yet.

Jeff's patch is
https://gcc.gnu.org/ml/gcc-patches/2015-09/msg02391.html

2015-09-30  Jeff Law  <law@redhat.com>

        * tree-ssa-dom.c (optimize_stmt): Collapse control flow statements
        with constant conditions.
        * tree-ssa-threadupdate.c (remove_jump_threads_starting_at): New.
        (remove_ctrl_stmt_and_useless_edges): No longer static.
        * tree-ssa-threadupdate.h (remove_jump_threads_starting_at): Prototype.
        (remove_ctrl_stmt_and_useless_edges): Likewise.

On the main branch, the first good git revision is
f1344f45994243478b1f4225e676446fedf18bf5.  The last bad git revision is
a4f58df11093c1253f9648e0ffa7fbdf604d05c2.