[Bug tree-optimization/68844] New: [6 regression] gcc.dg/tree-ssa/ssa-dom-thread-4.c fails since r228306

[wschmidt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68844

            Bug ID: 68844
           Summary: [6 regression] gcc.dg/tree-ssa/ssa-dom-thread-4.c
                    fails since r228306
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wschmidt at gcc dot gnu.org
                CC: law at gcc dot gnu.org, seurer at linux dot vnet.ibm.com
  Target Milestone: ---
              Host: powerpc64-*-linux*
            Target: powerpc64*-*-linux*
             Build: powerpc64*-*-linux*

Created attachment 36986
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36986&action=edit
dom1 dump from r228305

The subject test case has been failing as follows:

FAIL: gcc.dg/tree-ssa/ssa-dom-thread-4.c scan-tree-dump-times dom1 "Threaded" 4

since r228306, which is:

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.

My guess is that the test case just needs adjusting because the control flow
has changed sufficiently to remove some jump-threading opportunities, but I'm
not familiar enough with the code to be sure.  Jeff, would you mind taking a
quick look?

The removed threaded jumps are:

  Threaded jump 15 --> 3 to 22
  Threaded jump 5 --> 18 to 23
  Threaded jump 7 --> 18 to 23

Attaching the dom1 dumps from r228305 and r228306.

Apologies if this is a known issue, but I couldn't find a PR open on it.