[Bug tree-optimization/88405] Missed DSE opportunity

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88405

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-12-10
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |matz at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.

But in general a prerequesite to eliminate Section A is
fusion of both loop nests and then CSEing the loads from a and b and finally
eliminate the stores as dead because a and b are only written to.

For the testcase we eliminate Section B because 'c' is discovered as writeonly.
But we do not update the writeonly flag of a or b when removing uses (and
IMHO we cannot easily).  Note we perform this "DSE" in fixup_cfg.