[Bug tree-optimization/66502] SCCVN can't handle PHIs optimistically optimally

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66502

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2015-06-11
           Assignee|unassigned 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> ---
https://gcc.gnu.org/ml/gcc-patches/2015-06/msg00760.html was posted that
changes the current preference to handle the first testcase (but no longer
the second).  The first opportunity is frequently introduced by the vectorizer,
together with redundant loads, that are not eliminated in any way currently.

IVOPTs can remove redundant IVs but the issue is usually that it can't prove
the initial values are the same nor does it remove the redundant loads
leading to cost model issues (DOM might later remove the redundant loads).

The above mentioned patch passed bootstrap & regtest on
x86_64-unknown-linux-gnu.

Enlarging the SCCs to cover all candidates is a bit tricky in the current
framework, I still like to experiment with that idea.