[Bug tree-optimization/84737] [8 Regression] 20% degradation in CPU2000 172.mgrid starting with r256888

[pthaugen at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84737

--- Comment #14 from Pat Haugen <pthaugen at gcc dot gnu.org> ---
Created attachment 43928
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43928&action=edit
r256888 pcom dump

So the difference appears to be occurring in predictive commoning. In the
ipa-cp clone, resid.constprop, pcom is failing to hoist some loads/expressions
from the vectorized loop. This results in an additional 9 vector loads and 5
vector adds being executed each iteration of the loop.

I've attached a pcom dump of the original resid() and the clone
resid.constprop(). You can see that in the original resid(), pcom is moving
some loads/adds, but not in resid.constprop(). BB 6 is the vectorized loop in
resid(), BB 5 is the same loop in resid.constprop().

Not sure if this is a similar issue to pr55334 wrt losing restrict.