Redundant logical operations left after early splitting

[Jeff Law]

hutchinsonandy@aim.com wrote:
> If I understand correctly:
> 
> Prop. of "0" causes simplfy-rtx to create NOP from OR Rx,0
> This NOP (deletion?) creates another set of potential uses -  as now the 
> prior RHS def now passes straight thru to a new set of uses  - but we 
> miss those new uses. (which in the testcase are often 0)
> 
> I will try fwprop in a few different spots latter and see what, if any 
> changes occur.
Classic cascading.

I think some of this is an artifact of how your splitters are working.
You end up creating code which looks like


(set (reg1) (const_int 0)
(set (reg1) (ior (reg1) (other_reg)

What's important here is that reg1 is being set multiple times.  You'd
be better off if you can twiddle the splitters to avoid this behavior.
If you need a new pseudo, then get one :-)

Once you do that, local would propagate these things better.  That
still leaves the simplification & nop problem, but I'm pretty sure
that can be trivially fixed within local without resorting to running
another forwprop pass after splitting.

Jeff