[Bug tree-optimization/102540] [12 Regression] Dead Code Elimination Regression at -O3 since r12-476-gd846f225c25c5885

["amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102540

--- Comment #8 from Andrew Macleod <amacleod at redhat dot com> ---

> > 
> > It would probably resolve an entire class of things where we don't recognize an
> > equivalence between a cast and a bitmask of equivalent precision.
> > 
> > This would also mean the reverse would apply.. ie if we instead branched on _2
> > != 0 we would also understand that _6 will be non-zero.
> 
> I believe tracking known zero/one bits in addition to a range is more
> useful - would that help in this case?

Thats in plan and what I first thought would fix it.  Reflecting on this
problem however, it would only help on the zero side where all the bits are
known zero, but the non-zero property we are looking for can't be reflected by
known ones or zeros.. 

Unfortunately I don't see how we can solve this particular problem by tracking
known bit values.. there wont be any known 1s or 0s in a.0_1...  just a
particular bit pattern that cannot occur.

Another option would be if I can get a cheap enough low-opt pass of evrp (also
on my list) we could run it really early before things get rearranged and then
run the higher levels later.

Anyway, I'll keep thinking about this...