combine/dce patch for PR36003, PR42575

[Steven Bosscher]

On Sun, Jun 27, 2010 at 11:21 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
> I did some work on speeding up notes that I hope to get into mainline soonish
> and considered testing replacing fast_dce by the du/ud implementation.

The du/ud implementation has its drawbacks too.  Such as not working
very well for very large functions.


> I think the slowness, from large part, comes from two reasons.
> 1) DCE iterates and calls dataflow at each step.  This is naturaly slow but
>   this probably can be limited.  Pass don't get dead induction vars removed
>   and other loop carried liveness is not too important.

Originally DCE did not iterate, but this caused bugs, e.g. PR35805.


> Also perhaps byte-dce can replace one of existing DCE passes?

It doesn't remove the same insns.

Ciao!
Steven