Re: [ssaupdate] Local dominance info

[Zdenek Dvorak <rakdver@atrey.karlin.mff.cuni.cz>]

Hello,

> the claim is that it is cheap. I claim it will have a cost, perhaps it
> is small most of the time, but there is bound to be pathological cases
> which do have an impact.
> 
> > > I don't see any reason why it should be kept up to date all the time
> > > when virtually no-one else cares about it.
> > 
> > Cleanlyness of interfaces?  Less potential for funny bugs because the 
> > wrong inserters were used by some common code?
> 
> It depends. I've seen no reason for it. If it turns out that having it
> present *will* make a different in updating ssa on the fly, thats
> different. I've seen no argument why 70 other optimizations should keep
> the information up to date when they dont care about it.
> 
> The general renamer has to go through all the basic blocks *anyway* in
> order to calculate live-on-entry, so it seems pretty easy to do that
> local numbering on the fly since you are making a pass the the IL
> anyway.
> 
> If there is a more iterative into-ssa solution that can make use of this
> info and doesnt have to make a pass through the IL, then thats an
> arguement for it.  If there is a reason to keep it up to date other than
> that, perhaps that is an argument too. 
> 
> So far all Ive seen/heard is keeping the information up to date doesnt
> cost a lot, but I see few passes have a use for it.  Tell me how useful
> it is, and why its better than calculating it on the fly when you need
> it and maybe I am convinced. 

answer to your questions from my side is "I don't know."  I think it
will be useful (obviously :-), but it may turn out that I will be wrong.
Ssaupdate is a development branch, so the way things are done now do not
necessarily have to be the same (or even close to) those that will be in
final version.

Zdenek