[tree-ssa]: PRE updates and note on bug

[Andrew MacLeod]

On Tue, 2003-07-22 at 13:37, Daniel Berlin wrote:
> 

> > but....  are you sure you are looking at the right thing?
> >

> >
> > out_of_ssa doesnt use the reverse iterators, but it does use the
> > FOR_EACH_STMT_IN_REVERSE macro because its processes things backwards,
> > and its not having any problems. If things were screwed up by DCE,
> > something would get the wrong live range, or get missed, and I would
> > expect something to break later on.
> 
> As I said, it could be the reverse iterators.
> If the end of the bb is an empty statement, bsi = bsi_last (bb); 
> bsi_prev (bsi), doesn't work.
> If it should, then the reverse iterators are broken.

then thats the problem...

> If the end of the bb shouldn't be an empty statement, then the bb ends 
> aren't being updated.
> 
And thats not the problem.

> >
> > If the end of block pointer is actually set wrong, the forward 
> > iterators
> > would be missing stmts or something I would think.
> >
> Nope. They skip the empty statements, so they don't see the problem.
> 

And bsi_prev is implemented using bsi_next from the start of the block
to find the previous stmt, so the same limitations would be encountered.
Its an interaction between bsi_last and bsi_prev

> 1. The patch I pasted inserts the check when *DCE* runs, which is the 
> first thing that runs.
> Not when PRE would run.
> It has nothing to do with PRE at all.
> You will see the problem any time you use reverse iterators.
> which is #2
> 2. Nothing else uses reverse iterators.
> 

sooooooo.. wouldnt the natural conclusion be to look at the iterator?

Like I said, Im contemplating ditching bsi_prev anyway... no-one uses it
and its horribly inefficient.

Besides, since its the iterator thats broken, stop using it to check
your stmt count and move on. It shouldnt be affecting you since you dont
use it for any real work. Consider it removed, I'll take it out in my
round after next of changes. And/or fix it when I get a chance.

Andrew