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

[Daniel Berlin <dberlin at dberlin dot org>]

On Tuesday, July 22, 2003, at 1:37 PM, Daniel Berlin wrote:

> On Tuesday, July 22, 2003, at 1:21 PM, Andrew MacLeod wrote:
>
> > On Tue, 2003-07-22 at 12:57, Daniel Berlin wrote:
> > > > I thought we resolved this last week? You had a patch for
> > > > remove_stmt()...
> > > >
> > > > Or did it not fix your problem?
> > >
> > > If you apply the following patch, you can see where we still have
> > > problems (this includes when a cond_expr is the end of the bb, and we
> > > remove it, when we remove empty statements and the previous 
> > > statement is
> > > an empty statement, etc).
> > >
> > > We probably should use this patch (without the #if 0'd code, of 
> > > course, i
> > > was just trying to start attacking the various cases) once all the 
> > > cases
> > > are handled, or to force ourselves to fix the bugs.
> >
> > but....  are you sure you are looking at the right thing?
> It could be the reverse iterators, of course.
>
> > Im not convinced that it isn't something else.. like a bug in the
> > reverse iterators or a missing bb_for_stmt...
> Where, exactly?
>
> > 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.

As an FYI, the reason it doesn't work is because of bsi_last:

 /* If the last stmt pointer isn't something a BSI can represent (ie, an
     empty statement node), then find the last stmt the slow way.  */
  if (b.tp == NULL)
    {


This condition is wrong for the comment.
If BSI's can't represent empty statement nodes, it needs to check
at least
1. IS_EMPTY_STMT (*b.tp)
and I believe
2. If *b.tp == COMPOUND_EXPR, IS_EMPTY_STMT (the first operand).