Thoughts about semantics of BB_VISITED

[Zdenek Dvorak]

Hello,

> >>>>>If the state of the mark would be undefined, you would need either to
> >>>>>clean them up first (over whole cfg), or use bitmaps for the marks.
> >>>>>Both of these approaches bring a risk of quadratic behavior.
> >>>>>
> >>>>
> >>>>Quadratic behaviour?  How?
> >>>
> >>>
> >>>because the places where it is used now (most notably, get_loop_body)
> >>>do not look at whole cfg, but just at the loop body.
> >>>
> >>Oh, I see.  So, this is inside a helper, not a pass.  How unfortunate.
> >>Perhaps get_loop_body should not use BB_VISITED.  What if the pass
> >>calling get_loop_body is using BB_VISITED for its own processing?
> >
> >then it is doomed :-(.  Quite often there is no real reason why a pass
> >should use BB_VISITED -- it can simply use sbitmap of basic blocks instead.
> 
> Why can't one just pass an sbitmap to get_loop_body, instead of it 
> destroying visited flags without telling you?

because get_loop_body is often used on places where (lots of) new blocks
get created (like unroller).  Resizing the sbitmap would be quite
annoying.

Zdenek