[tree-ssa] Insert on edge comment

[Andrew MacLeod]

On Fri, 2003-05-30 at 19:13, law@redhat.com wrote:
> In message <3ED7E4C7.69416EBC@redhat.com>, "Vladimir N. Makarov" writes:
>  >I am ignorant too (never looked at ssa-tree branch).  As I understand the
>  >problem is in phi nodes which are based on dominator frontier analysis.
>  >Insertion of new BB is simple for general IR but for SSA it can significantly
>  >change dominator frontiers for many blocks and as consequence phi-nodes in
>  >the dominator frontiers. So updating phi-nodes is not a trivial task.
> That's certainly true as well.  If you look at how we do this at the
> RTL level, you'll find that we don't split edges while in SSA form.  
> Instead we queue things (such as edge insertions) and split the edge
> after we no longer need dominance info and PHI nodes.
> 

PRE will probably want to insert things on edge and commit them before
we are done with PHI nodes :-)  I suspect I'll need to add PHI node
updating as a flag to the commit routine. 

Andrew