[tree-ssa] Insert on edge comment

[Vladimir N. Makarov]

Michael Matz wrote:

> Hi,
>
> On 30 May 2003, Andrew MacLeod wrote:
>
> > edge feeding the phi will have to be updated.  My new switch edge
> > manipulation will be a real ugly one to update.. brrr.
> >
> > There may be other structures which need to be fixed, etc.
>
> Forgive me my ignorance (I never looked at tree-ssa carefully), but why is
> edge splitting in your infrastructure so hard?  It's a totally trivial
> operation for the usual IRs.  If this doesn't hold for tree-ssa it just
> doesn't sound quite "right".  Could you explain in simple words? ;-)
>

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.

Vlad