Loop optimizer issues

[Diego Novillo]

I've been thinking about this proposal and I am not convinced that it
would be a good idea to merge the two branches.

      * The tree-ssa branch is primarily trying to replace RTL passes. 
        This may include the loop optimizer.  So far, it is not clear
        whether we will actually need to have a sophisticated loop
        optimizer at the RTL level (if at all).  The only thing that
        comes to mind that may needed from RTL are:
              * instruction lengths/costs to guide the unroller
              * prefetching capabilities and memory costs to guide
                software pipelining and memory hierarchy optimizations.
      * While there still isn't much loop-related work on the branch,
        the loop discovery infrastructure that we inherited from
        mainline seems adequate for the basic things.
      * The branch is slowly becoming increasingly demanding wrt new
        contributions and optimization work.  Nightly tests on many
        platforms are very helpful.  I don't really want to add anything
        that's not related to the tree optimization infrastructure and
        may or may not be needed.
      * The tree-ssa branch is already very difficult to merge with
        mainline.  I don't want to make it more difficult.  I would
        prefer to see the rtlopt branch merged into mainline separately.

Having said that, there may be infrastructure from the rtlopt branch
that we may find useful for the tree optimizers.  I have personally not
looked into what's needed for the tree loop optimizer.  Some other folks
have.  Maybe we could take bits and pieces that are useful.


Diego.