Some thoughts and quetsions about the data flow infrastracture

[Richard Kenner]

> Combiner is an older approach of code selection.  

Combine can be considered both as code selection or optimization.  Likewise,
CSE.  In many cases, especially now that we have the tree optimizers, CSE
does more code selection (best choice of operand) than CSE.  So you could say
that CSE and Combine are, in some sense, doing the same thing but with
different data structures.

Well before GCC 4.x there was an attempt that a few of us worked on to try to
move the algebraic simplifications from combine to simplify-rtx.c, just like
we put tree folding in fold-const.c.  At that point, combine just becomes
"bookkeeping".  The problem with that approach was what to do with such
things as nonzero_bits and num sign bit copies.  But if those (and more!)
become part of a global DF infrastructure, then they are available in *all*
RTL passes.

Next, one can finish the task of moving the simplifications to simplify-rtx.c
(since the global information will always be around) and much of the rest of
combine is replaced by the incremental update part of DF.

That means we could merge CSE and combine and do a *lot* more than we ever
were able to do before while having the code for both passes be much simpler.