CSE optimization question

[law@redhat.com]

  > I asked this before, but why isn't GCSE making that obsolete now? It isn't,
  > but I don't understand *why* it isn't.
There's a lot of code in cse that doesn't exist in gcse -- for example cse
does a lot of simplifications that gcse knows nothing about.

But more importantly gcse is based on common subexpression elimination
while cse is actually a derivative of value numbering.  Meaning they're
based on completely different concepts for identifying expressions which
are redundant.  And finally, gcse doesn't handle redundancies which occur
within a single basic block.

jeff