[tree-ssa] RFC: Making control flow more explicit

[law@redhat.com]

In message <Pine.LNX.4.44.0308120047480.19843-100000@wotan.suse.de>, Michael Ma
tz writes:
 >> Very very rarely within the DCE optimizer.
 >
 >Control dependence or postdoms also are necessary in if-conversion and for
 >some edge probability predicators.
Yup.  One of the unaddressed questions in my mind is whether or not it makes
sense to compute postdoms once early in the SSA path and keep them up-to-date
through the SSA optimizers or not.  We compute them on-demand in a few
places, mostly in areas dealing with IF removal.


 > Hmm, but maybe you're right and some
 >other form of approximate but faster to calculate form of control
 >dependence is enough.
It's enough for tree-ssa-dce.c; I have no idea if it's suitable elsewhere.

Hell, it took me a while to convince myself that what tree-ssa-dom.c was
indeed safe.  It didn't take long to determine that what tree-ssa-dom.c
does was faster than using true control dependence...  Let's call it an
interesting surprise.

The good news is that if we want to move to using true control dependence
that it's a fairly simple change to lift the first few hundred lines out
of ssa-dce.c and put them in tree-ssa-dce.c and tweak tree-ssa-dce.c to use
them.

Jeff