[Bug middle-end/35545] virtual call specialization not happening with FDO

[law at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35545

--- Comment #8 from Jeffrey A. Law <law at redhat dot com> ---
It's not a matter of cost model, but if propagating the values to their uses. 
I haven't looked closely at the tracer, but wouldn't it benefit by having
constants in particular propagated to their uses?

Yes, DOM's duplication/isolation of paths exposes the degenerate PHIs.  So we
might have had:

One of the nice side effects of jump threading is that it isolates paths.  So
we might have had 

BBn
x = phi (a, b, c, constant, d, e, f)

duplication for threading might turn that into

BBn:
x = phi (a, b, c, d, e, f)  // The original


elsewhere

BBm:
x' = phi (constant)  // the duplicate


Propagating the constant for x' in BBm and eliminating the degenerate is what
the phi-only cprop pass does.  If the tracer generates similar things, then
running phi-only cprop after it might be useful as well.  It *should* be very
fast.