[Bug rtl-optimization/64164] [4.9/5 Regression] one more stack slot used due to one less inlining level

["law at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64164

--- Comment #28 from Jeffrey A. Law <law at redhat dot com> ---
So I've been thinking about how to integrate life/conflict analysis into the
uncprop code and it may not be that bad, both from an implementation and
computation standpoint.

Most importantly, we don't have to compute full life information.  We really
just need to compute the life of the equivalence.  Given the life of the
equivalence, if the equivalence is live in any block that contains the defining
statement for an SSA_NAME appearing in the target PHI, then the equivalence
conflicts and we don't want to unpropagate it.

Computing the life of the equivalence is pretty easy and should be reasonably
quick.  This is a cost we'd have to pay regardless of whether or not we
integrate uncprop with out-of-ssa since we won't have life information for the
expression.

Collecting the SSA_NAMEs appearing on the RHS of the PHI so that we don't test
for conflicts multiple times if an SSA_NAME shows up in multiple PHI
alternatives would help keep the cost down as well.

Ultimately I don't think we need to integrate uncprop and out-of-ssa to avoid
the unprofitable transformation during uncprop.