cmd/compile: Use Sreedhar+Gao phi building algorithm
Should be more asymptotically happy.
We process each variable in turn to find all the
locations where it needs a phi (the dominance frontier
of all of its definitions). Then we add all those phis.
This takes O(n * #variables), although hopefully much less.
Then we do a single tree walk to match all the
FwdRefs with the nearest definition or phi.
This takes O(n) time.
The one remaining inefficiency is that we might end up
introducing a bunch of dead phis in the first step.
A TODO is to introduce phis only where they might be
used by a read.
The old algorithm is still faster on small functions,
so there's a cutover size (currently 500 blocks).
This algorithm supercedes the David's sparse phi
placement algorithm for large functions.
Lowers compile time of example from #14934 from
~10 sec to ~4 sec.
Lowers compile time of example from #16361 from
~4.5 sec to ~3 sec.
Lowers #16407 from ~20 min to ~30 sec.
Update #14934
Update #16361
Fixes #16407
Change-Id: I1cff6364e1623c143190b6a924d7599e309db58f
Reviewed-on: https://go-review.googlesource.com/30163 Reviewed-by: David Chase <drchase@google.com>