[dev.ssa] cmd/compile: speed up liveness analysis

This reduces the wall time to run test/slice3.go
on my laptop from >10m to ~20s.

This could perhaps be further reduced by using
a worklist of blocks and/or implementing the
suggestion in the comment in this CL, but at this
point, it's fast enough that there is no need.

Change-Id: I741119e0c8310051d7185459f78be8b89237b85b
Reviewed-on: https://go-review.googlesource.com/12564Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/regalloc.go

@@ -419,13 +419,24 @@ func live(f *Func) [][]ID {
 
 	s := newSparseSet(f.NumValues())
 	t := newSparseSet(f.NumValues())
+
+	// Instead of iterating over f.Blocks, iterate over their postordering.
+	// Liveness information flows backward, so starting at the end
+	// increases the probability that we will stabilize quickly.
+	// TODO: Do a better job yet. Here's one possibility:
+	// Calculate the dominator tree and locate all strongly connected components.
+	// If a value is live in one block of an SCC, it is live in all.
+	// Walk the dominator tree from end to beginning, just once, treating SCC
+	// components as single blocks, duplicated calculated liveness information
+	// out to all of them.
+	po := postorder(f)
 	for {
-		for _, b := range f.Blocks {
+		for _, b := range po {
 			f.Logf("live %s %v\n", b, live[b.ID])
 		}
 		changed := false
 
-		for _, b := range f.Blocks {
+		for _, b := range po {
 			// Start with known live values at the end of the block
 			s.clear()
 			s.addAll(live[b.ID])