[dev.ssa] cmd/compile: detect rewrite loops of length > 1

Use a version of Floyd's cycle finding algorithm,
but advance by 1 and 1/2 steps per cycle rather
than by 1 and 2. It is simpler and should be cheaper
in the normal, acyclic case.

This should fix the 386 and arm builds,
which are currently hung.

Change-Id: If8bd443011b28a5ecb004a549239991d3dfc862b
Reviewed-on: https://go-review.googlesource.com/13473Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -44,9 +44,18 @@ func applyRewrite(f *Func, rb func(*Block) bool, rv func(*Value, *Config) bool)
 					}
 					// Rewriting can generate OpCopy loops.
 					// They are harmless (see removePredecessor),
-					// but take care not to loop forever.
-					for a.Op == OpCopy && a != a.Args[0] {
+					// but take care to stop if we find a cycle.
+					slow := a // advances every other iteration
+					var advance bool
+					for a.Op == OpCopy {
 						a = a.Args[0]
+						if slow == a {
+							break
+						}
+						if advance {
+							slow = a
+						}
+						advance = !advance
 					}
 					v.Args[i] = a
 				}