cmd/callgraph: update flag docstring

And add a TODO.

Change-Id: I51c63b32e9ac4309cdfb8228348a5d528f36a919
Reviewed-on: https://go-review.googlesource.com/10489
Reviewed-by: Robert Griesemer <gri@golang.org>

cmd/callgraph/main.go

@@ -42,7 +42,7 @@ import (
 )
 
 var algoFlag = flag.String("algo", "rta",
-	`Call graph construction algorithm, one of "rta" or "pta"`)
+	`Call graph construction algorithm (static, cha, rta, pta)`)
 
 var testFlag = flag.Bool("test", false,
 	"Loads test code (*_test.go) for imported packages")

go/callgraph/static/static.go

@@ -15,6 +15,8 @@ func CallGraph(prog *ssa.Program) *callgraph.Graph {
 	cg := callgraph.New(nil) // TODO(adonovan) eliminate concept of rooted callgraph
 
 	// TODO(adonovan): opt: use only a single pass over the ssa.Program.
+	// TODO(adonovan): opt: this is slower than RTA (perhaps because
+	// the lower precision means so many edges are allocated)!
 	for f := range ssautil.AllFunctions(prog) {
 		fnode := cg.CreateNode(f)
 		for _, b := range f.Blocks {