pass: naive implementation of liveness

pass/reg.go

@@ -0,0 +1,62 @@
+package pass
+
+import (
+	"github.com/mmcloughlin/avo"
+	"github.com/mmcloughlin/avo/reg"
+)
+
+// Liveness computes register liveness.
+func Liveness(fn *avo.Function) error {
+	is := fn.Instructions()
+
+	// Initialize to empty sets.
+	for _, i := range is {
+		i.LiveIn = map[reg.ID]bool{}
+		i.LiveOut = map[reg.ID]bool{}
+	}
+
+	// Iterative dataflow analysis.
+	for {
+		changes := false
+
+		for _, i := range is {
+			// in[n] = use[n] UNION (out[n] - def[n])
+			nin := len(i.LiveIn)
+			for _, r := range i.InputRegisters() {
+				i.LiveIn[r.ID()] = true
+			}
+			def := map[reg.ID]bool{}
+			for _, r := range i.OutputRegisters() {
+				def[r.ID()] = true
+			}
+			for id := range i.LiveOut {
+				if !def[id] {
+					i.LiveIn[id] = true
+				}
+			}
+			if len(i.LiveIn) != nin {
+				changes = true
+			}
+
+			// out[n] = UNION[s IN succ[n]] in[s]
+			nout := len(i.LiveOut)
+			for _, s := range i.Succ {
+				if s == nil {
+					continue
+				}
+				for id := range s.LiveIn {
+					i.LiveOut[id] = true
+				}
+			}
+			if len(i.LiveOut) != nout {
+				changes = true
+			}
+		}
+
+		if !changes {
+			break
+		}
+	}
+
+	return nil
+}