diff --git a/ast.go b/ast.go
index 026c4b8..1eeb2f8 100644
--- a/ast.go
+++ b/ast.go
@@ -47,6 +47,9 @@ func (i Instruction) TargetLabel() *Label {
 	if !i.IsBranch {
 		return nil
 	}
+	if len(i.Operands) == 0 {
+		return nil
+	}
 	if ref, ok := i.Operands[0].(operand.LabelRef); ok {
 		lbl := Label(ref)
 		return &lbl
diff --git a/pass/cfg.go b/pass/cfg.go
index 120888a..501e09e 100644
--- a/pass/cfg.go
+++ b/pass/cfg.go
@@ -75,7 +75,9 @@ func CFG(fn *avo.Function) error {
 	// Populate predecessors.
 	for _, i := range is {
 		for _, s := range i.Succ {
-			s.Pred = append(s.Pred, i)
+			if s != nil {
+				s.Pred = append(s.Pred, i)
+			}
 		}
 	}
 
diff --git a/pass/cfg_test.go b/pass/cfg_test.go
index eb0237c..adddc3d 100644
--- a/pass/cfg_test.go
+++ b/pass/cfg_test.go
@@ -2,8 +2,11 @@ package pass
 
 import (
 	"reflect"
+	"sort"
 	"testing"
 
+	"github.com/mmcloughlin/avo/operand"
+
 	"github.com/mmcloughlin/avo"
 )
 
@@ -68,10 +71,222 @@ func TestLabelTargetInstructionFollowLabel(t *testing.T) {
 	}
 }
 
-func TestCFG(t *testing.T) {
-	// TODO(mbm): jump backward
-	// TODO(mbm): jump forward
-	// TODO(mbm): multiple returns
-	// TODO(mbm): infinite loop
-	// TODO(mbm): very short infinite loop
+func TestCFGSingleBasicBlock(t *testing.T) {
+	f := avo.NewFunction("simple")
+	f.AddInstruction(&avo.Instruction{Opcode: "A"})
+	f.AddInstruction(&avo.Instruction{Opcode: "B"})
+	f.AddInstruction(Terminal("RET"))
+
+	if err := ComputeCFG(t, f); err != nil {
+		t.Fatal(err)
+	}
+
+	AssertSuccessors(t, f, map[string][]string{
+		"A":   {"B"},
+		"B":   {"RET"},
+		"RET": {},
+	})
+
+	AssertPredecessors(t, f, map[string][]string{
+		"A":   {},
+		"B":   {"A"},
+		"RET": {"B"},
+	})
+}
+
+func TestCFGCondBranch(t *testing.T) {
+	f := avo.NewFunction("condbranch")
+	f.AddInstruction(&avo.Instruction{Opcode: "A"})
+	f.AddLabel(avo.Label("lblB"))
+	f.AddInstruction(&avo.Instruction{Opcode: "B"})
+	f.AddInstruction(&avo.Instruction{Opcode: "C"})
+	f.AddInstruction(CondBranch("J", "lblB"))
+	f.AddInstruction(Terminal("RET"))
+
+	if err := ComputeCFG(t, f); err != nil {
+		t.Fatal(err)
+	}
+
+	AssertSuccessors(t, f, map[string][]string{
+		"A":   {"B"},
+		"B":   {"C"},
+		"C":   {"J"},
+		"J":   {"B", "RET"},
+		"RET": {},
+	})
+}
+
+func TestCFGUncondBranch(t *testing.T) {
+	f := avo.NewFunction("uncondbranch")
+	f.AddInstruction(&avo.Instruction{Opcode: "A"})
+	f.AddLabel(avo.Label("lblB"))
+	f.AddInstruction(&avo.Instruction{Opcode: "B"})
+	f.AddInstruction(UncondBranch("JMP", "lblB"))
+
+	if err := ComputeCFG(t, f); err != nil {
+		t.Fatal(err)
+	}
+
+	AssertSuccessors(t, f, map[string][]string{
+		"A":   {"B"},
+		"B":   {"JMP"},
+		"JMP": {"B"},
+	})
+}
+
+func TestCFGJumpForward(t *testing.T) {
+	f := avo.NewFunction("forward")
+	f.AddInstruction(&avo.Instruction{Opcode: "A"})
+	f.AddInstruction(CondBranch("J", "done"))
+	f.AddInstruction(&avo.Instruction{Opcode: "B"})
+	f.AddLabel(avo.Label("done"))
+	f.AddInstruction(Terminal("RET"))
+
+	if err := ComputeCFG(t, f); err != nil {
+		t.Fatal(err)
+	}
+
+	AssertSuccessors(t, f, map[string][]string{
+		"A":   {"J"},
+		"J":   {"B", "RET"},
+		"B":   {"RET"},
+		"RET": {},
+	})
+}
+
+func TestCFGMultiReturn(t *testing.T) {
+	f := avo.NewFunction("multireturn")
+	f.AddInstruction(&avo.Instruction{Opcode: "A"})
+	f.AddInstruction(CondBranch("J", "fork"))
+	f.AddInstruction(&avo.Instruction{Opcode: "B"})
+	f.AddInstruction(Terminal("RET1"))
+	f.AddLabel(avo.Label("fork"))
+	f.AddInstruction(&avo.Instruction{Opcode: "C"})
+	f.AddInstruction(Terminal("RET2"))
+
+	if err := ComputeCFG(t, f); err != nil {
+		t.Fatal(err)
+	}
+
+	AssertSuccessors(t, f, map[string][]string{
+		"A":    {"J"},
+		"J":    {"B", "C"},
+		"B":    {"RET1"},
+		"RET1": {},
+		"C":    {"RET2"},
+		"RET2": {},
+	})
+}
+
+func TestCFGShortLoop(t *testing.T) {
+	f := avo.NewFunction("shortloop")
+	f.AddLabel(avo.Label("cycle"))
+	f.AddInstruction(UncondBranch("JMP", "cycle"))
+
+	if err := ComputeCFG(t, f); err != nil {
+		t.Fatal(err)
+	}
+
+	AssertSuccessors(t, f, map[string][]string{
+		"JMP": {"JMP"},
+	})
+}
+
+func TestCFGUndefinedLabel(t *testing.T) {
+	f := avo.NewFunction("undeflabel")
+	f.AddInstruction(&avo.Instruction{Opcode: "A"})
+	f.AddInstruction(CondBranch("J", "undef"))
+
+	err := ComputeCFG(t, f)
+
+	if err == nil {
+		t.Fatal("expect error on undefined label")
+	}
+}
+
+func TestCFGMissingLabel(t *testing.T) {
+	f := avo.NewFunction("missinglabel")
+	f.AddInstruction(&avo.Instruction{Opcode: "A"})
+	f.AddInstruction(&avo.Instruction{Opcode: "J", IsBranch: true}) // no label operand
+	err := ComputeCFG(t, f)
+	if err == nil {
+		t.Fatal("expect error on missing label")
+	}
+}
+
+// Terminal builds a terminal instruction.
+func Terminal(opcode string) *avo.Instruction {
+	return &avo.Instruction{Opcode: opcode, IsTerminal: true}
+}
+
+// CondBranch builds a conditional branch instruction to the given label.
+func CondBranch(opcode, lbl string) *avo.Instruction {
+	return &avo.Instruction{
+		Opcode:        opcode,
+		Operands:      []operand.Op{operand.LabelRef(lbl)},
+		IsBranch:      true,
+		IsConditional: true,
+	}
+}
+
+// UncondBranch builds an unconditional branch instruction to the given label.
+func UncondBranch(opcode, lbl string) *avo.Instruction {
+	return &avo.Instruction{
+		Opcode:        opcode,
+		Operands:      []operand.Op{operand.LabelRef(lbl)},
+		IsBranch:      true,
+		IsConditional: false,
+	}
+}
+
+func ComputeCFG(t *testing.T, f *avo.Function) error {
+	t.Helper()
+	if err := LabelTarget(f); err != nil {
+		t.Fatal(err)
+	}
+	return CFG(f)
+}
+
+func AssertSuccessors(t *testing.T, f *avo.Function, expect map[string][]string) {
+	AssertEqual(t, "successors", OpcodeSuccessorGraph(f), expect)
+}
+
+func AssertPredecessors(t *testing.T, f *avo.Function, expect map[string][]string) {
+	AssertEqual(t, "predecessors", OpcodePredecessorGraph(f), expect)
+}
+
+func AssertEqual(t *testing.T, what string, got, expect interface{}) {
+	t.Logf("%s=%#v\n", what, got)
+	if reflect.DeepEqual(expect, got) {
+		return
+	}
+	t.Fatalf("bad %s; expected=%#v", what, expect)
+}
+
+// OpcodeSuccessorGraph builds a map from opcode name to successor opcode names.
+func OpcodeSuccessorGraph(f *avo.Function) map[string][]string {
+	return OpcodeGraph(f, func(i *avo.Instruction) []*avo.Instruction { return i.Succ })
+}
+
+// OpcodePredecessorGraph builds a map from opcode name to predecessor opcode names.
+func OpcodePredecessorGraph(f *avo.Function) map[string][]string {
+	return OpcodeGraph(f, func(i *avo.Instruction) []*avo.Instruction { return i.Pred })
+}
+
+// OpcodeGraph builds a map from opcode name to neighboring opcode names. Each list of neighbors is sorted.
+func OpcodeGraph(f *avo.Function, neighbors func(*avo.Instruction) []*avo.Instruction) map[string][]string {
+	g := map[string][]string{}
+	for _, i := range f.Instructions() {
+		opcodes := []string{}
+		for _, n := range neighbors(i) {
+			opcode := "<nil>"
+			if n != nil {
+				opcode = n.Opcode
+			}
+			opcodes = append(opcodes, opcode)
+		}
+		sort.Strings(opcodes)
+		g[i.Opcode] = opcodes
+	}
+	return g
 }