diff --git a/arm64_eval.go b/arm64_eval.go
index 7c230f9..61505b6 100644
--- a/arm64_eval.go
+++ b/arm64_eval.go
@@ -137,6 +137,12 @@ func (c *arm64Ctx) eval64(op Operand, postInc bool) (string, error) {
 		return c.imm64(op.Imm), nil
 	case OpReg:
 		return c.loadReg(op.Reg)
+	case OpRegExtend:
+		v, err := c.loadReg(op.Reg)
+		if err != nil {
+			return "", err
+		}
+		return c.extendReg64(v, op.Ext)
 	case OpRegShift:
 		v, err := c.loadReg(op.Reg)
 		if err != nil {
@@ -191,6 +197,38 @@ func (c *arm64Ctx) eval64(op Operand, postInc bool) (string, error) {
 	}
 }
 
+func (c *arm64Ctx) extendReg64(v string, ext ExtendOp) (string, error) {
+	switch ext {
+	case ExtendUXTX, ExtendSXTX:
+		return v, nil
+	}
+
+	var fromTy string
+	var extOp string
+	switch ext {
+	case ExtendUXTB:
+		fromTy, extOp = "i8", "zext"
+	case ExtendUXTH:
+		fromTy, extOp = "i16", "zext"
+	case ExtendUXTW:
+		fromTy, extOp = "i32", "zext"
+	case ExtendSXTB:
+		fromTy, extOp = "i8", "sext"
+	case ExtendSXTH:
+		fromTy, extOp = "i16", "sext"
+	case ExtendSXTW:
+		fromTy, extOp = "i32", "sext"
+	default:
+		return "", fmt.Errorf("arm64: unsupported register extension %q", ext)
+	}
+
+	tr := c.newTmp()
+	fmt.Fprintf(c.b, "  %%%s = trunc i64 %s to %s\n", tr, v, fromTy)
+	ex := c.newTmp()
+	fmt.Fprintf(c.b, "  %%%s = %s %s %%%s to i64\n", ex, extOp, fromTy, tr)
+	return "%" + ex, nil
+}
+
 func (c *arm64Ctx) evalFPValue64(op Operand) (string, error) {
 	slot, ok := c.fpParams[op.FPOffset]
 	if !ok {
diff --git a/arm64_helper_edge_test.go b/arm64_helper_edge_test.go
index fc05ee8..fe2a615 100644
--- a/arm64_helper_edge_test.go
+++ b/arm64_helper_edge_test.go
@@ -1485,6 +1485,14 @@ func TestARM64EvalCoverage(t *testing.T) {
 	ops := []Operand{
 		{Kind: OpImm, Imm: 7},
 		{Kind: OpReg, Reg: "R0"},
+		{Kind: OpRegExtend, Reg: "R2", Ext: ExtendUXTB},
+		{Kind: OpRegExtend, Reg: "R2", Ext: ExtendUXTH},
+		{Kind: OpRegExtend, Reg: "R2", Ext: ExtendUXTW},
+		{Kind: OpRegExtend, Reg: "R2", Ext: ExtendUXTX},
+		{Kind: OpRegExtend, Reg: "R3", Ext: ExtendSXTB},
+		{Kind: OpRegExtend, Reg: "R3", Ext: ExtendSXTH},
+		{Kind: OpRegExtend, Reg: "R3", Ext: ExtendSXTW},
+		{Kind: OpRegExtend, Reg: "R3", Ext: ExtendSXTX},
 		{Kind: OpRegShift, Reg: "R1", ShiftOp: ShiftLeft, ShiftAmount: 2},
 		{Kind: OpRegShift, Reg: "R1", ShiftOp: ShiftRight, ShiftAmount: 1},
 		{Kind: OpFP, FPOffset: 0},
@@ -1514,6 +1522,9 @@ func TestARM64EvalCoverage(t *testing.T) {
 	if _, err := c.eval64(Operand{Kind: OpRegShift, Reg: "R1", ShiftOp: ShiftRotate, ShiftReg: "R2"}, false); err == nil {
 		t.Fatalf("eval64(register shift) unexpectedly succeeded")
 	}
+	if _, err := c.eval64(Operand{Kind: OpRegExtend, Reg: "R1", Ext: ExtendOp("BAD")}, false); err == nil {
+		t.Fatalf("eval64(bad extension) unexpectedly succeeded")
+	}
 	if _, err := c.eval64(Operand{Kind: OpRegShift, Reg: "R1", ShiftOp: ShiftRotate, ShiftAmount: 1}, false); err == nil {
 		t.Fatalf("eval64(rotate) unexpectedly succeeded")
 	}
diff --git a/arm64_new_opfamilies_test.go b/arm64_new_opfamilies_test.go
index 5cac3e8..38c143e 100644
--- a/arm64_new_opfamilies_test.go
+++ b/arm64_new_opfamilies_test.go
@@ -78,3 +78,26 @@ TEXT featureprobe(SB),NOSPLIT,$0-0
 		t.Fatal(err)
 	}
 }
+
+func TestTranslateARM64CompareWithExtendedRegister(t *testing.T) {
+	src := `
+TEXT countcmp(SB),NOSPLIT,$0-0
+	CMP	R2.UXTB, R5
+	CINC	EQ, R11, R11
+	RET
+`
+	file, err := Parse(ArchARM64, src)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = Translate(file, Options{
+		TargetTriple: "aarch64-unknown-linux-gnu",
+		Sigs: map[string]FuncSig{
+			"countcmp": {Name: "countcmp", Ret: Void},
+		},
+		Goarch: "arm64",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/types.go b/types.go
index f4bfeb8..9ecf439 100644
--- a/types.go
+++ b/types.go
@@ -140,6 +140,7 @@ const (
 	OpInvalid OperandKind = iota
 	OpImm
 	OpReg
+	OpRegExtend
 	OpRegShift
 	OpFP
 	OpFPAddr
@@ -159,6 +160,22 @@ const (
 	ShiftRotate ShiftOp = "@>"
 )
 
+// ExtendOp is an ARM64 register-extension modifier (for example UXTB, SXTW).
+// The prefix U/S selects zero- or sign-extension; the suffix B/H/W/X selects
+// the source width (8/16/32/64 bits).
+type ExtendOp string
+
+const (
+	ExtendUXTB ExtendOp = "UXTB"
+	ExtendUXTH ExtendOp = "UXTH"
+	ExtendUXTW ExtendOp = "UXTW"
+	ExtendUXTX ExtendOp = "UXTX"
+	ExtendSXTB ExtendOp = "SXTB"
+	ExtendSXTH ExtendOp = "SXTH"
+	ExtendSXTW ExtendOp = "SXTW"
+	ExtendSXTX ExtendOp = "SXTX"
+)
+
 // Operand models a minimal subset of Plan 9 asm operands.
 //
 // Supported:
@@ -168,9 +185,10 @@ const (
 type Operand struct {
 	Kind OperandKind
 
-	Imm    int64  // OpImm
-	ImmRaw string // OpImm unresolved symbolic placeholder, including leading '$'
-	Reg    Reg    // OpReg
+	Imm    int64    // OpImm
+	ImmRaw string   // OpImm unresolved symbolic placeholder, including leading '$'
+	Reg    Reg      // OpReg
+	Ext    ExtendOp // OpRegExtend
 	// OpRegShift
 	ShiftOp     ShiftOp
 	ShiftAmount int64
@@ -210,6 +228,8 @@ func (o Operand) String() string {
 		return fmt.Sprintf("$%d", o.Imm)
 	case OpReg:
 		return string(o.Reg)
+	case OpRegExtend:
+		return fmt.Sprintf("%s.%s", o.Reg, o.Ext)
 	case OpRegShift:
 		suffix := fmt.Sprintf("%d", o.ShiftAmount)
 		if o.ShiftReg != "" {
@@ -516,6 +536,9 @@ func parseOperand(s string) (Operand, error) {
 	if name, off, ok := parseFPAddr(s); ok {
 		return Operand{Kind: OpFPAddr, FPName: name, FPOffset: off}, nil
 	}
+	if r, ext, ok := parseRegExtend(s); ok {
+		return Operand{Kind: OpRegExtend, Reg: r, Ext: ext}, nil
+	}
 	if base, sop, amt, shiftReg, ok := parseRegShift(s); ok {
 		return Operand{Kind: OpRegShift, Reg: base, ShiftOp: sop, ShiftAmount: amt, ShiftReg: shiftReg}, nil
 	}
@@ -578,6 +601,29 @@ func parseOperand(s string) (Operand, error) {
 	return Operand{}, fmt.Errorf("unsupported operand: %q", s)
 }
 
+func parseRegExtend(s string) (Reg, ExtendOp, bool) {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return "", "", false
+	}
+	dot := strings.LastIndexByte(s, '.')
+	if dot <= 0 || dot == len(s)-1 {
+		return "", "", false
+	}
+	r, ok := parseReg(s[:dot])
+	if !ok {
+		return "", "", false
+	}
+	ext := ExtendOp(strings.ToUpper(strings.TrimSpace(s[dot+1:])))
+	switch ext {
+	case ExtendUXTB, ExtendUXTH, ExtendUXTW, ExtendUXTX,
+		ExtendSXTB, ExtendSXTH, ExtendSXTW, ExtendSXTX:
+		return r, ext, true
+	default:
+		return "", "", false
+	}
+}
+
 func parseRegShift(s string) (base Reg, sop ShiftOp, amt int64, shiftReg Reg, ok bool) {
 	s = strings.TrimSpace(s)
 	if s == "" {
diff --git a/types_deep_test.go b/types_deep_test.go
index 10e32df..4b9c275 100644
--- a/types_deep_test.go
+++ b/types_deep_test.go
@@ -10,6 +10,7 @@ func TestTypeHelperCoverage(t *testing.T) {
 		{Operand{Kind: OpImm, Imm: 7}, "$7"},
 		{Operand{Kind: OpImm, ImmRaw: "$(A+1)"}, "$(A+1)"},
 		{Operand{Kind: OpReg, Reg: AX}, "AX"},
+		{Operand{Kind: OpRegExtend, Reg: "R2", Ext: ExtendUXTB}, "R2.UXTB"},
 		{Operand{Kind: OpRegShift, Reg: "R1", ShiftOp: ShiftLeft, ShiftAmount: 2}, "R1<<2"},
 		{Operand{Kind: OpRegShift, Reg: "R2", ShiftOp: ShiftRight, ShiftReg: "R3"}, "R2>>R3"},
 		{Operand{Kind: OpFP, FPName: "arg", FPOffset: 8}, "arg+8(FP)"},
@@ -203,12 +204,19 @@ func TestTypeParserEdgeCoverage(t *testing.T) {
 		{"MIDR_EL1", OpIdent},
 		{"helper<>(SB)", OpSym},
 		{"R1@>2", OpRegShift},
+		{"R2.UXTB", OpRegExtend},
 	} {
 		op, err := parseOperand(tc.in)
 		if err != nil || op.Kind != tc.want {
 			t.Fatalf("parseOperand(%q) = (%v, %v), want kind %v", tc.in, err, op.Kind, tc.want)
 		}
 	}
+	if reg, ext, ok := parseRegExtend("r3.sxtw"); !ok || reg != "R3" || ext != ExtendSXTW {
+		t.Fatalf("parseRegExtend(r3.sxtw) = (%q, %q, %v)", reg, ext, ok)
+	}
+	if _, _, ok := parseRegExtend("R2.BAD"); ok {
+		t.Fatalf("parseRegExtend(R2.BAD) unexpectedly succeeded")
+	}
 	if _, err := parseOperand("[]"); err == nil {
 		t.Fatalf("parseOperand([]) unexpectedly succeeded")
 	}