reg: support for register casting

Adds methods for referencing sub- or super-registers. For example, for
general purpose registers you can now reference As8(), As16(), ... and
for vector AsX(), AsY(), AsZ().

Closes #1

reg/reg_test.go

@@ -64,3 +64,74 @@ func TestAreConflicting(t *testing.T) {
 		}
 	}
 }
+
+func TestFamilyLookup(t *testing.T) {
+	cases := []struct {
+		Family *Family
+		ID     PID
+		Spec   Spec
+		Expect Physical
+	}{
+		{GeneralPurpose, 0, S8, AL},
+		{GeneralPurpose, 1, S8L, CL},
+		{GeneralPurpose, 2, S8H, DH},
+		{GeneralPurpose, 3, S16, BX},
+		{GeneralPurpose, 9, S32, R9L},
+		{GeneralPurpose, 13, S64, R13},
+		{GeneralPurpose, 13, S512, nil},
+		{GeneralPurpose, 133, S64, nil},
+		{Vector, 1, S128, X1},
+		{Vector, 13, S256, Y13},
+		{Vector, 27, S512, Z27},
+		{Vector, 1, S16, nil},
+		{Vector, 299, S256, nil},
+	}
+	for _, c := range cases {
+		got := c.Family.Lookup(c.ID, c.Spec)
+		if got != c.Expect {
+			t.Errorf("pid=%v spec=%v: lookup got %v expect %v", c.ID, c.Spec, got, c.Expect)
+		}
+	}
+}
+
+func TestPhysicalAs(t *testing.T) {
+	cases := []struct {
+		Register Physical
+		Spec     Spec
+		Expect   Physical
+	}{
+		{DX, S8L, DL},
+		{DX, S8H, DH},
+		{DX, S8, DL},
+		{DX, S16, DX},
+		{DX, S32, EDX},
+		{DX, S64, RDX},
+		{DX, S256, nil},
+	}
+	for _, c := range cases {
+		got := c.Register.as(c.Spec)
+		if got != c.Expect {
+			t.Errorf("%s.as(%v) = %v; expect %v", c.Register.Asm(), c.Spec, got, c.Expect)
+		}
+	}
+}
+
+func TestVirtualAs(t *testing.T) {
+	cases := []struct {
+		Virtual  Register
+		Physical Physical
+		Match    bool
+	}{
+		{GeneralPurpose.Virtual(0, B8), CL, true},
+		{GeneralPurpose.Virtual(0, B8), CH, true},
+		{GeneralPurpose.Virtual(0, B32).as(S8L), CL, true},
+		{GeneralPurpose.Virtual(0, B32).as(S8L), CH, false},
+		{GeneralPurpose.Virtual(0, B16).as(S32), R9L, true},
+		{GeneralPurpose.Virtual(0, B16).as(S32), R9, false},
+	}
+	for _, c := range cases {
+		if c.Virtual.(Virtual).SatisfiedBy(c.Physical) != c.Match {
+			t.Errorf("%s.SatisfiedBy(%v) != %v", c.Virtual.Asm(), c.Physical, c.Match)
+		}
+	}
+}