fix(arm64): reject bare BTI with a diagnostic and accept the full family

Assisted-by: GLM 5.3 Flash

asm/arm64_encode_test.go

@@ -675,6 +675,36 @@ func TestArm64AcquireRelease(t *testing.T) {
 	}
 }
 
+// TestArm64BTI pins the landing-pad family against the toolchain words:
+// only the uppercase C/J/JC spellings assemble, and bare BTI is a
+// diagnostic, never a panic.
+func TestArm64BTI(t *testing.T) {
+	got := arm64Words(t, "\tBTI C\n\tBTI J\n\tBTI JC\n")
+	want := []uint32{
+		0xd503245f, // BTI C
+		0xd503249f, // BTI J
+		0xd50324df, // BTI JC
+		0xd65f03c0, // RET
+	}
+	if len(got) != len(want) {
+		t.Fatalf("got %d words, want %d", len(got), len(want))
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Errorf("word %d = %#x, want %#x", i, got[i], want[i])
+		}
+	}
+	for _, src := range []string{"\tBTI\n", "\tBTI c\n", "\tBTI B\n"} {
+		f, errs := parser.Parse("test_arm64.s", "#include \"textflag.h\"\n\nTEXT ·f(SB), NOSPLIT, $0-0\n"+src+"\tRET\n")
+		if len(errs) > 0 {
+			continue
+		}
+		if _, err := AssembleFileARM64(f); err == nil {
+			t.Errorf("BTI spelling %q should be rejected, as go tool asm rejects it", src)
+		}
+	}
+}
+
 // TestArm64System pins BRK, SVC, the barriers, cache maintenance and the
 // system register accesses.
 func TestArm64System(t *testing.T) {