fix(asm): size the arm64 logical-immediate materialisation exactly

Assisted-by: GLM 5.3

asm/arm64_encode_test.go

@@ -2070,6 +2070,40 @@ func TestArm64ConRnRejections(t *testing.T) {
 	}
 }
 
+// TestArm64LogicalMaterialisationBranch pins a forward branch over the
+// three-word logical materialisation against `go tool asm -S` (Go 1.27,
+// arm64): the size pass must count the MOVZ/MOVK pair the encoder lays down,
+// or the label offsets desynchronise from the bytes and the branch lands a
+// word early.
+func TestArm64LogicalMaterialisationBranch(t *testing.T) {
+	// The body closes with its own RET under the end label, so the file is
+	// parsed as written rather than through arm64Words' appended RET.
+	f, errs := parser.Parse("test_arm64.s", "#include \"textflag.h\"\n\nTEXT ·f(SB), NOSPLIT, $0-0\n\tCBZ\tR2, end\n\tTST\t$0x4900000049, R0\nend:\tRET\n")
+	if len(errs) > 0 {
+		t.Fatalf("parse: %v", errs)
+	}
+	img, err := AssembleFileARM64(f)
+	if err != nil {
+		t.Fatalf("AssembleFileARM64: %v", err)
+	}
+	got := leWords(img.Code)
+	want := []uint32{
+		0xb4000082, // CBZ R2, +16 (four words ahead)
+		0xd280093b, // MOVZ $0x49, R27
+		0xf2c0093b, // MOVK $(0x49<<32), R27
+		0xea1b001f, // TST R27, R0
+		0xd65f03c0, // RET
+	}
+	if len(got) != len(want) {
+		t.Fatalf("word count = %d, want %d", len(got), len(want))
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Errorf("word %d = %08x, want %08x", i, got[i], want[i])
+		}
+	}
+}
+
 // TestArm64FPImmediate pins the FP immediate moves against `go tool asm`
 // words: the FMOV (immediate) instruction for the 8-bit encodable values and
 // the FMOV-from-ZR move for zero, plus the rejections the toolchain raises