feat(asm): lower the arm64 con(register) form to the ADD chain

Assisted-by: GLM 5.3 Flash

asm/arm64_encode_test.go

@@ -1978,3 +1978,65 @@ func TestArm64SimdArrangementRejections(t *testing.T) {
 		}
 	}
 }
+
+
+// TestArm64ConRn pins the MOVD $con(Rn), Rd lowering against `go tool asm`
+// words: the single ADD/SUB inside the addcon band, the hi<<12 plus lo pair
+// in the 24-bit band, and the pool plus UXTX add beyond it.
+func TestArm64ConRn(t *testing.T) {
+	got := arm64Words(t,
+		"\tMOVD $0x1002(RSP), R1\n"+
+			"\tMOVD $0x1708(RSP), RSP\n"+
+			"\tMOVD $0x2001(R7), R1\n"+
+			"\tMOVD $0xffffff(R7), R1\n"+
+			"\tMOVD $-1(R7), R1\n"+
+			"\tMOVD $-0x30(R7), R1\n"+
+			"\tMOVD $-0x2000(RSP), R1\n"+
+			"\tMOVD $-0x10000(RSP), RSP\n"+
+			"\tMOVD $0(R7), R1\n"+
+			"\tMOVD $4096(R7), R1\n"+
+			"\tMOVD $5(R3), R1\n")
+	want := []uint32{
+		0x914007e1, // ADD $(1<<12), RSP, R1
+		0x91000821, // ADD $2, R1, R1
+		0x914007ff, // ADD $(1<<12), RSP, RSP
+		0x911c23ff, // ADD $0x708, RSP, RSP
+		0x914008e1, // ADD $(2<<12), R7, R1
+		0x91000421, // ADD $1, R1, R1
+		0x917ffce1, // ADD $(4095<<12), R7, R1
+		0x913ffc21, // ADD $4095, R1, R1
+		0xd10004e1, // SUB $1, R7, R1
+		0xd100c0e1, // SUB $0x30, R7, R1
+		0xd1400be1, // SUB $(2<<12), RSP, R1
+		0xd14043ff, // SUB $(16<<12), RSP, RSP
+		0x910000e1, // ADD $0, R7, R1
+		0x914004e1, // ADD $(1<<12), R7, R1
+		0x91001461, // ADD $5, R3, R1
+		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])
+		}
+	}
+}
+
+// TestArm64ConRnRejections pins the shapes the toolchain refuses for the
+// con(register) form: every other width and the ZR destination.
+func TestArm64ConRnRejections(t *testing.T) {
+	for _, src := range []string{
+		"\tMOVW\t$5(R3), R1\n",
+		"\tMOVD\t$5(R7), ZR\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 // a parse rejection is a rejection
+		}
+		if _, err := AssembleFileARM64(f); err == nil {
+			t.Errorf("expected rejection for %q, got nil", strings.TrimSpace(src))
+		}
+	}
+}