test(asm): pin the ADDV16 immediate family the toolchain assembles

Assisted-by: GLM 5.3 Flash

asm/loong64_encode_test.go

@@ -1105,3 +1105,43 @@ TEXT ·d(SB), NOSPLIT, $0
 		})
 	}
 }
+
+// TestLOONG64_addv16 pins the ADDV16 immediate family, the toolchain's
+// assembler name for the addu16i.d hardware instruction (loong64enc1.s
+// spells it ADDV16; the spelling ADDU16I.D is the decoder's name for the
+// encoding and no assembler input at all).  The immediate must be a
+// multiple of 65536 and rides the instruction field shifted right by 16;
+// the oracle words are the toolchain's own loong64enc1.s rows, each also
+// confirmed against GOARCH=loong64 go tool asm.
+func TestLOONG64_addv16(t *testing.T) {
+	fn := firstTextLOONG64(t, `#include "textflag.h"
+TEXT ·a16(SB), NOSPLIT, $0
+	ADDV16 $(-32768<<16), R4, R5
+	ADDV16 $(0<<16), R4, R5
+	ADDV16 $(8<<16), R4, R5
+	ADDV16 $(32767<<16), R4, R5
+	ADDV16 $(16<<16), R4
+	RET
+`)
+	code := assembleLOONG64Helper(t, fn)
+	wantWords(t, code,
+		0x12000085, // addu16i.d r5, r4, -32768
+		0x10000085, // addu16i.d r5, r4, 0
+		0x10002085, // addu16i.d r5, r4, 8
+		0x11FFFC85, // addu16i.d r5, r4, 32767
+		0x10004084, // addu16i.d r4, r4, 16 (destination-only form)
+		0x4C000020,
+	)
+	// The non-multiples the loong64error.s catalogue pins (its rows 3 and
+	// 4): $1 and $65535 both refuse.
+	for _, src := range []string{
+		"ADDV16 $1, R4, R5",
+		"ADDV16 $65535, R4, R5",
+		"ADDV16 $(16<<16), F4",
+	} {
+		fn := firstTextLOONG64(t, "#include \"textflag.h\"\nTEXT ·e(SB), NOSPLIT, $0\n\t"+src+"\n\tRET\n")
+		if _, _, _, _, _, err := assembleLOONG64(fn); err == nil {
+			t.Errorf("%q: expected an error, got none", src)
+		}
+	}
+}