fix(parser): fold a signed parenthesised displacement expression

Assisted-by: GLM 5.3 Flash

parser/parser_test.go

@@ -634,3 +634,27 @@ func TestBareTrailingImmediate(t *testing.T) {
 		t.Errorf("jump target = %+v, want label loop", jmp.Operands[0])
 	}
 }
+
+// TestSignedParenDisplacement pins a sign before a parenthesised
+// displacement expression: -(24+8)(X6) negates the folded value and keeps
+// the base group, the shape GOROOT's riscv64 and loong64 files use.
+func TestSignedParenDisplacement(t *testing.T) {
+	file, errs := Parse("t.s", "TEXT \u00b7f(SB), NOSPLIT, $0-0\n\tMOV X7, -(24+8)(X6)\n\tMOV X7, +(16)(X6)\n\tRET\n")
+	if len(errs) > 0 {
+		t.Fatalf("parse: %v", errs)
+	}
+	text := file.Decls[0].(*ast.Text)
+	ins := text.Body[0].(*ast.Instr)
+	op := ins.Operands[1] // Plan 9 order: the destination address is last
+	if !op.Addr.HasOff || op.Addr.Offset != -32 {
+		t.Errorf("-(24+8): offset = %v hasOff=%v, want -32 true", op.Addr.Offset, op.Addr.HasOff)
+	}
+	if op.Addr.Base != "X6" {
+		t.Errorf("-(24+8): base = %q, want X6", op.Addr.Base)
+	}
+	ins = text.Body[1].(*ast.Instr)
+	op = ins.Operands[1]
+	if !op.Addr.HasOff || op.Addr.Offset != 16 || op.Addr.Base != "X6" {
+		t.Errorf("+(16): offset = %v hasOff=%v base=%q, want 16 true X6", op.Addr.Offset, op.Addr.HasOff, op.Addr.Base)
+	}
+}