parse_expr
static struct hist_field *parse_expr(struct hist_trigger_data *hist_data,
operand1 = parse_expr(hist_data, file, str, operand_flags, NULL, n_subexprs);
operand1 = parse_expr(hist_data, file, operand1_str, operand_flags, NULL, n_subexprs);
operand2 = parse_expr(hist_data, file, str, operand_flags, NULL, n_subexprs);
hist_field = parse_expr(hist_data, file, field_str, flags, var_name, &n_subexprs);
hist_field = parse_expr(hist_data, file, field_str, flags,