我用C语言定义了下面的结构

struct __attribute__((__packed__)) Input {
    enum {
        I_PE_left     = 0, 
        I_PE_up       = 1, 
        I_PE_right    = 2, 
        I_PE_down     = 3,
        I_LSU_left    = 4,
        I_LSU_up      = 5,
        I_LSU_right   = 6,
        I_LSU_down    = 7,
        I_LR_0        = 8,
        I_LR_1        = 9,
        I_LR_2        = 10,
        I_IMMI_0      = 11,
        I_IMMI_1      = 12,
        I_IMMI_2      = 13,
        I_UNUSE       = 15
    } src:4;
};  
struct __attribute__((__packed__)) Instr {
    struct Input inputx;
    struct Input inputy;
    struct Input inputz;
    struct Input inputzz;
};

在上面的示例中，sizeof(struct Instr)为4，这意味着即使是打包的结构体也至少需要1个字节。
然而，我想把sizeof(struct Instr)推到2，但不知道怎么做。
完整的定义如下。我可能需要递归地定义结构，因为我想多次使用结构定义。

struct __attribute__((__packed__)) Output {
    enum {
        O_S_PE_left     = 0, 
        O_S_PE_up       = 1, 
        O_S_PE_right    = 2, 
        O_S_PE_down     = 3,
        O_S_LSU_left    = 4,
        O_S_LSU_up      = 5,
        O_S_LSU_right   = 6,
        O_S_LSU_down    = 7,
        O_S_CALC_res    = 8,
        O_S_LR_0        = 9,
        O_S_LR_1        = 10,
        O_S_LR_2        = 11,
        O_S_IMMI_0      = 12,
        O_S_IMMI_1      = 13,
        O_S_IMMI_2      = 14,
        O_S_UNUSE       = 15
    } src:4;
    enum {
        O_I_LR_0        = 0,
        O_I_LR_1        = 1,
        O_I_LR_2        = 2,
        O_I_GR          = 3,
        O_I_FIFO        = 4,
        O_I_PE_DIR      = 5,
        O_I_LSU         = 6
    } dst:3;
};
struct __attribute__((__packed__)) Input {
    enum {
        I_PE_left     = 0, 
        I_PE_up       = 1, 
        I_PE_right    = 2, 
        I_PE_down     = 3,
        I_LSU_left    = 4,
        I_LSU_up      = 5,
        I_LSU_right   = 6,
        I_LSU_down    = 7,
        I_LR_0        = 8,
        I_LR_1        = 9,
        I_LR_2        = 10,
        I_IMMI_0      = 11,
        I_IMMI_1      = 12,
        I_IMMI_2      = 13,
        I_UNUSE       = 15
    } src:4;
};  
struct __attribute__((__packed__)) Instr {
    unsigned char un_use:6;
    unsigned char ctl_id:2;
    unsigned char output_ctl_right:2;
    unsigned char output_ctl_down:2;
    unsigned char output_ctl_left:2;
    unsigned char output_ctl_up:2;
    struct Output output_right; 
    struct Output output_down; 
    struct Output output_left; 
    struct Output output_up;
    struct Input inputx;
    struct Input inputy;
    struct Input inputz;
};