bitfield structures

[Alex Deucher]

As part of the amdgpu transition, we are moving to using database
generated register and packet headers.  We have a number of options
for formatting, some of which involve bitfields (don't worry there
will also be shift/mask style headers as well which is mainly what we
use in the code).  I think these formats are cleaner for a number of
cases, however, as far as I know, C does not define the ordering of
bits within bitfields.  That said, every compiler I've used seems to
do what you would expect.  It makes coding a lot cleaner as less
error-prone in certain cases.  Here are a couple of example of what
I'm talking about:

A register example:

union GRPH_SWAP_CNTL {
        struct {
#if BIG_ENDIAN
                unsigned int                                 : 20;
                unsigned int             GRPH_ALPHA_CROSSBAR : 2;
                unsigned int              GRPH_BLUE_CROSSBAR : 2;
                unsigned int             GRPH_GREEN_CROSSBAR : 2;
                unsigned int               GRPH_RED_CROSSBAR : 2;
                unsigned int                                 : 2;
                unsigned int                GRPH_ENDIAN_SWAP : 2;
#else
                unsigned int                GRPH_ENDIAN_SWAP : 2;
                unsigned int                                 : 2;
                unsigned int               GRPH_RED_CROSSBAR : 2;
                unsigned int             GRPH_GREEN_CROSSBAR : 2;
                unsigned int              GRPH_BLUE_CROSSBAR : 2;
                unsigned int             GRPH_ALPHA_CROSSBAR : 2;
                unsigned int                                 : 20;
#endif
        } bitfields, bits;
        unsigned int u32All;
        signed int i32All;
        float f32All;
};

A packet example:

typedef union PM4_TYPE_3_HEADER
{
    struct
    {
#if BIG_ENDIAN
        unsigned int type      : 2; ///< packet identifier. It should
be 3 for type 3 packets
        unsigned int count     : 14;///< number of DWORDs - 1 in the
information body.
        unsigned int opcode    : 8; ///< IT opcode
        unsigned int reserved1 : 6; ///< reserved
        unsigned int shaderType: 1; ///< 0: Graphics, 1: Compute Shader
        unsigned int predicate : 1; ///< predicated version of packet when set
#else
        unsigned int predicate : 1; ///< predicated version of packet when set
        unsigned int shaderType: 1; ///< 0: Graphics, 1: Compute Shader
        unsigned int reserved1 : 6; ///< reserved
        unsigned int opcode    : 8; ///< IT opcode
        unsigned int count     : 14;///< number of DWORDs - 1 in the
information body.
        unsigned int type      : 2; ///< packet identifier. It should
be 3 for type 3 packets
#endif
    };
    unsigned int u32All;
} PM4_TYPE_3_HEADER;

//--------------------MEM_SEMAPHORE--------------------

enum MEM_SEMAPHORE_signal_type_enum {
signal_type_mem_semaphore_SIGNAL_TYPE_INCREMENT_0 = 0,
signal_type_mem_semaphore_SIGNAL_TYPE_WRITE_1 = 1 };
enum MEM_SEMAPHORE_client_code_enum { client_code_mem_semaphore_CP_0 =
0, client_code_mem_semaphore_CB_1 = 1, client_code_mem_semaphore_DB_2
= 2, client_code_mem_semaphore_RESERVED_3 = 3 };
enum MEM_SEMAPHORE_sem_sel_enum {
sem_sel_mem_semaphore_SIGNAL_SEMAPHORE_6 = 6,
sem_sel_mem_semaphore_WAIT_SEMAPHORE_7 = 7 };

typedef struct _PM4_MEM_SEMAPHORE
{
    union
    {
        PM4_TYPE_3_HEADER   header;            ///header
        unsigned int        ordinal1;
    };

    union
    {
        struct
    {
#if BIG_ENDIAN
            unsigned int address_lo:29;
            unsigned int reserved1:3;
#else
            unsigned int reserved1:3;
            unsigned int address_lo:29;
#endif
        } bitfields2;
        unsigned int ordinal2;
    };

    union
    {
        struct
    {
#if BIG_ENDIAN
            MEM_SEMAPHORE_sem_sel_enum sem_sel:3;
            unsigned int reserved4:3;
            MEM_SEMAPHORE_client_code_enum client_code:2;
            unsigned int reserved3:3;
            MEM_SEMAPHORE_signal_type_enum signal_type:1;
            unsigned int reserved2:3;
            unsigned int use_mailbox:1;
            unsigned int address_hi:16;
#else
            unsigned int address_hi:16;
            unsigned int use_mailbox:1;
            unsigned int reserved2:3;
            MEM_SEMAPHORE_signal_type_enum signal_type:1;
            unsigned int reserved3:3;
            MEM_SEMAPHORE_client_code_enum client_code:2;
            unsigned int reserved4:3;
            MEM_SEMAPHORE_sem_sel_enum sem_sel:3;
#endif
        } bitfields3;
        unsigned int ordinal3;
    };

}  PM4MEM_SEMAPHORE, *PPM4MEM_SEMAPHORE;

Are there any strong objections to these sorts of structures?

Thanks,

Alex