Better pack struct zio_prop

[amotin]

By using precisely sized fields it is possible to reduce the size of this structure and respectively struct zio it is included into by 40 bytes (from 92 to 52).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

Very nice. For reviewers, after this commit here's the packed struct as reported by pahole.

struct zio_prop {
        enum zio_checksum          zp_checksum:8;        /*     0: 0  4 */
        enum zio_compress          zp_compress:8;        /*     0: 8  4 */

        /* Bitfield combined with next fields */

        uint8_t                    zp_complevel;         /*     2     1 */
        uint8_t                    zp_level;             /*     3     1 */
        uint8_t                    zp_copies;            /*     4     1 */
        uint8_t                    zp_gang_copies;       /*     5     1 */

        /* Bitfield combined with previous fields */

        dmu_object_type_t          zp_type:8;            /*     4:16  4 */
        dmu_object_type_t          zp_storage_type:8;    /*     4:24  4 */
        boolean_t                  zp_dedup:1;           /*     8: 0  4 */
        boolean_t                  zp_dedup_verify:1;    /*     8: 1  4 */
        boolean_t                  zp_nopwrite:1;        /*     8: 2  4 */
        boolean_t                  zp_brtwrite:1;        /*     8: 3  4 */
        boolean_t                  zp_encrypt:1;         /*     8: 4  4 */
        boolean_t                  zp_byteorder:1;       /*     8: 5  4 */
        boolean_t                  zp_direct_write:1;    /*     8: 6  4 */
        boolean_t                  zp_rewrite:1;         /*     8: 7  4 */

        /* XXX 24 bits hole, try to pack */

        uint32_t                   zp_zpl_smallblk;      /*    12     4 */
        uint8_t                    zp_salt[8];           /*    16     8 */
        uint8_t                    zp_iv[12];            /*    24    12 */
        uint8_t                    zp_mac[16];           /*    36    16 */

        /* size: 52, cachelines: 1, members: 20 */
        /* sum members: 44 */
        /* sum bitfield members: 40 bits, bit holes: 1, sum bit holes: 24 bits */
        /* last cacheline: 52 bytes */
};