Micro-OP: reduce UserItem class 72 to 64 bytes using align for 64-bit platforms

[GermanAizek]

@robert-ancell,

/* saved 8 bytes and 1 cacheline! */

The reduction allowed us to fit the cache line into one cache, this is an advantage, it is desirable to have structures and classes of exactly 64 bytes or less in size, now fewer CPU cycles are spent working with this structure.

This PR will decrease costs copying, moving, and creating object-structures only for common 64bit processors due to the 8-byte data alignment.

Smaller size structure or class, higher chance putting into CPU cache. Most processors are already 64 bit, so the change won't make it any worse.

Pahole example:

• Comment /* XXX {n} bytes hole, try to pack */ shows where optimization is possible by rearranging the order of fields structures and classes

About Linux Kernel tool Pahole: https://linux.die.net/man/1/pahole

Master branch

class UserItem {
public:

        class QString             name;                  /*     0     8 */
        class QString             realName;              /*     8     8 */
        class QString             homeDirectory;         /*    16     8 */
        class QString             image;                 /*    24     8 */
        class QString             background;            /*    32     8 */
        class QString             session;               /*    40     8 */
        bool                       isLoggedIn;           /*    48     1 */
        bool                       hasMessages;          /*    49     1 */

        /* XXX 6 bytes hole, try to pack */

        quint64                    uid;                  /*    56     8 */
        class QString displayName(const class UserItem  *);

        /* --- cacheline 1 boundary (64 bytes) --- */
        bool                       isLocked;             /*    64     1 */
        void UserItem(class UserItem *, const class UserItem  &);

        void ~UserItem(class UserItem *, int);

        void UserItem(class UserItem *);


        /* size: 72, cachelines: 2, members: 10 */
        /* sum members: 59, holes: 1, sum holes: 6 */
        /* padding: 7 */
        /* last cacheline: 8 bytes */
};

This PR

class UserItem {
public:

        class QString             name;                  /*     0     8 */
        class QString             realName;              /*     8     8 */
        class QString             homeDirectory;         /*    16     8 */
        class QString             image;                 /*    24     8 */
        class QString             background;            /*    32     8 */
        class QString             session;               /*    40     8 */
        bool                       isLoggedIn;           /*    48     1 */
        bool                       hasMessages;          /*    49     1 */
        bool                       isLocked;             /*    50     1 */

        /* XXX 5 bytes hole, try to pack */

        quint64                    uid;                  /*    56     8 */

        /* size: 64, cachelines: 1, members: 10 */
        /* sum members: 59, holes: 1, sum holes: 5 */
};   /* saved 8 bytes and 1 cacheline! */

Info about technique:

https://hpc.rz.rptu.de/Tutorials/AVX/alignment.shtml

https://wr.informatik.uni-hamburg.de/_media/teaching/wintersemester_2013_2014/epc-14-haase-svenhendrik-alignmentinc-presentation.pdf

https://en.wikipedia.org/wiki/Data_structure_alignment

https://stackoverflow.com/a/20882083

https://zijishi.xyz/post/optimization-technique/learning-to-use-data-alignment/