[Bug libdw/30967] Discriminator in Dwarf_Line_s may exceed 24 bits

["fche at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30967

--- Comment #8 from Frank Ch. Eigler <fche at redhat dot com> ---
> The issue here is that we create (very) large arrays of struct Dwarf_Line_s
> and we do that eagerly, see bug #27405
> So we would like to keep that struct as small as possible.

Do we have an estimate about the numbers we're talking about here?  The current
Dwarf_Line_s object is 40 bytes long (on x86-64).  Even if packing becomes less
efficient, it could cost us say 2 bytes more per record.  These records are
packed together in allocation via realloc() et al.  How many records do we see
in programs of interest?  readelf -wL /bin/emacs indicates about 800 thousand;
libxul.so about 8 million.  Is this about single digit megabytes more RAM in
grand total?

Note that bug #27405 was a request for optimization, not in order to save a few
percent of memory for used data, but to save ALL the related memory & time for
totally unused data.


struct Dwarf_Line_s {
        Dwarf_Files *              files;                /*     0     8 */
        Dwarf_Addr                 addr;                 /*     8     8 */
        unsigned int               file;                 /*    16     4 */
        int                        line;                 /*    20     4 */
        short unsigned int         column;               /*    24     2 */

        /* Bitfield combined with previous fields */

        unsigned int               is_stmt:1;            /*    24:16  4 */
        unsigned int               basic_block:1;        /*    24:17  4 */
        unsigned int               end_sequence:1;       /*    24:18  4 */
        unsigned int               prologue_end:1;       /*    24:19  4 */
        unsigned int               epilogue_begin:1;     /*    24:20  4 */
        unsigned int               op_index:8;           /*    24:21  4 */

        /* XXX 3 bits hole, try to pack */

        /* Force alignment to the next boundary: */
        unsigned int               :0;

        unsigned int               isa:8;                /*    28: 0  4 */
        unsigned int               discriminator:24;     /*    28: 8  4 */
        unsigned int               context;              /*    32     4 */
        unsigned int               function_name;        /*    36     4 */

        /* size: 40, cachelines: 1, members: 15 */
        /* sum members: 34 */
        /* sum bitfield members: 45 bits, bit holes: 1, sum bit holes: 3 bits
*/
        /* last cacheline: 40 bytes */
};

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