dwarf2out: Decrease dw_loc_descr_node and dw_attr_struct struct sizes [PR119711]HEADtrunkmaster

As noted by Richi on a large testcase, there are unnecessary paddings
in some heavily used dwarf2out.{h,cc} structures on 64-bit hosts.

struct dw_val_node {
        enum dw_val_class          val_class;            /*     0     4 */

        /* XXX 4 bytes hole, try to pack */

        struct addr_table_entry *  val_entry;            /*     8     8 */
        union dw_val_struct_union  v;                    /*    16    16 */

        /* size: 32, cachelines: 1, members: 3 */
        /* sum members: 28, holes: 1, sum holes: 4 */
        /* last cacheline: 32 bytes */
};
struct dw_loc_descr_node {
        dw_loc_descr_ref           dw_loc_next;          /*     0     8 */
        enum dwarf_location_atom   dw_loc_opc:8;         /*     8: 0  4 */
        unsigned int               dtprel:1;             /*     8: 8  4 */
        unsigned int               frame_offset_rel:1;   /*     8: 9  4 */

        /* XXX 22 bits hole, try to pack */

        int                        dw_loc_addr;          /*    12     4 */
        struct dw_val_node         dw_loc_oprnd1;        /*    16    32 */
        struct dw_val_node         dw_loc_oprnd2;        /*    48    32 */

        /* size: 80, cachelines: 2, members: 7 */
        /* sum members: 76 */
        /* sum bitfield members: 10 bits, bit holes: 1, sum bit holes: 22 bits */
        /* last cacheline: 16 bytes */
};
struct dw_attr_struct {
        enum dwarf_attribute       dw_attr;              /*     0     4 */

        /* XXX 4 bytes hole, try to pack */

        struct dw_val_node         dw_attr_val;          /*     8    32 */

        /* size: 40, cachelines: 1, members: 2 */
        /* sum members: 36, holes: 1, sum holes: 4 */
        /* last cacheline: 40 bytes */
};

The following patch is an (not very clean admittedly) attempt to decrease
size of dw_loc_descr_node from 80 bytes to 72 and (more importantly)
dw_attr_struct from 40 bytes to 32 by moving the dw_attr member from
dw_attr_struct into dw_attr_val's padding and similarly move
dw_loc_opc/dtprel/frame_offset_rel members into dw_loc_oprnd1 padding
and dw_loc_addr into dw_loc_oprnd2 padding.
All we need to ensure is that nothing tries to copy whole dw_val_node
structs unless it is copied as part of whole dw_loc_descr_node or
dw_attr_struct copy.

To verify that wasn't the case, I've temporarily added a deleted copy ctor
to dw_val_node and then looked at all the errors/warnings caused by that,
and those were just from memcpy/memmove or structure assignments of whole
dw_loc_descr_node/dw_attr_struct.

2025-04-24  Jakub Jelinek  <jakub@redhat.com>

	PR debug/119711
	* dwarf2out.h (struct dw_val_node): Add u member.
	(struct dw_loc_descr_node): Remove dw_loc_opc, dtprel,
	frame_offset_rel and dw_loc_addr members.
	(dw_loc_opc, dw_loc_dtprel, dw_loc_frame_offset_rel, dw_loc_addr):
	Define.
	(struct dw_attr_struct): Remove dw_attr member.
	(dw_attr): Define.
	* dwarf2out.cc (loc_descr_equal_p_1): Use dw_loc_dtprel instead of
	dtprel.
	(output_loc_operands, new_addr_loc_descr, loc_checksum,
	loc_checksum_ordered): Likewise.
	(resolve_args_picking_1): Use dw_loc_frame_offset_rel instead of
	frame_offset_rel.
	(loc_list_from_tree_1): Likewise.
	(resolve_addr_in_expr): Use dw_loc_dtprel instead of dtprel.
	(copy_deref_exprloc): Copy val_class, val_entry and v members
	instead of whole dw_loc_oprnd1 and dw_loc_oprnd2.
	(optimize_string_length): Copy val_class, val_entry and v members
	instead of whole dw_attr_val.
	(hash_loc_operands): Use dw_loc_dtprel instead of dtprel.
	(compare_loc_operands, compare_locs): Likewise.

0 files changed, 0 insertions, 0 deletions