[patch] dwarf2out: Drop the size + performance overhead of DW_AT_sibling

[Jan Kratochvil <jan.kratochvil@redhat.com>]

Hi,

dropping the optional DWARF attribute DW_AT_sibling has only advantages and no
disadvantages:

For files with .gdb_index GDB initial scan does not use DW_AT_sibling at all.
For files without .gdb_index GDB initial scan has 1.79% time _improvement_.
For .debug files it brings 3.49% size decrease (7.84% for rpm compressed files).

I guess DW_AT_sibling had real performance gains on CPUs with 1x (=no) clock
multipliers.  Nowadays mostly only the data size transferred over FSB matters.

I do not think there would be any DWARF consumers compatibility problems as
DW_AT_sibling has always been optional but I admit I have tested only GDB.


"clean" is FSF GCC+GDB, "ns" is FSF GCC with the patch applied.

gdbindex -readnow 100x warm:
clean:
56.975 57.161 57.738 58.243 57.52924999999999 seconds
ns:
57.799 58.008 58.202 58.473 58.120499999999993 seconds
+1.03% = performance decrease but it should be 0%, it is a measurement error
gdbindex -readnow 20x warm(gdb) cold(data):
clean:
57.989
ns:
58.538
+0.95% = performance decrease but it should be 0%, it is a measurement error
200x warm:
clean:
14.393 14.414 14.587 14.496 14.472499999999998 seconds
ns:
14.202 14.160 14.174 14.318 14.213499999999998 seconds
-1.79% = performance improvement of non-gdbindex scan (dwarf2_build_psymtabs_hard)

gdbindex .debug:
clean = 5589272 bytes
ns = 5394120 bytes
-3.49% = size improvement

gdbindex .debug.xz9:
clean = 1158696 bytes
ns = 1067900 bytes
-7.84% = size improvement

.debug_info + .debug_types:
clean = 0x1a11a0+0x08f389 bytes
ns = 0x184205+0x0833b0 bytes
-7.31% = size improvement

Intel i7-920 CPU and only libstdc++ from GCC 4.7.0 20111002 and `-O2 -gdwarf-4
-fdebug-types-section' were used for the benchmark.  GCC 4.7.0 20111002
--enable-languages=c++ was used for `make check' regression testing.


Thanks,
Jan


gcc/
2011-10-12  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Stop producing DW_AT_sibling.
	* dwarf2out.c (add_sibling_attributes): Remove the declaration.
	(add_sibling_attributes): Remove the function.
	(dwarf2out_finish): Remove calls of add_sibling_attributes.

--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -3316,7 +3316,6 @@ static int htab_cu_eq (const void *, const void *);
 static void htab_cu_del (void *);
 static int check_duplicate_cu (dw_die_ref, htab_t, unsigned *);
 static void record_comdat_symbol_number (dw_die_ref, htab_t, unsigned);
-static void add_sibling_attributes (dw_die_ref);
 static void build_abbrev_table (dw_die_ref);
 static void output_location_lists (dw_die_ref);
 static int constant_size (unsigned HOST_WIDE_INT);
@@ -7482,24 +7481,6 @@ copy_decls_for_unworthy_types (dw_die_ref unit)
   unmark_dies (unit);
 }
 
-/* Traverse the DIE and add a sibling attribute if it may have the
-   effect of speeding up access to siblings.  To save some space,
-   avoid generating sibling attributes for DIE's without children.  */
-
-static void
-add_sibling_attributes (dw_die_ref die)
-{
-  dw_die_ref c;
-
-  if (! die->die_child)
-    return;
-
-  if (die->die_parent && die != die->die_parent->die_child)
-    add_AT_die_ref (die, DW_AT_sibling, die->die_sib);
-
-  FOR_EACH_CHILD (die, c, add_sibling_attributes (c));
-}
-
 /* Output all location lists for the DIE and its children.  */
 
 static void
@@ -22496,14 +22477,6 @@ dwarf2out_finish (const char *filename)
       prune_unused_types ();
     }
 
-  /* Traverse the DIE's and add add sibling attributes to those DIE's
-     that have children.  */
-  add_sibling_attributes (comp_unit_die ());
-  for (node = limbo_die_list; node; node = node->next)
-    add_sibling_attributes (node->die);
-  for (ctnode = comdat_type_list; ctnode != NULL; ctnode = ctnode->next)
-    add_sibling_attributes (ctnode->root_die);
-
   /* Output a terminator label for the .text section.  */
   switch_to_section (text_section);
   targetm.asm_out.internal_label (asm_out_file, TEXT_END_LABEL, 0);