Re: gcc dwarf2out: Drop the size + performance overhead of DW_AT_sibling

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

Hi Mark,

(.debug size)
5059640 = -3.71% no DW_AT_sibling
stap	0m0.577s (real)
gdb	0m0.234s (real)

5061160 = -3.68% DW_AT_sibling if >= 256 total children
stap	0m0.572s
gdb	0m0.232s

5084040 = -3.25%; DW_AT_sibling if >= 16 total children
stap	0m0.545s
gdb	0m0.231s

5169888 = -1.62%; DW_AT_sibling if >= 4 total children
stap	0m0.540s
gdb	0m0.235s

5254792 = ------; all DW_AT_sibling
stap	0m0.536s
gdb	0m0.243s


So the stap vs. gdb performance is exactly the opposite.

I will redo the timings after another change (DW_FORM_ref_udata) which may yet
change the timing / magic threshold.


Thanks,
Jan


gcc/
2011-10-19  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* dwarf2out.c (add_sibling_attributes): New variables next_die_no and
	this_die_no.  Do not produce DW_AT_sibling for too few children and no
	-gstruct-dwarf.

--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -7490,14 +7490,21 @@ static void
 add_sibling_attributes (dw_die_ref die)
 {
   dw_die_ref c;
+  static unsigned long next_die_no;
+  unsigned long this_die_no = next_die_no++;
 
   if (! die->die_child)
     return;
 
+  FOR_EACH_CHILD (die, c, add_sibling_attributes (c));
+
+  /* -gstruct-dwarf can be used for unconditional DW_AT_sibling for backward
+     compatibility.  */
+  if (!dwarf_strict && next_die_no - this_die_no < 16)
+    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.  */