Re: Performance issues with -ffunction-sections and dwarf2 debug line info

[Alan Modra <amodra at bigpond dot net dot au>]

On Wed, Apr 26, 2006 at 12:29:18PM +0200, Nicolas Roche wrote:
> I am using GCC 3.4 and the -ffunction-sections -fdata-sections options 
> along with binutils 2.16.1 (for dead code elimination)
> 
> When using this functions the debug line info is no more sorted 
> (increasing line and increasing vma) and so on big files with lots of 
> functions you can observe a quadratic behavior for the time needed to 
> read this info (addr2line, ...). The reason is that we are almost always 
> triggering the worst case in add_line_info function in bfd/dwarf2.c.
> 
> The problem is in gas. Indeed in dwarf2dbg.c a list of sections is 
> maintained (all_segs), but each time a new section is created, it is 
> appended to the beginning of the list....
> 
> so if  you have the following source:
> 
> 1 begin function1
> 2
> 3 end function1
> 4 begin function2
> 5
> 6 end function2
> 
> the line information will be sorted in the following order:
> 4,5,6,1,2,3
> 
> In order to solve this issue we just have to append to the tail of all_segs.

Thanks for the analysis and patch.  It looks OK to me as all places that
traverse the list don't care too much about the order.  I'm going to
apply the following slightly different patch instead, so we don't need
another static var and to make line_seg and line_subseg list addition
look similar.

	* dwarf2dbg.c (get_line_subseg): Attach new struct line_seg to end
	of list rather than beginning.

Index: gas/dwarf2dbg.c
===================================================================
RCS file: /cvs/src/src/gas/dwarf2dbg.c,v
retrieving revision 1.81
diff -u -p -r1.81 dwarf2dbg.c
--- gas/dwarf2dbg.c	28 Feb 2006 00:38:19 -0000	1.81
+++ gas/dwarf2dbg.c	27 Apr 2006 03:18:54 -0000
@@ -209,21 +209,21 @@ get_line_subseg (segT seg, subsegT subse
   static subsegT last_subseg;
   static struct line_subseg *last_line_subseg;
 
-  struct line_seg *s;
+  struct line_seg **ps, *s;
   struct line_subseg **pss, *ss;
 
   if (seg == last_seg && subseg == last_subseg)
     return last_line_subseg;
 
-  for (s = all_segs; s; s = s->next)
+  for (ps = &all_segs; (s = *ps) != NULL; ps = &s->next)
     if (s->seg == seg)
       goto found_seg;
 
   s = (struct line_seg *) xmalloc (sizeof (*s));
-  s->next = all_segs;
+  s->next = NULL;
   s->seg = seg;
   s->head = NULL;
-  all_segs = s;
+  *ps = s;
 
  found_seg:
   for (pss = &s->head; (ss = *pss) != NULL ; pss = &ss->next)

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre