[PATCH] add splay tree for info_ptr -> CU mapping

Martin Liška mliska@suse.cz
Sun Aug 7 18:05:13 GMT 2022


While using perf top for MozillaThunderbird I noticed quite some slow
dissably call with source code involved. E.g.

time ./objdump --start-address=0x0000000004e0dcd0 --stop-address=0x0000000004e0df8b -l -d --no-show-raw-insn -S -C /usr/lib64/thunderbird/libxul.so

took 2.071s and I noticed quite some time is spent in
find_abstract_instance:

    33.46%  objdump  objdump               [.] find_abstract_instance
    18.22%  objdump  objdump               [.] arange_add
    13.77%  objdump  objdump               [.] read_attribute_value
     4.82%  objdump  objdump               [.] comp_unit_maybe_decode_line_info
     3.10%  objdump  libc.so.6             [.] __memset_avx2_unaligned_erms

where linked list of CU is iterated when searing for where info_ptr
belongs to:

         : 3452   for (u = unit->prev_unit; u != NULL; u = u->prev_unit)
    0.00 :   4c61f7: mov    0x10(%rbx),%rax
    0.00 :   4c61fb: test   %rax,%rax
    0.00 :   4c61fe: je     4c6215 <find_abstract_instance+0x365>
         : 3453   if (info_ptr >= u->info_ptr_unit && info_ptr < u->end_ptr)
    0.00 :   4c6200: cmp    0x60(%rax),%rdx
   83.20 :   4c6204: jb     4c620c <find_abstract_instance+0x35c>
    0.00 :   4c6206: cmp    0x78(%rax),%rdx
    6.89 :   4c620a: jb     4c6270 <find_abstract_instance+0x3c0>
         : 3452   for (u = unit->prev_unit; u != NULL; u = u->prev_unit)
    0.00 :   4c620c: mov    0x10(%rax),%rax
    7.90 :   4c6210: test   %rax,%rax
    0.00 :   4c6213: jne    4c6200 <find_abstract_instance+0x350>

The following scan can be replaced with search in a splay tree and with
that I can get to 1.5s and there are other symbols where the difference
is even bigger.

bfd/ChangeLog:

	* dwarf2.c (struct addr_range): New.
	(addr_range_intersects): Likewise.
	(splay_tree_compare_addr_range): Likewise.
	(splay_tree_free_addr_range): Likewise.
	(struct dwarf2_debug_file): Add comp_unit_tree.
	(find_abstract_instance): Use the splay tree when searching
	for a info_ptr.
	(stash_comp_unit): Insert to the splay tree.
	(_bfd_dwarf2_cleanup_debug_info): Clean up the splay tree.
---
 bfd/dwarf2.c | 77 +++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 67 insertions(+), 10 deletions(-)

diff --git a/bfd/dwarf2.c b/bfd/dwarf2.c
index 47618788513..0449877966a 100644
--- a/bfd/dwarf2.c
+++ b/bfd/dwarf2.c
@@ -36,6 +36,7 @@
 #include "elf-bfd.h"
 #include "dwarf2.h"
 #include "hashtab.h"
+#include "splay-tree.h"
 
 /* The data in the .debug_line statement prologue looks like this.  */
 
@@ -152,6 +153,45 @@ static struct trie_node *alloc_trie_leaf (bfd *abfd)
   return &leaf->head;
 }
 
+struct addr_range
+{
+  bfd_byte *start;
+  bfd_byte *end;
+};
+
+/* Return true if address range do intersect.  */
+
+static bool
+addr_range_intersects (struct addr_range *r1, struct addr_range *r2)
+{
+  return (r1->start <= r2->start && r2->start < r1->end)
+    || (r1->start <= (r2->end - 1) && (r2->end - 1) < r1->end);
+}
+
+/* Compare function for splay tree of addr_ranges.  */
+
+static int
+splay_tree_compare_addr_range (splay_tree_key xa, splay_tree_key xb)
+{
+  struct addr_range *r1 = (struct addr_range *) xa;
+  struct addr_range *r2 = (struct addr_range *) xb;
+
+  if (addr_range_intersects (r1, r2) || addr_range_intersects (r2, r1))
+    return 0;
+  else if (r1->end <= r2->start)
+    return -1;
+  else
+    return 1;
+}
+
+/* Splay tree release function for keys (addr_range).  */
+
+static void
+splay_tree_free_addr_range (splay_tree_key key)
+{
+  free ((struct addr_range *)key);
+}
+
 struct dwarf2_debug_file
 {
   /* The actual bfd from which debug info was loaded.  Might be
@@ -235,6 +275,9 @@ struct dwarf2_debug_file
 
   /* Root of a trie to map addresses to compilation units.  */
   struct trie_node *trie_root;
+
+  /* Splay tree to map info_ptr address to compilation units.  */
+  splay_tree comp_unit_tree;
 };
 
 struct dwarf2_debug
@@ -3447,16 +3490,12 @@ find_abstract_instance (struct comp_unit *unit,
       else
 	{
 	  /* Check other CUs to see if they contain the abbrev.  */
-	  struct comp_unit *u;
-
-	  for (u = unit->prev_unit; u != NULL; u = u->prev_unit)
-	    if (info_ptr >= u->info_ptr_unit && info_ptr < u->end_ptr)
-	      break;
-
-	  if (u == NULL)
-	    for (u = unit->next_unit; u != NULL; u = u->next_unit)
-	      if (info_ptr >= u->info_ptr_unit && info_ptr < u->end_ptr)
-		break;
+	  struct comp_unit *u = NULL;
+	  struct addr_range range = { info_ptr, info_ptr };
+	  splay_tree_node v = splay_tree_lookup (unit->file->comp_unit_tree,
+						 (splay_tree_key)&range);
+	  if (v != NULL)
+	    u = (struct comp_unit *)v->value;
 
 	  if (attr_ptr->form == DW_FORM_ref_addr)
 	    while (u == NULL)
@@ -5517,6 +5556,22 @@ stash_comp_unit (struct dwarf2_debug *stash, struct dwarf2_debug_file *file)
 						info_ptr_unit, offset_size);
       if (each)
 	{
+	  if (file->comp_unit_tree == NULL)
+	    file->comp_unit_tree
+	      = splay_tree_new (splay_tree_compare_addr_range,
+				splay_tree_free_addr_range, NULL);
+
+	  struct addr_range *r
+	    = (struct addr_range *)bfd_malloc (sizeof (struct addr_range));
+	  r->start = each->info_ptr_unit;
+	  r->end = each->end_ptr;
+	  splay_tree_node v = splay_tree_lookup (file->comp_unit_tree,
+						 (splay_tree_key)r);
+	  if (v != NULL || r->end <= r->start)
+	    abort ();
+	  splay_tree_insert (file->comp_unit_tree, (splay_tree_key)r,
+			     (splay_tree_value)each);
+
 	  if (file->all_comp_units)
 	    file->all_comp_units->prev_unit = each;
 	  else
@@ -5990,6 +6045,8 @@ _bfd_dwarf2_cleanup_debug_info (bfd *abfd, void **pinfo)
 	  free (file->line_table->dirs);
 	}
       htab_delete (file->abbrev_offsets);
+      if (file->comp_unit_tree != NULL)
+	splay_tree_delete (file->comp_unit_tree);
 
       free (file->dwarf_line_str_buffer);
       free (file->dwarf_str_buffer);
-- 
2.37.1



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