[PATCH 2/3] libdw: optimize dwarf_hasattr to just look at abbrev

Josh Stone jistone at redhat.com
Thu Dec 11 02:48:04 UTC 2014


To just check the presence of an attribute, we only need to look at the
abbreviation data.  This optimization avoids reading over die data at all,
except possibly just to get the abbrev code.

Signed-off-by: Josh Stone <jistone at redhat.com>
---
 libdw/ChangeLog       |  3 +++
 libdw/dwarf_hasattr.c | 42 ++++++++++++++++++++++++++++++++++++++----
 2 files changed, 41 insertions(+), 4 deletions(-)

diff --git a/libdw/ChangeLog b/libdw/ChangeLog
index a2a4e1d49747..2a69bea371fc 100644
--- a/libdw/ChangeLog
+++ b/libdw/ChangeLog
@@ -1,4 +1,7 @@
 2014-12-10  Josh Stone  <jistone at redhat.com>
+	* dwarf_hasattr.c (dwarf_hasattr): Just walk abbrev for presence.
+
+2014-12-10  Josh Stone  <jistone at redhat.com>
 
 	* libdwP.h (__libdw_dieabbrev): New die->abbrev lookup function.
 	* dwarf_child.c (__libdw_find_attr, dwarf_child): Use it.
diff --git a/libdw/dwarf_hasattr.c b/libdw/dwarf_hasattr.c
index fb7e1d5ed495..6ed1a98cbb49 100644
--- a/libdw/dwarf_hasattr.c
+++ b/libdw/dwarf_hasattr.c
@@ -43,10 +43,44 @@ dwarf_hasattr (die, search_name)
   if (die == NULL)
     return 0;
 
-  /* Search for the attribute with the given name.  */
-  unsigned int code;
-  unsigned char *addr = __libdw_find_attr (die, search_name, &code, NULL);
+  /* Find the abbreviation entry.  */
+  Dwarf_Abbrev *abbrevp = __libdw_dieabbrev (die, NULL);
+  if (unlikely (abbrevp == DWARF_END_ABBREV))
+    {
+    invalid_dwarf:
+      __libdw_seterrno (DWARF_E_INVALID_DWARF);
+      return 0;
+    }
 
-  return addr != NULL && code == search_name;
+  Dwarf *dbg = die->cu->dbg;
+
+  /* Search the name attribute.  */
+  unsigned char *const endp
+    = ((unsigned char *) dbg->sectiondata[IDX_debug_abbrev]->d_buf
+       + dbg->sectiondata[IDX_debug_abbrev]->d_size);
+
+  const unsigned char *attrp = abbrevp->attrp;
+  while (1)
+    {
+      /* Are we still in bounds?  This test needs to be refined.  */
+      if (unlikely (attrp + 1 >= endp))
+	goto invalid_dwarf;
+
+      /* Get attribute name and form.
+
+	 XXX We don't check whether this reads beyond the end of the
+	 section.  */
+      unsigned int attr_name;
+      get_uleb128 (attr_name, attrp);
+      unsigned int attr_form;
+      get_uleb128 (attr_form, attrp);
+
+      /* We can stop if we found the attribute with value zero.  */
+      if (attr_name == 0 || attr_form == 0)
+	return 0;
+
+      if (attr_name == search_name)
+	return 1;
+    }
 }
 INTDEF (dwarf_hasattr)
-- 
2.1.0



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