Skip the cache of DWARFInfo and CU.

Add DWARFInfo.skip_cache() and DWARFInfo.enable_cache() to give users
the ability of controlling cache.

For the case of parsing the DWARF of a large binary, we may want to
skip the cache to release the memory ASAP, avoiding extra CPU cycles
on maintaining a cache.

One of my use causes is to extract types, functions, and call sites
from the DWARF of a Linux kernel image. With caches, it takes about
573 seconds to go through all DIEs. Skipping caches reduces time to
448 seconds. It is about 27% faster. When going through every DIEs
sequentially, cache doesn't help use at all.

elftools/dwarf/compileunit.py

@@ -226,6 +226,9 @@ def _get_cached_DIE(self, offset):
         # the top DIE and obtain a reference to its stream.
         top_die_stream = self.get_top_DIE().stream
 
+        if self.dwarfinfo._skip_cache:
+            return DIE(cu=self, stream=top_die_stream, offset=offset)
+
         # `offset` is the offset in the stream of the DIE we want to return.
         # The map is maintined as a parallel array to the list.  We call
         # bisect each time to ensure new DIEs are inserted in the correct

elftools/dwarf/dwarfinfo.py

@@ -134,6 +134,8 @@ def __init__(self,
         self._cu_cache = []
         self._cu_offsets_map = []
 
+        self._skip_cache = False
+
     @property
     def has_debug_info(self):
         """ Return whether this contains debug information.
@@ -430,6 +432,10 @@ def _cached_CU_at_offset(self, offset):
 
             See get_CU_at().
         """
+        if self._skip_cache:
+            cu = self._parse_CU_at_offset(offset)
+            return cu
+
         # Find the insert point for the requested offset.  With bisect_right,
         # if this entry is present in the cache it will be the prior entry.
         i = bisect_right(self._cu_offsets_map, offset)
@@ -575,3 +581,10 @@ def parse_debugsupinfo(self):
             return suplink.sup_filename
         return None
 
+    def skip_cache(self):
+        self._skip_cache = True
+        pass
+
+    def enable_cache(self):
+        self._skip_cache = False
+        pass