[PATCH 2/3] Add caching to use_reduce function

This function is called extremely frequently with similar arguments, so
this optimization reduces "emerge -uDvpU --with-bdeps=y @world" runtime from
43.5 -> 34.5s -- a 25.8% speedup.
---
 lib/portage/dep/__init__.py | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/lib/portage/dep/__init__.py b/lib/portage/dep/__init__.py
index 72988357a..df296dd81 100644
--- a/lib/portage/dep/__init__.py
+++ b/lib/portage/dep/__init__.py
@@ -404,6 +404,8 @@ def paren_enclose(mylist, unevaluated_atom=False, opconvert=False):
 			mystrparts.append(x)
 	return " ".join(mystrparts)
 
+_use_reduce_cache = {}
+
 def use_reduce(depstr, uselist=(), masklist=(), matchall=False, excludeall=(), is_src_uri=False, \
 	eapi=None, opconvert=False, flat=False, is_valid_flag=None, token_class=None, matchnone=False,
 	subset=None):
@@ -440,6 +442,27 @@ def use_reduce(depstr, uselist=(), masklist=(), matchall=False, excludeall=(), i
 	@rtype: List
 	@return: The use reduced depend array
 	"""
+	uselist_key = None
+	masklist_key = None
+	excludeall_key = None
+	subset_key = None
+	if uselist is not None:
+	    uselist_key = tuple(uselist)
+	if masklist is not None:
+	    masklist_key = tuple(masklist)
+	if excludeall is not None:
+	    excludeall_key = tuple(excludeall)
+	if subset is not None:
+	    subset_key = tuple(subset)
+	cache_key = (depstr, uselist_key, masklist_key, matchall, excludeall_key, \
+		is_src_uri, eapi, opconvert, flat, is_valid_flag, token_class, \
+		matchnone, subset_key)
+
+	cache_entry = _use_reduce_cache.get(cache_key)
+	if cache_entry is not None:
+		# The list returned by this function may be modified, so return a copy.
+		return cache_entry[:]
+
 	if isinstance(depstr, list):
 		if portage._internal_caller:
 			warnings.warn(_("Passing paren_reduced dep arrays to %s is deprecated. " + \
@@ -767,6 +790,9 @@ def use_reduce(depstr, uselist=(), masklist=(), matchall=False, excludeall=(), i
 		raise InvalidDependString(
 			_("Missing file name at end of string"))
 
+	# The list returned by this function may be modified, so store a copy.
+	_use_reduce_cache[cache_key] = stack[0][:]
+
 	return stack[0]
 
 def dep_opconvert(deplist):
-- 
2.27.0.212.ge8ba1cc988-goog