diff --git a/project.py b/project.py
index 45b6c63..bc385f2 100644
--- a/project.py
+++ b/project.py
@@ -1698,6 +1698,11 @@
         if cb is None or name != cb:
           kill.append(name)
 
+    # Minor optimization: If there's nothing to prune, then don't try to read
+    # any project state.
+    if not kill and not cb:
+      return []
+
     rev = self.GetRevisionId(left)
     if cb is not None \
        and not self._revlist(HEAD + '...' + rev) \