diff --git a/hgext3rd/clindex.pyx b/hgext3rd/clindex.pyx
--- a/hgext3rd/clindex.pyx
+++ b/hgext3rd/clindex.pyx
@@ -12,6 +12,7 @@
 
     [clindex]
     # Use Rust childmap.
+    # It's currently used to speed up phases calculation.
     childmap = True
 
     # Use Rust nodemap
@@ -105,7 +106,42 @@
         return self._origindex[rev]
 
     def computephasesmapsets(self, roots):
-        return self._origindex.computephasesmapsets(roots)
+        if not self._config.childmap:
+            return self._origindex.computephasesmapsets(roots)
+        # using childmap
+        drafts = self._descendants(roots[0])
+        secrets = self._descendants(roots[1])
+        drafts -= secrets
+        sets = [None, drafts, secrets]
+        if self._config.verify:
+            origlen, origsets = self._origindex.computephasesmapsets(roots)
+            for phase in (1, 2):
+                origset = origsets[phase]
+                thisset = sets[phase]
+                added = thisset - origset
+                removed = origset - thisset
+                if added or removed:
+                    _logandraise(
+                        self._vfs,
+                        'phases: inconsistent result(%s):\n  added %r'
+                        '\n removed  %r' % (phase, added, removed))
+        return len(self), sets
+
+    cpdef _descendants(self, rootrevs):
+        assert self._config.childmap
+        queue = rootrevs[:]
+        revs = set()
+        while queue:
+            rev = queue.pop()
+            if rev in revs:
+                continue
+            revs.add(rev)
+            queue.extend(self._children(rev))
+        return revs
+
+    cpdef _children(self, int rev):
+        assert self._config.childmap
+        return self._childmap[rev]
 
     def reachableroots2(self, int minroot, heads, roots, includepath):
         return self._origindex.reachableroots2(minroot, heads, roots,