diff --git a/mercurial/dagop.py b/mercurial/dagop.py
--- a/mercurial/dagop.py
+++ b/mercurial/dagop.py
@@ -723,13 +723,18 @@
     revision number and returns an iterable of parent revision numbers, possibly
     including nullrev.
 
-    Returns a set of revision numbers that are DAG heads within the passed subset.
+    Returns a set of revision numbers that are DAG heads within the passed
+    subset.
+
+    ``nullrev`` is never included in the returned set, even if it is provided in
+    the input set.
     """
-    headrevs = set()
+    headrevs = set(revs)
 
     for rev in revs:
         for prev in parentsfn(rev):
-            if prev != node.nullrev:
-                headrevs.discard(prev)
+            headrevs.discard(prev)
+
+    headrevs.discard(node.nullrev)
 
     return headrevs
diff --git a/mercurial/dagutil.py b/mercurial/dagutil.py
--- a/mercurial/dagutil.py
+++ b/mercurial/dagutil.py
@@ -20,21 +20,6 @@
     def __init__(self, revlog):
         self._revlog = revlog
 
-    def parents(self, ix):
-        rlog = self._revlog
-        idx = rlog.index
-        revdata = idx[ix]
-        prev = revdata[5]
-        if prev != nullrev:
-            prev2 = revdata[6]
-            if prev2 == nullrev:
-                return [prev]
-            return [prev, prev2]
-        prev2 = revdata[6]
-        if prev2 != nullrev:
-            return [prev2]
-        return []
-
     def linearize(self, ixs):
         '''linearize and topologically sort a list of revisions
 
@@ -45,7 +30,7 @@
         parent, then adding the rev itself to the output list.
         '''
         sorted = []
-        visit = list(dagop.headrevs(ixs, self.parents))
+        visit = list(dagop.headrevs(ixs, self._revlog.parentrevs))
         visit.sort(reverse=True)
         finished = set()
 
@@ -58,7 +43,7 @@
                     finished.add(cur)
             else:
                 visit.append(-cur - 1)
-                visit += [p for p in self.parents(cur)
-                          if p in ixs and p not in finished]
+                visit += [p for p in self._revlog.parentrevs(cur)
+                          if p != nullrev and p in ixs and p not in finished]
         assert len(sorted) == len(ixs)
         return sorted