--- a/mercurial/revset.py	Mon Jun 18 19:41:54 2018 -0700
+++ b/mercurial/revset.py	Tue Jun 26 15:26:21 2018 -0700
@@ -608,6 +608,19 @@
     return subset.filter(lambda r: repo[r].closesbranch(),
                          condrepr='<branch closed>')
 
+# for internal use
+@predicate('_commonancestorheads(set)', safe=True)
+def _commonancestorheads(repo, subset, x):
+    # This is an internal method is for quickly calculating "heads(::x and
+    # ::y)"
+
+    # These greatest common ancestors are the same ones that the consesus bid
+    # merge will find.
+    h = heads(repo, fullreposet(repo), x, defineorder)
+
+    ancs = repo.changelog._commonancestorsheads(*list(h))
+    return subset & baseset(ancs)
+
 @predicate('commonancestors(set)', safe=True)
 def commonancestors(repo, subset, x):
     """Returns all common ancestors of the set.

--- a/mercurial/revsetlang.py	Mon Jun 18 19:41:54 2018 -0700
+++ b/mercurial/revsetlang.py	Tue Jun 26 15:26:21 2018 -0700
@@ -459,6 +459,12 @@
         f = getsymbol(x[1])
         wa, ta = _optimize(x[2])
         w = getattr(symbols.get(f), '_weight', 1)
+        m = _match('commonancestors(_)', ta)
+
+        # Optimize heads(commonancestors(_)) because we have a fast version
+        if f == 'heads' and m:
+            return w + wa, _build('_commonancestorheads(_)', m[1])
+
         return w + wa, (op, x[1], ta)
     raise ValueError('invalid operator %r' % op)
 

--- a/tests/test-merge-criss-cross.t	Mon Jun 18 19:41:54 2018 -0700
+++ b/tests/test-merge-criss-cross.t	Tue Jun 26 15:26:21 2018 -0700
@@ -410,6 +410,21 @@
   3 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
 
+Test the greatest common ancestor returning multiple changesets
+
+  $ hg log -r 'heads(commonancestors(head()))'
+  changeset:   1:0f6b37dbe527
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     1 first change f1
+  
+  changeset:   2:d1d156401c1b
+  parent:      0:40494bf2444c
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     2 first change f2
+  
+
   $ cd ..
 
 http://stackoverflow.com/questions/9350005/how-do-i-specify-a-merge-base-to-use-in-a-hg-merge/9430810

--- a/tests/test-revset2.t	Mon Jun 18 19:41:54 2018 -0700
+++ b/tests/test-revset2.t	Tue Jun 26 15:26:21 2018 -0700
@@ -1834,3 +1834,21 @@
         (keyvalue
           (symbol 'depth')
           (symbol '1')))))
+
+test commonancestors and its optimization
+
+  $ hg debugrevspec --verify -p analyzed -p optimized 'heads(commonancestors(head()))'
+  * analyzed:
+  (func
+    (symbol 'heads')
+    (func
+      (symbol 'commonancestors')
+      (func
+        (symbol 'head')
+        None)))
+  * optimized:
+  (func
+    (symbol '_commonancestorheads')
+    (func
+      (symbol 'head')
+      None))