diff --git a/hgext/split.py b/hgext/split.py
new file mode 100644
--- /dev/null
+++ b/hgext/split.py
@@ -0,0 +1,177 @@
+# split.py - split a changeset into smaller ones
+#
+# Copyright 2015 Laurent Charignon <lcharignon@fb.com>
+# Copyright 2017 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""command to split a changeset into smaller ones (EXPERIMENTAL)"""
+
+from __future__ import absolute_import
+
+from mercurial.i18n import _
+
+from mercurial.node import (
+    nullid,
+    short,
+)
+
+from mercurial import (
+    bookmarks,
+    cmdutil,
+    commands,
+    error,
+    hg,
+    obsolete,
+    phases,
+    registrar,
+    revsetlang,
+    scmutil,
+)
+
+# allow people to use split without explicitly enabling rebase extension
+from . import (
+    rebase,
+)
+
+cmdtable = {}
+command = registrar.command(cmdtable)
+
+# Note for extension authors: ONLY specify testedwith = 'ships-with-hg-core' for
+# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
+# be specifying the version(s) of Mercurial they are tested with, or
+# leave the attribute unspecified.
+testedwith = 'ships-with-hg-core'
+
+@command('^split',
+    [('r', 'rev', '', _("revision to split"), _('REV')),
+     ('', 'rebase', True, _('rebase descendants after split')),
+    ] + cmdutil.commitopts2,
+    _('hg split [--no-rebase] [[-r] REV]'))
+def split(ui, repo, *revs, **opts):
+    """split a changeset into smaller ones
+
+    Repeatedly prompt changes and commit message for new changesets until there
+    is nothing left in the original changeset.
+
+    If --rev was not given, split the working directory parent.
+
+    By default, rebase connected non-obsoleted descendants onto the new
+    changeset. Use --no-rebase to avoid the rebase.
+    """
+    revlist = []
+    if opts.get('rev'):
+        revlist.append(opts.get('rev'))
+    revlist.extend(revs)
+    with repo.wlock(), repo.lock(), repo.transaction('split') as tr:
+        revs = scmutil.revrange(repo, revlist or ['.'])
+        if len(revs) > 1:
+            raise error.Abort(_('cannot split multiple revisions'))
+
+        rev = revs.first()
+        ctx = repo[rev]
+        if rev is None or ctx.node() == nullid:
+            ui.status(_('nothing to split\n'))
+            return 1
+        if ctx.node() is None:
+            raise error.Abort(_('cannot split working directory'))
+
+        # rewriteutil.precheck is not very useful here because:
+        # 1. null check is done above and it's more friendly to return 1
+        #    instead of abort
+        # 2. mergestate check is done below by cmdutil.bailifchanged
+        # 3. unstable check is more complex here because of --rebase
+        #
+        # So only "public" check is useful and it's checked directly here.
+        if ctx.phase() == phases.public:
+            raise error.Abort(_('cannot split public changeset'),
+                              hint=_("see 'hg help phases' for details"))
+
+        descendants = list(repo.revs('(%d::) - (%d)', rev, rev))
+        alloworphaned = obsolete.isenabled(repo, obsolete.allowunstableopt)
+        if opts.get('rebase'):
+            # Skip obsoleted descendants and their descendants so the rebase
+            # won't cause conflicts for sure.
+            torebase = list(repo.revs('%ld - (%ld & obsolete())::',
+                                      descendants, descendants))
+            if not alloworphaned and len(torebase) != len(descendants):
+                raise error.Abort(_('split would leave orphaned changesets '
+                                    'behind'))
+        else:
+            if not alloworphaned and descendants:
+                raise error.Abort(
+                    _('cannot split changeset with children without rebase'))
+            torebase = ()
+
+        if len(ctx.parents()) > 1:
+            raise error.Abort(_('cannot split a merge changeset'))
+
+        cmdutil.bailifchanged(repo)
+
+        # Deactivate bookmark temporarily so it won't get moved unintentionally
+        bname = repo._activebookmark
+        if bname and repo._bookmarks[bname] != ctx.node():
+            bookmarks.deactivate(repo)
+
+        wnode = repo['.'].node()
+        top = None
+        try:
+            top = dosplit(ui, repo, tr, ctx, opts)
+        finally:
+            # top is None: split failed, need update --clean recovery.
+            # wnode == ctx.node(): wnode split, no need to update.
+            if top is None or wnode != ctx.node():
+                hg.clean(repo, wnode, show_stats=False)
+            if bname:
+                bookmarks.activate(repo, bname)
+        if torebase and top:
+            dorebase(ui, repo, torebase, top)
+
+def dosplit(ui, repo, tr, ctx, opts):
+    committed = [] # [ctx]
+
+    # Set working parent to ctx.p1(), and keep working copy as ctx's content
+    # NOTE: if we can have "update without touching working copy" API, the
+    # revert step could be cheaper.
+    hg.clean(repo, ctx.p1().node(), show_stats=False)
+    parents = repo.changelog.parents(ctx.node())
+    ui.pushbuffer()
+    cmdutil.revert(ui, repo, ctx, parents)
+    ui.popbuffer() # discard "reverting ..." messages
+
+    # Any modified, added, removed, deleted result means split is incomplete
+    incomplete = lambda repo: any(repo.status()[:4])
+
+    # Main split loop
+    while incomplete(repo):
+        if committed:
+            header = (_('HG: Splitting %s. So far it has been split into:\n')
+                      % short(ctx.node()))
+            for c in committed:
+                firstline = c.description().split('\n', 1)[0]
+                header += _('HG: - %s: %s\n') % (short(c.node()), firstline)
+            header += _('HG: Write commit message for the next split '
+                        'changeset.\n')
+        else:
+            header = _('HG: Splitting %s. Write commit message for the '
+                       'first split changeset.\n') % short(ctx.node())
+        opts.update({
+            'edit': True,
+            'interactive': True,
+            'message': header + ctx.description(),
+        })
+        commands.commit(ui, repo, **opts)
+        newctx = repo['.']
+        committed.append(newctx)
+
+    if not committed:
+        raise error.Abort(_('cannot split an empty revision'))
+
+    scmutil.cleanupnodes(repo, {ctx.node(): [c.node() for c in committed]},
+                         operation='split')
+
+    return committed[-1]
+
+def dorebase(ui, repo, src, dest):
+    rebase.rebase(ui, repo, rev=[revsetlang.formatspec('%ld', src)],
+                  dest=revsetlang.formatspec('%d', dest))
diff --git a/tests/test-split.t b/tests/test-split.t
new file mode 100644
--- /dev/null
+++ b/tests/test-split.t
@@ -0,0 +1,525 @@
+#testcases obsstore-on obsstore-off
+
+  $ cat > $TESTTMP/editor.py <<EOF
+  > #!$PYTHON
+  > import os, sys
+  > path = os.path.join(os.environ['TESTTMP'], 'messages')
+  > messages = open(path).read().split('--\n')
+  > prompt = open(sys.argv[1]).read()
+  > sys.stdout.write(''.join('EDITOR: %s' % l for l in prompt.splitlines(True)))
+  > sys.stdout.flush()
+  > with open(sys.argv[1], 'w') as f:
+  >    f.write(messages[0])
+  > with open(path, 'w') as f:
+  >    f.write('--\n'.join(messages[1:]))
+  > EOF
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > drawdag=$TESTDIR/drawdag.py
+  > split=
+  > [ui]
+  > interactive=1
+  > [diff]
+  > git=1
+  > unified=0
+  > [alias]
+  > glog=log -G -T '{rev}:{node|short} {desc} {bookmarks}\n'
+  > EOF
+
+#if obsstore-on
+  $ cat >> $HGRCPATH <<EOF
+  > [experimental]
+  > evolution=all
+  > EOF
+#endif
+
+  $ hg init a
+  $ cd a
+
+Nothing to split
+
+  $ hg split
+  nothing to split
+  [1]
+
+  $ hg commit -m empty --config ui.allowemptycommit=1
+  $ hg split
+  abort: cannot split an empty revision
+  [255]
+
+  $ rm -rf .hg
+  $ hg init
+
+Cannot split working directory
+
+  $ hg split -r 'wdir()'
+  abort: cannot split working directory
+  [255]
+
+Generate some content
+
+  $ $TESTDIR/seq.py 1 5 >> a
+  $ hg ci -m a1 -A a -q
+  $ hg bookmark -i r1
+  $ sed 's/1/11/;s/3/33/;s/5/55/' a > b
+  $ mv b a
+  $ hg ci -m a2 -q
+  $ hg bookmark -i r2
+
+Cannot split a public changeset
+
+  $ hg phase --public -r 'all()'
+  $ hg split .
+  abort: cannot split public changeset
+  (see 'hg help phases' for details)
+  [255]
+
+  $ hg phase --draft -f -r 'all()'
+
+Cannot split while working directory is dirty
+
+  $ touch dirty
+  $ hg add dirty
+  $ hg split .
+  abort: uncommitted changes
+  [255]
+  $ hg forget dirty
+  $ rm dirty
+
+Split a head
+
+  $ cp -R . ../b
+  $ cp -R . ../c
+
+  $ hg bookmark r3
+
+  $ hg split 'all()'
+  abort: cannot split multiple revisions
+  [255]
+
+  $ runsplit() {
+  > cat > $TESTTMP/messages <<EOF
+  > split 1
+  > --
+  > split 2
+  > --
+  > split 3
+  > EOF
+  > cat <<EOF | hg split "$@"
+  > y
+  > y
+  > y
+  > y
+  > y
+  > y
+  > EOF
+  > }
+
+  $ HGEDITOR=false runsplit
+  diff --git a/a b/a
+  1 hunks, 1 lines changed
+  examine changes to 'a'? [Ynesfdaq?] y
+  
+  @@ -5,1 +5,1 @@ 4
+  -5
+  +55
+  record this change to 'a'? [Ynesfdaq?] y
+  
+  transaction abort!
+  rollback completed
+  abort: edit failed: false exited with status 1
+  [255]
+  $ hg status
+
+  $ HGEDITOR="$PYTHON $TESTTMP/editor.py"
+  $ runsplit
+  diff --git a/a b/a
+  1 hunks, 1 lines changed
+  examine changes to 'a'? [Ynesfdaq?] y
+  
+  @@ -5,1 +5,1 @@ 4
+  -5
+  +55
+  record this change to 'a'? [Ynesfdaq?] y
+  
+  EDITOR: HG: Splitting 1df0d5c5a3ab. Write commit message for the first split changeset.
+  EDITOR: a2
+  EDITOR: 
+  EDITOR: 
+  EDITOR: HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  EDITOR: HG: Leave message empty to abort commit.
+  EDITOR: HG: --
+  EDITOR: HG: user: test
+  EDITOR: HG: branch 'default'
+  EDITOR: HG: changed a
+  created new head
+  diff --git a/a b/a
+  1 hunks, 1 lines changed
+  examine changes to 'a'? [Ynesfdaq?] y
+  
+  @@ -3,1 +3,1 @@ 2
+  -3
+  +33
+  record this change to 'a'? [Ynesfdaq?] y
+  
+  EDITOR: HG: Splitting 1df0d5c5a3ab. So far it has been split into:
+  EDITOR: HG: - e704349bd21b: split 1
+  EDITOR: HG: Write commit message for the next split changeset.
+  EDITOR: a2
+  EDITOR: 
+  EDITOR: 
+  EDITOR: HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  EDITOR: HG: Leave message empty to abort commit.
+  EDITOR: HG: --
+  EDITOR: HG: user: test
+  EDITOR: HG: branch 'default'
+  EDITOR: HG: changed a
+  diff --git a/a b/a
+  1 hunks, 1 lines changed
+  examine changes to 'a'? [Ynesfdaq?] y
+  
+  @@ -1,1 +1,1 @@
+  -1
+  +11
+  record this change to 'a'? [Ynesfdaq?] y
+  
+  EDITOR: HG: Splitting 1df0d5c5a3ab. So far it has been split into:
+  EDITOR: HG: - e704349bd21b: split 1
+  EDITOR: HG: - a09ad58faae3: split 2
+  EDITOR: HG: Write commit message for the next split changeset.
+  EDITOR: a2
+  EDITOR: 
+  EDITOR: 
+  EDITOR: HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  EDITOR: HG: Leave message empty to abort commit.
+  EDITOR: HG: --
+  EDITOR: HG: user: test
+  EDITOR: HG: branch 'default'
+  EDITOR: HG: changed a
+  saved backup bundle to $TESTTMP/a/.hg/strip-backup/1df0d5c5a3ab-8341b760-split.hg (glob) (obsstore-off !)
+
+#if obsstore-off
+  $ hg bookmark
+     r1                        0:a61bcde8c529
+     r2                        3:00eebaf8d2e2
+   * r3                        3:00eebaf8d2e2
+  $ hg glog -p
+  @  3:00eebaf8d2e2 split 3 r2 r3
+  |  diff --git a/a b/a
+  |  --- a/a
+  |  +++ b/a
+  |  @@ -1,1 +1,1 @@
+  |  -1
+  |  +11
+  |
+  o  2:a09ad58faae3 split 2
+  |  diff --git a/a b/a
+  |  --- a/a
+  |  +++ b/a
+  |  @@ -3,1 +3,1 @@
+  |  -3
+  |  +33
+  |
+  o  1:e704349bd21b split 1
+  |  diff --git a/a b/a
+  |  --- a/a
+  |  +++ b/a
+  |  @@ -5,1 +5,1 @@
+  |  -5
+  |  +55
+  |
+  o  0:a61bcde8c529 a1 r1
+     diff --git a/a b/a
+     new file mode 100644
+     --- /dev/null
+     +++ b/a
+     @@ -0,0 +1,5 @@
+     +1
+     +2
+     +3
+     +4
+     +5
+  
+#else
+  $ hg bookmark
+     r1                        0:a61bcde8c529
+     r2                        4:00eebaf8d2e2
+   * r3                        4:00eebaf8d2e2
+  $ hg glog
+  @  4:00eebaf8d2e2 split 3 r2 r3
+  |
+  o  3:a09ad58faae3 split 2
+  |
+  o  2:e704349bd21b split 1
+  |
+  o  0:a61bcde8c529 a1 r1
+  
+#endif
+
+Split a head while working parent is not that head
+
+  $ cd $TESTTMP/b
+
+  $ hg up 0 -q
+  $ hg bookmark r3
+
+  $ runsplit tip >/dev/null
+
+#if obsstore-off
+  $ hg bookmark
+     r1                        0:a61bcde8c529
+     r2                        3:00eebaf8d2e2
+   * r3                        0:a61bcde8c529
+  $ hg glog
+  o  3:00eebaf8d2e2 split 3 r2
+  |
+  o  2:a09ad58faae3 split 2
+  |
+  o  1:e704349bd21b split 1
+  |
+  @  0:a61bcde8c529 a1 r1 r3
+  
+#else
+  $ hg bookmark
+     r1                        0:a61bcde8c529
+     r2                        4:00eebaf8d2e2
+   * r3                        0:a61bcde8c529
+  $ hg glog
+  o  4:00eebaf8d2e2 split 3 r2
+  |
+  o  3:a09ad58faae3 split 2
+  |
+  o  2:e704349bd21b split 1
+  |
+  @  0:a61bcde8c529 a1 r1 r3
+  
+#endif
+
+Split a non-head
+
+  $ cd $TESTTMP/c
+  $ echo d > d
+  $ hg ci -m d1 -A d
+  $ hg bookmark -i d1
+  $ echo 2 >> d
+  $ hg ci -m d2
+  $ echo 3 >> d
+  $ hg ci -m d3
+  $ hg bookmark -i d3
+  $ hg up '.^' -q
+  $ hg bookmark d2
+  $ cp -R . ../d
+
+  $ runsplit -r 1 | grep rebasing
+  rebasing 2:b5c5ea414030 "d1" (d1)
+  rebasing 3:f4a0a8d004cc "d2" (d2)
+  rebasing 4:777940761eba "d3" (d3)
+#if obsstore-off
+  $ hg bookmark
+     d1                        4:c4b449ef030e
+   * d2                        5:c9dd00ab36a3
+     d3                        6:19f476bc865c
+     r1                        0:a61bcde8c529
+     r2                        3:00eebaf8d2e2
+  $ hg glog -p
+  o  6:19f476bc865c d3 d3
+  |  diff --git a/d b/d
+  |  --- a/d
+  |  +++ b/d
+  |  @@ -2,0 +3,1 @@
+  |  +3
+  |
+  @  5:c9dd00ab36a3 d2 d2
+  |  diff --git a/d b/d
+  |  --- a/d
+  |  +++ b/d
+  |  @@ -1,0 +2,1 @@
+  |  +2
+  |
+  o  4:c4b449ef030e d1 d1
+  |  diff --git a/d b/d
+  |  new file mode 100644
+  |  --- /dev/null
+  |  +++ b/d
+  |  @@ -0,0 +1,1 @@
+  |  +d
+  |
+  o  3:00eebaf8d2e2 split 3 r2
+  |  diff --git a/a b/a
+  |  --- a/a
+  |  +++ b/a
+  |  @@ -1,1 +1,1 @@
+  |  -1
+  |  +11
+  |
+  o  2:a09ad58faae3 split 2
+  |  diff --git a/a b/a
+  |  --- a/a
+  |  +++ b/a
+  |  @@ -3,1 +3,1 @@
+  |  -3
+  |  +33
+  |
+  o  1:e704349bd21b split 1
+  |  diff --git a/a b/a
+  |  --- a/a
+  |  +++ b/a
+  |  @@ -5,1 +5,1 @@
+  |  -5
+  |  +55
+  |
+  o  0:a61bcde8c529 a1 r1
+     diff --git a/a b/a
+     new file mode 100644
+     --- /dev/null
+     +++ b/a
+     @@ -0,0 +1,5 @@
+     +1
+     +2
+     +3
+     +4
+     +5
+  
+#else
+  $ hg bookmark
+     d1                        8:c4b449ef030e
+   * d2                        9:c9dd00ab36a3
+     d3                        10:19f476bc865c
+     r1                        0:a61bcde8c529
+     r2                        7:00eebaf8d2e2
+  $ hg glog
+  o  10:19f476bc865c d3 d3
+  |
+  @  9:c9dd00ab36a3 d2 d2
+  |
+  o  8:c4b449ef030e d1 d1
+  |
+  o  7:00eebaf8d2e2 split 3 r2
+  |
+  o  6:a09ad58faae3 split 2
+  |
+  o  5:e704349bd21b split 1
+  |
+  o  0:a61bcde8c529 a1 r1
+  
+#endif
+
+Split a non-head without rebase
+
+  $ cd $TESTTMP/d
+#if obsstore-off
+  $ runsplit -r 1 --no-rebase
+  abort: cannot split changeset with children without rebase
+  [255]
+#else
+  $ runsplit -r 1 --no-rebase >/dev/null
+  $ hg bookmark
+     d1                        2:b5c5ea414030
+   * d2                        3:f4a0a8d004cc
+     d3                        4:777940761eba
+     r1                        0:a61bcde8c529
+     r2                        7:00eebaf8d2e2
+
+  $ hg glog
+  o  7:00eebaf8d2e2 split 3 r2
+  |
+  o  6:a09ad58faae3 split 2
+  |
+  o  5:e704349bd21b split 1
+  |
+  | o  4:777940761eba d3 d3
+  | |
+  | @  3:f4a0a8d004cc d2 d2
+  | |
+  | o  2:b5c5ea414030 d1 d1
+  | |
+  | x  1:1df0d5c5a3ab a2
+  |/
+  o  0:a61bcde8c529 a1 r1
+  
+#endif
+
+Split a non-head with obsoleted descendants
+
+#if obsstore-on
+  $ hg init $TESTTMP/e
+  $ cd $TESTTMP/e
+  $ hg debugdrawdag <<'EOS'
+  >   H I   J
+  >   | |   |
+  >   F G1 G2  # amend: G1 -> G2
+  >   | |  /   # prune: F
+  >   C D E
+  >    \|/
+  >     B
+  >     |
+  >     A
+  > EOS
+  $ eval `hg tags -T '{tag}={node}\n'`
+  $ rm .hg/localtags
+  $ hg split $B --config experimental.evolution=createmarkers
+  abort: split would leave orphaned changesets behind
+  [255]
+  $ cat > $TESTTMP/messages <<EOF
+  > Split B
+  > EOF
+  $ cat <<EOF | hg split $B
+  > y
+  > y
+  > EOF
+  diff --git a/B b/B
+  new file mode 100644
+  examine changes to 'B'? [Ynesfdaq?] y
+  
+  @@ -0,0 +1,1 @@
+  +B
+  \ No newline at end of file
+  record this change to 'B'? [Ynesfdaq?] y
+  
+  EDITOR: HG: Splitting 112478962961. Write commit message for the first split changeset.
+  EDITOR: B
+  EDITOR: 
+  EDITOR: 
+  EDITOR: HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  EDITOR: HG: Leave message empty to abort commit.
+  EDITOR: HG: --
+  EDITOR: HG: user: test
+  EDITOR: HG: branch 'default'
+  EDITOR: HG: added B
+  created new head
+  rebasing 2:26805aba1e60 "C"
+  rebasing 3:be0ef73c17ad "D"
+  rebasing 4:49cb92066bfd "E"
+  rebasing 7:97a6268cc7ef "G2"
+  rebasing 10:e2f1e425c0db "J"
+  $ hg glog -r 'sort(all(), topo)'
+  o  16:556c085f8b52 J
+  |
+  o  15:8761f6c9123f G2
+  |
+  o  14:a7aeffe59b65 E
+  |
+  | o  13:e1e914ede9ab D
+  |/
+  | o  12:01947e9b98aa C
+  |/
+  o  11:0947baa74d47 Split B
+  |
+  | o  9:88ede1d5ee13 I
+  | |
+  | x  6:af8cbf225b7b G1
+  | |
+  | x  3:be0ef73c17ad D
+  | |
+  | | o  8:74863e5b5074 H
+  | | |
+  | | x  5:ee481a2a1e69 F
+  | | |
+  | | x  2:26805aba1e60 C
+  | |/
+  | x  1:112478962961 B
+  |/
+  o  0:426bada5c675 A
+  
+#endif