diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -993,6 +993,16 @@
             if self._repo._encodefilterpats:
                 # can't rely on size() because wdir content may be decoded
                 return self._filelog.cmp(self._filenode, fctx.data())
+            # filelog.size() has two special cases:
+            # - censored metadata
+            # - copy/rename tracking
+            # The first is detected by peaking into the delta,
+            # the second is detected by abusing parent order
+            # in the revlog index as flag bit. This leaves files using
+            # the dummy encoding and non-standard meta attributes.
+            # The following check is a special case for the empty
+            # metadata block used if the raw file content starts with '\1\n'.
+            # Cases of arbitrary metadata flags are currently mishandled.
             if self.size() - 4 == fctx.size():
                 # size() can match:
                 # if file data starts with '\1\n', empty metadata block is
diff --git a/mercurial/filelog.py b/mercurial/filelog.py
--- a/mercurial/filelog.py
+++ b/mercurial/filelog.py
@@ -33,6 +33,7 @@
             target=(revlog_constants.KIND_FILELOG, path),
             radix=b'/'.join((b'data', path)),
             censorable=True,
+            canonical_parent_order=False,  # see comment in revlog.py
         )
         # Full name of the user visible file, relative to the repository root.
         # Used by LFS.
@@ -208,6 +209,7 @@
             return 0
 
         # XXX if self.read(node).startswith("\1\n"), this returns (size+4)
+        # XXX See also basefilectx.cmp.
         return self._revlog.size(rev)
 
     def cmp(self, node, text):
diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -299,6 +299,7 @@
         persistentnodemap=False,
         concurrencychecker=None,
         trypending=False,
+        canonical_parent_order=True,
     ):
         """
         create a revlog object
@@ -374,6 +375,13 @@
 
         self._concurrencychecker = concurrencychecker
 
+        # parent order is supposed to be semantically irrelevant, so we
+        # normally resort parents to ensure that the first parent is non-null,
+        # if there is a non-null parent at all.
+        # filelog obuses the parent order as flag to mark some instances of
+        # meta-encoded files, so allow it to disable this behavior.
+        self.canonical_parent_order = canonical_parent_order
+
     def _init_opts(self):
         """process options (from above/config) to setup associated default revlog mode
 
@@ -899,7 +907,10 @@
                 raise error.WdirUnsupported
             raise
 
-        return entry[5], entry[6]
+        if self.canonical_parent_order and entry[5] == nullrev:
+            return entry[6], entry[5]
+        else:
+            return entry[5], entry[6]
 
     # fast parentrevs(rev) where rev isn't filtered
     _uncheckedparentrevs = parentrevs
@@ -921,6 +932,11 @@
         i = self.index
         d = i[self.rev(node)]
         return i[d[5]][7], i[d[6]][7]  # map revisions to nodes inline
+        # inline node() to avoid function call overhead
+        if self.canonical_parent_order and d[5] == nullid:
+            return i[d[6]][7], i[d[5]][7]
+        else:
+            return i[d[5]][7], i[d[6]][7]
 
     def chainlen(self, rev):
         return self._chaininfo(rev)[0]
diff --git a/tests/test-narrow-shallow-merges.t b/tests/test-narrow-shallow-merges.t
--- a/tests/test-narrow-shallow-merges.t
+++ b/tests/test-narrow-shallow-merges.t
@@ -179,7 +179,7 @@
   
 
   $ hg log -T '{if(ellipsis,"...")}{node|short} {p1node|short} {p2node|short} {desc}\n' | sort
-  ...2a20009de83e 000000000000 3ac1f5779de3 outside 10
+  ...2a20009de83e 3ac1f5779de3 000000000000 outside 10
   ...3ac1f5779de3 bb96a08b062a 465567bdfb2d merge a/b/c/d 9
   ...8d874d57adea 7ef88b4dd4fa 000000000000 outside 12
   ...b844052e7b3b 000000000000 000000000000 outside 2c