diff --git a/src/demangle/ItaniumDemangle.h b/src/demangle/ItaniumDemangle.h
index 98489e4..db65c60 100644
--- a/src/demangle/ItaniumDemangle.h
+++ b/src/demangle/ItaniumDemangle.h
@@ -6,8 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// Generic itanium demangler library. This file has two byte-per-byte identical
-// copies in the source tree, one in libcxxabi, and the other in llvm.
+// Generic itanium demangler library.
+// There are two copies of this file in the source tree.  The one under
+// libcxxabi is the original and the one under llvm is the copy.  Use
+// cp-to-llvm.sh to update the copy.  See README.txt for more details.
 //
 //===----------------------------------------------------------------------===//
 
diff --git a/src/demangle/README.txt b/src/demangle/README.txt
index 514ff6d..76470f6 100644
--- a/src/demangle/README.txt
+++ b/src/demangle/README.txt
@@ -4,41 +4,50 @@
 Introduction
 ------------
 
-This directory contains the generic itanium name demangler library. The main
-purpose of the library is to demangle C++ symbols, i.e. convert the string
-"_Z1fv" into "f()". You can also use the CRTP base ManglingParser to perform
-some simple analysis on the mangled name, or (in LLVM) use the opaque
-ItaniumPartialDemangler to query the demangled AST.
+This directory contains the generic itanium name demangler
+library. The main purpose of the library is to demangle C++ symbols,
+i.e. convert the string "_Z1fv" into "f()". You can also use the CRTP
+base ManglingParser to perform some simple analysis on the mangled
+name, or (in LLVM) use the opaque ItaniumPartialDemangler to query the
+demangled AST.
 
 Why are there multiple copies of the this library in the source tree?
 ---------------------------------------------------------------------
 
-This directory is mirrored between libcxxabi/demangle and
-llvm/include/llvm/Demangle. The simple reason for this is that both projects
-need to demangle symbols, but neither can depend on each other. libcxxabi needs
-the demangler to implement __cxa_demangle, which is part of the itanium ABI
-spec. LLVM needs a copy for a bunch of places, but doesn't want to use the
-system's __cxa_demangle because it a) might not be available (i.e., on Windows),
-and b) probably isn't that up-to-date on the latest language features.
+The canonical sources are in libcxxabi/src/demangle and some of the
+files are copied to llvm/include/llvm/Demangle.  The simple reason for
+this comes from before the monorepo, and both [sub]projects need to
+demangle symbols, but neither can depend on each other.
 
-The copy of the demangler in LLVM has some extra stuff that aren't needed in
-libcxxabi (ie, the MSVC demangler, ItaniumPartialDemangler), which depend on the
-shared generic components. Despite these differences, we want to keep the "core"
-generic demangling library identical between both copies to simplify development
-and testing.
+* libcxxabi needs the demangler to implement __cxa_demangle, which is
+  part of the itanium ABI spec.
 
-If you're working on the generic library, then do the work first in libcxxabi,
-then run the cp-to-llvm.sh script in src/demangle. This script takes as an
-argument the path to llvm, and re-copies the changes you made to libcxxabi over.
-Note that this script just blindly overwrites all changes to the generic library
-in llvm, so be careful.
+* LLVM needs a copy for a bunch of places, and cannot rely on the
+  system's __cxa_demangle because it a) might not be available (i.e.,
+  on Windows), and b) may not be up-to-date on the latest language
+  features.
 
-Because the core demangler needs to work in libcxxabi, everything needs to be
-declared in an anonymous namespace (see DEMANGLE_NAMESPACE_BEGIN), and you can't
-introduce any code that depends on the libcxx dylib.
+The copy of the demangler in LLVM has some extra stuff that aren't
+needed in libcxxabi (ie, the MSVC demangler, ItaniumPartialDemangler),
+which depend on the shared generic components. Despite these
+differences, we want to keep the "core" generic demangling library
+identical between both copies to simplify development and testing.
 
-Hopefully, when LLVM becomes a monorepo, we can de-duplicate this code, and have
-both LLVM and libcxxabi depend on a shared demangler library.
+If you're working on the generic library, then do the work first in
+libcxxabi, then run the cp-to-llvm.sh script in src/demangle. This
+script takes as an optional argument the path to llvm, and copies the
+changes you made to libcxxabi over.  Note that this script just
+blindly overwrites all changes to the generic library in llvm, so be
+careful.
+
+Because the core demangler needs to work in libcxxabi, everything
+needs to be declared in an anonymous namespace (see
+DEMANGLE_NAMESPACE_BEGIN), and you can't introduce any code that
+depends on the libcxx dylib.
+
+FIXME: Now that LLVM is a monorepo, it should be possible to
+de-duplicate this code, and have both LLVM and libcxxabi depend on a
+shared demangler library.
 
 Testing
 -------
diff --git a/src/demangle/StringView.h b/src/demangle/StringView.h
index 7c8cb48..90890e3 100644
--- a/src/demangle/StringView.h
+++ b/src/demangle/StringView.h
@@ -7,6 +7,9 @@
 //===----------------------------------------------------------------------===//
 //
 // FIXME: Use std::string_view instead when we support C++17.
+// There are two copies of this file in the source tree.  The one under
+// libcxxabi is the original and the one under llvm is the copy.  Use
+// cp-to-llvm.sh to update the copy.  See README.txt for more details.
 //
 //===----------------------------------------------------------------------===//
 
diff --git a/src/demangle/Utility.h b/src/demangle/Utility.h
index 587c0e4..3b27328 100644
--- a/src/demangle/Utility.h
+++ b/src/demangle/Utility.h
@@ -6,7 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// Provide some utility classes for use in the demangler(s).
+// Provide some utility classes for use in the demangler.
+// There are two copies of this file in the source tree.  The one in libcxxabi
+// is the original and the one in llvm is the copy.  Use cp-to-llvm.sh to update
+// the copy.  See README.txt for more details.
 //
 //===----------------------------------------------------------------------===//
 
diff --git a/src/demangle/cp-to-llvm.sh b/src/demangle/cp-to-llvm.sh
index e4342a0..faf6056 100755
--- a/src/demangle/cp-to-llvm.sh
+++ b/src/demangle/cp-to-llvm.sh
@@ -26,10 +26,11 @@
     chmod -w $LLVM_DEMANGLE_DIR/README.txt
     for I in $HDRS ; do
 	rm -f $LLVM_DEMANGLE_DIR/$I
-	cat - $I >$LLVM_DEMANGLE_DIR/$I <<EOF
-// Do not edit! -*- read-only -*-
-// See README.txt for instructions
-EOF
+	dash=$(echo "$I---------------------------" | cut -c -27 |\
+		   sed 's|[^-]*||')
+	sed -e '1s|^//=*-* .*\.h -*.*=*// *$|//===--- '"$I $dash"'-*- mode:c++;eval:(read-only-mode) -*-===//|' \
+	    -e '2s|^// *$|//       Do not edit! See README.txt.|' \
+	    $I >$LLVM_DEMANGLE_DIR/$I
 	chmod -w $LLVM_DEMANGLE_DIR/$I
     done
 fi