diff --git a/libcxx/include/__config b/libcxx/include/__config
index 45de2bc0e8a19..55ae1481ec7a9 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -45,10 +45,6 @@
 #  define _LIBCPP_CONCAT_IMPL(_X, _Y) _X##_Y
 #  define _LIBCPP_CONCAT(_X, _Y) _LIBCPP_CONCAT_IMPL(_X, _Y)
 
-// Valid C++ identifier that revs with every libc++ version. This can be used to
-// generate identifiers that must be unique for every released libc++ version.
-#  define _LIBCPP_VERSIONED_IDENTIFIER _LIBCPP_CONCAT(v, _LIBCPP_VERSION)
-
 #  if __STDC_HOSTED__ == 0
 #    define _LIBCPP_FREESTANDING
 #  endif
@@ -754,22 +750,54 @@ typedef __char32_t char32_t;
 #    define _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION _LIBCPP_ALWAYS_INLINE
 #  endif
 
+#  if _LIBCPP_ENABLE_HARDENED_MODE
+#    define _LIBCPP_HARDENING_SIG h
+#  elif _LIBCPP_ENABLE_ASSERTIONS
+#    define _LIBCPP_HARDENING_SIG s
+#  elif _LIBCPP_ENABLE_DEBUG_MODE
+#    define _LIBCPP_HARDENING_SIG d
+#  else
+#    define _LIBCPP_HARDENING_SIG u // for unchecked
+#  endif
+
+#  ifdef _LIBCPP_HAS_NO_EXCEPTIONS
+#    define _LIBCPP_EXCEPTIONS_SIG n
+#  else
+#    define _LIBCPP_EXCEPTIONS_SIG e
+#  endif
+
+#  define _LIBCPP_ODR_SIGNATURE                                                                                        \
+    _LIBCPP_CONCAT(_LIBCPP_CONCAT(_LIBCPP_HARDENING_SIG, _LIBCPP_EXCEPTIONS_SIG), _LIBCPP_VERSION)
+
 // This macro marks a symbol as being hidden from libc++'s ABI. This is achieved
 // on two levels:
 // 1. The symbol is given hidden visibility, which ensures that users won't start exporting
 //    symbols from their dynamic library by means of using the libc++ headers. This ensures
 //    that those symbols stay private to the dynamic library in which it is defined.
 //
-// 2. The symbol is given an ABI tag that changes with each version of libc++. This ensures
-//    that no ODR violation can arise from mixing two TUs compiled with different versions
-//    of libc++ where we would have changed the definition of a symbol. If the symbols shared
-//    the same name, the ODR would require that their definitions be token-by-token equivalent,
-//    which basically prevents us from being able to make any change to any function in our
-//    headers. Using this ABI tag ensures that the symbol name is "bumped" artificially at
-//    each release, which lets us change the definition of these symbols at our leisure.
-//    Note that historically, this has been achieved in various ways, including force-inlining
-//    all functions or giving internal linkage to all functions. Both these (previous) solutions
-//    suffer from drawbacks that lead notably to code bloat.
+// 2. The symbol is given an ABI tag that encodes the ODR-relevant properties of the library.
+//    This ensures that no ODR violation can arise from mixing two TUs compiled with different
+//    versions or configurations of libc++ (such as exceptions vs no-exceptions). Indeed, if the
+//    program contains two definitions of a function, the ODR requires them to be token-by-token
+//    equivalent, and the linker is allowed to pick either definition and discard the other one.
+//
+//    For example, if a program contains a copy of `vector::at()` compiled with exceptions enabled
+//    *and* a copy of `vector::at()` compiled with exceptions disabled (by means of having two TUs
+//    compiled with different settings), the two definitions are both visible by the linker and they
+//    have the same name, but they have a meaningfully different implementation (one throws an exception
+//    and the other aborts the program). This violates the ODR and makes the program ill-formed, and in
+//    practice what will happen is that the linker will pick one of the definitions at random and will
+//    discard the other one. This can quite clearly lead to incorrect program behavior.
+//
+//    A similar reasoning holds for many other properties that are ODR-affecting. Essentially any
+//    property that causes the code of a function to differ from the code in another configuration
+//    can be considered ODR-affecting. In practice, we don't encode all such properties in the ABI
+//    tag, but we encode the ones that we think are most important: library version, exceptions, and
+//    hardening mode.
+//
+//    Note that historically, solving this problem has been achieved in various ways, including
+//    force-inlining all functions or giving internal linkage to all functions. Both these previous
+//    solutions suffer from drawbacks that lead notably to code bloat.
 //
 // Note that we use _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION to ensure that we don't depend
 // on _LIBCPP_HIDE_FROM_ABI methods of classes explicitly instantiated in the dynamic library.
@@ -789,7 +817,7 @@ typedef __char32_t char32_t;
 #  ifndef _LIBCPP_NO_ABI_TAG
 #    define _LIBCPP_HIDE_FROM_ABI                                                                                      \
       _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION                                                       \
-          __attribute__((__abi_tag__(_LIBCPP_TOSTRING(_LIBCPP_VERSIONED_IDENTIFIER))))
+          __attribute__((__abi_tag__(_LIBCPP_TOSTRING(_LIBCPP_ODR_SIGNATURE))))
 #  else
 #    define _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 #  endif
diff --git a/libcxx/test/libcxx/odr_signature.exceptions.sh.cpp b/libcxx/test/libcxx/odr_signature.exceptions.sh.cpp
new file mode 100644
index 0000000000000..6bf60b5e82d3c
--- /dev/null
+++ b/libcxx/test/libcxx/odr_signature.exceptions.sh.cpp
@@ -0,0 +1,46 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// TODO: Investigate
+// XFAIL: msvc
+
+// Test that we encode whether exceptions are supported in an ABI tag to avoid
+// ODR violations when linking TUs that have different values for it.
+
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU1  -fno-exceptions -o %t.tu1.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU2  -fexceptions    -o %t.tu2.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DMAIN                 -o %t.main.o
+// RUN: %{cxx} %t.tu1.o %t.tu2.o %t.main.o %{flags} %{link_flags} -o %t.exe
+// RUN: %{exec} %t.exe
+
+// -fno-exceptions
+#ifdef TU1
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI inline int f() { return 1; }
+int tu1() { return f(); }
+#endif // TU1
+
+// -fexceptions
+#ifdef TU2
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI inline int f() { return 2; }
+int tu2() { return f(); }
+#endif // TU2
+
+#ifdef MAIN
+#  include <cassert>
+
+int tu1();
+int tu2();
+
+int main(int, char**) {
+  assert(tu1() == 1);
+  assert(tu2() == 2);
+  return 0;
+}
+#endif // MAIN
diff --git a/libcxx/test/libcxx/odr_signature.hardening.sh.cpp b/libcxx/test/libcxx/odr_signature.hardening.sh.cpp
new file mode 100644
index 0000000000000..fa682fe37804a
--- /dev/null
+++ b/libcxx/test/libcxx/odr_signature.hardening.sh.cpp
@@ -0,0 +1,72 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// TODO: Remove these UNSUPPORTED lines once we change how hardening is enabled to avoid
+//       mutually exclusive modes being enabled at the same time.
+// UNSUPPORTED: libcpp-has-hardened-mode
+// UNSUPPORTED: libcpp-has-debug-mode
+// UNSUPPORTED: libcpp-has-assertions
+
+// TODO: Investigate
+// XFAIL: msvc
+
+// Test that we encode the hardening mode in an ABI tag to avoid ODR violations
+// when linking TUs that have different values for it.
+
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU1  -D_LIBCPP_ENABLE_HARDENED_MODE -o %t.tu1.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU2  -D_LIBCPP_ENABLE_ASSERTIONS    -o %t.tu2.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU3  -D_LIBCPP_ENABLE_DEBUG_MODE    -o %t.tu3.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DTU4                                 -o %t.tu4.o
+// RUN: %{cxx} %s %{flags} %{compile_flags} -c -DMAIN                                -o %t.main.o
+// RUN: %{cxx} %t.tu1.o %t.tu2.o %t.tu3.o %t.tu4.o %t.main.o %{flags} %{link_flags} -o %t.exe
+// RUN: %{exec} %t.exe
+
+// hardened mode
+#ifdef TU1
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI inline int f() { return 1; }
+int tu1() { return f(); }
+#endif // TU1
+
+// safe mode
+#ifdef TU2
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI inline int f() { return 2; }
+int tu2() { return f(); }
+#endif // TU2
+
+// debug mode
+#ifdef TU3
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI inline int f() { return 3; }
+int tu3() { return f(); }
+#endif // TU3
+
+// unchecked mode
+#ifdef TU4
+#  include <__config>
+_LIBCPP_HIDE_FROM_ABI inline int f() { return 4; }
+int tu4() { return f(); }
+#endif // TU4
+
+#ifdef MAIN
+#  include <cassert>
+
+int tu1();
+int tu2();
+int tu3();
+int tu4();
+
+int main(int, char**) {
+  assert(tu1() == 1);
+  assert(tu2() == 2);
+  assert(tu3() == 3);
+  assert(tu4() == 4);
+  return 0;
+}
+#endif // MAIN