diff --git a/libcxxabi/src/cxa_default_handlers.cpp b/libcxxabi/src/cxa_default_handlers.cpp
--- a/libcxxabi/src/cxa_default_handlers.cpp
+++ b/libcxxabi/src/cxa_default_handlers.cpp
@@ -15,7 +15,7 @@
 #include "cxa_handlers.h"
 #include "cxa_exception.h"
 #include "private_typeinfo.h"
-#include "include/atomic_support.h"
+#include "src/include/atomic_support.h" // from libc++
 
 #if !defined(LIBCXXABI_SILENT_TERMINATE)
 
diff --git a/libcxxabi/src/cxa_exception.cpp b/libcxxabi/src/cxa_exception.cpp
--- a/libcxxabi/src/cxa_exception.cpp
+++ b/libcxxabi/src/cxa_exception.cpp
@@ -17,7 +17,7 @@
 #include "cxa_exception.h"
 #include "cxa_handlers.h"
 #include "fallback_malloc.h"
-#include "include/atomic_support.h"
+#include "src/include/atomic_support.h" // from libc++
 
 #if __has_feature(address_sanitizer)
 #include <sanitizer/asan_interface.h>
diff --git a/libcxxabi/src/cxa_guard_impl.h b/libcxxabi/src/cxa_guard_impl.h
--- a/libcxxabi/src/cxa_guard_impl.h
+++ b/libcxxabi/src/cxa_guard_impl.h
@@ -38,7 +38,7 @@
  */
 
 #include "__cxxabi_config.h"
-#include "include/atomic_support.h"
+#include "src/include/atomic_support.h" // from libc++
 #include <unistd.h>
 #if defined(__has_include)
 #  if __has_include(<sys/syscall.h>)
diff --git a/libcxxabi/src/cxa_handlers.cpp b/libcxxabi/src/cxa_handlers.cpp
--- a/libcxxabi/src/cxa_handlers.cpp
+++ b/libcxxabi/src/cxa_handlers.cpp
@@ -17,7 +17,7 @@
 #include "cxa_handlers.h"
 #include "cxa_exception.h"
 #include "private_typeinfo.h"
-#include "include/atomic_support.h"
+#include "src/include/atomic_support.h" // from libc++
 
 namespace std
 {
diff --git a/libcxxabi/src/include/atomic_support.h b/libcxxabi/src/include/atomic_support.h
deleted file mode 100644
--- a/libcxxabi/src/include/atomic_support.h
+++ /dev/null
@@ -1,180 +0,0 @@
-//===----------------------------------------------------------------------===////
-//
-// 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
-//
-//===----------------------------------------------------------------------===////
-
-// FIXME: This file is copied from libcxx/src/include/atomic_support.h. Instead
-// of duplicating the file in libc++abi we should require that the libc++
-// sources are available when building libc++abi.
-
-#ifndef ATOMIC_SUPPORT_H
-#define ATOMIC_SUPPORT_H
-
-#include "__config"
-#include "memory" // for __libcpp_relaxed_load
-
-#if defined(__clang__) && __has_builtin(__atomic_load_n)             \
-                       && __has_builtin(__atomic_store_n)            \
-                       && __has_builtin(__atomic_add_fetch)          \
-                       && __has_builtin(__atomic_exchange_n)         \
-                       && __has_builtin(__atomic_compare_exchange_n) \
-                       && defined(__ATOMIC_RELAXED)                  \
-                       && defined(__ATOMIC_CONSUME)                  \
-                       && defined(__ATOMIC_ACQUIRE)                  \
-                       && defined(__ATOMIC_RELEASE)                  \
-                       && defined(__ATOMIC_ACQ_REL)                  \
-                       && defined(__ATOMIC_SEQ_CST)
-#   define _LIBCXXABI_HAS_ATOMIC_BUILTINS
-#elif defined(_LIBCPP_COMPILER_GCC)
-#   define _LIBCXXABI_HAS_ATOMIC_BUILTINS
-#endif
-
-#if !defined(_LIBCXXABI_HAS_ATOMIC_BUILTINS) && !defined(_LIBCXXABI_HAS_NO_THREADS)
-# if defined(_LIBCPP_WARNING)
-    _LIBCPP_WARNING("Building libc++ without __atomic builtins is unsupported")
-# else
-#   warning Building libc++ without __atomic builtins is unsupported
-# endif
-#endif
-
-_LIBCPP_BEGIN_NAMESPACE_STD
-
-namespace {
-
-#if defined(_LIBCXXABI_HAS_ATOMIC_BUILTINS) && !defined(_LIBCXXABI_HAS_NO_THREADS)
-
-enum __libcpp_atomic_order {
-    _AO_Relaxed = __ATOMIC_RELAXED,
-    _AO_Consume = __ATOMIC_CONSUME,
-    _AO_Acquire = __ATOMIC_ACQUIRE,
-    _AO_Release = __ATOMIC_RELEASE,
-    _AO_Acq_Rel = __ATOMIC_ACQ_REL,
-    _AO_Seq     = __ATOMIC_SEQ_CST
-};
-
-template <class _ValueType, class _FromType>
-inline _LIBCPP_INLINE_VISIBILITY
-void __libcpp_atomic_store(_ValueType* __dest, _FromType __val,
-                           int __order = _AO_Seq)
-{
-    __atomic_store_n(__dest, __val, __order);
-}
-
-template <class _ValueType, class _FromType>
-inline _LIBCPP_INLINE_VISIBILITY
-void __libcpp_relaxed_store(_ValueType* __dest, _FromType __val)
-{
-    __atomic_store_n(__dest, __val, _AO_Relaxed);
-}
-
-template <class _ValueType>
-inline _LIBCPP_INLINE_VISIBILITY
-_ValueType __libcpp_atomic_load(_ValueType const* __val,
-                                int __order = _AO_Seq)
-{
-    return __atomic_load_n(__val, __order);
-}
-
-template <class _ValueType, class _AddType>
-inline _LIBCPP_INLINE_VISIBILITY
-_ValueType __libcpp_atomic_add(_ValueType* __val, _AddType __a,
-                               int __order = _AO_Seq)
-{
-    return __atomic_add_fetch(__val, __a, __order);
-}
-
-template <class _ValueType>
-inline _LIBCPP_INLINE_VISIBILITY
-_ValueType __libcpp_atomic_exchange(_ValueType* __target,
-                                    _ValueType __value, int __order = _AO_Seq)
-{
-    return __atomic_exchange_n(__target, __value, __order);
-}
-
-template <class _ValueType>
-inline _LIBCPP_INLINE_VISIBILITY
-bool __libcpp_atomic_compare_exchange(_ValueType* __val,
-    _ValueType* __expected, _ValueType __after,
-    int __success_order = _AO_Seq,
-    int __fail_order = _AO_Seq)
-{
-    return __atomic_compare_exchange_n(__val, __expected, __after, true,
-                                       __success_order, __fail_order);
-}
-
-#else // _LIBCPP_HAS_NO_THREADS
-
-enum __libcpp_atomic_order {
-    _AO_Relaxed,
-    _AO_Consume,
-    _AO_Acquire,
-    _AO_Release,
-    _AO_Acq_Rel,
-    _AO_Seq
-};
-
-template <class _ValueType, class _FromType>
-inline _LIBCPP_INLINE_VISIBILITY
-void __libcpp_atomic_store(_ValueType* __dest, _FromType __val,
-                           int = 0)
-{
-    *__dest = __val;
-}
-
-template <class _ValueType, class _FromType>
-inline _LIBCPP_INLINE_VISIBILITY
-void __libcpp_relaxed_store(_ValueType* __dest, _FromType __val)
-{
-    *__dest = __val;
-}
-
-template <class _ValueType>
-inline _LIBCPP_INLINE_VISIBILITY
-_ValueType __libcpp_atomic_load(_ValueType const* __val,
-                                int = 0)
-{
-    return *__val;
-}
-
-template <class _ValueType, class _AddType>
-inline _LIBCPP_INLINE_VISIBILITY
-_ValueType __libcpp_atomic_add(_ValueType* __val, _AddType __a,
-                               int = 0)
-{
-    return *__val += __a;
-}
-
-template <class _ValueType>
-inline _LIBCPP_INLINE_VISIBILITY
-_ValueType __libcpp_atomic_exchange(_ValueType* __target,
-                                    _ValueType __value, int  = _AO_Seq)
-{
-    _ValueType old = *__target;
-    *__target = __value;
-    return old;
-}
-
-template <class _ValueType>
-inline _LIBCPP_INLINE_VISIBILITY
-bool __libcpp_atomic_compare_exchange(_ValueType* __val,
-    _ValueType* __expected, _ValueType __after,
-    int = 0, int = 0)
-{
-    if (*__val == *__expected) {
-        *__val = __after;
-        return true;
-    }
-    *__expected = *__val;
-    return false;
-}
-
-#endif // _LIBCPP_HAS_NO_THREADS
-
-} // end namespace
-
-_LIBCPP_END_NAMESPACE_STD
-
-#endif // ATOMIC_SUPPORT_H
diff --git a/libcxxabi/src/stdlib_stdexcept.cpp b/libcxxabi/src/stdlib_stdexcept.cpp
--- a/libcxxabi/src/stdlib_stdexcept.cpp
+++ b/libcxxabi/src/stdlib_stdexcept.cpp
@@ -12,9 +12,7 @@
 #include <cstring>
 #include <cstdint>
 #include <cstddef>
-
-// This includes an implementation file from libc++.
-#include "src/include/refstring.h"
+#include "src/include/refstring.h" // from libc++
 
 static_assert(sizeof(std::__libcpp_refstring) == sizeof(const char *), "");