diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 19ae4cef9fa5..a94334f8ab16 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,5 +1,14 @@
 2020-02-26  Jonathan Wakely  <jwakely@redhat.com>
 
+	Backport from mainline
+	2020-01-17  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/92376
+	* include/bits/c++config: Only do PSTL config when the header is
+	present, to fix freestanding.
+	* libsupc++/new_opa.cc [!_GLIBCXX_HOSTED]: Declare allocation
+	functions if they were detected by configure.
+
 	Backport from mainline
 	2019-10-09  Jonathan Wakely  <jwakely@redhat.com>
 
diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index 7a7e78819ba7..7e0962edd199 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -668,6 +668,8 @@ namespace std
 // PSTL configuration
 
 #if __cplusplus >= 201703L
+// This header is not installed for freestanding:
+#if __has_include(<pstl/pstl_config.h>)
 // Preserved here so we have some idea which version of upstream we've pulled in
 // #define PSTL_VERSION 104
 // #define PSTL_VERSION_MAJOR (PSTL_VERSION/100)
@@ -686,6 +688,7 @@ namespace std
 # define __PSTL_ASSERT_MSG(_Condition, _Message) __glibcxx_assert(_Condition)
 
 #include <pstl/pstl_config.h>
+#endif // __has_include
+#endif // C++17
 
-#endif
 // End of prewritten config; the settings discovered at configure time follow.
diff --git a/libstdc++-v3/libsupc++/new_opa.cc b/libstdc++-v3/libsupc++/new_opa.cc
index 8143c129bc62..9c7f53d7073c 100644
--- a/libstdc++-v3/libsupc++/new_opa.cc
+++ b/libstdc++-v3/libsupc++/new_opa.cc
@@ -43,6 +43,21 @@ extern "C" void *memalign(std::size_t boundary, std::size_t size);
 using std::new_handler;
 using std::bad_alloc;
 
+#if ! _GLIBCXX_HOSTED
+extern "C"
+{
+# if _GLIBCXX_HAVE_ALIGNED_ALLOC
+  void *aligned_alloc(size_t alignment, size_t size);
+# elif _GLIBCXX_HAVE__ALIGNED_MALLOC
+  void *_aligned_malloc(size_t size, size_t alignment);
+# elif _GLIBCXX_HAVE_POSIX_MEMALIGN
+  void *posix_memalign(void **, size_t alignment, size_t size);
+# elif _GLIBCXX_HAVE_MEMALIGN
+  void *memalign(size_t alignment, size_t size);
+# endif
+}
+#endif
+
 namespace __gnu_cxx {
 #if _GLIBCXX_HAVE_ALIGNED_ALLOC
 using ::aligned_alloc;