diff --git a/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES b/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
index 447c7ed2cdb..5bb42933398 100644
--- a/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
+++ b/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
@@ -1,3 +1,4 @@
 r12-6647
 r12-6648
 r12-6664
+r12-6665
diff --git a/libstdc++-v3/src/c++17/fast_float/fast_float.h b/libstdc++-v3/src/c++17/fast_float/fast_float.h
index ee129309ba3..31fb88b8aba 100644
--- a/libstdc++-v3/src/c++17/fast_float/fast_float.h
+++ b/libstdc++-v3/src/c++17/fast_float/fast_float.h
@@ -128,7 +128,9 @@ from_chars_result from_chars_advanced(const char *first, const char *last,
 #define FASTFLOAT_VISUAL_STUDIO 1
 #endif
 
-#ifdef _WIN32
+#ifdef __BYTE_ORDER__
+#define FASTFLOAT_IS_BIG_ENDIAN (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
+#elif defined _WIN32
 #define FASTFLOAT_IS_BIG_ENDIAN 0
 #else
 #if defined(__APPLE__) || defined(__FreeBSD__)