diff --git a/clang/lib/Headers/unwind.h b/clang/lib/Headers/unwind.h
--- a/clang/lib/Headers/unwind.h
+++ b/clang/lib/Headers/unwind.h
@@ -14,12 +14,8 @@
 
 #if defined(__APPLE__) && __has_include_next(<unwind.h>)
 /* Darwin (from 11.x on) provide an unwind.h. If that's available,
- * use it. libunwind wraps some of its definitions in #ifdef _GNU_SOURCE,
- * so define that around the include.*/
-# ifndef _GNU_SOURCE
-#  define _SHOULD_UNDEFINE_GNU_SOURCE
-#  define _GNU_SOURCE
-# endif
+ * use it. */
+
 // libunwind's unwind.h reflects the current visibility.  However, Mozilla
 // builds with -fvisibility=hidden and relies on gcc's unwind.h to reset the
 // visibility to default and export its contents.  gcc also allows users to
@@ -33,10 +29,6 @@
 #  include_next <unwind.h>
 #  pragma GCC visibility pop
 # endif
-# ifdef _SHOULD_UNDEFINE_GNU_SOURCE
-#  undef _GNU_SOURCE
-#  undef _SHOULD_UNDEFINE_GNU_SOURCE
-# endif
 #else
 
 #include <stdint.h>