Index: llvm/lib/Support/Unix/Path.inc
===================================================================
--- llvm/lib/Support/Unix/Path.inc
+++ llvm/lib/Support/Unix/Path.inc
@@ -873,7 +873,16 @@
 void mapped_file_region::dontNeedImpl() {
   assert(Mode == mapped_file_region::readonly);
   if (Mapping)
+#if defined(__MVS__) || !defined(HAVE_SYS_MMAN_H)
+    // If we don't have either madvise or posix_madvise, treat this as a nop.
+    return;
+#elif defined(_AIX)
+    // Prefer the POSIX implementation on AIX, since the signature for madvise is non-standardised
+    // and only exists with certain feature test macros.
+    ::posix_madvise(Mapping, Size, POSIX_MADV_DONTNEED);
+#else
     ::madvise(Mapping, Size, MADV_DONTNEED);
+#endif
 }

 int mapped_file_region::alignment() {