Index: lib/builtins/CMakeLists.txt
===================================================================
--- lib/builtins/CMakeLists.txt
+++ lib/builtins/CMakeLists.txt
@@ -444,6 +444,7 @@
 endif()
 
 set(aarch64_SOURCES
+  aarch64/chkstk_darwin.S
   ${GENERIC_TF_SOURCES}
   ${GENERIC_SOURCES})
 
Index: lib/builtins/aarch64/chkstk_darwin.S
===================================================================
--- /dev/null
+++ lib/builtins/aarch64/chkstk_darwin.S
@@ -0,0 +1,33 @@
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+
+#include "../assembly.h"
+
+#ifdef __aarch64__
+
+#define PAGE_SZ 4096
+
+// ___chkstk_darwin - a generic stack probing function.
+// The number of bytes to probe is passed in w9.
+// This clobbers registers x9, x10 and x11.
+// Does not modify any memory or the stack pointer
+
+.balign 4
+DEFINE_COMPILERRT_FUNCTION(___chkstk_darwin)
+.weak_definition ___chkstk_darwin
+        cmp     w9, #PAGE_SZ
+        mov     x10, sp
+        b.lo    Lend
+Lloop:
+        sub     x10, x10, #PAGE_SZ
+        ldr     x11, [x10]
+        sub     w9, w9, #PAGE_SZ
+        cmp     w9, #PAGE_SZ
+        b.hi    Lloop
+Lend:
+        sub     x10, x10, x9
+        ldr     x11, [x10]
+        ret
+END_COMPILERRT_FUNCTION(___chkstk_darwin)
+
+#endif // __aarch64__
Index: lib/builtins/assembly.h
===================================================================
--- lib/builtins/assembly.h
+++ lib/builtins/assembly.h
@@ -18,6 +18,8 @@
 
 #if defined(__POWERPC__) || defined(__powerpc__) || defined(__ppc__)
 #define SEPARATOR @
+#elif defined(__aarch64__)
+#define SEPARATOR %%
 #else
 #define SEPARATOR ;
 #endif