diff --git a/config/external/ffi/moz.build b/config/external/ffi/moz.build
index 1d5a615a5f35..0c98a8639e74 100644
--- a/config/external/ffi/moz.build
+++ b/config/external/ffi/moz.build
@@ -78,7 +78,18 @@ else:
     elif CONFIG['FFI_TARGET'] == 'AARCH64':
         ffi_srcs = ('sysv.S', 'ffi.c')
     elif CONFIG['FFI_TARGET'] == 'ARM64_WIN64':
-        ffi_srcs = ('win64.asm', 'ffi.c')
+        ffi_srcs = ['ffi.c']
+
+        GENERATED_FILES += ['win64_aarch.asm']
+        asm = GENERATED_FILES['win64_aarch.asm']
+        asm.inputs = [
+            '/js/src/ctypes/libffi/src/aarch64/win64.asm',
+            '!../../../js/src/ctypes/libffi/fficonfig.h',
+            '!../../../js/src/ctypes/libffi/include/ffi.h',
+        ]
+        asm.script = 'preprocess_libffi_asm.py'
+        asm.flags = ['$(DEFINES)', '$(LOCAL_INCLUDES)']
+        SOURCES += ['!win64_aarch.asm']
     elif CONFIG['FFI_TARGET'] == 'X86':
         ffi_srcs = ('ffi.c', 'sysv.S', 'win32.S')
     elif CONFIG['FFI_TARGET'] == 'X86_64':
diff --git a/config/external/ffi/preprocess_libffi_asm.py b/config/external/ffi/preprocess_libffi_asm.py
index d88eec74a747..1abcfad8769e 100644
--- a/config/external/ffi/preprocess_libffi_asm.py
+++ b/config/external/ffi/preprocess_libffi_asm.py
@@ -15,7 +15,8 @@ def main(output, input_asm, ffi_h, ffi_config_h, defines, includes):
     defines = shlex.split(defines)
     includes = shlex.split(includes)
     # CPP uses -E which generates #line directives. -EP suppresses them.
-    cpp = buildconfig.substs['CPP'] + ['-EP']
+    # -TC forces the compiler to treat the input as C.
+    cpp = buildconfig.substs['CPP'] + ['-EP'] + ['-TC']
     input_asm = mozpath.relpath(input_asm, os.getcwd())
     args = cpp + defines + includes + [input_asm]
     print(' '.join(args))
diff --git a/js/src/ctypes/libffi/src/aarch64/win64.asm b/js/src/ctypes/libffi/src/aarch64/win64.asm
index 5cdb61edce70..4c29d8a7506c 100644
--- a/js/src/ctypes/libffi/src/aarch64/win64.asm
+++ b/js/src/ctypes/libffi/src/aarch64/win64.asm
@@ -19,9 +19,11 @@
 ;; TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 ;; SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
+#include "ksarm64.h"
+
 ;; Hand-converted from the sysv.S file in this directory.
 
-        AREA |.text|, CODE, ARM64
+        TEXTAREA
 
    ;; ffi_call_SYSV()
 
@@ -58,30 +60,26 @@
    ;; This function uses the following stack frame layout:
 
    ;; ==
-   ;;              saved x30(lr)
-   ;; x29(fp)->    saved x29(fp)
    ;;              saved x24
    ;;              saved x23
    ;;              saved x22
-   ;; sp'    ->    saved x21
+   ;;              saved x21
+   ;;              saved x20
+   ;;              saved x19
+   ;;              saved x30(lr)
+   ;; x29(fp)->    saved x29(fp)
    ;;              ...
    ;; sp     ->    (constructed callee stack arguments)
    ;; ==
 
    ;; Voila!
 
-	EXPORT |ffi_call_SYSV|
+        NESTED_ENTRY |ffi_call_SYSV|
 
-|ffi_call_SYSV| PROC
-;#define ffi_call_SYSV_FS (8 * 4)
-
-        stp     x29, x30, [sp, #-16]!
-
-        mov     x29, sp
-        sub     sp, sp, #32 	; ffi_call_SYSV_FS
-
-        stp     x21, x22, [sp, #0]
-        stp     x23, x24, [sp, #16]
+        PROLOG_SAVE_REG_PAIR x29, x30, #-64!
+        PROLOG_SAVE_REG_PAIR x19, x20, #16
+        PROLOG_SAVE_REG_PAIR x21, x22, #32
+        PROLOG_SAVE_REG_PAIR x23, x24, #48
 
         mov     x21, x1
         mov     x22, x2
@@ -145,19 +143,14 @@ noload_call
 
 nosave_call
         ; All done, unwind our stack frame.
-        ldp     x21, x22, [x29,  # - 32] ; ffi_call_SYSV_FS
+        EPILOG_STACK_RESTORE
+        EPILOG_RESTORE_REG_PAIR x19, x20, #16
+        EPILOG_RESTORE_REG_PAIR x21, x22, #32
+        EPILOG_RESTORE_REG_PAIR x23, x24, #48
+        EPILOG_RESTORE_REG_PAIR x29, x30, #64!
+        EPILOG_RETURN
 
-        ldp     x23, x24, [x29,  # - 32 + 16] ; ffi_call_SYSV_FS
-
-        mov     sp, x29
-
-        ldp     x29, x30, [sp], #16
-
-        ret
-
-	ENDP
-
-; #define ffi_closure_SYSV_FS (8 * 2 + AARCH64_CALL_CONTEXT_SIZE)
+        NESTED_END |ffi_call_SYSV|
 
    ;; ffi_closure_SYSV
 
@@ -186,10 +179,12 @@ nosave_call
    ;; This function uses the following stack frame layout:
 
    ;; ==
-   ;;              saved x30(lr)
-   ;; x29(fp)->    saved x29(fp)
    ;;              saved x22
    ;;              saved x21
+   ;;              saved x20
+   ;;              saved x19
+   ;;              saved x30(lr)
+   ;; x29(fp)->    saved x29(fp)
    ;;              ...
    ;; sp     ->    call_context
    ;; ==
@@ -197,16 +192,14 @@ nosave_call
    ;; Voila!
 
 	IMPORT |ffi_closure_SYSV_inner|
-	EXPORT |ffi_closure_SYSV|
 
-|ffi_closure_SYSV| PROC
-        stp     x29, x30, [sp, #-16]!
+        NESTED_ENTRY |ffi_closure_SYSV|
 
-        mov     x29, sp
+        PROLOG_SAVE_REG_PAIR fp, lr, #-48!
+        PROLOG_SAVE_REG_PAIR x19, x20, #16
+        PROLOG_SAVE_REG_PAIR x21, x22, #32
 
-        sub     sp, sp, #256+512+16
-
-        stp     x21, x22, [x29, #-16]
+        sub     sp, sp, #256+512
 
         ; Load x21 with &call_context.
         mov     x21, sp
@@ -260,13 +253,12 @@ noload_closure
         ; Note nothing useful is returned in x8.
 
         ; We are done, unwind our frame.
-        ldp     x21, x22, [x29,  #-16]
+        EPILOG_STACK_RESTORE
+        EPILOG_RESTORE_REG_PAIR x19, x20, #16
+        EPILOG_RESTORE_REG_PAIR x21, x22, #32
+        EPILOG_RESTORE_REG_PAIR x29, x30, #48!
+        EPILOG_RETURN
 
-        mov     sp, x29
+        NESTED_END |ffi_closure_SYSV|
 
-        ldp     x29, x30, [sp], #16
-
-        ret
-
-	ENDP
-	END
+        END