WIP add update_base_pointer macro

runtime/arm64.S

@@ -975,6 +975,30 @@ END_FUNCTION(caml_callback3_asm)
         LEAVE_FUNCTION
 .endm
 
+/* Updates the oldest saved frame pointer in the target fiber.
+
+   A fiber stack may need to grow, causing the reallocation of the entire fiber,
+   including stack_info and stack_handler structures.
+   caml_try_realloc_stack will not be able to update the linked list of
+   frame-pointers if it has been split (ie, in a continuation).
+   caml_resume and caml_reperform use this macro to update the oldest saved fp (x29)
+   (highest one in the stack) in case the fiber was reallocated to reattach the
+   frame-pointer linked list.
+
+   REG: Stack_handler(target_fiber)
+
+   The frame pointer will be pushed into the stack immediately after these
+   instructions. The offset of the oldest saved fp in a fiber from the stack
+   handler is 48 = 4 words (caml_runstack) + 2 words (lr (x30) and fp (x29)).
+   */
+#ifdef WITH_FRAME_POINTERS
+#define UPDATE_BASE_POINTER(REG)                          \
+        ;; ldr     TMP, [sp, -8];                            \
+        ;; str     TMP, [REG, -48]
+#else
+#define UPDATE_BASE_POINTER(REG)
+#endif
+
 
 /*
  * A continuation is a one word object that points to a fiber. A fiber [f] will
@@ -1070,7 +1094,7 @@ FUNCTION(caml_reperform)
         add     x3, x2, 1 /* x3 (last_fiber) := Val_ptr(old stack) */
     /* Need to update the oldest saved frame pointer here as the execution of
        the handler may have caused the current fiber stack to reallocate. */
-    /* TODO */
+    /* TODO call UPDATE_BASE_POINTERS (x2) */
         b       L(do_perform)
         CFI_ENDPROC
 END_FUNCTION(caml_reperform)
@@ -1121,7 +1145,10 @@ FUNCTION(caml_resume)
     /* Need to update the oldest saved frame pointer here as the current fiber
        stack may have been reallocated or we may be resuming a computation
        that was not originally run here. */
-    /* TODO */
+    /* TODO UPDATE_BASE_POINTER(x8)
+        When we switch stacks to x9, then we loose the placeholder for
+        where we were resumed from?
+        */
         SWITCH_OCAML_STACKS x9, x0
         mov     x0, x2
         br      x4