diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 97da6076239..973c65aa4fb 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -6531,7 +6531,9 @@ aarch64_layout_frame (void)
 	&& !crtl->abi->clobbers_full_reg_p (regno))
       frame.reg_offset[regno] = SLOT_REQUIRED;
 
-  /* With stack-clash, LR must be saved in non-leaf functions.  */
+  /* With stack-clash, LR must be saved in non-leaf functions.  The saving of
+     LR counts as an implicit probe which allows us to maintain the invariant
+     described in the comment at expand_prologue.  */
   gcc_assert (crtl->is_leaf
 	      || maybe_ne (frame.reg_offset[R30_REGNUM], SLOT_NOT_REQUIRED));