diff --git a/llvm/utils/update_llc_test_checks.py b/llvm/utils/update_llc_test_checks.py
--- a/llvm/utils/update_llc_test_checks.py
+++ b/llvm/utils/update_llc_test_checks.py
@@ -5,6 +5,12 @@
 This script is a utility to update LLVM 'llc' based test cases with new
 FileCheck patterns. It can either update all of the tests in the file or
 a single test function.
+
+If your function has runtime preemption specifiers (such as dso_local or
+dso_preemptable), then it will not match the regex this script uses when
+deciding which functions need to be updated. Often the runtime preemption
+specifiers are unnecessary for the test and you should delete them.
+FIXME: https://bugs.llvm.org/show_bug.cgi?id=45714.
 """
 
 from __future__ import print_function