diff --git a/cpu/cortexm_common/Makefile b/cpu/cortexm_common/Makefile
index 48422e909a..6b348c14e3 100644
--- a/cpu/cortexm_common/Makefile
+++ b/cpu/cortexm_common/Makefile
@@ -1 +1,5 @@
+# thread_arch.c's inline assembler breaks when compiling with link time
+# optimization. see #5774.
+SRC_NOLTO += thread_arch.c
+
 include $(RIOTBASE)/Makefile.base