1 files changed, 5 insertions, 0 deletions
diff --git a/py/py.mk b/py/py.mk
index 3cb505920..3e1d7989f 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -346,6 +346,11 @@ $(HEADER_BUILD)/qstrdefs.generated.h: $(PY_SRC)/makeqstrdata.py $(HEADER_BUILD)/
 
 $(PY_BUILD)/qstr.o: $(HEADER_BUILD)/qstrdefs.generated.h
 
+# Standard C functions like memset need to be compiled with special flags so
+# the compiler does not optimise these functions in terms of themselves.
+CFLAGS_BUILTIN ?= -ffreestanding -fno-builtin -fno-lto
+$(BUILD)/lib/libc/string0.o: CFLAGS += $(CFLAGS_BUILTIN)
+
 # Force nlr code to always be compiled with space-saving optimisation so
 # that the function preludes are of a minimal and predictable form.
 $(PY_BUILD)/nlr%.o: CFLAGS += -Os