py/py.mk: Use additional CFLAGS to compile string0.c.

dpgeorge · tannewt · commit f43834aba2ed · 2020-07-22T16:26:47.000-07:00
Otherwise functions like memset might get optimised to call themselves (eg with gcc 10). And provide CFLAGS_BUILTIN so these options can be changed by a port if needed. Fixes issue adafruit#6053.
diff --git 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
