>If I know that a small set of MI source files will break the compiler if
>compiled with optimization on, is there some easy way to turn it off in,
>say, sys/${ARCH}/conf/Makefile.${ARCH}?

Yes.  That's one of the reasons I added ODE's :U modifier.

sys.mk should not have

CFLAGS?= -O

I ususall make that
DBG?= -O
CFLAGS?= ${DBG}

but anyway, if you made it something like:

DEF_COPT?=	-O
CFLAGS?=	${${.TARGET}_COPT:U${DEF_COPT}}

you could have

fred.o_COPT=-O0

and -O0 would be used when .TARGET is fred.o

Change it around to suit your tastes, COPT_fred.o might look better.

--sjg