Taken from debian GNU/kFreeBSD.

Regretably, with GCC 4.4 or later this is the only flag combination
I could find that doesn't cause kernel panics CPU triple-faults or
other kind of severe breakage. This is Gentoo Bug #410945.

Clang can use -O2, so we do not force -O1 on it

--- a/sys/conf/kern.pre.mk
+++ b/sys/conf/kern.pre.mk
@@ -41,6 +41,9 @@
 .if !empty(COPTFLAGS:M-O[23s]) && empty(COPTFLAGS:M-fno-strict-aliasing)
 COPTFLAGS+= -fno-strict-aliasing
 .endif
+.if ${CC:T:Mclang} != "clang" && !empty(COPTFLAGS:M-O[23s])
+COPTFLAGS+= -O1
+.endif
 .if !defined(NO_CPU_COPTFLAGS)
 COPTFLAGS+= ${_CPUCFLAGS}
 .endif
--- a/sys/conf/kmod.mk
+++ b/sys/conf/kmod.mk
@@ -91,6 +91,9 @@
 .if !empty(CFLAGS:M-O[23s]) && empty(CFLAGS:M-fno-strict-aliasing)
 CFLAGS+=	-fno-strict-aliasing
 .endif
+.if ${CC:T:Mclang} != "clang" && !empty(CFLAGS:M-O[23s])
+CFLAGS+=	-O1
+.endif
 WERROR?=	-Werror
 CFLAGS+=	${WERROR}
 CFLAGS+=	-D_KERNEL