Index: sys/conf/kern.pre.mk
===================================================================
--- sys/conf/kern.pre.mk	(revision 228799)
+++ sys/conf/kern.pre.mk	(working copy)
@@ -29,15 +29,13 @@
 .else
 .if ${MACHINE_CPUARCH} == "powerpc"
 _MINUS_O=	-O	# gcc miscompiles some code at -O2
+.elif ${MACHINE_CPUARCH} == "amd64" && ${CC:T:Mclang} != "clang"
+_MINUS_O=	-O2 -frename-registers
 .else
 _MINUS_O=	-O2
 .endif
 .endif
-.if ${MACHINE_CPUARCH} == "amd64"
-COPTFLAGS?=-O2 -frename-registers -pipe
-.else
 COPTFLAGS?=${_MINUS_O} -pipe
-.endif
 .if !empty(COPTFLAGS:M-O[23s]) && empty(COPTFLAGS:M-fno-strict-aliasing)
 COPTFLAGS+= -fno-strict-aliasing
 .endif