Index: lib/libc/Makefile
===================================================================
--- lib/libc/Makefile
+++ lib/libc/Makefile
@@ -162,7 +162,7 @@
 
 # If there are no machine dependent sources, append all the
 # machine-independent sources:
-.if empty(MDSRCS)
+.if empty(MDSRCS) || ${MK_MACHDEP_OPTIMIZATIONS} == no
 SRCS+=	${MISRCS}
 .else
 # Append machine-dependent sources, then append machine-independent sources
Index: lib/libmd/Makefile
===================================================================
--- lib/libmd/Makefile
+++ lib/libmd/Makefile
@@ -103,7 +103,7 @@
 .PATH: ${SRCTOP}/sys/crypto/skein ${SRCTOP}/sys/crypto/skein/${MACHINE_ARCH}
 
 USE_ASM_SOURCES?=1
-.if defined(BOOTSTRAPPING)
+.if defined(BOOTSTRAPPING) || ${MK_MACHDEP_OPTIMIZATIONS} == no
 # Don't build ASM sources when bootstrapping to avoid toolchain dependencies
 USE_ASM_SOURCES:=0
 .endif
Index: lib/msun/Makefile
===================================================================
--- lib/msun/Makefile
+++ lib/msun/Makefile
@@ -151,7 +151,7 @@
 #COMMON_SRCS+=	s_fabs.c s_frexp.c s_isnan.c s_ldexp.c s_modf.c
 
 # Exclude the generic versions of what we provide in the MD area.
-.if defined(ARCH_SRCS)
+.if defined(ARCH_SRCS) && ${MK_MACHDEP_OPTIMIZATIONS} == yes
 .for i in ${ARCH_SRCS}
 COMMON_SRCS:=  ${COMMON_SRCS:N${i:R}.c}
 .endfor
Index: share/man/man5/src.conf.5
===================================================================
--- share/man/man5/src.conf.5
+++ share/man/man5/src.conf.5
@@ -1094,6 +1094,8 @@
 Build
 .Xr ls 1
 without support for colors to distinguish file types.
+.It Va WITHOUT_MACHDEP
+Prefer machine-independent non-assembler code across the code base.
 .It Va WITHOUT_MAIL
 Do not build any mail support (MUA or MTA).
 When set, it enforces these options:
Index: share/mk/src.opts.mk
===================================================================
--- share/mk/src.opts.mk
+++ share/mk/src.opts.mk
@@ -142,6 +142,7 @@
     LOCATE \
     LPR \
     LS_COLORS \
+    MACHDEP_OPTIMIZATIONS \
     MAIL \
     MAILWRAPPER \
     MAKE \
Index: tools/build/options/WITHOUT_MACHDEP_OPTIMIZATIONS
===================================================================
--- /dev/null
+++ tools/build/options/WITHOUT_MACHDEP_OPTIMIZATIONS
@@ -0,0 +1,2 @@
+.\" $FreeBSD$
+Prefer machine-independent non-assembler code across the code base.