Index: Makefile.inc1
===================================================================
--- Makefile.inc1
+++ Makefile.inc1
@@ -784,6 +784,7 @@
 WMAKEENV=	${CROSSENV} \
 		INSTALL="${INSTALL_CMD} -U" \
 		PATH=${TMPPATH} \
+		WORLD_SYSROOT=${WORLDTMP} \
 		SYSROOT=${WORLDTMP}
 
 # make hierarchy
Index: Makefile.libcompat
===================================================================
--- Makefile.libcompat
+++ Makefile.libcompat
@@ -12,6 +12,7 @@
 LIBCOMPATWMAKEENV+= \
 		INSTALL="${INSTALL_CMD} -U" \
 		PATH=${TMPPATH} \
+		WORLD_SYSROOT=${WORLDTMP} \
 		SYSROOT=${LIBCOMPATTMP} \
 		LIBDIR=/usr/lib${libcompat} \
 		SHLIBDIR=/usr/lib${libcompat} \
Index: share/mk/bsd.compat.mk
===================================================================
--- share/mk/bsd.compat.mk
+++ share/mk/bsd.compat.mk
@@ -142,6 +142,8 @@
 .endfor
 
 # Shared flags
+_libcompat_machine:=	${MACHINE}.${MACHINE_ARCH}
+LIBCOMPAT_REPRODUCIBLE_OBJTOP?=	${REPRODUCIBLE_OBJROOT}${_libcompat_machine}/obj-lib${libcompat}
 LIBCOMPAT_OBJTOP?=	${OBJTOP}/obj-lib${libcompat}
 LIBCOMPATTMP?=		${LIBCOMPAT_OBJTOP}/tmp
 
@@ -153,6 +155,7 @@
 
 LIBCOMPATWMAKEENV+=	MACHINE=${LIBCOMPAT_MACHINE}
 LIBCOMPATWMAKEENV+=	MACHINE_ARCH=${LIBCOMPAT_MACHINE_ARCH}
+LIBCOMPATWMAKEENV+=	REPRODUCIBLE_OBJTOP=${LIBCOMPAT_REPRODUCIBLE_OBJTOP}
 
 # -B is needed to find /usr/lib32/crti.o for GCC and /usr/libsoft/crti.o for
 # Clang/GCC.
@@ -166,6 +169,7 @@
 LDFLAGS+=	${CFLAGS} ${LIBCOMPATLDFLAGS}
 MACHINE=	${LIBCOMPAT_MACHINE}
 MACHINE_ARCH=	${LIBCOMPAT_MACHINE_ARCH}
+REPRODUCIBLE_OBJTOP=	${REPRODUCIBLE_OBJROOT}${_libcompat_machine}
 .endif
 
 .endif
Index: share/mk/bsd.own.mk
===================================================================
--- share/mk/bsd.own.mk
+++ share/mk/bsd.own.mk
@@ -283,6 +283,14 @@
 # overridden by Makefiles, but the user may choose to set this in src.conf(5).
 TESTSBASE?= /usr/tests
 
+# Canonical source and object directories, used to fake src roots in reproducible
+# builds
+REPRODUCIBLE_SRCTOP?=	/usr/src
+REPRODUCIBLE_SYSDIR?=	${REPRODUCIBLE_SRCTOP}/sys
+REPRODUCIBLE_OBJDIRPREFIX?=	/usr/obj
+REPRODUCIBLE_OBJROOT?=	${REPRODUCIBLE_OBJDIRPREFIX}${REPRODUCIBLE_SRCTOP}/
+REPRODUCIBLE_OBJTOP?=	${REPRODUCIBLE_OBJROOT}${MACHINE}.${MACHINE_ARCH}
+
 DEPENDFILE?=	.depend
 
 # Compat for the moment -- old bsd.own.mk only included this when _WITHOUT_SRCCONF
Index: share/mk/bsd.sys.mk
===================================================================
--- share/mk/bsd.sys.mk
+++ share/mk/bsd.sys.mk
@@ -247,6 +247,31 @@
 CFLAGS+=	${SSP_CFLAGS}
 .endif # SSP && !ARM && !MIPS
 
+.if ${COMPILER_FEATURES:Mfileprefixmap}
+CFLAGS+=	-ffile-prefix-map=${OBJTOP}/tmp/=/
+.if !empty(SYSROOT:N${OBJTOP}/tmp)
+CFLAGS+=	-ffile-prefix-map=${SYSROOT}/=/
+.endif
+.if !empty(WORLD_SYSROOT:N${OBJTOP}/tmp:N${SYSROOT})
+CFLAGS+=	-ffile-prefix-map=${WORLD_SYSROOT}/=/
+.endif
+.if !empty(LIBCOMPATTMP:N${OBJTOP}/tmp)
+CFLAGS+=	-ffile-prefix-map=${LIBCOMPATTMP}=/
+.endif
+.if ${MK_REPRODUCIBLE_BUILD:Uno} != "no"
+.if ${OBJTOP} != ${REPRODUCIBLE_OBJTOP}
+CFLAGS+=	-ffile-prefix-map=${OBJTOP}=${REPRODUCIBLE_OBJTOP}
+.endif
+.if ${SRCTOP} != ${REPRODUCIBLE_SRCTOP}
+CFLAGS+=	-ffile-prefix-map=${SRCTOP}=${REPRODUCIBLE_SRCTOP}
+.endif
+#.if defined(LIBCOMPAT_OBJTOP) && \
+#	${LIBCOMPAT_OBJTOP} != ${LIBCOMPAT_REPRODUCIBLE_OBJTOP}
+#CFLAGS+=	-ffile-prefix-map=${LIBCOMPAT_OBJTOP}=${LIBCOMPAT_REPRODUCIBLE_OBJTOP}
+#.endif
+.endif
+.endif
+
 # Additional flags passed in CFLAGS and CXXFLAGS when MK_DEBUG_FILES is
 # enabled.
 DEBUG_FILES_CFLAGS?= -g