diff --git a/lib/libsys/Makefile.sys b/lib/libsys/Makefile.sys
--- a/lib/libsys/Makefile.sys
+++ b/lib/libsys/Makefile.sys
@@ -135,15 +135,28 @@
 FEATURE_NOTE=''
 .endif
 
-${SASM}:
-	printf '/* %sgenerated by libc/sys/Makefile.inc */\n' @ > ${.TARGET}
+# Add this file as a dependency of the generated assembly along with
+# the two included files compat.h and SYS.h.  Depending on this Makefile
+# will cause some needless regenerations, but handles both changes in
+# generated assembly and movement between MIASM and PSEUDO/INTERPOSED.
+# The dependency on compat.h and SYS.h should properly be on the
+# <foo>.S-><foo>.o rules, but there are too many .o variants for it to
+# be easy and touching the geneated source files has the same effect in
+# practice.
+__makefile_sys:=	${.PARSEDIR}/${.PARSEFILE}
+__asm_deps=	${__makefile_sys} \
+		${LIBC_SRCTOP}/include/compat.h \
+		${LIBSYS_SRCTOP}/${LIBC_ARCH}/SYS.h
+
+${SASM}: ${__asm_deps}
+	printf '/* %sgenerated by libsys/Makefile.sys */\n' @ > ${.TARGET}
 	printf '#include "compat.h"\n' >> ${.TARGET}
 	printf '#include "SYS.h"\nRSYSCALL(${.PREFIX})\n' >> ${.TARGET}
 	printf  ${NOTE_GNU_STACK} >>${.TARGET}
 	printf  ${FEATURE_NOTE} >> ${.TARGET}
 
-${SPSEUDO}:
-	printf '/* %sgenerated by libc/sys/Makefile.inc */\n' @ > ${.TARGET}
+${SPSEUDO}: ${__asm_deps}
+	printf '/* %sgenerated by libsys/Makefile.sys */\n' @ > ${.TARGET}
 	printf '#include "compat.h"\n' >> ${.TARGET}
 	printf '#include "SYS.h"\nPSEUDO(${.PREFIX:S/_//})\n' \
 	    >> ${.TARGET}