Add a SUBDIR_PARALLEL option to bsd.subdir.mk, to allow make to process

all the SUBDIR entries in parallel, instead of serially.  Apply this
option to a selected number of Makefiles, which can greatly speed up the
build on multi-core machines, when using make -j.

This can be extended to more Makefiles later on, whenever they are
verified to work correctly with parallel building.

I tested this on a 24-core machine, with make -j48 buildworld (N = 6):

                before    stddev       after    stddev
                =======   ======       =======  ======
real time        1741.1     16.5         959.8     2.7
user time       12468.7     16.4       14393.0    16.8
sys  time        1825.0     54.8        2110.6    22.8

(user+sys)/real     8.2                   17.1

E.g. the build was approximately 45% faster in real time.  On machines
with less cores, or with lower -j settings, the speedup will not be as
impressive.  But at least you can now almost max out a machine with
buildworld!

Submitted by:	jilles
MFC after:	2 weeks

bin/Makefile

@@ -60,4 +60,6 @@ SUBDIR+=	tests
 
 SUBDIR:=	${SUBDIR:O}
 
+SUBDIR_PARALLEL=
+
 .include <bsd.subdir.mk>

lib/Makefile

@@ -276,4 +276,8 @@ afterinstall:
 	${INSTALL_SYMLINK} ../include ${DESTDIR}/usr/lib/include
 .endif
 
+.if !make(install)
+SUBDIR_PARALLEL=
+.endif
+
 .include <bsd.subdir.mk>

lib/clang/Makefile

@@ -147,4 +147,6 @@ SUBDIR+=liblldb \
 
 SUBDIR+= include
 
+SUBDIR_PARALLEL=
+
 .include <bsd.subdir.mk>

sbin/Makefile

@@ -126,4 +126,6 @@ SUBDIR+=	tests
 
 SUBDIR:=	${SUBDIR:O}
 
+SUBDIR_PARALLEL=
+
 .include <bsd.subdir.mk>

share/mk/bsd.subdir.mk

@@ -71,7 +71,26 @@ ${SUBDIR}: .PHONY .MAKE
 .for __target in all all-man checkdpadd clean cleandepend cleandir \
     cleanilinks depend distribute lint maninstall manlint obj objlink \
     realinstall regress tags ${SUBDIR_TARGETS}
+.ifdef SUBDIR_PARALLEL
+.for __dir in ${SUBDIR}
+${__target}: ${__target}_subdir_${__dir}
+${__target}_subdir_${__dir}: .MAKE
+	@${_+_}set -e; \
+		if test -d ${.CURDIR}/${__dir}.${MACHINE_ARCH}; then \
+			${ECHODIR} "===> ${DIRPRFX}${__dir}.${MACHINE_ARCH} (${__target:realinstall=install})"; \
+			edir=${__dir}.${MACHINE_ARCH}; \
+			cd ${.CURDIR}/$${edir}; \
+		else \
+			${ECHODIR} "===> ${DIRPRFX}${__dir} (${__target:realinstall=install})"; \
+			edir=${__dir}; \
+			cd ${.CURDIR}/$${edir}; \
+		fi; \
+		${MAKE} ${__target:realinstall=install} \
+		    DIRPRFX=${DIRPRFX}$$edir/
+.endfor
+.else
 ${__target}: _SUBDIR
+.endif
 .endfor
 
 .for __target in files includes

usr.bin/Makefile

@@ -379,4 +379,6 @@ SUBDIR+=	svn
 
 SUBDIR:=	${SUBDIR:O}
 
+SUBDIR_PARALLEL=
+
 .include <bsd.subdir.mk>

usr.sbin/Makefile

@@ -320,4 +320,6 @@ SUBDIR+=	wpa
 
 SUBDIR:=	${SUBDIR:O}
 
+SUBDIR_PARALLEL=
+
 .include <bsd.subdir.mk>