kbuild: create object directories simpler and faster

For the out-of-tree build, scripts/Makefile.build creates output
directories, but this operation is not efficient.

scripts/Makefile.lib calculates obj-dirs as follows:

  obj-dirs := $(dir $(multi-objs) $(obj-y))

Please notice $(sort ...) is not used here.  Usually the result is
as many "./" as objects here.

For a lot of duplicated paths, the following command is invoked.

  _dummy := $(foreach d,$(obj-dirs), $(shell [ -d $(d) ] || mkdir -p $(d)))

Then, the costly shell command is run over and over again.

I see many points for optimization:

[1] Use $(sort ...) to cut down duplicated paths before passing them
    to system call
[2] Use single $(shell ...) instead of repeating it with $(foreach ...)
    This will reduce forking.
[3] We can calculate obj-dirs more simply.  Most of objects are already
    accumulated in $(targets).  So, $(dir $(targets)) is fine and more
    comprehensive.

I also removed ugly code in arch/x86/entry/vdso/Makefile.  This is now
really unnecessary.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Douglas Anderson <dianders@chromium.org>

arch/x86/entry/vdso/Makefile

@@ -129,10 +129,6 @@ $(obj)/vdsox32.so.dbg: $(src)/vdsox32.lds $(vobjx32s) FORCE
 CPPFLAGS_vdso32.lds = $(CPPFLAGS_vdso.lds)
 VDSO_LDFLAGS_vdso32.lds = -m32 -Wl,-m,elf_i386 -Wl,-soname=linux-gate.so.1
 
-# This makes sure the $(obj) subdirectory exists even though vdso32/
-# is not a kbuild sub-make subdirectory.
-override obj-dirs = $(dir $(obj)) $(obj)/vdso32/
-
 targets += vdso32/vdso32.lds
 targets += vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
 targets += vdso32/vclock_gettime.o

scripts/Makefile.build

@@ -64,15 +64,6 @@ ifneq ($(hostprogs-y)$(hostprogs-m)$(hostlibs-y)$(hostlibs-m)$(hostcxxlibs-y)$(h
 include scripts/Makefile.host
 endif
 
-ifneq ($(KBUILD_SRC),)
-# Create output directory if not already present
-_dummy := $(shell [ -d $(obj) ] || mkdir -p $(obj))
-
-# Create directories for object files if directory does not exist
-# Needed when obj-y := dir/file.o syntax is used
-_dummy := $(foreach d,$(obj-dirs), $(shell [ -d $(d) ] || mkdir -p $(d)))
-endif
-
 ifndef obj
 $(warning kbuild: Makefile.build is included improperly)
 endif
@@ -589,6 +580,12 @@ ifneq ($(cmd_files),)
   include $(cmd_files)
 endif
 
+ifneq ($(KBUILD_SRC),)
+# Create directories for object files if they do not exist
+obj-dirs := $(sort $(obj) $(patsubst %/,%, $(dir $(targets))))
+$(shell mkdir -p $(obj-dirs))
+endif
+
 # Declare the contents of the .PHONY variable as phony.  We keep that
 # information in a variable se we can use it in if_changed and friends.
 

scripts/Makefile.host

@@ -48,15 +48,6 @@ host-cxxobjs	:= $(sort $(foreach m,$(host-cxxmulti),$($(m)-cxxobjs)))
 host-cshobjs	:= $(sort $(foreach m,$(host-cshlib),$($(m:.so=-objs))))
 host-cxxshobjs	:= $(sort $(foreach m,$(host-cxxshlib),$($(m:.so=-objs))))
 
-# output directory for programs/.o files
-# hostprogs-y := tools/build may have been specified.
-# Retrieve also directory of .o files from prog-objs or prog-cxxobjs notation
-host-objdirs := $(dir $(__hostprogs) $(host-cobjs) $(host-cxxobjs))
-
-host-objdirs := $(strip $(sort $(filter-out ./,$(host-objdirs))))
-
-
-__hostprogs     := $(addprefix $(obj)/,$(__hostprogs))
 host-csingle	:= $(addprefix $(obj)/,$(host-csingle))
 host-cmulti	:= $(addprefix $(obj)/,$(host-cmulti))
 host-cobjs	:= $(addprefix $(obj)/,$(host-cobjs))
@@ -66,9 +57,6 @@ host-cshlib	:= $(addprefix $(obj)/,$(host-cshlib))
 host-cxxshlib	:= $(addprefix $(obj)/,$(host-cxxshlib))
 host-cshobjs	:= $(addprefix $(obj)/,$(host-cshobjs))
 host-cxxshobjs	:= $(addprefix $(obj)/,$(host-cxxshobjs))
-host-objdirs    := $(addprefix $(obj)/,$(host-objdirs))
-
-obj-dirs += $(host-objdirs)
 
 #####
 # Handle options to gcc. Support building with separate output directory

scripts/Makefile.lib

@@ -50,15 +50,11 @@ single-used-m := $(sort $(filter-out $(multi-used-m),$(obj-m)))
 # objects depend on those (obviously)
 multi-objs-y := $(foreach m, $(multi-used-y), $($(m:.o=-objs)) $($(m:.o=-y)))
 multi-objs-m := $(foreach m, $(multi-used-m), $($(m:.o=-objs)) $($(m:.o=-y)))
-multi-objs   := $(multi-objs-y) $(multi-objs-m)
 
 # $(subdir-obj-y) is the list of objects in $(obj-y) which uses dir/ to
 # tell kbuild to descend
 subdir-obj-y := $(filter %/built-in.o, $(obj-y))
 
-# $(obj-dirs) is a list of directories that contain object files
-obj-dirs := $(dir $(multi-objs) $(obj-y))
-
 # Replace multi-part objects by their individual parts, look at local dir only
 real-objs-y := $(foreach m, $(filter-out $(subdir-obj-y), $(obj-y)), $(if $(strip $($(m:.o=-objs)) $($(m:.o=-y))),$($(m:.o=-objs)) $($(m:.o=-y)),$(m))) $(extra-y)
 real-objs-m := $(foreach m, $(obj-m), $(if $(strip $($(m:.o=-objs)) $($(m:.o=-y)) $($(m:.o=-m))),$($(m:.o=-objs)) $($(m:.o=-y)) $($(m:.o=-m)),$(m)))
@@ -81,7 +77,6 @@ multi-used-m	:= $(addprefix $(obj)/,$(multi-used-m))
 multi-objs-y	:= $(addprefix $(obj)/,$(multi-objs-y))
 multi-objs-m	:= $(addprefix $(obj)/,$(multi-objs-m))
 subdir-ym	:= $(addprefix $(obj)/,$(subdir-ym))
-obj-dirs	:= $(addprefix $(obj)/,$(obj-dirs))
 
 # These flags are needed for modversions and compiling, so we define them here
 # $(modname_flags) defines KBUILD_MODNAME as the name of the module it will