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>

1 files changed, 6 insertions, 9 deletions

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 6f60377..496ecd8 100644
--- a/scripts/Makefile.build
+++ b/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.