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[PATCH v2 5/6] kbuild: optimize object directory creation for incremental build

The previous commit largely optimized the object directory creation.
We can optimize it more for incremental build.

There are already *.cmd files in the output directory.  The existing
*.cmd files have been picked up by $(wildcard ...).  Obviously,
directories containing them exist too, so we can skip "mkdir -p".

With this, Kbuild runs almost zero "mkdir -p" in incremental building.

Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>

Changes in v2:
  - Remove "." from the filter-out list

 scripts/Makefile.build | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 496ecd8..8624924 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -583,8 +583,13 @@ endif
 ifneq ($(KBUILD_SRC),)
 # Create directories for object files if they do not exist
 obj-dirs := $(sort $(obj) $(patsubst %/,%, $(dir $(targets))))
+# If cmd_files exist, their directories apparently exist.  Skip mkdir.
+exist-dirs := $(sort $(patsubst %/,%, $(dir $(cmd_files))))
+obj-dirs := $(strip $(filter-out $(exist-dirs), $(obj-dirs)))
+ifneq ($(obj-dirs),)
 $(shell mkdir -p $(obj-dirs))
 # 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.