[PATCH] Kbuild: Update config_data.gz only if KCONFIG_CONFIG materially changed

[Elliot Berman]

If you update the timestamp of KCONFIG_CONFIG without actually changing
anything, config_data.gz is re-generated and causes vmlinux to re-link.
When Link Time Optimization is enabled, unnecessary re-linking of
vmlinux is highly desirable since it adds several minutes to build time.

Avoid touching config_data.gz by using filechk to compare the existing
config_data.gz and update only if it changed.

The .config can be touched, for instance, by a build script which
installs the default defconfig and then applies a defconfig fragment on
top.

For a simple example on my x86 machine, I modified x86 default defconfig to set
CONFIG_IKCONFIG=y and run:
  make -j50 defconfig tiny.config vmlinux
  make -j50 defconfig tiny.config vmlinux
With this patch, vmlinux is not re-built as a result of config_data.gz
change.

Signed-off-by: Elliot Berman <eberman@xxxxxxxxxxxxxx>
---
 kernel/Makefile      | 2 +-
 scripts/Makefile.lib | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/Makefile b/kernel/Makefile
index 320f1f3..bd4e558 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -140,7 +140,7 @@ $(obj)/configs.o: $(obj)/config_data.gz
 
 targets += config_data.gz
 $(obj)/config_data.gz: $(KCONFIG_CONFIG) FORCE
-	$(call if_changed,gzip)
+	$(call filechk,gzip)
 
 $(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz
 
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index a4fbaf8..81d3ec1 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -282,6 +282,8 @@ cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $(OBJCOPYFLAGS_$(@F)) $< $@
 quiet_cmd_gzip = GZIP    $@
       cmd_gzip = cat $(real-prereqs) | $(KGZIP) -n -f -9 > $@
 
+filechk_gzip = cat $(real-prereqs) | $(KGZIP) -n -f -9
+
 # DTC
 # ---------------------------------------------------------------------------
 DTC ?= $(objtree)/scripts/dtc/dtc
-- 
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