[RFC PATCH] Don't reset timestamps in include/generated if not needed

[Valdis Kletnieks <Valdis.Kletnieks@vt.edu>]

Kbuild regenerates bounds.h and asm-offsets.h, resetting the timestamps
and forcing rebuilds even if the contents haven't changed.  Add a bit of
shell magic to only replace the file if the contents have in fact changed,
which should speed up git bisects and similar.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>

---
So I was doing a git bisect, and getting towards the end, and wondering why
it's taking a long time for each build, and I notice that netfilter modules
are being rebuilt, even though 'git bisect visualize' doesn't show anything
that should have touched netfilter.  Turns out the offender is bounds.h

Fully 3/4 of my modules have a dependency on bounds.h - and we touch it
all the time, even if the contents haven't changed.

RFC because I can't wrap my head around why this wasn't done ages ago.
If I'm missing something something obvious, please apply a cluestick. :)

Lightly tested - if I rm one of those two files, it gets rebuilt. If I
insert some whitespace, it gets replaced. If I don't touch it, the datestamp
doesn't change.

--- linux-next/Kbuild.dist	2015-03-03 19:50:45.175673346 -0500
+++ linux-next/Kbuild	2015-03-03 20:03:20.107820199 -0500
@@ -15,7 +15,7 @@
 
 quiet_cmd_offsets = GEN     $@
 define cmd_offsets
-	(set -e; \
+	( (set -e; \
 	 echo "#ifndef $2"; \
 	 echo "#define $2"; \
 	 echo "/*"; \
@@ -26,7 +26,10 @@
 	 echo ""; \
 	 sed -ne $(sed-y) $<; \
 	 echo ""; \
-	 echo "#endif" ) > $@
+	 echo "#endif" ) > $@.tmp; \
+	 if [ ! -f $@ ]; then mv $@.tmp $@; \
+	 elif cmp -s $@ $@.tmp ; then rm $@.tmp; \
+	 else mv -f $@.tmp $@; fi )
 endef
 
 #####