[PATCH] Avoid recompilation of unrelated files

[Bean <bean123@126.com>]

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A problem of the grub2 make system is that even the smallest change of source
code can trigger a huge recompilation. Try this:

touch commands/cmp.c
make

The ideal situation is that only files related to cmp module is recompiled. 
But unfortunately, this is not the case. The problem lies in file dependence.

cmp.mod: pre-cmp.o mod-cmp.o

pre-cmp.o: cmp_mod-commands_cmp.o

cmp_mod-commands_cmp.o: cmp.c

mod-cmp.o: mod-cmp.c

mod-cmp.c:  moddep.lst genmodsrc.sh

def-cmp.lst: pre-cmp.o

und-cmp.lst: pre-cmp.o

moddep.lst: def-cmp.lst und-cmp.lst

cmp.c => cmp_mod-commands_cmp.o => pre-cmp.o => def-cmp.lst and und-cmp.lst
=> moddep.lst

When cmp.c changes, moddep.lst changes. But every mod-*.c depends on 
moddep.lst, they all need to be recreated. This will lead to a huge
recompilation process.

def-cmp.lst and und-cmp.lst contains the exported symbol and unresolved symbol
of the module. Their content rarely change. So we can update them only when 
it's necessary, not every time we recompile the module.

The solution:

Merge these three rules:

pre-cmp.o: cmp_mod-commands_cmp.o

def-cmp.lst: pre-cmp.o

und-cmp.lst: pre-cmp.o

The output of def-cmp.lst is saved as tmp_pre-cmp.lst. When tmp_pre-cmp.lst
and the old pre-cmp.lst is different, move tmp_pre-cmp.lst to pre-cmp.lst, 
otherwise, just delete tmp_pre-cmp.lst. und-cmp.lst can be handled in exactly
the same way.

-- 
Bean

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	* genmk.rb: Avoid recompilation of unrelated files.


Index: genmk.rb
===================================================================
RCS file: /sources/grub/grub2/genmk.rb,v
retrieving revision 1.27
diff -u -r1.27 genmk.rb
--- genmk.rb	22 Sep 2006 00:27:38 -0000	1.27
+++ genmk.rb	26 Jun 2007 18:06:28 -0000
@@ -120,6 +120,13 @@
 #{pre_obj}: $(#{prefix}_DEPENDENCIES) #{objs_str}
 	-rm -f $@
 	$(TARGET_CC) $(#{prefix}_LDFLAGS) $(TARGET_LDFLAGS) -Wl,-r,-d -o $@ #{objs_str}
+ifneq ($(#{prefix}_EXPORTS),no)
+	$(NM) -g --defined-only -P -p $< | sed 's/^\\([^ ]*\\).*/\\1 #{mod_name}/' > tmp_#{defsym}
+	if cmp -s tmp_#{defsym} #{defsym} ; then rm tmp_#{defsym} ; else mv tmp_#{defsym} #{defsym} ; fi 
+endif
+	echo '#{mod_name}' > tmp_#{undsym}
+	$(NM) -u -P -p $< | cut -f1 -d' ' >> tmp_#{undsym}
+	if cmp -s tmp_#{undsym} #{undsym} ; then rm tmp_#{undsym} ; else mv tmp_#{undsym} #{undsym} ; fi
 
 #{mod_obj}: #{mod_src}
 	$(TARGET_CC) $(TARGET_CPPFLAGS) $(TARGET_CFLAGS) $(#{prefix}_CFLAGS) -c -o $@ $<
@@ -127,15 +134,6 @@
 #{mod_src}: moddep.lst genmodsrc.sh
 	sh $(srcdir)/genmodsrc.sh '#{mod_name}' $< > $@ || (rm -f $@; exit 1)
 
-ifneq ($(#{prefix}_EXPORTS),no)
-#{defsym}: #{pre_obj}
-	$(NM) -g --defined-only -P -p $< | sed 's/^\\([^ ]*\\).*/\\1 #{mod_name}/' > $@
-endif
-
-#{undsym}: #{pre_obj}
-	echo '#{mod_name}' > $@
-	$(NM) -u -P -p $< | cut -f1 -d' ' >> $@
-
 " + objs.collect_with_index do |obj, i|
       src = sources[i]
       fake_obj = File.basename(src).suffix('o')