[PATCH/RFC 2/2] Makefile: automatically compute header dependencies

[Jonathan Nieder <jrnieder@gmail.com>]

Use the gcc -MMD -MP -MF options to generate dependency rules as a
byproduct when building .o files.

A bit remains to be done:

 - add the same support to the .c.s rule
 - make this optional (not all compilers support this, and not all
   developers necessarily want to litter the directory with .*.o.d
   files)
 - document what gcc version introduced these options
 - find equivalent options for other compilers (e.g., Intel C,
   SunWSPro, MSVC)

but this should give the idea.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
Good idea?  Bad idea?

Good night,
Jonathan

 .gitignore |    1 +
 Makefile   |   15 ++++++++++++++-
 2 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/.gitignore b/.gitignore
index ac02a58..c7b2736 100644
--- a/.gitignore
+++ b/.gitignore
@@ -170,6 +170,7 @@
 *.exe
 *.[aos]
 *.py[co]
+.*.o.d
 *+
 /config.mak
 /autom4te.cache
diff --git a/Makefile b/Makefile
index ed0f461..af3f874 100644
--- a/Makefile
+++ b/Makefile
@@ -488,6 +488,7 @@ LIB_H += unpack-trees.h
 LIB_H += userdiff.h
 LIB_H += utf8.h
 LIB_H += wt-status.h
+LIB_H :=
 
 LIB_OBJS += abspath.o
 LIB_OBJS += advice.o
@@ -1559,13 +1560,23 @@ git.o git.spec \
 	$(patsubst %.perl,%,$(SCRIPT_PERL)) \
 	: GIT-VERSION-FILE
 
+dep_file = $(dir $@).$(notdir $@).d
+dep_args = -MF $(dep_file) -MMD -MP
+
 %.o: %.c GIT-CFLAGS
-	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) $<
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $<
 %.s: %.c GIT-CFLAGS
 	$(QUIET_CC)$(CC) -S $(ALL_CFLAGS) $<
 %.o: %.S
 	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) $<
 
+objects := $(wildcard *.o block-sha1/*.o arm/*.o ppc/*.o \
+		compat/*.o compat/*/*.o xdiff/*.o)
+dep_files := $(wildcard $(foreach f,$(objects),$(dir $f).$(notdir $f).d))
+ifneq ($(dep_files),)
+include $(dep_files)
+endif
+
 exec_cmd.o: exec_cmd.c GIT-CFLAGS
 exec_cmd.o: ALL_CFLAGS += \
 	'-DGIT_EXEC_PATH="$(gitexecdir_SQ)"' \
@@ -1875,6 +1886,8 @@ distclean: clean
 clean:
 	$(RM) *.o block-sha1/*.o arm/*.o ppc/*.o compat/*.o compat/*/*.o xdiff/*.o \
 		$(LIB_FILE) $(XDIFF_LIB)
+	$(RM) .*.o.d block-sha1/.*.o.d arm/.*.o.d ppc/.*.o.d compat/.*.o.d \
+		compat/*/.*.o.d xdiff/.*.o.d
 	$(RM) $(ALL_PROGRAMS) $(BUILT_INS) git$X
 	$(RM) $(TEST_PROGRAMS)
 	$(RM) *.spec *.pyc *.pyo */*.pyc */*.pyo common-cmds.h TAGS tags cscope*
-- 
1.6.5.3