[PATCH/RFC 2/2 v3] Makefile: lazily compute header dependencies

[Jonathan Nieder <jrnieder@gmail.com>]

Use the gcc -MMD -MP -MF options to generate dependencies as a
byproduct of building .o files.

This feature has to be optional (I don’t think MSVC, for example,
supports anything like this), so unless someone hooks in a rule
to check the static header dependencies for correctness, this
won’t help much with header dependency maintainance.  It is
enabled by setting the COMPUTE_HEADER_DEPENDENCIES variable,
unset by default.

The scope of the %.o: %.c pattern rule has been restricted to
make it easier to tell if a new object file has not been hooked
into the dependency generation machinery.

An unrelated fix also snuck in: the %.s: %.c pattern rule to
generate an assembler listing did not have correct dependencies.
It is meant to be invoked by hand and should always run.

To avoid litering the build directory with even more build
products, the generated Makefile fragments are squirreled away
into deps/ subdirectories of each directory containing object
files.  These directories are currently generated as a
side-effect of the GIT-CFLAGS rule, to guarantee they will be
available whenever the %.o: %.c and %.o: %.S pattern rules are
being used.  This is really not ideal, especially because it
requires hard-coding the list of directories with objects.

gcc learned the -MMD -MP -MF options in version 3.0, so most gcc
users should have them by now.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
I’ll send the .c.s and dependency fixes separately.

Thoughts? Advice?

Thanks,
Jonathan

 .gitignore |    1 +
 Makefile   |   63 ++++++++++++++++++++++++++++++++++++++++++-----------------
 2 files changed, 46 insertions(+), 18 deletions(-)

diff --git a/.gitignore b/.gitignore
index ac02a58..803247f 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..fb20302 100644
--- a/Makefile
+++ b/Makefile
@@ -216,6 +216,9 @@ all::
 #   DEFAULT_EDITOR='~/bin/vi',
 #   DEFAULT_EDITOR='$GIT_FALLBACK_EDITOR',
 #   DEFAULT_EDITOR='"C:\Program Files\Vim\gvim.exe" --nofork'
+#
+# Define COMPUTE_HEADER_DEPENDENCIES if you want to avoid rebuilding objects
+# when an unrelated header file changes and your compiler supports it.
 
 GIT-VERSION-FILE: .FORCE-GIT-VERSION-FILE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -1559,12 +1562,42 @@ git.o git.spec \
 	$(patsubst %.perl,%,$(SCRIPT_PERL)) \
 	: GIT-VERSION-FILE
 
-%.o: %.c GIT-CFLAGS
-	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) $<
-%.s: %.c GIT-CFLAGS
+GIT_OBJS := http.o http-walker.o http-push.o \
+	$(LIB_OBJS) $(BUILTIN_OBJS) \
+	$(patsubst git-%$X,%.o,$(PROGRAMS)) git.o
+
+OBJECTS := $(GIT_OBJS) $(XDIFF_OBJS)
+
+ifndef COMPUTE_HEADER_DEPENDENCIES
+$(GIT_OBJS): $(LIB_H)
+
+$(XDIFF_OBJS): xdiff/xinclude.h xdiff/xmacros.h xdiff/xdiff.h xdiff/xtypes.h \
+	xdiff/xutils.h xdiff/xprepare.h xdiff/xdiffi.h xdiff/xemit.h
+
+http.o http-walker.o http-push.o: http.h
+
+builtin-revert.o wt-status.o: wt-status.h
+
+$(patsubst git-%$X,%.o,$(PROGRAMS)) git.o: $(wildcard */*.h)
+else
+dep_files := $(wildcard $(foreach f,$(OBJECTS),$(dir $f)deps/$(notdir $f).d))
+
+ifneq ($(dep_files),)
+include $(dep_files)
+endif
+
+# Take advantage of gcc's dependency generation.
+# See <http://gcc.gnu.org/gcc-3.0/features.html>.
+dep_args = -MF $(dep_file) -MMD -MP
+dep_file = $(dir $@)deps/$(notdir $@).d
+endif
+
+$(OBJECTS): %.o: %.c GIT-CFLAGS
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $<
+%.s: %.c GIT-CFLAGS .FORCE-LISTING
 	$(QUIET_CC)$(CC) -S $(ALL_CFLAGS) $<
-%.o: %.S
-	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) $<
+$(OBJECTS): %.o: %.S GIT-CFLAGS
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $<
 
 exec_cmd.o: exec_cmd.c GIT-CFLAGS
 exec_cmd.o: ALL_CFLAGS += \
@@ -1594,10 +1627,6 @@ git-imap-send$X: imap-send.o $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIBS) $(OPENSSL_LINK) $(OPENSSL_LIBSSL)
 
-http.o http-walker.o http-push.o: http.h
-
-http.o http-walker.o: $(LIB_H)
-
 git-http-fetch$X: revision.o http.o http-walker.o http-fetch.o $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIBS) $(CURL_LIBCURL)
@@ -1609,22 +1638,15 @@ git-remote-curl$X: remote-curl.o http.o http-walker.o $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(LIBS) $(CURL_LIBCURL) $(EXPAT_LIBEXPAT)
 
-$(LIB_OBJS) $(BUILTIN_OBJS): $(LIB_H)
-$(patsubst git-%$X,%.o,$(PROGRAMS)) git.o: $(LIB_H) $(wildcard */*.h)
-builtin-revert.o wt-status.o: wt-status.h
-
 $(LIB_FILE): $(LIB_OBJS)
 	$(QUIET_AR)$(RM) $@ && $(AR) rcs $@ $(LIB_OBJS)
 
 XDIFF_OBJS=xdiff/xdiffi.o xdiff/xprepare.o xdiff/xutils.o xdiff/xemit.o \
 	xdiff/xmerge.o xdiff/xpatience.o
-$(XDIFF_OBJS): xdiff/xinclude.h xdiff/xmacros.h xdiff/xdiff.h xdiff/xtypes.h \
-	xdiff/xutils.h xdiff/xprepare.h xdiff/xdiffi.h xdiff/xemit.h
 
 $(XDIFF_LIB): $(XDIFF_OBJS)
 	$(QUIET_AR)$(RM) $@ && $(AR) rcs $@ $(XDIFF_OBJS)
 
-
 doc:
 	$(MAKE) -C Documentation all
 
@@ -1657,6 +1679,9 @@ TRACK_CFLAGS = $(subst ','\'',$(ALL_CFLAGS)):\
              $(bindir_SQ):$(gitexecdir_SQ):$(template_dir_SQ):$(prefix_SQ)
 
 GIT-CFLAGS: .FORCE-GIT-CFLAGS
+	mkdir -p deps block-sha1/deps ppc/deps compat/deps \
+		compat/regex/deps compat/nedmalloc/deps compat/fnmatch/deps \
+		xdiff/deps
 	@FLAGS='$(TRACK_CFLAGS)'; \
 	    if test x"$$FLAGS" != x"`cat GIT-CFLAGS 2>/dev/null`" ; then \
 		echo 1>&2 "    * new build flags or prefix"; \
@@ -1873,8 +1898,10 @@ distclean: clean
 	$(RM) configure
 
 clean:
-	$(RM) *.o block-sha1/*.o arm/*.o ppc/*.o compat/*.o compat/*/*.o xdiff/*.o \
+	$(RM) *.o block-sha1/*.o ppc/*.o compat/*.o compat/*/*.o xdiff/*.o \
 		$(LIB_FILE) $(XDIFF_LIB)
+	$(RM) -r deps block-sha1/deps ppc/deps compat/deps \
+		compat/*/deps xdiff/deps
 	$(RM) $(ALL_PROGRAMS) $(BUILT_INS) git$X
 	$(RM) $(TEST_PROGRAMS)
 	$(RM) *.spec *.pyc *.pyo */*.pyc */*.pyo common-cmds.h TAGS tags cscope*
@@ -1899,7 +1926,7 @@ endif
 .PHONY: all install clean strip
 .PHONY: shell_compatibility_test please_set_SHELL_PATH_to_a_more_modern_shell
 .PHONY: .FORCE-GIT-VERSION-FILE TAGS tags cscope .FORCE-GIT-CFLAGS
-.PHONY: .FORCE-GIT-BUILD-OPTIONS
+.PHONY: .FORCE-GIT-BUILD-OPTIONS .FORCE-LISTING
 
 ### Check documentation
 #
-- 
1.6.5.3