[PATCH 4/5] Makefile: lazily 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.

This feature has to be optional (MSVC does not seem to support
anything like this), so unfortunately it does not make the
Makefile much easier to maintain.  The feature is enabled by the
COMPUTE_HEADER_DEPENDENCIES variable, which is unset by default.

The generated Makefile fragments are saved in deps/
subdirectories of each directory containing object files.  These
directories are generated if missing at the start of each build.
A dependency on $(filter-out $(wildcard $(dep_dirs)),$(dep_dirs))
avoids needlessly regenerating files when the directories'
timestamps change.

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

The dependencies this option computes are more specific than the
rough estimate hard-coded in the Makefile, greatly speeding up
rebuilds when only a little-used header file has changed.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
Timings:

for arg in YesPlease ""
do
	{
		echo NO_CURL=1
		echo NO_TCLTK=1
		echo NO_PERL=1
		echo COMPUTE_HEADER_DEPENDENCIES="$arg"
	} >config.mak
	make
	make clean

	time -p make
	touch levenshtein.h
	time -p make

	make clean
done >/dev/null

Build	COMPUTE_HEADER_DEPENDENCIES	real	user	sys
first	YesPlease			55.06	45.13	5.23
second	YesPlease			3.13	2.04	0.79
first					55.45	45.49	4.99
second					53.14	43.19	4.70

 .gitignore |    1 +
 Makefile   |   40 ++++++++++++++++++++++++++++++++++++----
 2 files changed, 37 insertions(+), 4 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 87de3c3..578843c 100644
--- a/Makefile
+++ b/Makefile
@@ -221,6 +221,9 @@ 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
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -1653,15 +1656,38 @@ ASM_SRC := $(wildcard $(OBJECTS:o=S))
 ASM_OBJ := $(ASM_SRC:S=o)
 C_OBJ := $(filter-out $(ASM_OBJ),$(OBJECTS))
 
+ifdef COMPUTE_HEADER_DEPENDENCIES
+dep_dirs := $(addsuffix deps,$(sort $(dir $(OBJECTS))))
+dep_dir_dep := $(filter-out $(wildcard $(dep_dirs)),$(dep_dirs))
+
+$(dep_dirs):
+	mkdir -p $@
+else
+dep_dirs =
+dep_dir_dep =
+endif
+
 .SUFFIXES:
 
-$(C_OBJ): %.o: %.c GIT-CFLAGS
-	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) $<
+$(C_OBJ): %.o: %.c GIT-CFLAGS $(dep_dir_dep)
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $<
 %.s: %.c GIT-CFLAGS FORCE
 	$(QUIET_CC)$(CC) -S $(ALL_CFLAGS) $<
-$(ASM_OBJ): %.o: %.S GIT-CFLAGS
-	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) $<
+$(ASM_OBJ): %.o: %.S GIT-CFLAGS $(dep_dir_dep)
+	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(ALL_CFLAGS) $<
+
+ifdef COMPUTE_HEADER_DEPENDENCIES
+# Take advantage of gcc's dependency generation
+# See <http://gcc.gnu.org/gcc-3.0/features.html>.
+dep_files := $(wildcard $(foreach f,$(OBJECTS),$(dir f)deps/$(notdir $f).d))
 
+ifneq ($(dep_files),)
+include $(dep_files)
+endif
+
+dep_file = $(dir $@)deps/$(notdir $@).d
+dep_args = -MF $(dep_file) -MMD -MP
+else
 $(GIT_OBJS): $(LIB_H)
 http.o http-walker.o http-push.o: http.h
 $(patsubst git-%$X,%.o,$(PROGRAMS)) git.o: $(wildcard */*.h)
@@ -1670,6 +1696,9 @@ builtin-revert.o wt-status.o: wt-status.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
 
+dep_args =
+endif
+
 exec_cmd.s exec_cmd.o: ALL_CFLAGS += \
 	'-DGIT_EXEC_PATH="$(gitexecdir_SQ)"' \
 	'-DBINDIR="$(bindir_relative_SQ)"' \
@@ -1794,7 +1823,9 @@ test-delta$X: diff-delta.o patch-delta.o
 
 test-parse-options$X: parse-options.o
 
+ifndef COMPUTE_HEADER_DEPENDENCIES
 test-parse-options.o: parse-options.h
+endif
 
 .PRECIOUS: $(patsubst test-%$X,test-%.o,$(TEST_PROGRAMS))
 
@@ -1954,6 +1985,7 @@ clean:
 		$(LIB_FILE) $(XDIFF_LIB)
 	$(RM) $(ALL_PROGRAMS) $(BUILT_INS) git$X
 	$(RM) $(TEST_PROGRAMS)
+	$(RM) -r $(dep_dirs)
 	$(RM) *.spec *.pyc *.pyo */*.pyc */*.pyo common-cmds.h TAGS tags cscope*
 	$(RM) -r autom4te.cache
 	$(RM) config.log config.mak.autogen config.mak.append config.status config.cache
-- 
1.6.6.rc2