On Fri, Feb 01, 2013 at 01:36:38AM -0500, Jeff King wrote:

> It seems like building each object file should depend on its dependency
> file (but only when COMPUTE_HEADER_DEPENDENCIES is on, of course), since
> otherwise we cannot know if we have the right dependencies or not.
> Something like this almost works, I think:
> [...]
> +$(C_OBJ): %.o: %.c GIT-CFLAGS $(missing_dep_dirs) $(call dep_file, %.o)

Actually that would not work, as we do not have a rule to create
.depend/foo.o.d. We can add one, but it gets pretty hairy (and
replicates much of the normal build rule). A much simpler way is to just
find the missing dep files and force compilation of their matching
objects. Like:

diff --git a/Makefile b/Makefile
index 6b42f66..f94e8b9 100644
--- a/Makefile
+++ b/Makefile
@@ -1843,8 +1843,14 @@ dep_args = -MF $(dep_file) -MMD -MP
        @mkdir -p $@
 missing_dep_dirs := $(filter-out $(wildcard $(dep_dirs)),$(dep_dirs))
+missing_dep_files := $(filter-out $(wildcard $(dep_files)),$(dep_files))
+# we want to rewrite "foo/.depend/bar.o.d" into "foo/bar.o", but
+# make's patsubst is not powerful enough to remove something from the middle of
+# a string. Hack around it by shelling out.
+obj_files_with_missing_deps := $(shell echo $(missing_dep_files:.d=) | tr ' ' 
'\n' | sed 's,.depend/,,')
 dep_file = $(dir $@).depend/$(notdir $@).d
 dep_args = -MF $(dep_file) -MMD -MP
+$(obj_files_with_missing_deps): FORCE
 $(error cannot compute header dependencies outside a normal build. \
 Please unset CHECK_HEADER_DEPENDENCIES and try again)

which does solve the problem, but that shell hack is nasty. It would be
much simpler if we stored the dependency for foo/bar.o as
".depend/foo/bar.o.d", rather than "foo/.depend/bar.o.d", as then we
would patsubst it away. Or maybe there is some clever way to convince
make to do what I want here. Suggestions welcome.

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