Re: [PATCH 3/3] Makefile: split prefix flags from GIT-CFLAGS

On Tue, Jun 19, 2012 at 05:04:26PM -0400, Jeff King wrote:

quoted

The damage is not too bad from the point of view of linecount, but
this embeds the implicit knowledge of dependencies from $prefix to
various path variables to selected object files that embed these
paths variables by scattering dependencies on GIT-PREFIX in the
Makefile, which does not seem to scale very well.  I wonder if it
makes sense to have a single default-paths.o file that holds these
strings and recompile only that file when any of the paths change,
to localize the damage.

Of course, the current users of GIT_HTML_PATH that expect they can
do sizeof(GIT_HTML_PATH)-1 in place of strlen(GIT_HTML_PATH) may
need to be adjusted if we go that route.

Yeah, I think that would be nicer overall. If we move to a link-time
dependency, then we can even put _all_ of the Makefile-based strings in
there without ever having to care about who uses it. Of course, it won't
work for things that truly need to be preprocessor macros (for
conditional compilation), so we'd still be stuck with those (most of
them just end up in CFLAGS and trigger a full rebuild, which is probably
OK).

I started on this, but it is a little bit trickier than that. We can
cover C compilation with one strategy, but make variables end up going
lots of other places, too, like shell scripts, the perl Makefile
generation, etc. Those places all need to individually respect a
separate sentinel-file dependency. So I don't think there is an easy way
out of the sprinkling of dependencies.

At the very least, I tried to keep the dependency close to the
point-of-use, like:

  foo.o: GIT-PREFIX
  foo.o: EXTRA_CPPFLAGS = \
          '-DPREFIX=$(prefix_SQ)'

even if the actual build rules for foo.o are found elsewhere (and
typically they are, as they are part of a big pattern-based rule).

Some of the existing locations did not do a great job of that, and
instead it like this:

  version.o git.spec \
          $(patsubst %.sh,%,$(SCRIPT_SH)) \
          $(patsubst %.perl,%,$(SCRIPT_PERL)) \
          : GIT-VERSION-FILE

  [... 500 lines later ...]

  git.spec: git.spec.in
          sed -e 's/@@VERSION@@/$(GIT_VERSION)/g' <$< >$@+

If you read the latter hunk and wanted to emulate it, there is nothing
to indicate that you must also modify the earlier hunk. I think putting
them together makes more sense (even though it technically takes more
lines).

-Peff