Re: [PATCHv2 1/8] Makefile: apply dependencies consistently to sparse/asm targets

[Jeff King <peff@peff.net>]

On Tue, Jun 19, 2012 at 10:50:15PM -0500, Jonathan Nieder wrote:

> Making .sp and .s targets depend on generated .h files like
> common-cmds.h is very important.  Otherwise, I would not be able to
> generate my git.s assembler listing or sparse-check git.c unless
> common-cmds.h has already been generated as a side-effect of some
> earlier build process.

I suspect in most cases that the earlier build process has happened, and
that's why nobody really complained.

> On the other hand, making .sp and .s targets depend on preexisting .h
> files and files like GIT-CFLAGS would not have any effect at all,
> because:
> 
>  - .sp targets are phony --- there is no stamp file that certifies
>    a given file has been checked by a "make sparse" run.  Maybe that
>    will change some day.
> 
>  - .s targets are rebuilt every time.  Maybe I am just weird, but I
>    find myself upgrading my compiler pretty often, so when I manually
>    ask for an assembler listing I am happy to see it regenerated
>    unconditionally using the new code generation rules.

I don't have a strong opinion, as I don't use either feature. I noticed
the generated header file was a problem, and didn't realize that we
force .s builds.

My counters to the above points (and again, I don't really care
that much) would be:

  1. The .sp and .s targets _do_ need the same -D macros that the .o
     targets get. So it ends up being very obvious that you are omitting
     them in something like:

       foo.o: GIT-VERSION-FILE
       foo.o foo.sp foo.s: EXTRA_CPPFLAGS = \
              -DGIT_VERSION='$(GIT_VERSION)'

     I tend to think it is more readable to simply specify the
     dependencies fully (even if they end up being irrelevant because we
     force-build) than to confuse a reader who is not aware of our
     force-build '.s' rule that is 500 lines away (I was not aware of it
     until you mentioned it).

  2. You describe the behavior now, and I certainly have no plans to
     change it. But there also plausible reasons for both cases to stop
     force-building, in which case these dependencies would become
     relevant.

In other words, I think relying on the force-build is a bit of a
layering violation. Of course, it is a Makefile, which is not exactly
structured programming, but I like to try.

> It turns out that this patch is only about common-cmds.h, which was
> the straightforward case.  Why not say so and save the reader from
> having to think so hard? ;)

Because I didn't realize it was the case at all. :) My intent was
actually to clean up these lines so that they would be correct when I
added GIT-VERSION-FILES and the like to them later.

So I think my preference would be to tack on a note to the commit
message saying "yeah, this doesn't do anything for meta-dependencies,
but it doesn't hurt either". OK?

-Peff