Date: Thu, 13 Feb 2014 02:49:44 -0500
From: Austin Clements <amdragon@MIT.EDU>
To: Tomi Ollila <tomi.ollila@iki.fi>
Subject: Re: [PATCH 1/1] emacs: always write emacs/.eldeps when the target is
	remade
Message-ID: <20140213074944.GY4375@mit.edu>
References: <1390645309-2326-1-git-send-email-tomi.ollila@iki.fi>
	<20140213071026.GX4375@mit.edu> <m2sirn1nik.fsf@guru.guru-group.fi>
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Quoth Tomi Ollila on Feb 13 at  9:26 am:
> On Thu, Feb 13 2014, Austin Clements <amdragon@MIT.EDU> wrote:
> 
> > Quoth Tomi Ollila on Jan 25 at 12:21 pm:
> >> So that the target is newer than its prerequisites.
> >> ---
> >>  emacs/Makefile.local | 3 +--
> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >> 
> >> diff --git a/emacs/Makefile.local b/emacs/Makefile.local
> >> index 42bfbd9..d5d402e 100644
> >> --- a/emacs/Makefile.local
> >> +++ b/emacs/Makefile.local
> >> @@ -32,8 +32,7 @@ emacs_bytecode = $(emacs_sources:.el=.elc)
> >>  ifeq ($(HAVE_EMACS),1)
> >>  $(dir)/.eldeps: $(dir)/Makefile.local $(dir)/make-deps.el $(emacs_sources)
> >>  	$(call quiet,EMACS) --directory emacs -batch -l make-deps.el \
> >> -		-f batch-make-deps $(emacs_sources) > $@.tmp && \
> >> -		(cmp -s $@.tmp $@ || mv $@.tmp $@)
> >> +		-f batch-make-deps $(emacs_sources) > $@.tmp && mv $@.tmp $@
> >>  -include $(dir)/.eldeps
> >>  endif
> >>  CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp
> >
> > Is this just so the rule doesn't get run again on the next make
> > invocation (unless, of course, a dependent changed)?
> 
> Basically yes. I did multiple builds in rapid succession when developing
> something and this thing confused me quite a lot in the beginning...
> 
> > Interestingly, if any of the dependents have changed, but in ways that
> > don't affect .eldeps, this change will make the build more expensive
> > because it will trigger a make restart after .eldeps is updated.
> 
> I wondered what was the reason for the recipe you've chosen here, has
> it something to do with inodes or something ;) (and were going to ask
> an alternative to touch (or even : > $@) the target...
> 
> ... but now I understand. E.g. change in notmuch-lib.el will cause *all* 
> .el files to be re-bytecompiled after this change. 

This is unrelated to the cmp in the recipe.  A change in
notmuch-lib.el *must* cause almost all .el files to be recompiled
because they almost all require it and may use macros from it.  If you
touch, e.g., notmuch-hello.el, you'll see that very little is rebuilt
(with or without this patch).

> An alternative to this patch would be adding a message to the rule
> which informs user to touch .eldeps to avoid re-doing .eldeps if
> that irritates one :D
> 
> something like:
> 
> (cmp -s $@.tmp $@ && \
>      echo "touch $@ to avoid redoing this target" || mv $@.tmp $@)

I think this patch is good as it is; I was just pointing out that the
change also has a performance drawback in some situations.  (Knowing
make, there's probably *some* way to get the best of both worlds and
it's probably really ugly.)

> Thanks, 
> 
> Tomi