[PATCH 1/1] emacs: always write emacs/.eldeps when the target is remade

[Tomi Ollila]

On Wed, Feb 19 2014, Austin Clements <amdragon at MIT.EDU> wrote:

> Quoth myself on Feb 14 at  4:58 pm:
>> Quoth Tomi Ollila on Feb 14 at  9:24 pm:
>> > On Thu, Feb 13 2014, Austin Clements <amdragon at MIT.EDU> wrote:
>> > 
>> > > Quoth Tomi Ollila on Feb 13 at  9:26 am:
>> > >> On Thu, Feb 13 2014, Austin Clements <amdragon at 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.)
>> > 
>> > Ah, now I understand, because emacs/Makefile.local includes emacs/.eldeps
>> > when it notices mtime of emacs/.eldeps changes it restarts itself...
>> > 
>> > ... to compare with current behaviour:
>> > 
>> > $ make
>> > ...
>> > $ touch emacs/notmuch.el
>> > $ make
>> > ...
>> > $ make
>> > Use "make V=1" to see the verbose compile lines.
>> > EMACS emacs/.eldeps
>> > make: Nothing to be done for `all'.
>> > $ make
>> > Use "make V=1" to see the verbose compile lines.
>> > EMACS emacs/.eldeps
>> > make: Nothing to be done for `all'.
>> > $ make
>> > Use "make V=1" to see the verbose compile lines.
>> > EMACS emacs/.eldeps
>> > make: Nothing to be done for `all'.
>> > 
>> > It will rerun emacs -L emacs -batch -l make-deps.el -f batch-make-deps ...
>> > over and over again...
>> > 
>> > I thought I figured out a solution involving .eldeps.stamp file but
>> > that turned out to be no-go... (circular dependency netween .eldeps.stamp
>> > and Makefile.local).
>> > 
>> > So, with this if one edits emacs/* files and keeps building byte-compiled
>> > versions often, make will both run EMACS emacs/.eldeps and restarting
>> > -- and when moving to edit other files both of the above stops. Without
>> > this change after editing emacs/* files EMACS emacs/.eldeps will be
>> > executed until it is touched or make clean executed (I wonder how make
>> > handles rebuilding emacs/.eldeps while doing make clean ;)
>> > 
>> > 
>> > Tomi
>> 
>> Here's a way that works, but it's more complicated than your patch and
>> doesn't seem to perform any better:
>> 
>> diff --git a/emacs/Makefile.local b/emacs/Makefile.local
>> index 42bfbd9..8a1f1c2 100644
>> --- a/emacs/Makefile.local
>> +++ b/emacs/Makefile.local
>> @@ -29,14 +29,28 @@ emacs_bytecode = $(emacs_sources:.el=.elc)
>>  # the byte compiler may load an old .elc file when processing a
>>  # "require" or we may fail to rebuild a .elc that depended on a macro
>>  # from an updated file.
>> +#
>> +# We do this in two rules.  .eldeps is the "real" dependencies file
>> +# and will be rebuilt if and only if a .el source file changes.  We
>> +# could include this directly, but then whenever a .el file changed
>> +# (and .eldeps's mtime changed), make would restart immediately after
>> +# the include below.  To avoid this, we create a second file,
>> +# .eldeps.x, that we only update when its *contents* differ from
>> +# .eldeps.  .eldeps will often have an mtime older than .eldeps.x (any
>> +# .el file change that does not change dependencies will cause this),
>> +# so the .eldeps.x rule will often run, but it executes quickly and
>> +# quietly and will not trigger a make restart unless the dependencies
>> +# actually change.
>
> Here's a version of this comment that gets to the point directly:
>
> # We could include .eldeps directly, but that would cause a make
> # restart whenever any .el file was modified, even if dependencies
> # didn't change, because the mtime of .eldeps will change.  Instead,
> # we include a second file, .eldeps.x, which we ensure always has the
> # same content as .eldeps, but its mtime only changes when dependency
> # information changes, in which case a make restart is necessary
> # anyway.

I think this behaviour is tolerable: instead of verbosely re-run emacs to
re-create .eldeps every time just to see the new .eldeps is not going to be
used we just silently re-run only lightweight cmp.

Tomi

>
>>  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 $@)
>> --include $(dir)/.eldeps
>> +		mv $@.tmp $@
>> +$(dir)/.eldeps.x: $(dir)/.eldeps
>> +	@cmp -s $^ $@ || cp $^ $@
>> +-include $(dir)/.eldeps.x
>>  endif
>> -CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp
>> +CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp $(dir)/.eldeps.x
>>  
>>  ifeq ($(HAVE_EMACS),1)
>>  %.elc: %.el $(global_deps)
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