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

[Austin Clements]

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 = $(emacs_sources:.el=.elc)
 # the byte compiler may load an old .elc file when processing a
 # "require" or we may fail to rebuild a .elc that depended on a macro
 # from an updated file.
+#
+# We do this in two rules.  .eldeps is the "real" dependencies file
+# and will be rebuilt if and only if a .el source file changes.  We
+# could include this directly, but then whenever a .el file changed
+# (and .eldeps's mtime changed), make would restart immediately after
+# the include below.  To avoid this, we create a second file,
+# .eldeps.x, that we only update when its *contents* differ from
+# .eldeps.  .eldeps will often have an mtime older than .eldeps.x (any
+# .el file change that does not change dependencies will cause this),
+# so the .eldeps.x rule will often run, but it executes quickly and
+# quietly and will not trigger a make restart unless the dependencies
+# actually change.
 ifeq ($(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)