Re: [PATCH] emacs: Avoid rebuilding .eldeps even when there's nothing to do

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+From: Tomi Ollila <tomi.ollila@iki.fi>\r
+To: Austin Clements <amdragon@MIT.EDU>, notmuch@notmuchmail.org\r
+Subject: Re: [PATCH] emacs: Avoid rebuilding .eldeps even when there's nothing\r
+       to do\r
+In-Reply-To: <1392870264-11156-1-git-send-email-amdragon@mit.edu>\r
+References: <m2iosade2y.fsf@guru.guru-group.fi>\r
+       <1392870264-11156-1-git-send-email-amdragon@mit.edu>\r
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+Date: Thu, 20 Feb 2014 19:00:39 +0200\r
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+On Thu, Feb 20 2014, Austin Clements <amdragon@MIT.EDU> wrote:\r
+\r
+> Previously, we updated .eldeps only if the file contents actually\r
+> needed to change.  This was done to avoid unnecessary make restarts\r
+> (if the .eldeps rule changes the mtime of .eldeps, make has to restart\r
+> to collect the new dependencies).  However, this meant that, after a\r
+> modification to any .el file that did not change dependencies, .eldeps\r
+> would always be out of date, so every make invocation would run the\r
+> .eldeps rule, which is both expensive because it starts up Emacs and\r
+> noisy.  This was true even when there was nothing to do.  E.g.,\r
+>\r
+> $ make clean && make\r
+> ...\r
+> $ touch emacs/notmuch-lib.el && make\r
+> ...\r
+> $ make\r
+> Use "make V=1" to see the verbose compile lines.\r
+> EMACS emacs/.eldeps\r
+> make: Nothing to be done for `all'.\r
+> $ make\r
+> Use "make V=1" to see the verbose compile lines.\r
+> EMACS emacs/.eldeps\r
+> make: Nothing to be done for `all'.\r
+>\r
+> Fix this by replacing .eldeps with two files with identical content.\r
+> One tracks the mtime of the dependency information and triggers the\r
+> Emacs call to rebuild dependencies only when it may be necessary.  The\r
+> other tracks the content only; this rule over-triggers in the same way\r
+> the old rule did, but this rule is cheap and quiet.\r
+> ---\r
+\r
+LGTM. Works as expected.\r
+\r
+Tomi\r
+\r
+>  emacs/Makefile.local | 15 ++++++++++++---\r
+>  1 file changed, 12 insertions(+), 3 deletions(-)\r
+>\r
+> diff --git a/emacs/Makefile.local b/emacs/Makefile.local\r
+> index 42bfbd9..6a39b32 100644\r
+> --- a/emacs/Makefile.local\r
+> +++ b/emacs/Makefile.local\r
+> @@ -33,10 +33,19 @@ ifeq ($(HAVE_EMACS),1)\r
+>  $(dir)/.eldeps: $(dir)/Makefile.local $(dir)/make-deps.el $(emacs_sources)\r
+>      $(call quiet,EMACS) --directory emacs -batch -l make-deps.el \\r
+>              -f batch-make-deps $(emacs_sources) > $@.tmp && \\r
+> -            (cmp -s $@.tmp $@ || mv $@.tmp $@)\r
+> --include $(dir)/.eldeps\r
+> +            mv $@.tmp $@\r
+> +# We could include .eldeps directly, but that would cause a make\r
+> +# restart whenever any .el file was modified, even if dependencies\r
+> +# didn't change, because the mtime of .eldeps will change.  Instead,\r
+> +# we include a second file, .eldeps.x, which we ensure always has the\r
+> +# same content as .eldeps, but its mtime only changes when dependency\r
+> +# information changes, in which case a make restart is necessary\r
+> +# anyway.\r
+> +$(dir)/.eldeps.x: $(dir)/.eldeps\r
+> +    @cmp -s $^ $@ || cp $^ $@\r
+> +-include $(dir)/.eldeps.x\r
+>  endif\r
+> -CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp\r
+> +CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp $(dir)/.eldeps.x\r
+>  \r
+>  ifeq ($(HAVE_EMACS),1)\r
+>  %.elc: %.el $(global_deps)\r
+> -- \r
+> 1.8.4.rc3\r
+>\r
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