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38 id 1WDr3I-0004i2-MB; Thu, 13 Feb 2014 02:49:44 -0500
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39 Date: Thu, 13 Feb 2014 02:49:44 -0500
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40 From: Austin Clements <amdragon@MIT.EDU>
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41 To: Tomi Ollila <tomi.ollila@iki.fi>
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42 Subject: Re: [PATCH 1/1] emacs: always write emacs/.eldeps when the target is
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44 Message-ID: <20140213074944.GY4375@mit.edu>
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45 References: <1390645309-2326-1-git-send-email-tomi.ollila@iki.fi>
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46 <20140213071026.GX4375@mit.edu> <m2sirn1nik.fsf@guru.guru-group.fi>
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81 Quoth Tomi Ollila on Feb 13 at 9:26 am:
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82 > On Thu, Feb 13 2014, Austin Clements <amdragon@MIT.EDU> wrote:
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84 > > Quoth Tomi Ollila on Jan 25 at 12:21 pm:
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85 > >> So that the target is newer than its prerequisites.
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87 > >> emacs/Makefile.local | 3 +--
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88 > >> 1 file changed, 1 insertion(+), 2 deletions(-)
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90 > >> diff --git a/emacs/Makefile.local b/emacs/Makefile.local
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91 > >> index 42bfbd9..d5d402e 100644
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92 > >> --- a/emacs/Makefile.local
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93 > >> +++ b/emacs/Makefile.local
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94 > >> @@ -32,8 +32,7 @@ emacs_bytecode = $(emacs_sources:.el=.elc)
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95 > >> ifeq ($(HAVE_EMACS),1)
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96 > >> $(dir)/.eldeps: $(dir)/Makefile.local $(dir)/make-deps.el $(emacs_sources)
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97 > >> $(call quiet,EMACS) --directory emacs -batch -l make-deps.el \
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98 > >> - -f batch-make-deps $(emacs_sources) > $@.tmp && \
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99 > >> - (cmp -s $@.tmp $@ || mv $@.tmp $@)
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100 > >> + -f batch-make-deps $(emacs_sources) > $@.tmp && mv $@.tmp $@
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101 > >> -include $(dir)/.eldeps
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103 > >> CLEAN+=$(dir)/.eldeps $(dir)/.eldeps.tmp
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105 > > Is this just so the rule doesn't get run again on the next make
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106 > > invocation (unless, of course, a dependent changed)?
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108 > Basically yes. I did multiple builds in rapid succession when developing
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109 > something and this thing confused me quite a lot in the beginning...
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111 > > Interestingly, if any of the dependents have changed, but in ways that
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112 > > don't affect .eldeps, this change will make the build more expensive
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113 > > because it will trigger a make restart after .eldeps is updated.
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115 > I wondered what was the reason for the recipe you've chosen here, has
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116 > it something to do with inodes or something ;) (and were going to ask
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117 > an alternative to touch (or even : > $@) the target...
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119 > ... but now I understand. E.g. change in notmuch-lib.el will cause *all*
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120 > .el files to be re-bytecompiled after this change.
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122 This is unrelated to the cmp in the recipe. A change in
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123 notmuch-lib.el *must* cause almost all .el files to be recompiled
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124 because they almost all require it and may use macros from it. If you
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125 touch, e.g., notmuch-hello.el, you'll see that very little is rebuilt
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126 (with or without this patch).
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128 > An alternative to this patch would be adding a message to the rule
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129 > which informs user to touch .eldeps to avoid re-doing .eldeps if
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130 > that irritates one :D
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134 > (cmp -s $@.tmp $@ && \
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135 > echo "touch $@ to avoid redoing this target" || mv $@.tmp $@)
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137 I think this patch is good as it is; I was just pointing out that the
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138 change also has a performance drawback in some situations. (Knowing
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139 make, there's probably *some* way to get the best of both worlds and
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140 it's probably really ugly.)
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