(Emailing directly since mail sent by codereview.chromium.org for
@amazon.com addresses gets stuck in spam filters. My apologies if you get
this twice.)
> Q: I wonder whether we need to / should modify the "preparse data". Would
> it be
> better to keep it as is, so that it just contains the positions of the
>lazy
> top-level functions and no identifiers? (I don't know the answer.)
Adding the identifiers to the preparse data gets us a pretty big win on
warm parsing times. I don't have access to morejs so I can't try that, but
what I have been doing is running `d8 --cache=code --trace_parse` with
some popular JavaScript files from the web and looking at how long parsing
takes with and without the preparse data, with and without my change (on
my desktop, times in milliseconds, median of five runs):
before after
cold warm cold warm
http://connect.facebook.net/en_US/all.js 8.2 7.9 5.1 0.7
http://[1]/analytics.js 1.9 1.7 1.7 0.5
http://b.scorecardresearch.com/beacon.js 0.1 0.02 0.1 0.02
http://a.disquscdn.com/embed.js 1 0.04 1 0.04
http://www.google-analytics.com/ga.js 3 2.8 2.6 0.6
http://[2]/jquery/1.7.2/jquery.min.js 5.8 5.6 4.1 0.9
http://[3]/pagead/js/lidar.js 3.6 3.3 2.8 0.7
http://[3]/pagead/osd.js 3.1 2.9 2.5 0.6
http://platform.twitter.com/widgets.js 2.5 0.25 2.5 0.25
[1] www.google-analytics.com
[2] ajax.googleapis.com/ajax/libs
[3] pagead2.googlesyndication.com
As you can see, it's the same or better in every case (0-38% speedup cold,
0-91% speedup with the preparse data). It's a particularly large win on
modern JavaScript libraries like jQuery which make extensive use of
closures to avoid polluting the global scope.
Here's the numbers for the size of the preparse data (in bytes):
before after
http://connect.facebook.net/en_US/all.js 20 15260
http://[1]/analytics.js 20 14500
http://b.scorecardresearch.com/beacon.js 60 76
http://a.disquscdn.com/embed.js 340 468
http://www.google-analytics.com/ga.js 20 23300
http://[2]/jquery/1.7.2/jquery.min.js 20 17820
http://[3]/pagead/js/lidar.js 20 21384
http://[3]/pagead/osd.js 20 18744
http://platform.twitter.com/widgets.js 1120 1560
The extra preparse data is only significant in the cases where it's buying
us a large speedup.
>https://codereview.chromium.org/641283003/
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