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Javascript var vs let (de)optimization/slowdown issue in FF/SpiderMonkey

  • 3 replies
  • 2 have this problem
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  • Last reply by olvap80

olvap80
olvap80
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Firefox ver 53 (32 bit) on Windows - using modern JavaScript slows down execution speed drastically in some cases. Solution for FF is needed, assuming I have access to FF command line and able to tune registry, edit local FF settings, etc.

Exact code with slowdown

https://jsfiddle.net/nhppmzov/ and http://stackoverflow.com/questions/43153889/javascript-var-vs-let-deoptimization-slowdown-issue-in-v8-and-spidermonkey

without_let: 98.08ms let_below: 94.21ms let_above_and_in_loop: 877.85ms <---------- Slowdown let_in_loop: 96.63ms

=============

without_let: 289.560ms let_below: 296.265ms let_above_and_in_loop: 880.340ms <---------- Slowdown let_in_loop: 314.395ms

Firefox ver 53 (32 bit) on Windows - using modern JavaScript slows down execution speed drastically in some cases. Solution for FF is needed, assuming I have access to FF command line and able to tune registry, edit local FF settings, etc. Exact code with slowdown https://jsfiddle.net/nhppmzov/ and http://stackoverflow.com/questions/43153889/javascript-var-vs-let-deoptimization-slowdown-issue-in-v8-and-spidermonkey without_let: 98.08ms let_below: 94.21ms let_above_and_in_loop: 877.85ms <---------- Slowdown let_in_loop: 96.63ms ========================= without_let: 289.560ms let_below: 296.265ms let_above_and_in_loop: 880.340ms <---------- Slowdown let_in_loop: 314.395ms

All Replies (3)

philipp
philipp
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hi olvap80, thanks for your investigation! https://bugzilla.mozilla.org would be the right place to file your findings though if you want to reach firefox developers, as the site here is mostly user to user support.

Modified by philipp

jscher2000 - Support Volunteer
jscher2000 - Support Volunteer
  • Top 10 Contributor
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It's probably on someone's to do list to optimize, but... I couldn't tell you. This is one of the most challenging topics I've ever tried to search because the word "let" is everywhere.

olvap80
olvap80 Question owner
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philipp said

hi olvap80, thanks for your investigation! https://bugzilla.mozilla.org would be the right place to file your findings though if you want to reach firefox developers, as the site here is mostly user to user support.

Thanks, here is corresponding bugzilla item just opened https://bugzilla.mozilla.org/show_bug.cgi?id=1362930