What's the Fastest Way to Code a Loop in Javascript? | Hacker News Reader

What's the Fastest Way to Code a Loop in Javascript?

blogs.sun.com

22 pointsrbxbx15 comments

Given the amount of work that has gone into optimizing Javascript speed in browsers during the past two years, I'd say these results are completely obsolete.

Would be interesting to see some benchmarks against modern browsers, I don't have the bandwidth at the moment though.

So, anyone have results for browsers that were released within the past 2 years?

http://blogs.sun.com/greimer/resource/loop-test.html

Exceeds my wildest hopes and dreams. Thank you.

What surprises me is are some of the results like this:

  IE 7.0.5730.11 / WinXP SP2
  Native Array (length=1000, looped 100 times)

While loop in reverse, simplifying the test condition.

  var i = arr.length; while (i--) {}

Took 0ms to execute... very unexpected

It makes sense that a zero comparison is faster, but 0ms does seem strange. It's 31ms for a basic for loop on the same platform, whereas in Safari the times are 2ms and 5ms respectively, which seems a lot more reasonable.

Is it possible that IE 7 is deleting the whole loop as a no-op?

On IA-32, a decrementing-until-zero loop gets its own instruction (called—unsurprisingly—LOOP.) If it optimized in that one case, it would take microseconds to execute the whole test, which would show up as 0ms.

Isn't LOOP one of those microcoded instructions that actually runs slower on modern processors than a RISC-style separate decrement and compare?

It's a slow instruction, but it's a single instruction—in a loop with an empty body, this creates the best possible situation for memory locality: you just keep reading the same memory address/cache line over and over.

Some very interesting results -- I didn't expect the while loop in reverse to be so fast, and this is consistent across browsers.

The presentation needs work, though. Would some charts be so difficult to make? Much easier to compare results that way.

edit: note this post is from 2008.

I'd also like to see the same results after the code had been compiled with something like Google Closure.

well, i'd hope that in most cases closure would compile to var i = arr.length, since the body of the loop was nil.

I personally always (when applicable, of course) go with a reversed while-loop, because of its neat and tidy nature, and being "easy on the eye".

Using emacs :-)

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