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Re: Save and restore jslint settings

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  • aseem.kishore@ymail.com
    For what it s worth, some of the code I write does indeed run in a tight loop many times (striving for 60 frames/sec), and low-level optimizations have proven
    Message 1 of 13 , Sep 4, 2009
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      For what it's worth, some of the code I write does indeed run in a tight loop many times (striving for 60 frames/sec), and low-level optimizations have proven to help (especially in IE).

      For curiosity's sake, I did some testing between bit-shifting and regular math, and it seems bit-shifting is in fact faster for the cases I use.

      Using these two functions, with i going from 0 to 50000:

      function pow2BitShifting(i) {
      var x = 1 << (i % 30);
      }

      function pow2RegularMath(i) {
      var x = Math.pow(2, (i % 30));
      }

      In Firefox:

      Powers of 2 w/ bit shifting: 38 msecs
      Powers of 2 w/ regular math: 69 msecs

      In IE:

      Powers of 2 w/ bit shifting: 56 msecs
      Powers of 2 w/ regular math: 107 msecs

      In Chrome and Safari (nearly identical here):

      Powers of 2 w/ bit shifting: 2 msecs (!!)
      Powers of 2 w/ regular math: 16 msecs

      So anyway, it's clear that using bitwise operators has its place, so to get back to the original topic, it would be great if I could opt-in on a per-function basis. =)

      Aseem

      --- In jslint_com@yahoogroups.com, "christian.wirkus" <christian.wirkus@...> wrote:
      >
      > Sure. Write what is understood best. That's what code quality is about, I think.
      > I just wanted to mention that bitwise operators in Javascript are not as efficient as in other languages (because I read bitwise and optimized so close to one another). And often are used for gaining performance only.
      > And I thought of something like this:
      > var x = 2;
      > x * 2 === x << 1;
      >
      > If there is a context where bitwise is a clear and short solution to a problem, use it.
      >
      > Christian
      >
      >
      > --- In jslint_com@yahoogroups.com, Paul Novitski <paul@> wrote:
      > >
      > >
      > > >--- In jslint_com@yahoogroups.com, "christian.wirkus"
      > > ><christian.wirkus@> wrote:
      > > > > Are you sure bitwise operators optimize anything in Javascript?
      > > > > Javascript isn't C, and the data type number is 64 bit float, not integer.
      > >
      > > At 9/2/2009 10:24 AM, aseem.kishore@ wrote:
      > > ...
      > > >I'm also not sure about whether that indeed optimizes anything. I
      > > >haven't done profiling comparisons; I'm just reusing well-known algorithms.
      > >
      > >
      > >
      > > Christian, my guess is that the reasoning behind your comment is an
      > > assumption that Aseem would choose bit-manipulation primarily to
      > > optimize the code on a low level. From Aseem's reply that doesn't
      > > appear to be the case. He's using algorithms because they're
      > > dependable and do the job without regard for how they work on a low
      > > level, and that's interesting.
      > >
      > > I offer the opinion that, unless a particular language structure
      > > occurs in a loop that iterates an enormous number of times, low-level
      > > optimization is irrelevant compared to the importance of its
      > > usability in the high-level script.
      > >
      > > (Can we get away with calling usability high-level or human-level
      > > optimization?)
      > >
      > > If a bit of syntax is easy to use, read, proofread, and modify, then
      > > it wins. The whole purpose of high-level scripts such as JavaScript
      > > is their human benefit only; the computers don't care, they would run
      > > binary machine code much more quickly. But these days the chips are
      > > so bloody fast that it takes a lot of inefficiency to make their
      > > round trips perceptible to our own slow-poke bags of brain jelly.
      > >
      > > Regards,
      > >
      > > Paul
      > > __________________________
      > >
      > > Paul Novitski
      > > Juniper Webcraft Ltd.
      > > http://juniperwebcraft.com
      > >
      >
    • Stefan Weiss
      ... That s not really a fair comparison. The second function has to do a lookup for Math , find its pow property, and execute a method call to Math.pow. So
      Message 2 of 13 , Sep 4, 2009
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        On 04/09/09 18:10, aseem.kishore@... wrote:
        > For curiosity's sake, I did some testing between bit-shifting and
        > regular math, and it seems bit-shifting is in fact faster for the
        > cases I use.
        >
        > Using these two functions, with i going from 0 to 50000:
        >
        > function pow2BitShifting(i) {
        > var x = 1 << (i % 30);
        > }
        >
        > function pow2RegularMath(i) {
        > var x = Math.pow(2, (i % 30));
        > }

        That's not really a fair comparison. The second function has to do a
        lookup for 'Math', find its 'pow' property, and execute a method call to
        Math.pow.

        So yes, in this case, bit shifting is faster than a call to Math.pow
        with base 2, but that doesn't prove that the bit operations themselves
        are handled efficiently in JS.


        cheers,
        stefan
      • Aseem Kishore
        Fair enough. If you can think of a better way to test this, let me know. I m all for finding a more efficient alternative. =) Aseem ... [Non-text portions of
        Message 3 of 13 , Sep 4, 2009
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          Fair enough. If you can think of a better way to test this, let me know. I'm
          all for finding a more efficient alternative. =)
          Aseem

          On Fri, Sep 4, 2009 at 9:36 AM, Stefan Weiss <weiss@...> wrote:

          >
          >
          > On 04/09/09 18:10, aseem.kishore@... <aseem.kishore%40ymail.com>wrote:
          > > For curiosity's sake, I did some testing between bit-shifting and
          > > regular math, and it seems bit-shifting is in fact faster for the
          > > cases I use.
          > >
          > > Using these two functions, with i going from 0 to 50000:
          > >
          > > function pow2BitShifting(i) {
          > > var x = 1 << (i % 30);
          > > }
          > >
          > > function pow2RegularMath(i) {
          > > var x = Math.pow(2, (i % 30));
          > > }
          >
          > That's not really a fair comparison. The second function has to do a
          > lookup for 'Math', find its 'pow' property, and execute a method call to
          > Math.pow.
          >
          > So yes, in this case, bit shifting is faster than a call to Math.pow
          > with base 2, but that doesn't prove that the bit operations themselves
          > are handled efficiently in JS.
          >
          > cheers,
          > stefan
          >
          >


          [Non-text portions of this message have been removed]
        • aseem.kishore@ymail.com
          ... Fair point. Since that s my use case, it s a valid comparison for my scenario (even if I cache a local reference to Math.pow first, the numbers are almost
          Message 4 of 13 , Sep 4, 2009
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            --- In jslint_com@yahoogroups.com, Stefan Weiss <weiss@...> wrote:
            >
            > On 04/09/09 18:10, aseem.kishore@... wrote:
            > > For curiosity's sake, I did some testing between bit-shifting and
            > > regular math, and it seems bit-shifting is in fact faster for the
            > > cases I use.
            > >
            > > Using these two functions, with i going from 0 to 50000:
            > >
            > > function pow2BitShifting(i) {
            > > var x = 1 << (i % 30);
            > > }
            > >
            > > function pow2RegularMath(i) {
            > > var x = Math.pow(2, (i % 30));
            > > }
            >
            > That's not really a fair comparison. The second function has to do a
            > lookup for 'Math', find its 'pow' property, and execute a method call to
            > Math.pow.
            >
            > So yes, in this case, bit shifting is faster than a call to Math.pow
            > with base 2, but that doesn't prove that the bit operations themselves
            > are handled efficiently in JS.
            >
            >
            > cheers,
            > stefan
            >

            Fair point. Since that's my use case, it's a valid comparison for my scenario (even if I cache a local reference to Math.pow first, the numbers are almost identical, so apparently the bottleneck isn't the lookups).

            But to make a more fair comparison for the purposes of the underlying question, I changed the test to this:

            function times2BitShifting(i) {
            var x = i << 1;
            }

            function times2RegularMath(i) {
            var x = 2 * i;
            }

            Again, i goes from 0 to 50000. Across all browsers, the two take essentially the same amount of time. I sometimes just get a 1 ms difference in favor of bit-shifting.

            Doubling w/ bit-shifting: 31
            Doubling w/ regular math: 32

            So is this a fair enough comparison? Using bitwise operators seems to have no performance penalty, and in cases where it's actually useful (e.g. powers of 2), it's significantly [in a statistical sense] faster than regular math.

            Aseem
          • Arthur Blake
            One would think that modern JIT compiler implementations of JS would make this whole discussion moot... On Fri, Sep 4, 2009 at 4:01 PM,
            Message 5 of 13 , Sep 4, 2009
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              One would think that modern JIT compiler implementations of JS would
              make this whole discussion moot...

              On Fri, Sep 4, 2009 at 4:01 PM,
              aseem.kishore@...<aseem.kishore@...> wrote:
              >
              >
              > --- In jslint_com@yahoogroups.com, Stefan Weiss <weiss@...> wrote:
              >
              >>
              >> On 04/09/09 18:10, aseem.kishore@... wrote:
              >> > For curiosity's sake, I did some testing between bit-shifting and
              >> > regular math, and it seems bit-shifting is in fact faster for the
              >> > cases I use.
              >> >
              >> > Using these two functions, with i going from 0 to 50000:
              >> >
              >> > function pow2BitShifting(i) {
              >> > var x = 1 << (i % 30);
              >> > }
              >> >
              >> > function pow2RegularMath(i) {
              >> > var x = Math.pow(2, (i % 30));
              >> > }
              >>
              >> That's not really a fair comparison. The second function has to do a
              >> lookup for 'Math', find its 'pow' property, and execute a method call to
              >> Math.pow.
              >>
              >> So yes, in this case, bit shifting is faster than a call to Math.pow
              >> with base 2, but that doesn't prove that the bit operations themselves
              >> are handled efficiently in JS.
              >>
              >>
              >> cheers,
              >> stefan
              >>
              >
              > Fair point. Since that's my use case, it's a valid comparison for my
              > scenario (even if I cache a local reference to Math.pow first, the numbers
              > are almost identical, so apparently the bottleneck isn't the lookups).
              >
              > But to make a more fair comparison for the purposes of the underlying
              > question, I changed the test to this:
              >
              > function times2BitShifting(i) {
              > var x = i << 1;
              > }
              >
              > function times2RegularMath(i) {
              > var x = 2 * i;
              > }
              >
              > Again, i goes from 0 to 50000. Across all browsers, the two take essentially
              > the same amount of time. I sometimes just get a 1 ms difference in favor of
              > bit-shifting.
              >
              > Doubling w/ bit-shifting: 31
              > Doubling w/ regular math: 32
              >
              > So is this a fair enough comparison? Using bitwise operators seems to have
              > no performance penalty, and in cases where it's actually useful (e.g. powers
              > of 2), it's significantly [in a statistical sense] faster than regular math.
              >
              > Aseem
              >
              >
            • Stefan Weiss
              ... Much better, thank you. I didn t have the time to cook up a test myself earlier, and this pretty much confirms what I expected. ... I wasn t expecting to
              Message 6 of 13 , Sep 4, 2009
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                On 04/09/09 22:01, aseem.kishore@... wrote:
                > --- In jslint_com@yahoogroups.com, Stefan Weiss <weiss@...> wrote:
                >> That's not really a fair comparison. The second function has to do a
                >> lookup for 'Math', find its 'pow' property, and execute a method call to
                >> Math.pow.
                >>
                >> So yes, in this case, bit shifting is faster than a call to Math.pow
                >> with base 2, but that doesn't prove that the bit operations themselves
                >> are handled efficiently in JS.
                >
                > Fair point. Since that's my use case, it's a valid comparison for my
                > scenario (even if I cache a local reference to Math.pow first, the
                > numbers are almost identical, so apparently the bottleneck isn't the
                > lookups).
                >
                > But to make a more fair comparison for the purposes of the underlying
                > question, I changed the test to this:
                >
                > function times2BitShifting(i) {
                > var x = i << 1;
                > }
                >
                > function times2RegularMath(i) {
                > var x = 2 * i;
                > }
                >
                > Again, i goes from 0 to 50000. Across all browsers, the two take
                > essentially the same amount of time. I sometimes just get a 1 ms
                > difference in favor of bit-shifting.
                >
                > Doubling w/ bit-shifting: 31
                > Doubling w/ regular math: 32
                >
                > So is this a fair enough comparison?

                Much better, thank you. I didn't have the time to cook up a test myself
                earlier, and this pretty much confirms what I expected.

                > Using bitwise operators seems to have no performance penalty, and in
                > cases where it's actually useful (e.g. powers of 2), it's significantly
                > [in a statistical sense] faster than regular math.

                I wasn't expecting to see a performance penalty. My point is that
                JavaScript isn't C, and micro-optimizations using bit shifts won't bring
                any real advantage over normal arithmetic operations. (They're not very
                helpful in C anymore either, because modern compilers are all about
                optimization, and can do this sort of thing in their sleep.) It won't
                make much of a difference whether you write "i << 10" or "i * 1024".

                In your specific case, the performance gain is real, and apparently it's
                mostly due to avoiding the method call (unless you wrap the bit shifting
                it in a function, of course).

                When I'm not in a tight loop, I tend to go for the most readable
                implementation, even if it means using objects and properties instead of
                bit fields.


                cheers,
                stefan
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