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Re: Save and restore jslint settings

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  • pauanyu
    ... I had the same idea, but with different syntax: use strict ; var prop, foo; /*options forin: true */ for (prop in foo) { foo[prop] = foo ; } /*end
    Message 1 of 13 , Sep 2, 2009
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      --- In jslint_com@yahoogroups.com, "woomla" <woomla@...> wrote:
      >
      > Every know and then I want to change a /*jslint*/ setting for a particular statement. After the statement I want to restore the setting to its previous state. It would be nice if I can restore the setting without knowing its previous state/value (i.e. indent).
      >
      > A solution would be something to push the options and afterwards pop the options:
      >
      > "use strict";
      > var prop, foo;
      >
      > /*pushoptions*/
      > /*jslint forin:true*/
      > for (prop in foo) {
      > foo[prop] = "foo";
      > }
      > /*jslint forin:false*/
      > /*popoptions*/
      >
      >
      > I've implemented this in my pspad plugin and it works great. I would love to see it in the official script though.
      >

      I had the same idea, but with different syntax:

      "use strict";
      var prop, foo;

      /*options forin: true */
      for (prop in foo) {
      foo[prop] = "foo";
      }
      /*end options */
    • christian.wirkus
      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
      Message 2 of 13 , Sep 3, 2009
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        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
        >
      • 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 3 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 4 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 5 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 6 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 7 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 8 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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