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Re: "i = i|0"

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Followups directed to: comp.lang.javascript

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[F'up2 comp.lang.javascript]

Stefan Ram wrote:

>   I have read this in a World-Wide Web encyclopedia:
> 
> For example, given the following C code:
> 
> int f(int i) {
>   return i + 1;
> }
> 
> Emscripten would output the following JS code:
> 
> function f(i) {
>   i = i|0;
>   return (i + 1)|0;
> }
> 
>   Do you think that the »|0« is necessary to express the
>   C semantics in JavaScript, or could the speed of the
>   generated code be improved by omitting it?

First of all, this does _not_ express the C semantics in JavaScript, and 
there is no other way.  “int” is a *generic* type in C/C++; IIUC, the result 
could be a 32-bit integer when compiled for a 32-bit platform or a 64-bit 
integer when compiled for a 64-bit platform.

<http://en.wikibooks.org/wiki/C_Programming/Reference_Tables#Table_of_Data_Types>
<http://stackoverflow.com/questions/11438794/is-the-size-of-c-int-2-bytes-or-4-bytes>

By contrast, using the binary bitwise OR operator, as with all ECMAScript 
binary bitwise operators, *always* creates an IEEE-754 double-precision 
*floating-point* value representing a *32-bit* integer value.  Also, not 
only the result will be such a value, but also the operands are converted
to such values internally, before the operation is performed.

<http://ecma-international.org/ecma-262/5.1/#sec-11.10>


Conversion to an integer value of the ECMAScript Number type (i.e., where 
the fractional part of the mantissa is zero), which appears to be the goal 
here, can be better achieved with the Math.floor() and Math.ceil() 
functions, e.g.:

  if (typeof Number.prototype.toInteger != "undefined")
  {
    Number.prototype.toInteger = function () {
      return (this < 0 ? Math.ceil(this) : Math.floor(this));
    };
  }

  /**
   * Frobnicates this value
   *
   * @return {int} i
   *   The value to be frobnicated
   * @return {int}
   *   The frobnicated value
   */
  function f (i)
  {
    return (+i).toInteger() + 42;
  }

[JSX:array.js features another converter, for 
jsx.array.BigArray.prototype.slice() & friends¹, that for practical reasons 
more closely matches the Specification (ToInt32), but does not convert to 
32-bit floating-point integer.]

Of course, this still does not remotely implement the C semantics.  One 
aspect of it is that code where you pass a non-integer would not compile.  
Since ECMAScript uses dynamic type-checking, it is not possible to prevent 
compilation.  But at the very least passing unsuitable values should cause 
an exception to be thrown, so that it becomes unnecessary to handle them, 
e.g.:

  function f (i)
  {
    if (i % 1 != 0)
    {
      /* JSX:object.js provides jsx.InvalidArgumentError instead */
      throw new TypeError('f: Invalid argument for "i": ' + i + ':'
        + typeof i + '[' + _getClass(i) + ']'
        + (i != null ? ' by ' + (_getFunctionName(i.constructor) || '?')
                     : '')
        + '; expected integer');
    }

    return i + 42;
  }

_________
¹ supports arrays with up to 2⁵³−1 numerically indexed elements²
² Because 2⁵³+1 is indistinguishable from 2⁵³ due to precision limits,
  so that element overflow could not be detected, I had to reduce
  jsx.array.BigArray.MAX_LENGTH to Math.pow(2, 53) - 1 recently.
  (This is also the reason why standard Array instances can hold only
  up to 2³²_−1_ elements, so that the largest possible index is
  Math.pow(2, 32) _- 2_.)
-- 
PointedEars
FAQ: <http://PointedEars.de/faq> | SVN: <http://PointedEars.de/wsvn/>
Twitter: @PointedEars2 | ES Matrix: <http://PointedEars.de/es-matrix>
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Re: "i = i|0" Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2014-06-12 00:20 +0200
  Re: "i = i|0" Noob <root@127.0.0.1> - 2014-06-12 10:43 +0200
  Re: "i = i|0" Thomas Richter <thor@math.tu-berlin.de> - 2014-06-12 19:52 +0200

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