cheald on Reporting a bug on a fragile analysis

Comment on Reporting a bug on a fragile analysis parent

I changed variable names and declaration order, the number of loops in that inner for loop, and other such things that could possibly change the bytecode (to what effect, I don't know - I'm not a JS VM engineer, obviously) without changing the operations actually performed.

I don't have an explanation for this, though (maybe variable initialization counts as "run up until here"?):

Runs fast (11ms):

  --- tests/sunspider-0.9.1/math-cordic.js        2010-11-17 00:55:29.000000000 -0700
  +++ tests/sunspider-0.9.1-deadcode/math-cordic.js       2010-11-17 01:42:43.000000000 -0700
  @@ -56,12 +56,14 @@
       var TargetAngle;
       var CurrAngle;
       var Step;
  +    var foo;

       X = FIXED(AG_CONST);         /* AG_CONST * cos(0) */
       Y = 0;                       /* AG_CONST * sin(0) */

       TargetAngle = FIXED(28.027);
       CurrAngle = 0;
  +    foo = 1;
       for (Step = 0; Step < 12; Step++) {
           var NewX;
           if (TargetAngle > CurrAngle) {

But if I assign foo after the for loop, it runs slow:

  --- tests/sunspider-0.9.1/math-cordic.js        2010-11-17 00:55:29.000000000 -0700
  +++ tests/sunspider-0.9.1-deadcode/math-cordic.js       2010-11-17 01:43:20.000000000 -0700
  @@ -56,6 +56,7 @@
       var TargetAngle;
       var CurrAngle;
       var Step;
  +    var foo;

       X = FIXED(AG_CONST);         /* AG_CONST * cos(0) */
       Y = 0;                       /* AG_CONST * sin(0) */
  @@ -76,6 +77,7 @@
               CurrAngle -= Angles[Step];
           }
       }
  +    foo = 1;
   }

If I assign foo inside the for loop, it runs fast:

  --- tests/sunspider-0.9.1/math-cordic.js        2010-11-17 00:55:29.000000000 -0700
  +++ tests/sunspider-0.9.1-deadcode/math-cordic.js       2010-11-17 01:44:41.000000000 -0700
  @@ -56,6 +56,7 @@
       var TargetAngle;
       var CurrAngle;
       var Step;
  +    var foo;

       X = FIXED(AG_CONST);         /* AG_CONST * cos(0) */
       Y = 0;                       /* AG_CONST * sin(0) */
  @@ -63,6 +64,7 @@
       TargetAngle = FIXED(28.027);
       CurrAngle = 0;
       for (Step = 0; Step < 12; Step++) {
  +        foo = 1;
           var NewX;
           if (TargetAngle > CurrAngle) {
               NewX = X - (Y >> Step);

Color me boggled.

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