javascript - Firefox does not seem to be faster using the asm.js profile, yet Chrome is - Stack Overflow

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I am trying to understand how exactly ASM works and when it kicks in.

I took a small function from the asm.js website. I wrap it using the module pattern: once for asm, once with the same syntax but without the "use asm" annotation, and once like vanilla-javascript.

 var add_asm = (function MyAOTMod(stdlib, foreign, heap) {
  "use asm";
  var sqrt = stdlib.Math.sqrt;

  function square(x) {
    x = +x;
    return +(x * x);
  }
  return function(x, y) {
    x = +x; // x has type double
    y = +y; // y has type double
    return +sqrt(square(x) + square(y));
  };
}(window));

var add_reg_asmstyle = (function MyAsmLikeRegularMod() {

  function square(x) {
    x = +x;
    return +(x * x);
  }
  return function(x, y) {
    x = +x; // x has type double
    y = +y; // y has type double
    return +Math.sqrt(square(x) + square(y));
  };
}());


var add_reg = (function MyStrictProfile() {
  "use strict";
  return function(x, y) {
    return Math.sqrt(x * x + y * y);
  };
}())

I created a small jsperf: the jsperf code is slightly different from the above, incorporating tips from the discussion thread below

The performance shows that firefox 22 is slowest with the asm-syntax (with or without the "use asm" annotation), and chrome is fastest in asm-mode.

So my question is: how is this possible? I would expect Firefox to be fastest in asm mode. I would not expect to see a difference for Chrome. Do I use a wrong asm syntax? What am I missing?

Any advice or clarification is greatly appreciated. Thanks,

I am trying to understand how exactly ASM works and when it kicks in.

I took a small function from the asm.js website. I wrap it using the module pattern: once for asm, once with the same syntax but without the "use asm" annotation, and once like vanilla-javascript.

 var add_asm = (function MyAOTMod(stdlib, foreign, heap) {
  "use asm";
  var sqrt = stdlib.Math.sqrt;

  function square(x) {
    x = +x;
    return +(x * x);
  }
  return function(x, y) {
    x = +x; // x has type double
    y = +y; // y has type double
    return +sqrt(square(x) + square(y));
  };
}(window));

var add_reg_asmstyle = (function MyAsmLikeRegularMod() {

  function square(x) {
    x = +x;
    return +(x * x);
  }
  return function(x, y) {
    x = +x; // x has type double
    y = +y; // y has type double
    return +Math.sqrt(square(x) + square(y));
  };
}());


var add_reg = (function MyStrictProfile() {
  "use strict";
  return function(x, y) {
    return Math.sqrt(x * x + y * y);
  };
}())

I created a small jsperf: the jsperf code is slightly different from the above, incorporating tips from the discussion thread below http://jsperf./asm-simple/7

The performance shows that firefox 22 is slowest with the asm-syntax (with or without the "use asm" annotation), and chrome is fastest in asm-mode.

So my question is: how is this possible? I would expect Firefox to be fastest in asm mode. I would not expect to see a difference for Chrome. Do I use a wrong asm syntax? What am I missing?

Any advice or clarification is greatly appreciated. Thanks,

• javascript
• performance
• google-chrome
• firefox
• asm.js

Share Follow edited Sep 3, 2013 at 19:55 c69 21.6k88 gold badges5555 silver badges8383 bronze badges asked Jul 30, 2013 at 15:38 ThomasThomas 39133 silver badges66 bronze badges 23

• 3 @Ian: OP is paring the ASM and non-ASM code in each browser to see the difference it makes. Chrome doesn't support ASM optimizations AFAIK, so the annotation shouldn't make a difference, while FireFox does support it. OP isn't paring FF to Chrome. – user2437417 Commented Jul 30, 2013 at 15:49
• 2 @Thomas: I'd be curious to know why you have small variations in each function. The first one caches Math.sqrt, the second one doesn't, and the third doesn't use a function for rounding, and doesn't use the unary + operator. I don't know that any of that would make a difference, but I'd think you'd want to keep it consistent. – user2437417 Commented Jul 30, 2013 at 15:53
• 6 You are not supposed to write ASM.js code by hand, because you will almost certainly be unable to (note: using "use asm" in non-ASM.js-pliant code, will fallback silently to regular JS execution). It is supposed to be a target for code generated by a piler like emscripten. – CAFxX Commented Jul 30, 2013 at 15:57
• 8 @CAFxX if it is just javascript, you should be able to write it by hand. Just like how you can (and people do) write x86 assembly directly. – SheetJS Commented Jul 30, 2013 at 16:12
• 1 Implementation details of browsers can have non-intuitive effects. – Peter Trypsteen Commented Sep 3, 2013 at 17:14

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1 Answer 1

Sorted by: Reset to default 8

When you run code in Firefox, you can often see huge drop in speed for asm.js calls, which is most probably caused either by repeated pilation (which is visible in console) or by cost of js-to-asm calls. This hypothesis is further strenghtened by Luke Wagner, implementor of asm.js:

one performance fault that we already know trips up people trying to benchmark asm.js is that calling from non-asm.js into asm.js and vice versa is much slower than normal calls due to general-purpose enter/exit routines. We plan to fix this in the next few months but, in the meantime, for benchmarking purposes, try to keep the whole putation happening inside a single asm.js module, not calling in and out.

To see it for yourself - look at the fiddle: http://jsperf./asm-simple/10

• Firefox 26: 22,600K ops/sec in asm-asm case vs 300(!) in asm-js case.
• Chrome 28: 18K vs 13K
• IE11: ~7.5K for all tests, no big difference observed, except for dead code ellimination, where it shines ;)

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