What does “asm.js optimizations” mean?

https://blog.mozilla.org/luke/2015/02/18/microsoft-announces-asm-js-optimizations/

by ndesaulniers • 11 years ago

101 18 11 years ago

blog.mozilla.org

CJefferson4• 11 years ago

I am getting increasingly annoyed by the limitations of asm.js, in particular the "allocate all the memory you'll ever need up front", which requires compiling in your own malloc, and makes it hard not to massively waste memory by overallocating a huge buffer.

Therefore I greatly prefer optimisations which speed up 'asm.js-like code', rather than the all-or-nothing optimisation one gets from firefox.

rinon• 11 years ago

Coming from the compiler side of things, you're sacrificing in usability to make it possible for the JIT compiler to optimize the code further. If you allocate all memory ahead-of-time, the GC never needs to operate on that region, which gives you substantial performance improvements.

This is definitely a trade-off, but it's my understanding that asm.js makes more sense as a target for compilers to emit code for, rather than for hand-writing. Would you hand write an entire application in x86 assembly? Probably not...

CJefferson4• 11 years ago

Couldn't the JIT provide me a way of allocating new blocks of memory? and freeing blocks of memory back to the 'OS'? At least allocating new blocks?

I don't mind no GC, but having to allocate the maximum memory your program will use before it starts can be painful.

andhow• 11 years ago

asm.js now allows the heap to be resized (by replacing the heap's ArrayBuffer with a newer, bigger ArrayBuffer that was either copied or produced via ES7-proposed ArrayBuffer.transfer [1]). Heap resizing currently has to be enabled in Emscripten (by passing -s ALLOW_MEMORY_GROWTH=1), but may become the default in the future. The main asm.js spec page hasn't been updated yet, but the extension was discussed publicly with comments from Microsoft engineers [2].

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe... [2] http://discourse.specifiction.org/t/request-for-comments-swi...

CJefferson4• 11 years ago

That is nice to hear. I don't keep enough track to see that this was being discussed, but it does deal with the only major issue I had with C++ -> asm.js code conversion, particularly for mobile.

ndesaulniers2• 11 years ago

which requires compiling in your own malloc

dlmalloc is cross compiled for you. I might be mincing words, but you make it sound like you have to bring your own allocator. Not so.

makes it hard not to massively waste memory by overallocating a huge buffer.

Non issue. there are compiler flags `-s TOTAL_MEMORY=X` and `-s TOTAL_STACK=X`. Of course, you have to play around with these values otherwise you might not have enough space. I'm working on memory profiling tools to visualize the state of the heap in emscriptenized code [0] and getting them merged upstream. [1] The docs could be better on those flags.

Therefore I greatly prefer optimisations which speed up 'asm.js-like code', rather than the all-or-nothing optimisation one gets from firefox.

Why must they be mutually exclusive? It's a straw man to paint it like we can only have one or the other but not both, and therefor we must choose. All-or-nothing is also an incorrect categorization; Firefox's JIT makes the same optimizations for non ASM code after it warms up.

[0] https://github.com/nickdesaulniers/emscripten-memprof [1] https://github.com/kripken/emscripten/pull/3178

CJefferson4• 11 years ago

dlmalloc is cross compiled for you. I might be mincing words, but you make it sound like you have to bring your own allocator. Not so.

Wait, so I don't have to include an allocator in every asm.js project? How do I avoid doing that?

Non issue. there are compiler flags `-s TOTAL_MEMORY=X` and `-s TOTAL_STACK=X`. Of course, you have to play around with these values otherwise you might not have enough space.

How does that make my complaint a non-issue? I shouldn't have to, in this day and age, manually tune the maximum amount of memory my program will use up front. That what my OS is for, I ask for more memory when I want it. What happens when some user decides to use my app in a sensible, but more memory hungry way, and hits so arbitrary limit I had to set for mobile users?

Of course, someone else replied this second point is now invalid, as asm.js is gaining the ability to grow the heap.

jvilk6• 11 years ago

Are there any tutorials at all on how to use ASM.js's Foreign Function Interface (FFI)? The specification lacks examples, and seems to be very underspecified. :(

Note that the FFI is a crucial component of ASM.js, as everything in the browser is external to ASM.js (DOM/any strings/any objects/etc). It seems weird that there's so much hype around ASM.js, yet there's very little written about the FFI or how to use it. Any project that uses ASM.js is going to need to interact with it.

Also, are there any projects besides Emscripten that use ASM.js, or is the push to put ASM.js into these browsers due purely to Emscripten?

azakai3• 11 years ago

There is almost nothing to the FFI interface: Basically, the asm FFI can only have asm types, as you would expect. Then you can just call normal JS methods that were imported. So stuff like

    function asmFunc(import) {
      'use asm';

      var externFunc = import.externFunc;

      function asmFunc() {
        externFunc(123, -0.5);
      }

      return asmFunc;
    }

What extra info are you looking for?

jvilk6• 11 years ago

What extra info are you looking for?

Essentially, a complete executable example of a simple ASM.js program that involves a foreign function, including processing return values. (I imagine if it's this simple, then return values are handled using the same annotations as ASM.js functions?)

While I understand that specs don't necessarily have to have examples, I believe it would benefit the community to have one in the ASM.js spec.

Thanks for clarifying here, though! I now understand.

jvilk6• 11 years ago

Follow-up question.

How does it deduce the return type of a foreign function? Can foreign functions return values?

Normally, ASM.js functions require an annotation on the return statement [0].

[0] http://asmjs.org/spec/latest/#return-type-annotations

ndesaulniers2• 11 years ago

The specification lacks examples, and seems to be very underspecified.

See "Putting It All Together" http://asmjs.org/spec/latest/

From that example, the list of return types from operators (section 8), and the graph of value types (section 2.1), it is simple to figure out, if not a little foreign at first.

jvilk6• 11 years ago

The example doesn't use the FFI, though. And I understand the graph of value types and the restrictions that ASM.js imposes on ASM.js code, but there's no clear linkage between that and the FFI.

Some other FFIs require you to encode type information in some manner to uphold the guarantees of the language. It's not clear from scanning the specification if that is the case here, as the FFI is only briefly mentioned in passing.

Finally, ASM.js doesn't validate the return expression of external functions (it does for ASM.js code [0]). How does it deal with foreign functions that return values? What type does the return value take? Or does there need to be an annotation at the call site?

[0] http://asmjs.org/spec/latest/#return-type-annotations

CmonDev• 11 years ago

I hope this means that Microsoft has given up on pushing JavaScript to developers and we will get a proper C#/F#+XAML to whatever-legacy-highly-compatible-language (HTML, CSS, JS) toolchain.

What does “asm.js optimizations” mean?

Related Stories