I'm trying to better understand dart's effect on performance. On the dart website, their benchmarks show that Dart code compiled to Javascript is faster than just Javascript. How is this possible?
I understand how the Dart VM is faster than v8, but what I don't get is how dart2js-generated javascript is faster than plain old javascript, when both are running in the same environment, v8.
dart2js is able to perform optimizations that would not normally be manually added in JavaScript code.
There is nothing particularly special about Dart being the source language in this case: any automated tool that generates JavaScript should be able to do this, for instance the GWT compiler (Java to JavaScript) does this as well. Of course, you can run automated tools on JavaScript source to generate better JavaScript as well, this is what the Closure compiler does.
Technically, you can manually achieve the same speed with handwritten JavaScript if you know all the tricks.
One example is function inlining. If you need a code fragment called repeatedly you would refactor it out in a function/method. Dart2js often does the opposite. Method calls often get replaced with the code fragment contained by the called function/method which is called inlining. If you would do this manually this would lead to unmaintainable code.
I think many of the optimizations go in that direction.
Writing the code that way would just be unreadable and thus unmaintainable.
This doesn't mean it's sloppy.
There is a great presentation by Vyacheslav Egorov from the dart team where he explains in detail some of the optimizations including in lining..
http://www.infoq.com/presentations/dart-compiler
Summary Vyacheslav Egorov details how some of Dart's language features affected the design of a new JIT Dart compiler and how the V8
JavaScript engine influenced the overall design.
There is an interesting video by Seth Ladd and Kasper Lund.
Kasper is involved in creating the Dart2js compiler and gives some code examples on how the compiler optimizes the Javascript code.
Related
There is quite a bit of excitement on asm.js and how it will be able to run some very heavy applications. However, it is compiled from C++ code. Is it still possible to get the benefit of current improvements without knowing C++ or other low level languages?
Here is the thought that I had: Is it at all possible that we can write the code in Js, have it recompiled for asm.js for optimisation?
If you have small function that is very computation-heavy (crunching numbers rather than manipulating DOM) you could rewrite it in asm.js style yourself, manually. It's possible (I've done it), but tedious.
There are other asm.js compilers, e.g. LLJS that you may use instead of C++.
However, asm.js is not magic. You will only get performance benefit when you use language that is much better suited for ahead-of-time optimization than JS. You can't take fully-featured JS and make it faster by running JS VM on top of JS VM, just like you can't make ZIP files smaller by zipping them.
However, it is compiled from C++ code.
It is not. It's a language. Any program can emit text files which contain asm.js code. Emscripten compiles LLVM IR into asm.js, and there are compilers from C and C++ to LLVM IR, but this is only one possible way of getting asm.js code. Admittedly, it's currently the most mature, practical and popular way, but I would not be surprised at all if other asm.js compilers for other languages pop up some time in the future.
Is it still possible to get the benefit of current improvements without knowing C++ or other low level languages?
Well, in theory any language that can efficiently be compiled to machine code ahead-of-time can be implemented efficiently using asm.js, and that includes some rather high-level ones (e.g. Haskell).
But currently, nobody has a working implementation, and I don't expect this to ever become very popular. Right now, if you want asm.js performance, you'd probably write C or C++ code and compile it to asm.js, yes.
Note that the above excludes (among many others) Javascript.The fact that asm.js is a subset of Javascript is convenient in that asm.js code will run on unmodified browsers, but it's not of much use for anyone writing Javascript. asm.js is basically just a thin layer above machine code, with some amends for security and JS interoperability. Compiling JS to asm.js is as hard as compiling it to machine code: Easy if you don't give a damn about performance (just always used boxed dynamically-typed values like an interpreter, and emit calls to runtime library functions), very hard when you do.
In fact, after decades of research into the subject, there's still no example of a highly dynamic language like Javascript, Ruby or Python being compiled into machine code ahead of time and running much faster than a clever interpreter. Just-in-time compilation, on the other hand, is very much practical -- but the major JS engines already do that, in a less roundabout way than compiling to asm.js, then parsing it again and compiling it to machine code.
Asm.js isn't a separate language, but a subset of Javascript. It's just Javascript with a lot stripped out for performance. This means that you don't need to learn another language, though in this case knowing C/C++ might be useful to understand it.
Asm.js is a very strict subset of JavaScript, that can be generated relatively easily when compiling from C/C++ to JavaScript. Asm.js code is much closer to machine code than ordinary JavaScript code, which allowed browsers to heavily optimise for any code written in asm.js. In browsers that implemented those optimizations, your code will typically run about 50% of the speed of a C/C++ program that is compiled to machine code... which may seem slow, but is a hell of a lot faster than any ordinary JavaScript!
However, precisely because it's optimized for machines rather than humans, asm.js is practically impossible to handcode by any human developer... even though it's just JavaScript. While it is technically possible to convert - at least a subset of - ordinary JavaScript to an asm.js equivalent, such a conversion is not an easy task and I haven't encountered any project yet that made any attempt to achieve this.
Until someone achieves such a Herculean task, the best approach to producing asm.js code remains writing your code in C/C++ and converting it to JavaScript.
For more info on asm.js, see eg. John Resig's article from 2013 or the official specs.
Is it possible to create a simpler language by restricting the Javascript support in Google's V8? I'd like to embed the V8 engine in my own tool to run dynamic scripts, and like the idea of V8 precomiling the source for speed. However I need to drastically restrict what is possible within the language.
That means no dynamic allocation of data containers (e.g. arrays), no imported libraries, no recursion, no threads. It's more similar in philosophy to Renderman Shading Language than a general purpose language. The 'new' language is thus much simpler, and I'm only considering JS due to familiar syntax and the fact there's a good 'compiler' already (V8). I might also want it to run script code from within Chrome's native code (NaCl) environment, which Google seems to be working to support in V8.
How easy is it to redefine the JS 'grammar', or whatever other code define the language?
My other option is to create a new compiled language from scratch (maybe using LLVM stuff).
For all the features restriction you want, you would need to carry out a major surgery on V8 as V8 is never designed for such a radical modification.
An alternative solution is to invent a JavaScript-like language (with all the limitations you can impose) and compile it into normal JavaScript which then you can run with V8 (or any other JavaScript engine, for that matter). Well-known examples of such an approach are GWT (from Java), Dart, and TypeScript.
Take a closer look at squirrel language :
http://squirrel-lang.org
from description overview :
"both compiler and virtual machine fit together in about 7k lines of C++ code and add only around 100kb-150kb the executable size."
Enjoy!
I've read the article about Google's upcoming DASH/DART language, which I found quite interesting.
One thing I stumbled upon is that they say they will remove the inherent performance problems of JavaScript. But what are these performance problems exactly? There aren't any examples in the text. This is all it says:
Performance -- Dash is designed with performance characteristics in
mind, so that it is possible to create VMs that do not have the performance
problems that all EcmaScript VMs must have.
Do you have any ideas about what those inherent performance problems are?
This thread is a must read for anyone interested in dynamic language just in time compilers:
http://lambda-the-ultimate.org/node/3851
The participants of this thread are the creator of luajit, the pypy folks, Mozilla's javascript developers and many more.
Pay special attention to Mike Pall's comments (he is the creator of luajit) and his opinions about javascript and python in particular.
He says that language design affects performance. He gives importance to simplicity and orthogonality, while avoiding the crazy corner cases that plague javascript, for example.
Many different techiques and approaches are discussed there (tracing jits, method jits, interpreters, etc).
Check it out!
Luis
The article is referring to the optimization difficulties that come from extremely dynamic languages such as JavaScript, plus prototypal inheritance.
In languages such as Ruby or JavaScript, the program structure can change at runtime. Classes can get a new method, functions can be eval()'ed into existence, and more. This makes it harder for runtimes to optimize their code, because the structure is never guaranteed to be set.
Prototypal inheritance is harder to optimize than more traditional class-based languages. I suspect this is because there are many years of research and implementation experience for class-based VMs.
Interestingly, V8 (Chrome's JavaScript engine) uses hidden classes as part of its optimization strategy. Of course, JS doesn't have classes, so object layout is more complicated in V8.
Object layout in V8 requires a minimum of 3 words in the header. In contrast, the Dart VM requires just 1 word in the header. The size and structure of a Dart object is known at compile time. This is very useful for VM designers.
Another example: in Dart, there are real lists (aka arrays). You can have a fixed length list, which is easier to optimize than JavaScript's not-really-arrays and always variable lengths.
Read more about compiling Dart (and JavaScript) to efficient code with this presentation: http://www.dartlang.org/slides/2013/04/compiling-dart-to-efficient-machine-code.pdf
Another performance dimension is start-up time. As web apps get more complex, the number of lines of code goes up. The design of JavaScript makes it harder to optimize startup, because parsing and loading the code also executes the code. In Dart, the language has been carefully designed to make it quick to parse. Dart does not execute code as it loads and parses the files.
This also means Dart VMs can cache a binary representation of the parsed files (known as a snapshot) for even quicker startup.
One example is tail call elimination (I'm sure some consider it required for high-performance functional programming). A feature request was put in for Google's V8 Javascript VM, but this was the response:
Tail call elimination isn't compatible with JavaScript as it is used in the real
world.
By now, most mainstream browsers have started integrating optimizing JIT compilers to their JavaScript interpreters/virtual machines. That's good for everyone. Now, I'd be hard-pressed to know exactly which optimizations they do perform and how to best take advantage of them. What are references on optimizations in each of the major JavaScript engines?
Background:
I'm working on a compiler that generates JavaScript from a higher-level & safer language (shameless plug: it's called OPA and it's very cool) and, given the size of applications I'm generating, I'd like my JavaScript code to be as fast and as memory-efficient as possible. I can handle high-level optimizations, but I need to know more about which runtime transformations are performed, so as to know which low-level code will produce best results.
One example, from the top of my mind: the language I'm compiling will soon integrate support for laziness. Do JIT engines behave well with lazy function definitions?
This article series discusses the optimisations of V8. In summary:
It generates native machine code - not bytecode (V8 Design Elements)
Precise garbage collection (Wikipedia)
Inline caching of called methods (Wikipedia)
Storing class transition information so that objects with the same properties are grouped together (V8 Design Elements)
The first two points might not help you very much in this situation. The third might show insight into getting things cached together. The last might help you create objects with same properties so they use the same hidden classes.
This blog post discusses some of the optimisations of SquirrelFish Extreme:
Bytecode optimizations
Polymorphic inline cache (like V8)
Context threaded JIT (introduction of native machine code generation, like V8)
Regular expression JIT
TraceMonkey is optimised via tracing. I don't know much about it but it looks like it detects the type of a variable in some "hot code" (code run in loops often) and creates optimised code based on what the type of that variable is. If the type of the variable changes, it must recompile the code - based off of this, I'd say you should stay away from changing the type of a variable within a loop.
I found an additional resource:
What does V8 do with that loop?
Recently several tools have been released such as pyjamas and Scheme2js that allow one to take code in a language such as Python or Scheme and compile it to Javascript.
But how practical is such a tool?
I could see debugging being quite painful as you would have to debug the compiled javascript code itself, and correlate any bugs in that code with the corresponding lines in the original python/scheme/etc source code. Even with an intelligent stack trace such as the pyjamas -d option provides, this still seems tedious.
Also, libraries such as jQuery make writing Javascript much more fun and productive. But for many developers Javascript is still a new language to learn.
Has anyone worked with compiled Javascript in a production environment? Any recommendations or comments on the practicality of compiling to Javascript instead of writing your code directly in Javascript?
I believe GWT, based on Java, may be the most popular product of this kind, though I wouldn't describing it as "compiling Java to JS" but rather as "generating JS code". While I personally share some of your doubts, and would rather code JS directly, I have to admit that it is indeed an extremely practical as well as popular tool, entirely production-ready: I observe that, internally, many web apps that are rich and complex enough to warrant a front-end / back-end split are more and more often ending up as a Python back-end and a Java front-end -- the latter specifically to allow GWT (of course there are also plenty of Python front-ends, and plenty of Python back-ends, but I think this is a trend).
Google Wave uses GWT and is probably the most talked-about web app using it so far; together with the huge number of GWT-using web apps listed here, I think it establishes beyond any doubt that the approach is practical (as well as popular;-). Whether it's optimal (vs. writing actual javascript with a good framework in support) is a harder question to answer.
One of the more heavily used JavaScript compilers is GWT. This compiles Java to JavaScript, and is definitely used in production. The web interface to Google Wave is written in this system.
Also, Skydeck wrote Ocamljs, in order to make it easy for them to write FireFox extensions. That also worked quite well.
In summary, if you can write a good compiler, there is no showstoppers keeping you from writing a good JavaScript compiler.
Google Web Toolkit does it (Java to Javascript compiling), and GWT is used widely by Google (duh) and many others, so it definitely is practical.
Since the code is autogenerated, you debug problems in Java - assumption that the problem is in your code, and not in the compiler code, is true in 99% of all cases.
List of languages that compile to JS
As an another example Haxe could be mentioned. Haxe is a independent language and compiles to Flash 6-10, JavaScript, NekoVM and also to c++ - source code. Why is this practical?
you could use features the language itself could not offer
recompile code on multiple platforms (e.g.: form check in JavaScript and on server side)
there is a remoting package for communication between the platforms, and its genius.
autocompletion through the compiler
compile time type checks
If you are interested, you could start reading here.