Cheerp Blog

Cheerp 1.1 - C++ for the Web with fast startup times, dynamic memory and now, more speed!

Cheerp is a C++ compiler for the Web platform. Roughly a year ago we released Cheerp 1.0 with the promise of making C++ a first class language for the Web, with full access to DOM and HTML5 APIs (including WebGL) and great performance. At that time, we could only partially meet that promise.

With our early adopters starting to use Cheerp on real world, large scale applications, we were proud to see that Cheerp could indeed be used to seamlessly integrate C++ code into HTML5 apps. But we also realized that the performance of the compiled code was disappointing on real codebases.

As an example, our first benchmarking result on our first large-scale (~1M sloc) customer code was around forty times (40x) slower than native. Not only was this result disappointing, but it also was much worse than what we were expecting based on our internal benchmarks.

One year later, after significant effort focused on performance optimizations, we are here today to announce Cheerp 1.1.

Now we can fulfill our original promise.

Cheerp 1.1 includes a large number of optimizations allowing the output JavaScript code to clock in around 7 times slower than native on average, and something around 3x to 10x depending on your specific codebase.

This four- to six-fold improvement in speed has been validated not only on publicly available benchmarks, but also verified on real-world commercial code.

Cheerp 1.1 is now robust, battle-hardened and ready to be used on complex, large-scale codebases. Download it from our website

In this long post, we draw a renewed comparison with Emscripten, present recent performance benchmarks against native and analyze future steps for Cheerp.

How is Cheerp different from Emscripten?

Emscripten solves the problem of porting C++ to the Web by emulating the flat, byte addressable address space of traditional architectures (such as ARM and x86) using a large, monolithic, JavaScript TypedArray.

This approach makes it possible to port code with small changes and a high output performance. It also comes at a cost, as the typed array that stores all the data needs to be pre-allocated to its maximum size at the beginning of the program.

This means that the developer must be able to decide what the maximum memory size is at build time, with no possibility of scaling it with the capabilities of the users’ devices, and no dynamic memory allocation.

This is, in our opinion, a far from ideal approach, since it both creates a bad incentive for developers - preallocating a large amount of memory such as 512MB or 1GB is the easiest solution to this problem, and it basically prevents the coexistence of multiple applications created following this paradigm. This is especially true in web environments with bigger memory limitations such as mobile devices.

Last but not least, currently asm.js code limits by specification the size of typed arrays, and hence the maximum allocable memory for each application, to 2GB.

Cheerp, on the hand, solves the problem through the more sophisticated (but complex) approach of mapping C++ objects to JavaScript objects.

To make this possible, Cheerp requires the developer to apply stricter than usual type safety to C++, which results in some additional porting work.

But it turns out that most C++ is usually well behaved and type safe already, and on our customers codebases we have been able to fully port the code by changing around 0.1% of the original code, which we believe is a reasonable porting effort.

This is especially true since most of the changes tend to amount to a few repeated interventions on type safety and a limited amount of architectural changes required to bring an application to the Web, such as interfacing with DOM events and removing threading code. Most of these architectural changes would be needed anyway, as the web platform has some intrinsic differences which cannot be emulated (e.g. no real multi-threading).

As Cheerp uses JavaScript objects to store all program data the memory usage is fully flexible and will scale up or down depending on what the user is doing, with no intrinsic memory limitations. Moreover, this model is arguably more secure as using a flat array for all data brings a whole new class of application side vulnerabilities to the Web, such as stack and heap overflows.

With Cheerp, out of bounds accesses on an object will not give access to any other data structure. We believe that the security implications of Emscripten have not been properly understood and we intend to make a detailed post about this in the future.

On asm.js and startup time

We think that the role of asm.js is often misunderstood. The core idea of Asm.js is that you can improve the performance of JavaScript by making it closer to a typed language. This can be done by deciding a set of syntactical conventions that signals the type semantics to the browser.

Being fans of typed languages, we really like this idea.

What we don’t like is that currently, asm.js is strictly tied to the flat memory model of Emscripten and we can’t quite take advantage of it. We believe that it could be possible to extend asm.js specifications to support JavaScript objects or multiple heaps and we plan to propose possible extensions in the medium term.

One interesting side effect of using asm.js is that the browser needs to fully validate the entire code before being able to execute it, which introduces a large start time penalty (as analyzed by the Emscripten team here).

Code compiled by Cheerp can take full advantage of the lazy parsing supported by modern JavaScript engines.

As an example, we evaluated the startup time of a suite of C++ benchmarks, compiled using Cheerp, Emscripten with asm.js and Emscripten without asm.js. The chart below shows the time passed between the execution of the first line of the JavaScript script and the first line of main (i.e. the application itself).

It is very apparent that Cheerp code starts execution much faster than Emscripten+ams.js (approximately 80% faster) and also of Emscripten code compiled without asm.js (40% faster).

Performance, finally.

We have benchmarked Cheerp against Emscripten with Asm.js on a bunch of small scale micro-benchmarks and against two larger scale macro-benchmarks included in the emscripten distribution.

As expected, macro-benchmarks required some amount of porting. You can find a patch that can applied to the emscripten repository to reproduce the results here.

The figure below shows the performance of these benchmarks compiled with Cheerp 1.1 and Emscripten + asm.js, relative to native. The line at the top represents the approximate performance of our previous release of Cheerp - version 1.0 on real world codebases.

Methodological note: All times are normalized to native performance. For copy the native version is built using gcc instead of clang as the code generated by clang was very poor. Benchmarks were compiled using Emscripten v 1.32.0. Execution times were evaluated using the test runner from the emscripten project in a 64-bit linux environment. Spidermonkey as shipped with Firefox 38.0.1. V8 git commit cc2d376f11503646e278ebe1e65fd3a9fee7c91e.

The first thing that jumps out is the massive improvement we have achieved since Cheerp 1.0.

This was possible by developing a deep and thorough understanding of the internals of JavaScript engines, and implementing optimizations with that in mind. We will have some follow-up posts on this blog detailing some of the optimizations that we implemented.

Code compiled using Cheerp is always below or at the 10x slowdown compared to native, while the maximum slowdown under V8 is 6.4x. In many cases, compiled code runs below 5x native. Performances are overall very close to Emscripten on small tests (copy to skinning), and slower than Emscripten on larger scale tests (bullet and box2d).

The figure below focuses on the performance of Cheerp 1.1 compared to Emscripten on micro-benchmarks

On micro benchmarks Cheerp is between 1 and 5 times slower than native, and 1 to 2 times slower than Emscripten + Asm.js (sometimes faster, see memops under V8).

In general terms, we believe that there is small to no difference on pure computational code between Cheerp and Emscripten.

It is interesting to see that the best result comes sometimes from V8 and other times from Spidermonkey. Great food for thought for future discussions.

The figure below shows the performance results on two larger scale codebases, bullet and box2d.

Cheerp clearly has more overhead on more complex code which makes heavy use of memory, but it still somewhere around 3 to 5 times slower than Emscripten.

Moreover as you can see, in general, Cheerp performs better on Chrome (V8) than on Firefox (Spidermonkey). This is partially expected as we have implemented some optimizations which are specific for V8 architecture, but we have not yet started doing similar in depth studies on Spidermonkey.

On larger scale benchmarks making larger use of dynamic memory, Cheerp on V8 performs at less than 7x slowdown, without using static memory allocation and without using asm.js. We know that this is for the most part caused by code paths which are especially heavy on Cheerp, e.g. allocating large and complex objects inside frequently called functions.

It is important to note that these effects can be mitigated not only with better optimizations within Cheerp, but also by partially reworking the source code keeping in mind that the target architecture will be a Web browser.

Future steps

While we are happy with the results obtained in the last year, we now know for sure that we can and will do much better in future releases.

Thanks to months spent on optimizing the performance of generated code we now have a clear understanding of how we can reduce this performance gap even more and get closer to native performance.

We have now reached the point where we need to delve deep into the JavaScript engines implementation to find out how to generate better code, mostly by looking at the machine code generated by the engines. Which makes the task hard, but all the more interesting.

We also plan to enable at some point support for asm.js, even with its current highly limited specifications. We want to allow developers to selectively use asm.js on a type-by-type or method-by-method basis, deliberately accepting the advantages and limitations that this implies. We believe that asm.js is not needed on the whole program to achieve good performance (as this post by the Emscripten team suggests as well) and that a selective use is the best compromise.

From now on, we will also release more often so you can always access a better programming experience. Stay tuned.

Can I use it?

Cheerp is free and open source software, with some components release under the GPL license.

You are welcome to use it within the GPL restrictions.

We offer proprietary-friendly licensing as well, visit our website for details on our license model.

You can get Cheerp 1.1 release binaries on Launchpad. Take a look at our wiki for info and tutorials on Cheerp.

Cheerp is available for Linux, Windows and MacOSX.

About us

Leaning Technologies Ltd. is the startup company behind Cheerp. Beside our C++ compiler for the Web, we provide consulting services to help your business bringing your application to the Web. For more information, drop us a line at

You can follow us on twitter (@leaningtech) and visit our website to receive all our updates.

For additional technical information on Cheerp, please visit our wiki or our techblog.

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