Technology Temple

We have seen a lot of hype on the Web recently after the announce­ment of Asm.js by Mozilla: a new “extra­or­di­nar­ily opti­miz­able, low-level sub­set of JavaScript”. The sys­tem builds on the work that has been done on Emscripten: a LLVM-based solu­tion which com­piles C++ to JavaScript, allow­ing for an easy port of appli­ca­tions and games to the Web. The excite­ment for Asm.js stems from the fact that, by using a spe­cial vir­tual machine inte­grated in Fire­fox, it can improve the per­for­mance of emscripten-generated code and get it even closer to native performance.

We (Lean­ing Tech­nolo­gies Ltd) would like to intro­duce Duetto, our own LLVM-based solu­tion for pro­gram­ming the Web using C++. And by the Web, we mean both the client and server side of it, but let’s talk about the client side first.

Emscripten han­dles C++ code by emu­lat­ing a full byte-addressable address space. This is def­i­nitely a good solu­tion, but sub­op­ti­mal. Javascript is not based on a byte-addressable address model, but on an object-addressable model: all the acces­si­ble mem­ory is con­tained in some object. But when you think about it, C++ is not that different.

Our solu­tion inte­grates with clang and the LLVM tool­chain and is able to map C++ object-oriented con­structs to native JavaScript objects. It turns out that access­ing objects on mod­ern JavaScript engines is faster than access­ing arrays. By using this (and a few more) tricks we man­aged to get the fol­low­ing, pre­lim­i­nary, results on micro benchmarks.

For each bench­mark the best time in 10 runs has been selected. The V8 and Spi­der­monky JavaScript shells has been used. The respec­tive com­mits are b13921fa78ce7d7a94ce74f6198db79e075a2e03 and b9d56a1e0a61. *The fasta bench­mark has been mod­i­fied by remov­ing a memory

We man­aged to do this by real­iz­ing that by dis­al­low­ing some unsafe C++ capa­bil­i­ties (such as type unsafe pointer cast­ing and pointer arith­metics inside struc­tures) it’s actu­ally pos­si­ble to cre­ate more effi­cient, smaller and faster JavaScript code from C++. Inter­est­ingly enough, we dis­cov­ered that, in most cases, the needed lim­i­ta­tions on the lan­guage are actu­ally spec­i­fied as unde­fined behav­iour!

So, yes, Duetto does need some min­i­mal port­ing to bring C++ code to the Web while emscripten makes it mostly free. What you get in exchange for that is faster per­for­mance with no need of a spe­cial VM and deep inte­gra­tion with the browser. Duetto cre­ates a really seam­less C++ pro­gram­ming expe­ri­ence for the Web:

• Seam­less inte­gra­tion with the browser envi­ron­ment, com­plete access to the DOM and HTML5 tech­nolo­gies includ­ing WebGL. You can even access and use your favourite JavaScript library or exist­ing JavaScript from C++ by declar­ing the avail­able inter­faces in the C++ code using a sim­ple convention.
• Seam­less client/server pro­gram­ming, using trans­par­ent RPCs in sin­gle code­base. The com­piler will split the code auto­mat­i­cally in the client part (com­piled to JavaScript) and server part (com­piled to native code).

The Duetto back­end is already in a very advanced state, and we believe it’s already suit­able to bring the first appli­ca­tions to the Web. Espe­cially games, which are our pri­mary tar­get. Unfor­tu­nately our front end is not yet as pol­ished as we would like, as we want to improve the error report­ing to make the port­ing expe­ri­ence as smooth as possible.

We are not yet ready to release Duetto, but we are eager to start open­ing col­lab­o­ra­tions, so if you are inter­ested in bring­ing your C++ appli­ca­tion or game to the web, feel free to con­tact me (alessandro@leaningtech.com). We believe that in six months or less from now we will be able to release a robust prod­uct, most prob­a­bly capa­ble of gen­er­at­ing even faster code. And we want to release it as open source.

For more infor­ma­tion please visit our site: http://www.leaningtech.com