WebKit Apocrypha

ARM-NEON is a SIMD (Single instruction, multiple data) instruction set for the latest ARM cores. The NEON instruction set focuses on integer (8, 16 and 32 bit) and single precision floating point arithmetic. It also has some unique features like using the same register array as the Vector Floating Point (VFP) unit which allows mixing SIMD and common floating point instructions.

When I tried to cross compile QtWebKit for ARM-Linux, I didn't find any clear description, so I have decided to write a blogpost about it. It's not too hard, but if you have never done something like this, it can be a little bit confusing. In this post, I will describe this method in some simple steps.

There are a lot of make specifications in Qt, but none of them is suitable for RVCT compilation. So, if you want to compile QtWebKit with RVCT, you have two options: either you create a new specification or you can use an existing one with wrapper scripts. In this post I'll describe the second option.

WebKitGtk+ cross compilation consists of two parts. First, you need a cross compiler and the library dependencies of WebKitGtk+. Next, you have to setup the build system properly. You can reach this with some wrapper scripts.

There was a post about Nitro Extreme some time ago on this site, so it is time to recollect what happened so far. Nitro Extreme is not a branch anyomore, it went to mainline, and replaced the old JavaScript Value (JSValue) representation on 32 bit machines. To be more precise, it happened only on x86.

If we would like to build WebKitGTK+ for ARM platform, Scratchbox could be a good choice. In this post I will show, how we can install and setup Scratchbox to build WebKitGTK+. First, we have to install the Scratchbox and its ARM target. Next, we install some libraries to WebKitGTK+.
Let's see how we can do that.

Dynamic languages like JavaScript have a lot of interesting fetures: we can create or destroy new classes during runtime or assign anything to any variable regardless of its type. These features makes them popular and since computers are getting faster, more and more tasks are performed by these languages. The fact that a language is dynamic does not necessarily mean that programs written in it have to be slow. Perhaps they will never be as fast as a compiled language, but there are some nice optimization algorithms for them. Those algorithms are different form static compiler optimizations.

Among other things, one interesting advantage of dynamically generated code is that constants can be embedded into the instruction stream. Think about it: there are several constants (usually pointers), which are unknown at compile time, but behave as constant once a value is assigned to them. Those constants can be embedded into the generated code, so several load-from-memory operations can be eliminated. WebKit JIT goes one step further: you can also rewrite constants which are not even known at JIT compilation time. Those constants typically hold cached values used by some fast cases.

Recently, a new branch has appeared in the WebKit trac, called Nitro-Extreme. I know the work has not been finished yet, but it never hurts to take a look at the current revision.

We have announced the ARM port of WebKit on webkit-dev mailing list in April. Additionally, a Bugzilla entry was opened for the technical discussion. After that we received several feedbacks which encouraged us to continue our work, but some feedbacks reported technical difficulties on getting and building our code. This post is going to summarize how to get and build the ARM port for your device.