One of the major challenges in the design of lightspark is the ActionScript execution engine. Most of the more recent flash content is almost completely build out of the ActionScript technology, which with version 3.0 matured enough to become a foundational block of the current, and probably future web. The same technology is going to become also widespread offline if the Adobe AIR platform succeedes as a cross platform application framework.
But what is ActionScript? Basically it is an almost ECMAscript complaiant language; the specification covers the language itself, a huge library of components and the bytecode format that is used to deliver code to the clients, usually as part of a SWF (flash) file.
The bytecode models a stack-machine as most of the arguments are passed on the stack and not as operands in the code. This operational description — although quite dense — requires a lot of stack traffic, even for simple computations. It should be noted that modern x86/amd64 processors employ specific stack tracing units to optimize out such traffic, but this is highly architecture dependent and not guaranteed.
LLVM (which stands fot Low-Level Virtual Machine) is on the other hand based on an Intermediate Language in SSA form. This means that each symbol can be assigned only one time. This form is extremely useful when doing optimization over the code. LLVM offers a nice interface for a bunch of feature, most notably sophisticated optimization of the code and Just-In-Time compilation to native assemply.
The challenge is: how to exploit llvm power to build a fast ActionScript engine.
The answer is, as usual, a matter of compromises. Quite a lot of common usage patterns of the stack-machine can be heavily optimized with limited work, for example most of the data pushed on the stack is going to be used right away! More details on this on the next issue...