LuaJIT as backend for Vult

I have been keeping an eye on LuaJIT (http://luajit.org) for some time. It’s a very interesting project and I have read very good things about it. Some time ago I made a small benchmark comparing an optimized algorithm written in C++11 against a lazy coded version in OCaml and LuaJIT. In case you are curious here are the results:

Language	Execution Time	Lines of Code
C++11	0.355 s	180
OCaml	0.54 s	63
LuaJIT	0.78 s	85

Here’s a chart that plots the lines of code against the execution time.

I have to remark again: The OCaml and Lua code were not written with optimization in mind. My intention was to see how fast they program could go without putting too much effort coding.

One can see that the C++ code is much larger. On the other hand, the OCaml code is very compact. The main reason was that in OCaml I used algebraic data types which neither Lua or C++ have (https://en.wikipedia.org/wiki/Algebraic_data_type). Therefore, all the complex data types needed to be implemented with classes or tables in the case of Lua.

Recently I implemented a prototype of code generation Vult -> Lua in order to check if LuaJIT could be used to create a better live coding environment. I took the JavaScript generator and with a few modifications I got Lua support.

To generate Lua code you need to call Vult as follows:

$ ./vultc -luacode code.vult -o code

In order to check the performance I took one of the examples I have in Vult and made a test rendering 1000 s of audio. Here are the results:

Language	Execution Time
C++11	2.77 s
LuaJIT	3.55 s

Here’s the graphic view:

The results are quite promising. The code was pure Lua with exception of the arrays which are implemented using the LuaJIT FFI as double[]. One important thing to notice is that the LuaJIT performs all operations in double precision, while the C/C++ uses float. I’m not sure if this has a big impact or not in modern processors.

As follow up in this topic, I’m planning to do more testing to see if I can generate code that is faster in LuaJIT taking advantage of the FFI features. I’m gonna try as well to find a nice way of integrating LuaJIT + Vult into a VST or a PD external.