LuaJIT Hacking: Getting next() out of the NYI list

At Cloudflare we’re heavy users of LuaJIT and in the past have sponsored many improvements to its performance.

LuaJIT is a powerful piece of software, maybe the highest performing JIT in the industry. But it’s not always easy to get the most out of it, and sometimes a small change in one part of your code can negatively impact other, already optimized, parts.

One of the first pieces of advice anyone receives when writing Lua code to run quickly using LuaJIT is “avoid the NYIs”: the language or library features that can’t be compiled because they’re NYI (not yet implemented). And that means they run in the interpreter.

CC BY-SA 2.0 image by Dwayne Bent

Another very attractive feature of LuaJIT is the FFI library, which allows Lua code to directly interface with C code and memory structures. The JIT compiler weaves these memory operations in line with the generated machine language, making it much more efficient than using the traditional Lua C API.

Unfortunately, if for any reason the Lua code using the FFI library has to run under the interpreter, it takes a very heavy performance hit. As it happens, under the interpreter the FFI is usually Continue reading