Luau's Performance

Summary

One of main goals of Luau is to enable high performance code, with gameplay code being the main use case. This can be viewed as two separate goals: Make idiomatic code that wasn’t tuned faster Enable even higher performance through careful tuning Both of these goals are important - it’s insufficient to just focus on the highly tuned code, and all things being equal we prefer to raise all boats by implementing general optimizations. However, in some cases it’s important to be aware of optimizations that Luau does and doesn’t do. Worth noting is that Luau is focused on, first and foremost, stable high performance code in interpreted context. This is because JIT compilation is not available on many platforms Luau runs on, and AOT compilation would only work for code that Roblox ships (and even that does not always work). This is in stark contrast with LuaJIT that, while providing an excellent interpreter as well, focuses a lot of the attention on JIT (with many optimizations unavailable in the interpreter). Having said that, Luau has been updated to include an optional JIT component for x64 and arm64 platforms. This component can compile a selected set of functions, including limiting compilation to functions or modules marked explicitly by the user. While functions can be compiled at any time, automated JIT compilation decisions based on statistics/tracing are not performed. Luau JIT takes into account the type annotations present in the source code to specialize code paths and at this time, doesn’t include runtime analysis of the types/values flowing through the program. The rest of this document goes into some optimizations that Luau employs and how to best leverage them when writing code. The document is not complete - a lot of optimizations are transparent to the user and involve detailed low-level tuning of various parts that is not described here - and all of this is subject to change without notice, as it doesn’t affect the semantics of valid code. Fast bytecod...